diff --git a/docs/agent-environments.md b/docs/agent-environments.md
index a8fb4dfd3d..2912c2c257 100644
--- a/docs/agent-environments.md
+++ b/docs/agent-environments.md
@@ -871,8 +871,9 @@ names a `document` view once, at its initial document's `path`. The `version`,
 and the `state_evidence_id` and `types` that `data/get` reports, must match
 the served state, so a `data/get` result loads back unchanged. A number without
 an exact native value, or whose serialized spelling would not be read as exact
-(an integer from 2^53 below 1e21, such as `1e20`, in any spelling), or a
-`__proto__` member is refused rather than altered.
+(an integer from 2^53 below 1e21, such as `1e20`, in any spelling), is refused
+rather than altered. Caller data preserves all own JSON members, including
+`__proto__`; unknown members of the state document's structure are refused.
 An invalid document is `-32602` naming the first problem by JSON pointer, as in
 `/rea_state/entities/3/collection` (also in `error.data.path`); a problem
 inside caller data (`fields`, `value`, `types`) is located at that member, so
diff --git a/docs/specs/agent-environments/state-serve/spec.md b/docs/specs/agent-environments/state-serve/spec.md
index 53f830ee69..3ebf83c42b 100644
--- a/docs/specs/agent-environments/state-serve/spec.md
+++ b/docs/specs/agent-environments/state-serve/spec.md
@@ -110,7 +110,9 @@ this increment: write tools are not served from the state.
   differs from the entity's id (refused at that view's `fields`),
   documents naming document views once at the initial path, and the optional
   `state_evidence_id` and `types` that `data/get` reports equal to the served
-  ones. Refuse numbers without an exact native value and `__proto__` members.
+  ones. Refuse numbers without an exact native value. Preserve all own JSON
+  members in caller data, including `__proto__`, and reject unknown structural
+  members before projecting the document's declared fields.
   Name the first problem by JSON pointer, never deeper than a caller-data
   member (`fields`, `value`, `types`), and leave the state unchanged.
 - Keep record citations Evidence provenance: refuse `record_ordinals` or a
diff --git a/docs/specs/agent-environments/state-view/spec.md b/docs/specs/agent-environments/state-view/spec.md
index c7a602e6da..5534f7c0df 100644
--- a/docs/specs/agent-environments/state-view/spec.md
+++ b/docs/specs/agent-environments/state-view/spec.md
@@ -133,8 +133,8 @@ later increments.
 - Keep an item or list plan only when it reproduces each request line's latest
   initial recording from the state, comparing JSON values without regard to
   member order; otherwise record the tool with a reason. Parse bodies only when
-  JSON-like, UTF-8, at most 256 KiB, exactly representable and free of
-  `__proto__` members.
+  JSON-like, UTF-8, at most 256 KiB and exactly representable. Preserve all own
+  JSON members, including `__proto__`, through extraction and normalization.
 - Mark the Evidence inferred; state unknowns for authorization, response
   semantics and write effects.
 - Charge the manifest's admission and grouping, every record line, every body
diff --git a/docs/specs/agent-environments/state-writes/spec.md b/docs/specs/agent-environments/state-writes/spec.md
index f45e8eb464..688ac66399 100644
--- a/docs/specs/agent-environments/state-writes/spec.md
+++ b/docs/specs/agent-environments/state-writes/spec.md
@@ -150,7 +150,7 @@ the exemplar.
 
 - **Request body.** The exemplar's request body must be a JSON object that
   could enter the state: a JSON-like media type, complete UTF-8 text within
-  256 KiB, exact numbers, no `__proto__` member. Otherwise the reason is
+  256 KiB and exact numbers. All own JSON member names are retained. Otherwise the reason is
   `body-unusable`. A `DELETE` takes no body. A create's or update's body
   whose only member wraps one object in a one-element array (Ghost's
   `{"posts":[{...}]}`) is that object, and the plan records the member as
@@ -587,7 +587,7 @@ changes it atomically, so every MCP session and `data/get` see one state.
 - a create in another collection when the plan is tied to its own;
 - a document write at a path other than the one the state holds it at;
 - a body member or a resulting value `data/add` would refuse (a number not
-  exact as `data/get` writes it, a `__proto__` member, nesting past the
+  exact as `data/get` writes it or nesting past the
   limit), increments included, so `data/get` always loads back;
 - a create, update or document write that would take what `data/get` reports
   past the 32 MiB `data/add` intake limit, as a `resource_constraint` error;
diff --git a/docs/web-network-captures.md b/docs/web-network-captures.md
index b309312e0c..125de760ad 100644
--- a/docs/web-network-captures.md
+++ b/docs/web-network-captures.md
@@ -696,8 +696,9 @@ more than one query) and `rendered-oversize` (a document value whose JSON
 renders to more than 256 KiB). A document holds the value of its one `path`, so for such
 a template it says nothing about the template's other paths. A body is usable
 when its content type is
-JSON-like, it is UTF-8 text of at most 256 KiB, every number survives native
-conversion exactly, and no member is named `__proto__`.
+JSON-like, it is UTF-8 text of at most 256 KiB, and every number survives native
+conversion exactly. JSON member names, including `__proto__`, remain ordinary
+own data members through extraction and serving.
 
 A list view also learns, from its checked lines that do not list the whole
 collection, what each query parameter does, and names the result in `query`.
diff --git a/src/application/environment/environmentArgumentMembers.test.ts b/src/application/environment/environmentArgumentMembers.test.ts
new file mode 100644
index 0000000000..20352ddcef
--- /dev/null
+++ b/src/application/environment/environmentArgumentMembers.test.ts
@@ -0,0 +1,116 @@
+import { expect, it } from "vitest";
+import { servedFrom } from "../../../tests/fixtures/environmentStateServing.js";
+import { matchProbeRequest } from "./environmentCompareMatch.js";
+import {
+  selectsRecordedLine,
+  selectReplayExchange,
+} from "./environmentReplay.js";
+import { callQuery } from "./environmentStateCallLines.js";
+import { pinnedRequest } from "./environmentStateWriteQuery.js";
+
+it.each(["__proto__", "constructor", "toString"])(
+  "preserves an own %s probe argument and treats omission as absent",
+  (name) => {
+    const { source } = servedFrom();
+    const original = source.manifest.tools[0]!;
+    const tool = {
+      ...original,
+      method: "GET",
+      path_template: "/api/books",
+      params: [],
+      query_parameter_names: [name],
+    };
+    const manifest = { ...source.manifest, tools: [tool] };
+    const call = matchProbeRequest(manifest, {
+      method: "GET",
+      path: "/api/books",
+      query: `?${name}=selected`,
+      body: undefined,
+    });
+    expect(call).toBeDefined();
+    expect(Object.hasOwn(call!.args, name)).toBe(true);
+    expect(call!.args[name]).toBe("selected");
+    expect([...callQuery(tool, {})]).toEqual([]);
+    expect([...callQuery(tool, call!.args)]).toEqual([[name, "selected"]]);
+    const exchange = {
+      ...source.exchanges.values().next().value!,
+      path: "/api/books",
+      query: `?${name}=selected`,
+    };
+    const exchanges = new Map([[exchange.ordinal, exchange]]);
+    expect(selectsRecordedLine(tool, exchange, {})).toBe(false);
+    expect(selectReplayExchange(tool, exchanges, call!.args)).toMatchObject({
+      exact: true,
+    });
+  },
+);
+
+it.each(["__proto__", "constructor"])(
+  "fills an omitted pinned %s argument as an own member",
+  (name) => {
+    const serving = servedFrom();
+    const tool = {
+      ...serving.source.manifest.tools[0]!,
+      params: [],
+      query_parameter_names: [name],
+    };
+    const exchange = {
+      ...serving.source.exchanges.values().next().value!,
+      query: `?${name}=selected`,
+    };
+    const source = {
+      ...serving.source,
+      exchanges: new Map([[exchange.ordinal, exchange]]),
+    };
+    const plan = {
+      ...serving.state.writes.values().next().value!,
+      exemplar_ordinal: exchange.ordinal,
+    };
+    for (const args of [{}, Object.fromEntries([[name, "selected"]])]) {
+      const result = pinnedRequest(
+        { source, store: serving.store },
+        { tool, plan, args },
+      );
+      expect(result.ok).toBe(true);
+      if (!result.ok) throw result.error;
+      expect(Object.hasOwn(result.value.args, name)).toBe(true);
+      expect(result.value.args[name]).toBe("selected");
+    }
+  },
+);
+
+it("keeps a prototype-named path argument and every own body member in a probe", () => {
+  const { source } = servedFrom();
+  const tool = {
+    ...source.manifest.tools[0]!,
+    method: "PATCH",
+    path_template: "/api/books/{__proto__}",
+    params: [
+      {
+        name: "__proto__",
+        index: 2,
+        rule: "numeric" as const,
+        basis: "inferred" as const,
+        observed_values: ["1"],
+      },
+    ],
+    query_parameter_names: [],
+  };
+  const body = JSON.parse(
+    '{"__proto__":{"selected":true},"constructor":"value"}',
+  );
+  const call = matchProbeRequest(
+    { ...source.manifest, tools: [tool] },
+    {
+      method: "PATCH",
+      path: "/api/books/3",
+      query: null,
+      body,
+    },
+  );
+  expect(call).toBeDefined();
+  expect(Object.hasOwn(call!.args, "__proto__")).toBe(true);
+  expect(call!.args["__proto__"]).toBe("3");
+  expect(call!.args.body).toBe(body);
+  expect(JSON.stringify(call!.args.body)).toBe(JSON.stringify(body));
+});
diff --git a/src/application/environment/environmentCompare.ts b/src/application/environment/environmentCompare.ts
index fac79aea58..1fa4eb3cf7 100644
--- a/src/application/environment/environmentCompare.ts
+++ b/src/application/environment/environmentCompare.ts
@@ -1,4 +1,5 @@
 import {
+  ownMember,
   decodedSegment,
   same,
 } from "../../domain/analysisView/environmentStateRules.js";
@@ -240,7 +241,7 @@ const answersCall = (
   const body = probeArguments(call.tool, run.bodies).body;
   // A tool with no body parameter matches; one that takes a body matches only the body this call sent, including none.
   if (body === undefined) return true;
-  return same(run.recordedBodies.get(exchange.ordinal), args[body]);
+  return same(run.recordedBodies.get(exchange.ordinal), ownMember(args, body));
 };
 
 /** A served tool's input check: the first problem with a call's arguments, or undefined. */
@@ -348,7 +349,7 @@ const compareStep = (
     });
   const held = heldIds(run.store);
   const unknownPath = call.pathArguments.some((name) => {
-    const value = call.args[name];
+    const value = ownMember(call.args, name);
     if (typeof value !== "string") return false;
     const id = decodedSegment(value);
     return !run.map.hasEnv(id) && !held.has(id);
diff --git a/src/application/environment/environmentCompareMatch.ts b/src/application/environment/environmentCompareMatch.ts
index 1d7704c76d..ec7e3ab5e6 100644
--- a/src/application/environment/environmentCompareMatch.ts
+++ b/src/application/environment/environmentCompareMatch.ts
@@ -215,12 +215,12 @@ export const matchProbeRequest = (
   if (best === undefined) return undefined;
   const { tool } = best;
   const names = probeArguments(tool, bodies);
-  const args: Record<string, JsonValue> = {};
+  const args = new Map<string, JsonValue>();
   const pathArguments: string[] = [];
   for (const param of tool.params) {
     const name = names.path.get(param.name) ?? param.name;
     // The environment decodes a path id once. Passing the raw segment keeps `book%2531` from becoming `book1`.
-    args[name] = segments[param.index]!;
+    args.set(name, segments[param.index]!);
     pathArguments.push(name);
   }
   const ignoredQuery: string[] = [];
@@ -238,12 +238,12 @@ export const matchProbeRequest = (
       }
     }
   for (const [name, values] of queryValues)
-    args[name] = values.length === 1 ? values[0]! : values;
+    args.set(name, values.length === 1 ? values[0]! : values);
   if (names.body !== undefined && request.body !== undefined)
-    args[names.body] = request.body;
+    args.set(names.body, request.body);
   return {
     tool,
-    args,
+    args: Object.fromEntries(args),
     pathArguments,
     pagingArguments,
     ignoredQuery,
diff --git a/src/application/environment/environmentReplay.ts b/src/application/environment/environmentReplay.ts
index c7210eb547..88f8d6ed1a 100644
--- a/src/application/environment/environmentReplay.ts
+++ b/src/application/environment/environmentReplay.ts
@@ -1,6 +1,9 @@
 import type { AnalysisError } from "../../domain/analysisErrorBase.js";
 import { AnalysisInputError } from "../../domain/analysisErrorCore.js";
-import { pathSegmentMatches } from "../../domain/analysisView/environmentStateRules.js";
+import {
+  ownMember,
+  pathSegmentMatches,
+} from "../../domain/analysisView/environmentStateRules.js";
 import type { JsonValue } from "../../domain/jsonValue.js";
 import {
   iterateUrlQueryParameters,
@@ -62,7 +65,7 @@ const scoreExchange = (
 ): MatchScore => {
   const providedPaths: { index: number; value: string }[] = [];
   for (const param of tool.params) {
-    const value = args[pathArguments.get(param.name) ?? param.name];
+    const value = ownMember(args, pathArguments.get(param.name) ?? param.name);
     if (typeof value === "string")
       providedPaths.push({ index: param.index, value });
   }
@@ -84,7 +87,7 @@ const scoreExchange = (
   }
   const supplied = new Map<string, readonly string[]>();
   for (const [name, argument] of queryArguments) {
-    const value = args[argument];
+    const value = ownMember(args, argument);
     if (typeof value === "string") supplied.set(name, [value]);
     else if (
       Array.isArray(value) &&
@@ -128,7 +131,8 @@ const scoreExchange = (
             const occurrence = observedOccurrences.get(name) ?? 0;
             observedOccurrences.set(name, occurrence + 1);
             const argument = queryArguments.get(name);
-            const value = argument === undefined ? undefined : args[argument];
+            const value =
+              argument === undefined ? undefined : ownMember(args, argument);
             const normalized =
               typeof value === "string"
                 ? occurrence === 0
@@ -201,7 +205,7 @@ export const selectReplayExchange = (
   if (best === undefined) return undefined;
   const suppliedQueryNames = new Set(
     [...argumentsMap.query].flatMap(([name, argument]) =>
-      args[argument] === undefined ? [] : [name],
+      ownMember(args, argument) === undefined ? [] : [name],
     ),
   );
   const queryWasSupplied = suppliedQueryNames.size > 0;
@@ -266,8 +270,10 @@ export const selectsRecordedLine = (
   if (
     !tool.params.every(
       (param) =>
-        typeof args[argumentsMap.path.get(param.name) ?? param.name] ===
-        "string",
+        typeof ownMember(
+          args,
+          argumentsMap.path.get(param.name) ?? param.name,
+        ) === "string",
     ) ||
     !scoreExchange(
       tool,
@@ -287,7 +293,7 @@ export const selectsRecordedLine = (
   }
   for (const [name, values] of recorded) {
     const argument = argumentsMap.query.get(name);
-    const supplied = args[argument ?? ""];
+    const supplied = ownMember(args, argument ?? "");
     if (
       argument === undefined ||
       (typeof supplied === "string"
diff --git a/src/application/environment/environmentState.test.ts b/src/application/environment/environmentState.test.ts
index 9f6ec8988e..bc58e3fb0e 100644
--- a/src/application/environment/environmentState.test.ts
+++ b/src/application/environment/environmentState.test.ts
@@ -29,7 +29,10 @@ import {
   loadEnvironmentState,
   prepareServedState,
 } from "./environmentState.js";
-import { STATE_INTAKE_LIMIT_BYTES } from "./environmentStateDocument.js";
+import {
+  STATE_INTAKE_LIMIT_BYTES,
+  parseStateDocument,
+} from "./environmentStateDocument.js";
 import {
   EnvironmentStateStore,
   ENVIRONMENT_COLLECTION_HISTORY_LIMIT_BYTES,
@@ -679,24 +682,20 @@ describe("rea-state/v1 validation", () => {
       path: "rea_state/types",
       message: "differs from the served types",
     });
-    const prototypeMember = (at: "entity" | "fields") =>
+    const prototypeMember = () =>
       firstProblem(
         JSON.parse(
           JSON.stringify(documentWith(() => undefined)).replace(
-            at === "entity" ? '"type":' : '"fields":{',
-            at === "entity"
-              ? '"__proto__":{"x":1},"type":'
-              : '"fields":{"__proto__":{"x":1},',
+            '"type":',
+            '"__proto__":{"x":1},"type":',
           ),
         ),
       );
-    expect(prototypeMember("entity")).toEqual({
+    expect(prototypeMember()).toEqual({
       path: "rea_state/entities/0",
       message: "has a member rea-state/v1 does not define",
     });
-    expect(prototypeMember("fields")).toMatchObject({
-      path: "rea_state/entities/0/fields",
-    });
+
     expect(firstProblem([])).toMatchObject({ path: "" });
     expect(
       firstProblem({ rea_state: { version: 2, entities: [], documents: [] } }),
@@ -736,3 +735,70 @@ describe("wide state value validation", () => {
     expect(retainedObject).toEqual(wideObject);
   });
 });
+
+it("retains arbitrary dictionary keys and owned JSON references at state intake", () => {
+  const dictionary = JSON.parse(
+    '{"__proto__":{"constructor":{"prototype":"kept"}},"constructor":7,"prototype":8}',
+  );
+  const document = documentWith((value) => {
+    const entity = value.rea_state.entities[0]!;
+    entity.fields = { ...(entity.fields as object), ...dictionary };
+  });
+  const entity = document.rea_state.entities[0]!;
+  const fields = entity.fields;
+  const owned = parseStateDocument(state, document, true);
+  expect(owned.ok).toBe(true);
+  if (!owned.ok) throw owned.error;
+  expect(owned.value.entities[0]?.fields).toBe(fields);
+  const copied = parseStateDocument(state, document);
+  expect(copied.ok).toBe(true);
+  if (!copied.ok) throw copied.error;
+  expect(copied.value.entities[0]?.fields).not.toBe(fields);
+  expect(copied.value.entities[0]?.fields).toEqual(fields);
+  expect(Object.hasOwn(copied.value.entities[0]!.fields, "__proto__")).toBe(
+    true,
+  );
+  const store = new EnvironmentStateStore(state);
+  expect(store.replace(document).ok).toBe(true);
+  const snapshot = JSON.stringify(store.snapshot());
+  expect(store.replace(JSON.parse(snapshot)).ok).toBe(true);
+  expect(JSON.stringify(store.snapshot())).toBe(snapshot);
+});
+
+it.each(["__proto__", "constructor", "prototype"])(
+  "refuses unknown structural member %s before projection",
+  (key) => {
+    const document = documentWith(() => undefined);
+    const structures = [
+      document,
+      document.rea_state,
+      document.rea_state.entities[0]!,
+    ];
+    const view = { tool: "fixture", fields: {} };
+    document.rea_state.entities[0]!.view_fields = [view];
+    const held = { tool: "fixture", path: "/fixture", value: {} };
+    document.rea_state.documents = [held];
+    structures.push(view, held);
+    for (const structure of structures) {
+      Object.defineProperty(structure, key, {
+        value: 1,
+        configurable: true,
+        enumerable: true,
+      });
+      expect(firstProblem(document).message).toBe(
+        "has a member rea-state/v1 does not define",
+      );
+      Reflect.deleteProperty(structure, key);
+    }
+  },
+);
+
+it("keeps missing fields distinct from invalid JSON field contents", () => {
+  const document = documentWith((value) => {
+    Reflect.deleteProperty(value.rea_state.entities[0]!, "fields");
+  });
+  expect(firstProblem(document)).toEqual({
+    path: "rea_state/entities/0/fields",
+    message: "is required",
+  });
+});
diff --git a/src/application/environment/environmentStateAnswer.ts b/src/application/environment/environmentStateAnswer.ts
index 27155071ca..4a4b329349 100644
--- a/src/application/environment/environmentStateAnswer.ts
+++ b/src/application/environment/environmentStateAnswer.ts
@@ -1,6 +1,7 @@
 import { hasRepeatedQueryArgument } from "./environmentTools.js";
 import { stateBodyJson } from "../../domain/analysisView/EnvironmentStateAccumulator.js";
 import {
+  ownMember,
   collectionMatch,
   collectionPathOf,
   decodedSegment,
@@ -439,7 +440,7 @@ const listSelection = (
   }
   const idField = store.type(view.type)?.id_field ?? "";
   const members = line.recorded.flatMap((item) => {
-    const id = scalarText(item[idField]);
+    const id = scalarText(ownMember(item, idField));
     const entity =
       id === undefined ? undefined : store.entity(view.type, collection, id);
     return entity === undefined ? [] : [entity];
diff --git a/src/application/environment/environmentStateBulkAnswer.ts b/src/application/environment/environmentStateBulkAnswer.ts
index 8ce6d25434..aa44a69358 100644
--- a/src/application/environment/environmentStateBulkAnswer.ts
+++ b/src/application/environment/environmentStateBulkAnswer.ts
@@ -9,9 +9,10 @@ import {
   writtenBulkResponse,
 } from "../../domain/analysisView/environmentStateWriteApply.js";
 import type { FieldRule } from "../../domain/analysisView/environmentStateWriteRules.js";
-import type {
-  JsonObject,
-  StateEntity,
+import {
+  ownMember,
+  type JsonObject,
+  type StateEntity,
 } from "../../domain/analysisView/environmentStateRules.js";
 import type { AnalysisError } from "../../domain/analysisErrorBase.js";
 import type { JsonValue } from "../../domain/jsonValue.js";
@@ -57,7 +58,8 @@ const namedTargets = (
   const { address } = plan;
   const type = plan.target ?? "";
   const name = environmentToolArguments(tool, true).body;
-  const body = name === undefined ? undefined : (args[name] as JsonValue);
+  const body =
+    name === undefined ? undefined : (ownMember(args, name) as JsonValue);
   if (body !== undefined && !isLoadableValue(body))
     return err(unloadable(tool, "The body"));
   const named =
diff --git a/src/application/environment/environmentStateCallLines.ts b/src/application/environment/environmentStateCallLines.ts
index b661031076..e6c93b4f14 100644
--- a/src/application/environment/environmentStateCallLines.ts
+++ b/src/application/environment/environmentStateCallLines.ts
@@ -1,4 +1,5 @@
 import {
+  ownMember,
   allLines,
   collectionPathOf,
   collectionMatch,
@@ -35,7 +36,7 @@ export const callSegments = (
       segments.push({ text, supplied: false });
       continue;
     }
-    const value = args[names.get(param.name) ?? param.name];
+    const value = ownMember(args, names.get(param.name) ?? param.name);
     if (typeof value !== "string") return undefined;
     segments.push({ text: value, supplied: true });
   }
@@ -74,7 +75,9 @@ export const unfilteredLine = (
   const segments = callSegments(tool, args);
   if (
     segments === undefined ||
-    [...query.values()].some((argument) => args[argument] !== undefined)
+    [...query.values()].some(
+      (argument) => ownMember(args, argument) !== undefined,
+    )
   )
     return undefined;
   const lines = new Map<number, RecordedExchange>();
@@ -98,7 +101,7 @@ export const callQuery = (
   new Map(
     [...environmentToolArguments(tool).query].flatMap(
       ([name, argument]): [string, string][] => {
-        const value = args[argument];
+        const value = ownMember(args, argument);
         return value === undefined ? [] : [[name, String(value)]];
       },
     ),
diff --git a/src/application/environment/environmentStateDocument.ts b/src/application/environment/environmentStateDocument.ts
index 985b138fa5..85febafb2e 100644
--- a/src/application/environment/environmentStateDocument.ts
+++ b/src/application/environment/environmentStateDocument.ts
@@ -20,6 +20,8 @@ import { canonicalJson } from "../../domain/comparisonSemantics.js";
 import { parseExactJson } from "../../domain/exactJsonNumber.js";
 import { projectInputIssues } from "../../domain/inputIssueProjection.js";
 import {
+  jsonObjectSchema,
+  ownedJsonObjectSchema,
   jsonValueSchema,
   jsonValueValidationIssue,
   ownedJsonValueSchema,
@@ -45,30 +47,58 @@ export interface StateContent {
   readonly documents: readonly HeldDocument[];
 }
 
-const stateDocumentSchemaFor = (jsonValue: z.ZodType<JsonValue, unknown>) => {
-  const fieldsSchema = z.record(z.string(), jsonValue);
+/** Check source-owned structural keys before Zod projects the declared fields. */
+const strictStateObject = <Shape extends z.ZodRawShape>(shape: Shape) => {
+  const schema = z.strictObject(shape);
+  return z
+    .unknown()
+    .superRefine((value, context) => {
+      if (typeof value !== "object" || value === null || Array.isArray(value))
+        return;
+      if (types.isProxy(value)) {
+        context.addIssue({
+          code: "custom",
+          message: "Expected an ordinary state object",
+        });
+        return;
+      }
+      const keys = Object.getOwnPropertyNames(value).filter(
+        (key) => !Object.hasOwn(schema.shape, key),
+      );
+      if (keys.length > 0)
+        context.addIssue({ code: "unrecognized_keys", keys });
+    })
+    .pipe(schema);
+};
+
+const stateDocumentSchemaFor = (
+  jsonValue: z.ZodType<JsonValue, unknown>,
+  fieldsSchema: z.ZodType<JsonObject, unknown>,
+) => {
   const ordinalSchema = z.number().int().nonnegative();
 
-  return z.strictObject({
-    rea_state: z.strictObject({
+  return strictStateObject({
+    rea_state: strictStateObject({
       version: z.literal(1),
       // `data/get` reports these; accepting them back lets its output be loaded unchanged.
       state_evidence_id: z.string().optional(),
       types: z.array(jsonValue).optional(),
       entities: z.array(
-        z.strictObject({
+        strictStateObject({
           type: z.string(),
           collection: z.string(),
           id: z.string(),
           fields: fieldsSchema,
           view_fields: z
-            .array(z.strictObject({ tool: z.string(), fields: fieldsSchema }))
+            .array(
+              strictStateObject({ tool: z.string(), fields: fieldsSchema }),
+            )
             .optional(),
           record_ordinals: z.array(ordinalSchema).optional(),
         }),
       ),
       documents: z.array(
-        z.strictObject({
+        strictStateObject({
           tool: z.string(),
           path: z.string(),
           value: jsonValue,
@@ -79,8 +109,14 @@ const stateDocumentSchemaFor = (jsonValue: z.ZodType<JsonValue, unknown>) => {
   });
 };
 
-const stateDocumentSchema = stateDocumentSchemaFor(jsonValueSchema);
-const ownedStateDocumentSchema = stateDocumentSchemaFor(ownedJsonValueSchema);
+const stateDocumentSchema = stateDocumentSchemaFor(
+  jsonValueSchema,
+  jsonObjectSchema,
+);
+const ownedStateDocumentSchema = stateDocumentSchemaFor(
+  ownedJsonValueSchema,
+  ownedJsonObjectSchema,
+);
 
 type ParsedDocument = z.output<typeof stateDocumentSchema>["rea_state"];
 type ParsedEntity = ParsedDocument["entities"][number];
@@ -116,99 +152,14 @@ const memberProblem = (path: Path): AnalysisInputIssue => {
       );
 };
 
-/** The path of the first `__proto__` member in document order; schema parsing would drop such a member rather than refuse it. */
-const prototypeMemberPath = (root: unknown): Path | undefined => {
-  interface Cursor {
-    readonly value: unknown;
-    readonly parent: Cursor | undefined;
-    readonly key: string | number;
-    readonly entries?: Generator<string>;
-    index: number;
-  }
-  const pathOf = (node: Cursor): (string | number)[] => {
-    const path: (string | number)[] = [];
-    for (let at: Cursor = node; at.parent !== undefined; at = at.parent)
-      path.push(at.key);
-    return path.reverse();
-  };
-  function* ownKeys(value: Record<string, unknown>): Generator<string> {
-    // `for...in` is lazy. The ownership check keeps inherited enumerable
-    // properties out, matching Object.entries without allocating all entries.
-    for (const key in value) if (Object.hasOwn(value, key)) yield key;
-  }
-  const cursor = (
-    value: unknown,
-    parent: Cursor | undefined,
-    key: string | number,
-  ): Cursor => ({
-    value,
-    parent,
-    key,
-    index: 0,
-    ...(!Array.isArray(value) && typeof value === "object" && value !== null
-      ? { entries: ownKeys(value as Record<string, unknown>) }
-      : {}),
-  });
-  const pending: Cursor[] = [cursor(root, undefined, "")];
-  while (pending.length > 0) {
-    const node = pending[pending.length - 1]!;
-    const { value } = node;
-    if (typeof value !== "object" || value === null) {
-      pending.pop();
-      continue;
-    }
-    // The canonical JSON validator reports proxies without invoking their traps.
-    if (types.isProxy(value)) {
-      pending.pop();
-      continue;
-    }
-    const array = Array.isArray(value);
-    if (!array && Object.hasOwn(value, "__proto__"))
-      return [...pathOf(node), "__proto__"];
-    if (array) {
-      if (node.index >= value.length) {
-        pending.pop();
-        continue;
-      }
-      const index = node.index++;
-      const descriptor = Object.getOwnPropertyDescriptor(value, index);
-      if (
-        descriptor?.enumerable !== true ||
-        !Object.hasOwn(descriptor, "value")
-      )
-        continue;
-      const child: unknown = descriptor.value;
-      if (typeof child === "object" && child !== null)
-        pending.push(cursor(child, node, index));
-      continue;
-    }
-    const next = node.entries?.next();
-    if (next === undefined || next.done) {
-      pending.pop();
-      continue;
-    }
-    const key = next.value;
-    const descriptor = Object.getOwnPropertyDescriptor(value, key);
-    if (descriptor?.enumerable !== true || !Object.hasOwn(descriptor, "value"))
-      continue;
-    const child: unknown = descriptor.value;
-    if (typeof child === "object" && child !== null)
-      pending.push(cursor(child, node, key));
-  }
-  return undefined;
-};
-
-/** Whether `data/add` accepts a value as a `fields` member or a document's `value`: bounded nesting, no `__proto__` member, and numbers exact as `data/get` writes them. */
+/** Whether `data/add` accepts a value as a `fields` member or a document's `value`: bounded nesting and numbers exact as `data/get` writes them. */
 export const isLoadableValue = (value: JsonValue): boolean =>
   jsonValueValidationIssue(value) === undefined &&
-  prototypeMemberPath(value) === undefined &&
   parseExactJson(JSON.stringify(value)) !== undefined;
 
-/** The first member of an object `data/add` would refuse as `fields`, if any: a `__proto__` member, or one whose value does not load. */
+/** The first member of an object `data/add` would refuse as `fields`, if any: one whose value does not load. */
 export const unloadableMember = (object: JsonObject): string | undefined =>
-  Object.keys(object).find(
-    (member) => member === "__proto__" || !isLoadableValue(object[member]!),
-  );
+  Object.keys(object).find((member) => !isLoadableValue(object[member]!));
 
 /** The first schema problem, located without naming the caller's data. */
 const schemaProblem = (
@@ -224,6 +175,15 @@ const schemaProblem = (
     first.reason === "unknown_argument"
   )
     return memberProblem(first.path);
+  if (
+    content >= 0 &&
+    first.path[content] === "fields" &&
+    first.reason === "invalid_value"
+  )
+    return problem(
+      first.path.slice(0, content + 1),
+      "holds a value that is not exactly representable JSON",
+    );
   if (first.reason === "missing_argument")
     return { ...first, message: "is required" };
   return {
@@ -443,19 +403,15 @@ export const parseStateDocument = (
   value: unknown,
   owned = false,
 ): Result<StateContent, AnalysisInputError> => {
-  const prototype = prototypeMemberPath(value);
   const parsed = (
     owned ? ownedStateDocumentSchema : stateDocumentSchema
   ).safeParse(value);
   const initial = new Map(
     state.entities.map((entity) => [entityKey(entity), entity]),
   );
-  const issue =
-    prototype !== undefined
-      ? memberProblem(prototype)
-      : parsed.success
-        ? contentProblem(state, parsed.data.rea_state, initial)
-        : schemaProblem(parsed.error.issues, value);
+  const issue = parsed.success
+    ? contentProblem(state, parsed.data.rea_state, initial)
+    : schemaProblem(parsed.error.issues, value);
   if (issue !== undefined || !parsed.success)
     return err(
       new AnalysisInputError(
diff --git a/src/application/environment/environmentStateWriteAnswer.ts b/src/application/environment/environmentStateWriteAnswer.ts
index 838e679590..0eb24a170a 100644
--- a/src/application/environment/environmentStateWriteAnswer.ts
+++ b/src/application/environment/environmentStateWriteAnswer.ts
@@ -38,6 +38,7 @@ import {
   spelledLike,
 } from "../../domain/analysisView/environmentStateWriteValues.js";
 import {
+  ownMember,
   collectionMatch,
   decodedSegment,
   isObject,
@@ -97,7 +98,7 @@ export const pathValues = (
     .split("/");
   const values = new Map<string, string>();
   for (const param of tool.params) {
-    const value = args[names.get(param.name) ?? param.name];
+    const value = ownMember(args, names.get(param.name) ?? param.name);
     const segment = template[param.index] ?? "";
     if (typeof value === "string") values.set(segment.slice(1, -1), value);
   }
@@ -121,7 +122,7 @@ const callBody = ({
   plan,
 }: WriteRequest): Result<JsonObject | undefined, AnalysisError> => {
   const name = environmentToolArguments(tool).body;
-  const sent = name === undefined ? undefined : args[name];
+  const sent = name === undefined ? undefined : ownMember(args, name);
   if (sent === undefined) return ok(undefined);
   const envelope = plan.body?.envelope ?? "";
   const value = unenveloped(sent as JsonValue, plan.body?.envelope);
@@ -240,7 +241,7 @@ export const generator = (
     make: (attempt: number) => JsonValue,
   ): Result<JsonValue, string> => {
     const held = new Set(
-      ofType.map((entity) => scalarText(entity.fields[field])),
+      ofType.map((entity) => scalarText(ownMember(entity.fields, field))),
     );
     for (let attempt = 0; attempt < GENERATION_ATTEMPTS; attempt += 1) {
       const value = make(attempt);
@@ -254,7 +255,7 @@ export const generator = (
     target: GeneratedTarget,
     rule: GeneratedRule,
   ): Result<JsonValue, string> => {
-    const exemplar = target.observed[rule.field] ?? "";
+    const exemplar = ownMember(target.observed, rule.field) ?? "";
     const seed = (attempt: number) =>
       `${store.state.evidenceId}:${request.tool.name}:${rule.field}:${String(write.count)}:${String(attempt)}`;
     switch (rule.rule) {
@@ -268,7 +269,7 @@ export const generator = (
           );
         return rule.rule === "nested-clock"
           ? nestedClockValue(
-              (target.current ?? target.observed)[rule.field],
+              ownMember(target.current ?? target.observed, rule.field),
               rule,
               value,
             )
@@ -299,7 +300,7 @@ export const generator = (
       case "renewed":
         return ok(
           renewUuids(
-            target.current?.[rule.field] ?? null,
+            ownMember(target.current, rule.field) ?? null,
             rule.pointers,
             renewed,
           ),
@@ -384,7 +385,7 @@ const pathArguments = (
   const names = environmentToolArguments(tool).path;
   return new Map(
     tool.params.flatMap((param) => {
-      const value = args[names.get(param.name) ?? param.name];
+      const value = ownMember(args, names.get(param.name) ?? param.name);
       return typeof value === "string" ? [[param.index, value] as const] : [];
     }),
   );
@@ -497,7 +498,7 @@ const answerCreate = (
   if (!fields.ok) return err(refused(tool, fields.error));
   const field = unloadableMember(fields.value);
   if (field !== undefined) return err(unloadable(tool, `Field ${field}`));
-  const id = scalarText(fields.value[idField]);
+  const id = scalarText(ownMember(fields.value, idField));
   if (
     id === undefined ||
     serving.store.entity(type, collection, id) !== undefined
diff --git a/src/application/environment/environmentStateWriteQuery.ts b/src/application/environment/environmentStateWriteQuery.ts
index 23e1925d3b..ede0857e4c 100644
--- a/src/application/environment/environmentStateWriteQuery.ts
+++ b/src/application/environment/environmentStateWriteQuery.ts
@@ -28,8 +28,10 @@ export const pinnedRequest = (
   );
   if (!query.ok) return err(refused(tool, `Tool ${tool.name} ${query.error}`));
   const names = environmentToolArguments(tool).query;
-  const filled = { ...args };
-  for (const [name, value] of query.value)
-    filled[names.get(name) ?? name] ??= value;
-  return ok({ ...request, args: filled });
+  const filled = new Map(Object.entries(args));
+  for (const [name, value] of query.value) {
+    const argument = names.get(name) ?? name;
+    if (filled.get(argument) == null) filled.set(argument, value);
+  }
+  return ok({ ...request, args: Object.fromEntries(filled) });
 };
diff --git a/src/application/environment/environmentStateWriteRefusals.test.ts b/src/application/environment/environmentStateWriteRefusals.test.ts
index c989c6777c..4c2d36f7b1 100644
--- a/src/application/environment/environmentStateWriteRefusals.test.ts
+++ b/src/application/environment/environmentStateWriteRefusals.test.ts
@@ -120,20 +120,27 @@ describe("served document writes", () => {
 });
 
 describe("values the state can load back", () => {
+  it("preserves nested prototype-named data through writes and reload", () => {
+    const settings = servedFrom(SETTINGS_ROWS);
+    const meta = JSON.parse(
+      '{"__proto__":{"n":1},"constructor":2,"prototype":3}',
+    );
+    expect(
+      settings.answer(PATCH_SETTINGS, { id: "1", body: { meta } }).body,
+    ).toEqual({ theme: "dark", meta });
+    expect(settings.answer(READ_SETTINGS, { id: "1" }).body).toEqual({
+      theme: "dark",
+      meta,
+    });
+    expectReloadsUnchanged(settings);
+  });
+
   it("refuses a body value data/add would refuse, whatever member carries it", () => {
     const settings = servedFrom(SETTINGS_ROWS);
     const unexact =
       "Member meta would hold a value that is not exactly representable JSON";
     expectRefusedUnchanged(settings, [
       [PATCH_SETTINGS, { id: "1", body: { meta: { n: 1e20 } } }, unexact],
-      [
-        PATCH_SETTINGS,
-        {
-          id: "1",
-          body: JSON.parse('{"meta":{"__proto__":{"n":1}}}') as JsonValue,
-        },
-        unexact,
-      ],
       [
         PATCH_SETTINGS,
         { id: "1", body: { meta: { n: nested(MAX_JSON_DEPTH) } } },
diff --git a/src/application/environment/environmentTools.ts b/src/application/environment/environmentTools.ts
index 1cda2c6e89..f334d9b503 100644
--- a/src/application/environment/environmentTools.ts
+++ b/src/application/environment/environmentTools.ts
@@ -1,4 +1,5 @@
 import {
+  ownMember,
   isObject,
   isQueryPost,
   listItems,
@@ -318,7 +319,7 @@ export const hasRepeatedQueryArgument = (
   args: Readonly<Record<string, unknown>>,
 ): boolean => {
   for (const argument of environmentToolArguments(tool).query.values()) {
-    const value = args[argument];
+    const value = ownMember(args, argument);
     if (Array.isArray(value) && value.length > 1) return true;
   }
   return false;
diff --git a/src/domain/analysisView/EnvironmentStateAccumulator.ts b/src/domain/analysisView/EnvironmentStateAccumulator.ts
index 98f3d56916..a8dd690bf8 100644
--- a/src/domain/analysisView/EnvironmentStateAccumulator.ts
+++ b/src/domain/analysisView/EnvironmentStateAccumulator.ts
@@ -22,7 +22,6 @@ import { EnvironmentStateExtraction } from "./EnvironmentStateExtraction.js";
 import type { TemplateRule } from "./environmentManifestRules.js";
 import {
   deeperThan,
-  hasPrototypeKey,
   isStateWrite,
   STATE_BODY_LIMIT_BYTES,
   type ParsedBody,
@@ -68,7 +67,7 @@ type StateFields = Pick<
  */
 const STATE_NESTING_DEPTH = 16;
 
-/** A recorded response body as state JSON: a JSON-like media type, complete UTF-8 text within the state body limit, exact numbers and no `__proto__` member; otherwise undefined. */
+/** A recorded response body as state JSON: a JSON-like media type, complete UTF-8 text within the state body limit, exact numbers; otherwise undefined. */
 export const stateBodyJson = (
   contentType: string | null,
   body: RecordedResponseBody,
@@ -84,7 +83,6 @@ export const stateBodyJson = (
     return undefined;
   const value = parseExactJson(body.bodyText);
   return value === undefined ||
-    hasPrototypeKey(value) ||
     deeperThan(value, MAX_JSON_DEPTH - STATE_NESTING_DEPTH)
     ? undefined
     : value;
diff --git a/src/domain/analysisView/EnvironmentStateExtraction.ts b/src/domain/analysisView/EnvironmentStateExtraction.ts
index c423cd5db1..18e7bbbd1e 100644
--- a/src/domain/analysisView/EnvironmentStateExtraction.ts
+++ b/src/domain/analysisView/EnvironmentStateExtraction.ts
@@ -5,10 +5,10 @@ import {
   compositeKey,
 } from "../unicodeCodePointOrder.js";
 import {
+  ownMember,
   apiBaseLengths,
   at,
   collectionPathOf,
-  hasPrototypeKey,
   identifies,
   IDENTITY_NAMES,
   identityCandidates,
@@ -675,7 +675,8 @@ export class EnvironmentStateExtraction {
     const field = this.#identity.get(plan.type)!.field;
     let changed = previous.ids.size !== objects.length;
     for (const object of objects)
-      if (!previous.ids.delete(scalarText(object[field])!)) changed = true;
+      if (!previous.ids.delete(scalarText(ownMember(object, field))!))
+        changed = true;
     if (previous.ids.size > 0) changed = true;
     if (
       earlier !== undefined &&
@@ -691,7 +692,8 @@ export class EnvironmentStateExtraction {
       });
     }
     previous.ids.clear();
-    for (const object of objects) previous.ids.add(scalarText(object[field])!);
+    for (const object of objects)
+      previous.ids.add(scalarText(ownMember(object, field))!);
     previous.record = record;
   }
 
@@ -703,7 +705,7 @@ export class EnvironmentStateExtraction {
     index: number,
   ): void {
     const field = this.#identity.get(plan.type)!.field;
-    const id = scalarText(object[field]);
+    const id = scalarText(ownMember(object, field));
     if (id === undefined) return;
     const collection = collectionPathOf(record, plan.item);
     const key = entityKey(plan.type, collection, id);
diff --git a/src/domain/analysisView/environmentStateGate.ts b/src/domain/analysisView/environmentStateGate.ts
index 53321d7464..f1209064ba 100644
--- a/src/domain/analysisView/environmentStateGate.ts
+++ b/src/domain/analysisView/environmentStateGate.ts
@@ -2,6 +2,7 @@ import { identifierWords } from "./environmentStateWords.js";
 import type { JsonValue } from "../jsonValue.js";
 import { compareUnicodeCodePoints as compare } from "../unicodeCodePointOrder.js";
 import {
+  ownMember,
   at,
   collectionPathOf,
   isObject,
@@ -246,7 +247,7 @@ const listQuery = (
         ordinal: record.ordinal,
         values,
         choices,
-        listed: items.map((item) => scalarText(item[field]) ?? ""),
+        listed: items.map((item) => scalarText(ownMember(item, field)) ?? ""),
         entities: context.inCollection(
           plan.type,
           collectionPathOf(record, false),
@@ -317,7 +318,7 @@ const listReproduces = (
       body === undefined ? undefined : listItems(body, plan.pointer);
     if (body === undefined || items === undefined) return false;
     const observed = new Map(
-      items.map((item) => [scalarText(item[field]) ?? "", item]),
+      items.map((item) => [scalarText(ownMember(item, field)) ?? "", item]),
     );
     const computed = view.query?.record_ordinals.includes(record.ordinal)
       ? evaluateQuery(
@@ -333,7 +334,7 @@ const listReproduces = (
           const entity = context.entity(
             plan.type,
             collection,
-            scalarText(item[field]) ?? "",
+            scalarText(ownMember(item, field)) ?? "",
           );
           return entity === undefined ? [] : [entity];
         }));
diff --git a/src/domain/analysisView/environmentStateListFacts.ts b/src/domain/analysisView/environmentStateListFacts.ts
index efbaf96824..ecd7bfeb84 100644
--- a/src/domain/analysisView/environmentStateListFacts.ts
+++ b/src/domain/analysisView/environmentStateListFacts.ts
@@ -8,6 +8,7 @@ import {
   mintedFor,
 } from "./environmentStateResponseMembers.js";
 import {
+  ownMember,
   collectionPathOf,
   isObject,
   listItems,
@@ -62,7 +63,7 @@ export const listFacts = (
         Object.keys(parent).some((name) => continues(name, parent[name]))
       )
         return;
-      const ids = items.map((item) => scalarText(item[field]) ?? "");
+      const ids = items.map((item) => scalarText(ownMember(item, field)) ?? "");
       const expected = context
         .inCollection(plan.type, collectionPathOf(record, false))
         .map((entity) => entity.id);
diff --git a/src/domain/analysisView/environmentStateListQuery.ts b/src/domain/analysisView/environmentStateListQuery.ts
index d7c0cbac87..387877a1a8 100644
--- a/src/domain/analysisView/environmentStateListQuery.ts
+++ b/src/domain/analysisView/environmentStateListQuery.ts
@@ -3,6 +3,7 @@ import { compareUnicodeCodePoints as compare } from "../unicodeCodePointOrder.js
 import { literalSubstringPredicate } from "../analysisSearch.js";
 import { directionOf, orderRole } from "./environmentStateOrderParameters.js";
 import {
+  ownMember,
   isObject,
   recordedItemCount,
   formatItemCount,
@@ -81,7 +82,8 @@ const textOf = (value: JsonValue | undefined): string | undefined =>
 /** Whether a field holds a value: equal as text, or in an array directly or as an object's id, slug or name. */
 const holds = (field: JsonValue | undefined, value: string): boolean => {
   const names = (item: JsonValue | undefined) =>
-    isObject(item) && IDENTIFYING.some((key) => textOf(item[key]) === value);
+    isObject(item) &&
+    IDENTIFYING.some((key) => textOf(ownMember(item, key)) === value);
   return Array.isArray(field)
     ? field.some((item) => textOf(item) === value || names(item))
     : textOf(field) === value || names(field);
@@ -95,7 +97,7 @@ const contains = (
   const matches = literalSubstringPredicate(term, false);
   return (entity) =>
     fields.some((field) => {
-      const value = entity.fields[field];
+      const value = ownMember(entity.fields, field);
       return typeof value === "string" && matches(value);
     });
 };
@@ -181,7 +183,7 @@ const readCall = (
     if (PAGING_ROLES.has(parameter.role)) taken.add(parameter.role);
     if (parameter.role === "filter")
       call.selections.push((entity) =>
-        holds(entity.fields[parameter.field], value),
+        holds(ownMember(entity.fields, parameter.field), value),
       );
     else if (parameter.role === "search") {
       if (!plainSearch(value)) return undefined;
@@ -234,8 +236,8 @@ const sorted = (
       (left, right) =>
         sign *
           ascending(
-            left.entity.fields[key.field],
-            right.entity.fields[key.field],
+            ownMember(left.entity.fields, key.field),
+            ownMember(right.entity.fields, key.field),
           ) || left.index - right.index,
     )
     .map(({ entity }) => entity);
@@ -245,7 +247,10 @@ const sorted = (
   ) =>
     left !== undefined &&
     right !== undefined &&
-    ascending(left.fields[key.field], right.fields[key.field]) === 0;
+    ascending(
+      ownMember(left.fields, key.field),
+      ownMember(right.fields, key.field),
+    ) === 0;
   return {
     entities: ordered,
     positional: ordered.filter(
@@ -331,7 +336,9 @@ const filterFields = (
   items.length === 0
     ? []
     : Object.keys(items[0]!)
-        .filter((field) => items.every((item) => holds(item[field], value)))
+        .filter((field) =>
+          items.every((item) => holds(ownMember(item, field), value)),
+        )
         .sort(
           (left, right) =>
             Number(normalized(right) === normalized(name)) -
@@ -428,7 +435,9 @@ const withoutIdleFilters = (
     Object.entries(chosen).map(([name, role]) => {
       const value = line.values.get(name)!;
       return role.role === "filter" &&
-        line.entities.every((entity) => holds(entity.fields[role.field], value))
+        line.entities.every((entity) =>
+          holds(ownMember(entity.fields, role.field), value),
+        )
         ? [name, { role: "inert", values: [value] }]
         : [name, role];
     }),
@@ -547,7 +556,9 @@ const trustedRoles = (
       if (role.role !== "filter" || namedLike(role.field, name)) return true;
       const values = new Set(
         explained
-          .filter((entry) => entry.parameters[name]?.role === "filter")
+          .filter(
+            (entry) => ownMember(entry.parameters, name)?.role === "filter",
+          )
           .map((entry) => entry.line.values.get(name)),
       );
       return values.size >= 2;
diff --git a/src/domain/analysisView/environmentStateRules.ts b/src/domain/analysisView/environmentStateRules.ts
index a4d1c0c342..e4746e88ad 100644
--- a/src/domain/analysisView/environmentStateRules.ts
+++ b/src/domain/analysisView/environmentStateRules.ts
@@ -33,7 +33,7 @@ export interface StateToolFacts {
   readonly record_ordinals: readonly number[];
 }
 
-/** A record's response body as JSON, or unusable when absent, cut, not JSON, not exactly representable or carrying a `__proto__` member. */
+/** A record's response body as JSON, or unusable when absent, cut, not JSON, not exactly representable. */
 export type ParsedBody =
   | { readonly kind: "json"; readonly value: JsonValue }
   | { readonly kind: "unusable" };
@@ -265,6 +265,29 @@ export const scalarText = (value: JsonValue | undefined): string | undefined =>
     : typeof value === "number"
       ? String(value)
       : undefined;
+/** A JSON dictionary lookup observes only its own members, preserving absent fields. */
+export const ownMember = <Value>(
+  object: Readonly<Record<string, Value>> | undefined,
+  member: string,
+): Value | undefined =>
+  object !== undefined && Object.hasOwn(object, member)
+    ? object[member]
+    : undefined;
+
+/** Write one JSON member without interpreting its name as an inherited setter. */
+export const setOwnMember = (
+  object: JsonObject,
+  member: string,
+  value: JsonValue,
+): void => {
+  Object.defineProperty(object, member, {
+    value,
+    enumerable: true,
+    configurable: true,
+    writable: true,
+  });
+};
+
 /** Equal JSON values; object member order carries no meaning. */
 export const same = (
   left: JsonValue | undefined,
@@ -463,15 +486,6 @@ export const deeperThan = (value: JsonValue, limit: number): boolean => {
   return false;
 };
 
-/** JSON parsers keep a `__proto__` member that plain object construction would turn into a prototype. */
-export const hasPrototypeKey = (value: JsonValue): boolean =>
-  Array.isArray(value)
-    ? value.some(hasPrototypeKey)
-    : isObject(value) &&
-      Object.entries(value).some(
-        ([name, item]) => name === "__proto__" || hasPrototypeKey(item),
-      );
-
 export const pointerTo = (name: string): string =>
   `/${name.replaceAll("~", "~0").replaceAll("/", "~1")}`;
 /** The unescaped reference tokens of a pointer. */
@@ -487,7 +501,7 @@ const memberOf = (
   body: JsonValue | undefined,
   name: string,
 ): JsonValue | undefined =>
-  isObject(body) && Object.hasOwn(body, name) ? body[name] : undefined;
+  isObject(body) ? ownMember(body, name) : undefined;
 
 /** The element of a one-element array. */
 const soleElement = (value: JsonValue | undefined): JsonValue | undefined =>
@@ -570,7 +584,7 @@ export const identifies = (
   // Stop at the first missing or repeated value, so a sparse list costs one read per field.
   const seen = new Set<string>();
   for (const item of items) {
-    const value = scalarText(item[name]);
+    const value = scalarText(ownMember(item, name));
     if (value === undefined || seen.has(value)) return false;
     seen.add(value);
   }
@@ -603,7 +617,9 @@ export const locateList = (
   if (Array.isArray(body)) return body.every(isObject) ? "" : undefined;
   if (
     !isObject(body) ||
-    IDENTITY_NAMES.some((name) => scalarText(body[name]) !== undefined)
+    IDENTITY_NAMES.some(
+      (name) => scalarText(ownMember(body, name)) !== undefined,
+    )
   )
     return undefined;
   const arrays = Object.entries(body).filter(
diff --git a/src/domain/analysisView/environmentStateView.test.ts b/src/domain/analysisView/environmentStateView.test.ts
index 970d429001..a10be1cc3c 100644
--- a/src/domain/analysisView/environmentStateView.test.ts
+++ b/src/domain/analysisView/environmentStateView.test.ts
@@ -1790,3 +1790,32 @@ it("validates every own render-metadata dictionary entry through the public sche
   expect(parse({ kind: "epoch", unit: "invalid" }, ["id"]).success).toBe(false);
   expect(parse(clock, [1]).success).toBe(false);
 });
+
+it("preserves own prototype-named entity and document fields through capture projection and public normalization", () => {
+  const first = JSON.parse(
+    '{"id":1,"__proto__":{"label":"one"},"constructor":"first","meta":{"__proto__":"nested"}}',
+  ) as JsonObject;
+  const second = JSON.parse(
+    '{"id":2,"__proto__":{"label":"two"},"constructor":"second","meta":{"__proto__":"nested"}}',
+  ) as JsonObject;
+  const document = JSON.parse('{"__proto__":{"theme":"dark"}}') as JsonObject;
+  const projected = stateOf(
+    captureOf([
+      ["GET", "/api/items", [first, second]],
+      ["GET", "/api/items/1", first],
+      ["GET", "/api/settings", document],
+    ]),
+  );
+  const state = analysisViewResultSchema.parse(
+    JSON.parse(JSON.stringify(projected)),
+  );
+  if (state.kind !== "environment-state") throw new Error("Expected state");
+  const entity = state.entities.find((entry) => entry.id === "1")!;
+  expect(JSON.stringify(entity.fields)).toBe(JSON.stringify(first));
+  expect(
+    Object.getOwnPropertyDescriptor(entity.fields, "__proto__")?.value,
+  ).toEqual({ label: "one" });
+  expect(state.documents).toContainEqual(
+    expect.objectContaining({ value: document }),
+  );
+});
diff --git a/src/domain/analysisView/environmentStateView.ts b/src/domain/analysisView/environmentStateView.ts
index cf1b4e09ba..cfb4a95cbf 100644
--- a/src/domain/analysisView/environmentStateView.ts
+++ b/src/domain/analysisView/environmentStateView.ts
@@ -1,6 +1,6 @@
 import { z } from "zod";
 import type { AnalysisError } from "../analysisErrorBase.js";
-import { jsonValueSchema } from "../jsonValue.js";
+import { jsonObjectSchema, jsonValueSchema } from "../jsonValue.js";
 import { jsonRecordSchema } from "../jsonRecord.js";
 import { ok, type Result } from "../result.js";
 import type {
@@ -23,7 +23,6 @@ export const environmentStateViewRequestSchema = z.strictObject({
   ),
 });
 
-const jsonObjectFieldsSchema = z.record(z.string(), jsonValueSchema);
 const queryParameterSchema = z.discriminatedUnion("role", [
   z.strictObject({
     role: z.literal("filter"),
@@ -471,9 +470,9 @@ export const environmentStateFields = {
         type: z.string(),
         collection: z.string(),
         id: z.string(),
-        fields: jsonObjectFieldsSchema,
+        fields: jsonObjectSchema,
         view_fields: z.array(
-          z.strictObject({ tool: z.string(), fields: jsonObjectFieldsSchema }),
+          z.strictObject({ tool: z.string(), fields: jsonObjectSchema }),
         ),
         record_ordinals: z.array(z.number().int().nonnegative()),
       }),
diff --git a/src/domain/analysisView/environmentStateViewSpellings.ts b/src/domain/analysisView/environmentStateViewSpellings.ts
index 488dc216c8..3c85ea01f0 100644
--- a/src/domain/analysisView/environmentStateViewSpellings.ts
+++ b/src/domain/analysisView/environmentStateViewSpellings.ts
@@ -1,5 +1,10 @@
 import type { JsonValue } from "../jsonValue.js";
-import { isObject, same, type JsonObject } from "./environmentStateRules.js";
+import {
+  ownMember,
+  isObject,
+  same,
+  type JsonObject,
+} from "./environmentStateRules.js";
 import { readClock, type ClockFormat } from "./environmentStateWriteValues.js";
 import { ENTRY_BYTES, textBytes } from "./webNetworkRouteTemplates.js";
 
@@ -82,7 +87,7 @@ export class ClockSpellings {
 
   note(stored: JsonObject, shown: JsonObject): void {
     for (const [field, value] of Object.entries(shown)) {
-      if (isObject(value)) this.#shape(field, value, stored[field]);
+      if (isObject(value)) this.#shape(field, value, ownMember(stored, field));
       const now = readClock(value);
       if (now === undefined) continue;
       const was = Object.hasOwn(stored, field)
diff --git a/src/domain/analysisView/environmentStateWriteAddress.ts b/src/domain/analysisView/environmentStateWriteAddress.ts
index ae5a98cff5..e50fa830fe 100644
--- a/src/domain/analysisView/environmentStateWriteAddress.ts
+++ b/src/domain/analysisView/environmentStateWriteAddress.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   at,
   requestJson,
   isObject,
@@ -156,7 +157,8 @@ export const addressOf = (
     )
       return { by: "ids", member, kind };
     return value.every(
-      (entry) => isObject(entry) && scalarText(entry[idField]) !== undefined,
+      (entry) =>
+        isObject(entry) && scalarText(ownMember(entry, idField)) !== undefined,
     )
       ? { by: "elements", member }
       : undefined;
@@ -235,7 +237,9 @@ export const namedEntities = (
   )
     return undefined;
   const entries = (
-    address.member === null ? body : (body as JsonObject)[address.member]
+    address.member === null
+      ? body
+      : ownMember(body as JsonObject, address.member)
   ) as JsonValue[];
   const shared =
     address.member === null || !isObject(body)
@@ -246,7 +250,10 @@ export const namedEntities = (
   const named = entries.map((entry) =>
     address.by === "ids"
       ? { id: scalarText(entry)!, body: shared }
-      : { id: scalarText((entry as JsonObject)[idField])!, body: entry },
+      : {
+          id: scalarText(ownMember(entry as JsonObject, idField))!,
+          body: entry,
+        },
   ) as NamedEntity[];
   return new Set(named.map(({ id }) => id)).size === named.length
     ? named
diff --git a/src/domain/analysisView/environmentStateWriteAdoption.ts b/src/domain/analysisView/environmentStateWriteAdoption.ts
index 93c52ac5d5..9272e2165d 100644
--- a/src/domain/analysisView/environmentStateWriteAdoption.ts
+++ b/src/domain/analysisView/environmentStateWriteAdoption.ts
@@ -1,5 +1,6 @@
 import { ENTRY_BYTES, textBytes } from "./webNetworkRouteTemplates.js";
 import {
+  ownMember,
   at,
   isObject,
   stateValueRetentionBytes,
@@ -88,7 +89,7 @@ export class UnseenEntities {
     if (
       !isObject(object) ||
       idField === undefined ||
-      scalarText(object[idField]) !== id
+      scalarText(ownMember(object, idField)) !== id
     )
       return undefined;
     const entity = {
diff --git a/src/domain/analysisView/environmentStateWriteApply.test.ts b/src/domain/analysisView/environmentStateWriteApply.test.ts
index bce091a9c2..d8d65d7149 100644
--- a/src/domain/analysisView/environmentStateWriteApply.test.ts
+++ b/src/domain/analysisView/environmentStateWriteApply.test.ts
@@ -754,3 +754,46 @@ it("refuses a create whose composed address reads a member the call does not sen
   );
   expect(create({ title: "New Folder" })).toEqual(err(UNSPELLED));
 });
+
+it.each(["__proto__", "constructor"])(
+  "creates and updates an absent own %s field without changing object inheritance",
+  (field) => {
+    const plan = {
+      rules: [{ field, rule: "echo" as const }],
+      body: {
+        accepted: [field],
+        fixed: [],
+        required: [],
+        identity: null,
+        kinds: [],
+      },
+    };
+    const sent = Object.fromEntries([[field, { label: "stored" }]]);
+    const call = callWith(sent);
+    const created = createdFields(plan, { id: 1 }, call, generator().generate);
+    const updated = updatedFields(
+      plan,
+      {
+        before: { id: 1 },
+        observed: Object.fromEntries([
+          ["id", 1],
+          [field, { label: "old" }],
+        ]),
+      },
+      { ...call, id: "1" },
+      generator().generate,
+    );
+    for (const result of [created, updated]) {
+      expect(result.ok).toBe(true);
+      if (!result.ok) throw new Error(result.error);
+      expect(
+        Object.getOwnPropertyDescriptor(result.value, field)?.value,
+      ).toEqual({ label: "stored" });
+      expect(Object.getPrototypeOf(result.value)).toBe(Object.prototype);
+      expect(JSON.parse(JSON.stringify(result.value))).toEqual({
+        id: 1,
+        ...sent,
+      });
+    }
+  },
+);
diff --git a/src/domain/analysisView/environmentStateWriteApply.ts b/src/domain/analysisView/environmentStateWriteApply.ts
index 28b552e12e..1d414538bb 100644
--- a/src/domain/analysisView/environmentStateWriteApply.ts
+++ b/src/domain/analysisView/environmentStateWriteApply.ts
@@ -1,6 +1,8 @@
 import type { JsonValue } from "../jsonValue.js";
 import { err, ok, type Result } from "../result.js";
 import {
+  ownMember,
+  setOwnMember,
   at,
   isObject,
   pointerTo,
@@ -74,13 +76,6 @@ export type GenerateValue = (
 const kindOf = (value: JsonValue | undefined): string =>
   value === null ? "null" : Array.isArray(value) ? "array" : typeof value;
 
-/** JSON objects contain only their own members, even when a name also exists on Object.prototype. */
-const ownMember = (
-  object: JsonObject,
-  member: string,
-): JsonValue | undefined =>
-  Object.hasOwn(object, member) ? object[member] : undefined;
-
 /** The JSON types `stored` holds members in, for those `observed` shows no value of. */
 export const storedKinds = (
   members: readonly string[],
@@ -197,7 +192,9 @@ export const recordedGeneratedValue = (
   value: JsonValue | undefined,
 ): Applied<JsonValue | undefined> => {
   if (rule.rule !== "nested-clock") return ok(value);
-  const clock = isObject(value) ? value[rule.members[0]!] : undefined;
+  const clock = isObject(value)
+    ? ownMember(value, rule.members[0]!)
+    : undefined;
   if (
     clock === undefined ||
     JSON.stringify(readClock(clock)?.format) !== JSON.stringify(rule.format)
@@ -251,7 +248,7 @@ const ruledValue = (
         ? err(
             `The call names no ${rule.from} path value, which ${rule.field} takes.`,
           )
-        : ok(pathValue(observed[rule.field], value));
+        : ok(pathValue(ownMember(observed, rule.field), value));
     }
     default:
       return ok(undefined);
@@ -264,9 +261,9 @@ const templated = (
   written: JsonObject,
   rule: Extract<FieldRule, { rule: "template" }>,
 ): JsonValue | undefined => {
-  const text = observed[rule.field];
-  const token = scalarText(observed[rule.from]);
-  const replacement = scalarText(written[rule.from]);
+  const text = ownMember(observed, rule.field);
+  const token = scalarText(ownMember(observed, rule.from));
+  const replacement = scalarText(ownMember(written, rule.from));
   return typeof text === "string" &&
     token !== undefined &&
     replacement !== undefined
@@ -283,14 +280,16 @@ const derive = (
 ): string | undefined => {
   for (const rule of rules) {
     if (rule.rule === "slug") {
-      const from = sent[rule.from];
-      if (typeof from === "string") written[rule.field] = slugOf(from);
+      const from = ownMember(sent, rule.from);
+      if (typeof from === "string")
+        setOwnMember(written, rule.field, slugOf(from));
     } else if (rule.rule === "template") {
       const value = templated(observed, written, rule);
-      if (value !== undefined) written[rule.field] = value;
-    } else if (rule.rule === "stamp") written[rule.field] = rule.value;
+      if (value !== undefined) setOwnMember(written, rule.field, value);
+    } else if (rule.rule === "stamp")
+      setOwnMember(written, rule.field, rule.value);
     else if (rule.rule === "derived" && Object.hasOwn(sent, rule.member))
-      written[rule.field] = rule.value;
+      setOwnMember(written, rule.field, rule.value);
   }
   // Composed strings read the fields the rules above wrote.
   for (const rule of rules)
@@ -302,7 +301,7 @@ const derive = (
         );
         return `The write composes ${JSON.stringify(rule.field)} from ${listed([...sources])}, and the entity holds no value one of them can spell.`;
       }
-      written[rule.field] = value;
+      setOwnMember(written, rule.field, value);
     }
   return undefined;
 };
@@ -341,9 +340,10 @@ export const createdFields = (
   for (const rule of plan.rules) {
     const value = ruledValue(rule, observed, call, generate, referenced);
     if (!value.ok) return value;
-    if (value.value !== undefined) written[rule.field] = value.value;
+    if (value.value !== undefined)
+      setOwnMember(written, rule.field, value.value);
     else if (rule.rule === "echo" && defaults.has(rule.field))
-      written[rule.field] = defaults.get(rule.field)!;
+      setOwnMember(written, rule.field, defaults.get(rule.field)!);
     else if (rule.rule === "echo" || rule.rule === "reference")
       delete written[rule.field];
   }
@@ -404,15 +404,20 @@ export const updatedFields = (
   const written: JsonObject = { ...before };
   for (const rule of plan.rules) {
     const stated = statedValue(rule, sent);
-    if (stated !== undefined) written[rule.field] = stated;
+    if (stated !== undefined) setOwnMember(written, rule.field, stated);
     else if (rule.rule === "reference" && Object.hasOwn(sent, rule.member)) {
       const value = referencedValue(rule, sent, referenced);
       if (!value.ok) return value;
-      if (value.value !== undefined) written[rule.field] = value.value;
+      if (value.value !== undefined)
+        setOwnMember(written, rule.field, value.value);
     } else if (rule.rule === "increment") {
-      const was = integerOf(before[rule.field]);
+      const was = integerOf(ownMember(before, rule.field));
       if (was !== undefined)
-        written[rule.field] = spelledLike(before[rule.field]!, was + 1);
+        setOwnMember(
+          written,
+          rule.field,
+          spelledLike(ownMember(before, rule.field)!, was + 1),
+        );
     } else if (
       regenerated(rule) ||
       // Child rows are renewed only on an entity that holds them.
@@ -420,7 +425,8 @@ export const updatedFields = (
     ) {
       const value = generate(rule);
       if (!value.ok) return value;
-      if (value.value !== undefined) written[rule.field] = value.value;
+      if (value.value !== undefined)
+        setOwnMember(written, rule.field, value.value);
     }
   }
   const unspelled = derive(
@@ -532,10 +538,10 @@ export const writtenResponse = (
       pointer,
       Object.fromEntries(
         fields.map((field) => {
-          const value = written.fields![field]!;
+          const value = ownMember(written.fields!, field)!;
           const clock =
             written.clocks?.includes(field) === true
-              ? readClock(shown[field])
+              ? readClock(ownMember(shown, field))
               : undefined;
           return [
             field,
diff --git a/src/domain/analysisView/environmentStateWriteAttribution.ts b/src/domain/analysisView/environmentStateWriteAttribution.ts
index 7791bfc0b4..119ccca5f4 100644
--- a/src/domain/analysisView/environmentStateWriteAttribution.ts
+++ b/src/domain/analysisView/environmentStateWriteAttribution.ts
@@ -1,4 +1,5 @@
 import {
+  ownMember,
   renderStateEntity,
   same,
   type JsonObject,
@@ -412,7 +413,9 @@ export const writtenFieldChecks = (
     if (fields === null || compared.length === 0) continue;
     outcomes.set(
       tool,
-      compared.every((field) => same(shown[field], observed[field])),
+      compared.every((field) =>
+        same(ownMember(shown, field), ownMember(observed, field)),
+      ),
     );
   }
   return outcomes;
diff --git a/src/domain/analysisView/environmentStateWriteBlame.ts b/src/domain/analysisView/environmentStateWriteBlame.ts
index 1bbd2a1060..d285b733c1 100644
--- a/src/domain/analysisView/environmentStateWriteBlame.ts
+++ b/src/domain/analysisView/environmentStateWriteBlame.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   at,
   isObject,
   locateItem,
@@ -271,7 +272,7 @@ const fieldBlame = (
           !Object.hasOwn(read.object, field) ||
           read.volatile.has(field) ||
           others.has(field) ||
-          same(response[field], read.object[field]),
+          same(response[field], ownMember(read.object, field)),
       )
     );
   };
@@ -323,11 +324,13 @@ const lackingFields = (
   const unruled = new Set<string>();
   let unowned = false;
   for (const field of fields) {
-    const value = read.object[field] ?? null;
+    const value = ownMember(read.object, field) ?? null;
     const owners = [...writes.partial.keys()].filter((name) => {
       const response = writes.responses.get(name);
       return (
-        value !== null && response !== undefined && same(response[field], value)
+        value !== null &&
+        response !== undefined &&
+        same(ownMember(response, field), value)
       );
     });
     if (owners.length === 0) unowned = true;
@@ -349,13 +352,13 @@ const names = (
       ? EMBED_SHORT_ID_MATCH_LENGTH
       : EMBED_MATCH_MIN_LENGTH;
   return (
-    scalarText(item[idField]) === id &&
+    scalarText(ownMember(item, idField)) === id &&
     Object.entries(item).some(
       ([key, member]) =>
         key !== idField &&
         typeof member === "string" &&
         member.length >= length &&
-        entity[key] === member,
+        ownMember(entity, key) === member,
     )
   );
 };
@@ -383,7 +386,7 @@ const embeds = (
   entity: JsonObject,
   idField: string,
 ): boolean => {
-  const id = scalarText(entity[idField]);
+  const id = scalarText(ownMember(entity, idField));
   let found = false;
   if (id !== undefined)
     eachObject(value, (item) => {
@@ -422,7 +425,9 @@ const embeddingCreates = (
     if (entity.type === read.type && entity.id === read.entity?.id) continue;
     const held = heldOf(entity);
     const id =
-      held === undefined ? undefined : scalarText(held.fields[held.idField]);
+      held === undefined
+        ? undefined
+        : scalarText(ownMember(held.fields, held.idField));
     if (held === undefined || id === undefined) continue;
     idFields.add(held.idField);
     const key = `${held.idField}\u0000${id}`;
@@ -434,11 +439,11 @@ const embeddingCreates = (
   const tools = new Set<string>();
   if (byId.size === 0) return [];
   for (const [field, value] of Object.entries(read.object)) {
-    const before = read.entity?.fields[field];
+    const before = ownMember(read.entity?.fields, field);
     if (same(before, value)) continue;
     eachObject(value, (item) => {
       for (const idField of idFields) {
-        const id = scalarText(item[idField]);
+        const id = scalarText(ownMember(item, idField));
         for (const entity of id === undefined
           ? []
           : (byId.get(`${idField}\u0000${id}`) ?? []))
diff --git a/src/domain/analysisView/environmentStateWriteCalls.ts b/src/domain/analysisView/environmentStateWriteCalls.ts
index ff958dfec3..68fcfdb866 100644
--- a/src/domain/analysisView/environmentStateWriteCalls.ts
+++ b/src/domain/analysisView/environmentStateWriteCalls.ts
@@ -1,7 +1,7 @@
 import { jsonTextCodeUnits } from "../jsonSerialization.js";
 import { digestCanonicalValue } from "../canonicalDigest.js";
 import type { JsonValue } from "../jsonValue.js";
-import { same, type JsonObject } from "./environmentStateRules.js";
+import { ownMember, same, type JsonObject } from "./environmentStateRules.js";
 import { storedKinds } from "./environmentStateWriteApply.js";
 import { composedValue } from "./environmentStateWriteComposed.js";
 import type { explainUpdate } from "./environmentStateWriteExplain.js";
@@ -48,7 +48,7 @@ export const disagreesWithCall = (
     (rule) =>
       (unsentChange(rule) &&
         Object.hasOwn(call.after, rule.field) &&
-        !same(call.written[rule.field], call.after[rule.field])) ||
+        !same(ownMember(call.written, rule.field), call.after[rule.field])) ||
       (regenerates(rule) && call.kept.includes(rule.field)),
   );
 
@@ -60,8 +60,8 @@ export const steadyFields = (
 ): string[] =>
   rules.flatMap((rule) =>
     rule.rule === "set" &&
-    same(before[rule.field], rule.value) &&
-    same(after[rule.field], rule.value)
+    same(ownMember(before, rule.field), rule.value) &&
+    same(ownMember(after, rule.field), rule.value)
       ? [rule.field]
       : [],
   );
@@ -265,7 +265,8 @@ export const appliedUpdate = (
   );
   const derived = Object.keys(written).filter(
     (field) =>
-      !Object.hasOwn(before, field) || !same(written[field], before[field]),
+      !Object.hasOwn(before, field) ||
+      !same(written[field], ownMember(before, field)),
   );
   const changed = {
     fields: [...new Set([...sent, ...derived])],
diff --git a/src/domain/analysisView/environmentStateWriteChanges.ts b/src/domain/analysisView/environmentStateWriteChanges.ts
index c45b496360..35a85160ae 100644
--- a/src/domain/analysisView/environmentStateWriteChanges.ts
+++ b/src/domain/analysisView/environmentStateWriteChanges.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   isObject,
   same,
   type JsonObject,
@@ -52,8 +53,8 @@ export const changeRule = (
   field: string,
   slugs: ReadonlyMap<string, string>,
 ): FieldRule | undefined => {
-  const before = evidence.before[field];
-  const after = evidence.after[field]!;
+  const before = ownMember(evidence.before, field);
+  const after = ownMember(evidence.after, field)!;
   if (readClock(after) !== undefined && isClockNear(after, evidence.startMs))
     return { field, rule: "clock", format: readClock(after)!.format };
   if (freshUuid(evidence, field)) return { field, rule: "unique" };
@@ -93,7 +94,7 @@ const actionRule = (
   evidence: UpdateEvidence,
   field: string,
 ): FieldRule | undefined => {
-  const after = evidence.after[field]!;
+  const after = ownMember(evidence.after, field)!;
   return evidence.action === true &&
     (after === null || typeof after !== "object") &&
     readClock(after) === undefined
@@ -123,8 +124,8 @@ export const heldClock = (
   sent: JsonValue,
 ): boolean =>
   inCallSecond(sent, evidence.startMs) &&
-  same(evidence.before[member], sent) &&
-  same(evidence.after[member], sent);
+  same(ownMember(evidence.before, member), sent) &&
+  same(ownMember(evidence.after, member), sent);
 
 /** A field naming when its entity was made (`createdAt`, `row_created_timestamp`), which no update stamps. */
 const CREATION_CLOCK = /creat(?:ed|ion)/iu;
@@ -145,13 +146,13 @@ const sentSlugSource = (
   evidence: UpdateEvidence,
   member: string,
 ): string | undefined => {
-  const stored = evidence.after[member];
+  const stored = ownMember(evidence.after, member);
   if (typeof stored !== "string") return undefined;
   return Object.entries(evidence.call.body).find(
     ([other, value]) =>
       other !== member &&
       typeof value === "string" &&
-      same(evidence.after[other], value) &&
+      same(ownMember(evidence.after, other), value) &&
       slugOf(value) === stored,
   )?.[0];
 };
@@ -164,9 +165,9 @@ const restampedClock = (
 ): boolean =>
   !CREATION_CLOCK.test(member) &&
   readClock(sent) !== undefined &&
-  same(evidence.before[member], sent) &&
-  readClock(evidence.after[member]) !== undefined &&
-  isClockNear(evidence.after[member]!, evidence.startMs);
+  same(ownMember(evidence.before, member), sent) &&
+  readClock(ownMember(evidence.after, member)) !== undefined &&
+  isClockNear(ownMember(evidence.after, member)!, evidence.startMs);
 
 /** How an update stored a sent member: stamped anew as the one held clock it sent back other than a creation clock, echoed, stamped anew or slugged from another member in place of what it sent, renewed as a fresh UUID, or as a constant. */
 export const sentRule = (
@@ -174,26 +175,30 @@ export const sentRule = (
   member: string,
   held: readonly string[],
 ): FieldRule => {
-  const sent = evidence.call.body[member]!;
+  const sent = ownMember(evidence.call.body, member)!;
   if (held.length === 1 && held[0] === member && !CREATION_CLOCK.test(member))
     return {
       field: member,
       rule: "held-clock",
       format: readClock(sent)!.format,
     };
-  if (same(evidence.after[member], sent))
+  if (same(ownMember(evidence.after, member), sent))
     return { field: member, rule: "echo" };
-  if (same(evidence.after[member], sortedStrings(sent)))
+  if (same(ownMember(evidence.after, member), sortedStrings(sent)))
     return { field: member, rule: "echo", sorted: true };
   if (restampedClock(evidence, member, sent))
     return {
       field: member,
       rule: "clock",
-      format: readClock(evidence.after[member])!.format,
+      format: readClock(ownMember(evidence.after, member))!.format,
     };
   const from = sentSlugSource(evidence, member);
   if (from !== undefined) return { field: member, rule: "slug", from };
   return freshUuid(evidence, member)
     ? { field: member, rule: "unique" }
-    : { field: member, rule: "constant", value: evidence.after[member]! };
+    : {
+        field: member,
+        rule: "constant",
+        value: ownMember(evidence.after, member)!,
+      };
 };
diff --git a/src/domain/analysisView/environmentStateWriteChecks.ts b/src/domain/analysisView/environmentStateWriteChecks.ts
index 4fd3a01aa8..f0eabd8b35 100644
--- a/src/domain/analysisView/environmentStateWriteChecks.ts
+++ b/src/domain/analysisView/environmentStateWriteChecks.ts
@@ -2,6 +2,7 @@ import { ServedOrder } from "./environmentStateWriteOrder.js";
 import type { JsonValue } from "../jsonValue.js";
 import { compositeKey } from "../unicodeCodePointOrder.js";
 import {
+  ownMember,
   at,
   decodedSegment,
   isObject,
@@ -408,7 +409,10 @@ const listCheck = (
   )
     return { pass: false };
   const observed = new Map(
-    check.items.map((item) => [scalarText(item[check.idField]) ?? "", item]),
+    check.items.map((item) => [
+      scalarText(ownMember(item, check.idField)) ?? "",
+      item,
+    ]),
   );
   const rendered = members.map((entity) =>
     renderStateEntity(entity, view, check.volatile, observed.get(entity.id)),
@@ -450,7 +454,7 @@ const listedTargets = (
   const targets: string[] = [];
   const unheld: string[] = [];
   const ids = [...check.baseItems, ...check.items].map(
-    (item) => scalarText(item[check.idField]) ?? "",
+    (item) => scalarText(ownMember(item, check.idField)) ?? "",
   );
   for (const id of new Set(ids))
     (state.entity(view.type, check.collection, id) === undefined
@@ -525,7 +529,7 @@ const checkAllList = (
       ),
     ],
     unheld: check.items
-      .map((item) => scalarText(item[check.idField]) ?? "")
+      .map((item) => scalarText(ownMember(item, check.idField)) ?? "")
       .filter((id) => !held.has(id))
       .map((id) => entityTarget(view.type, collection, id)),
   };
@@ -538,7 +542,8 @@ const checkRecordedList = (
   check: ListBodies,
 ): CheckOutcome => {
   const { collection } = check;
-  const idOf = (item: JsonObject) => scalarText(item[check.idField]) ?? "";
+  const idOf = (item: JsonObject) =>
+    scalarText(ownMember(item, check.idField)) ?? "";
   const members = check.baseItems.flatMap((item) => {
     const entity = state.entity(view.type, collection, idOf(item));
     return entity === undefined ? [] : [entity];
diff --git a/src/domain/analysisView/environmentStateWriteComposed.ts b/src/domain/analysisView/environmentStateWriteComposed.ts
index 0017aa905f..5572c6b296 100644
--- a/src/domain/analysisView/environmentStateWriteComposed.ts
+++ b/src/domain/analysisView/environmentStateWriteComposed.ts
@@ -1,5 +1,10 @@
 import type { JsonValue } from "../jsonValue.js";
-import { same, scalarText, type JsonObject } from "./environmentStateRules.js";
+import {
+  ownMember,
+  same,
+  scalarText,
+  type JsonObject,
+} from "./environmentStateRules.js";
 import { COPY_MIN_LENGTH } from "./environmentStateWriteCopies.js";
 import { GENERATED } from "./environmentStateWriteGenerated.js";
 import type { ComposedPart, FieldRule } from "./environmentStateWriteRules.js";
@@ -39,7 +44,7 @@ export const composedPartText = (
   fields: JsonObject,
 ): string | undefined => {
   if ("text" in part) return part.text;
-  const value = fields[part.from];
+  const value = ownMember(fields, part.from);
   if (part.as === "clock") return clockDigits(value, part.pattern);
   if (part.as === "value") return scalarText(value);
   return typeof value === "string" ? slugOf(value) : undefined;
@@ -208,14 +213,14 @@ export const createdComposer = (
       zones.get(left.from) === zones.get(right.from));
   let index: CandidateIndex | undefined;
   return (field) => {
-    const text = entity[field];
+    const text = ownMember(entity, field);
     if (typeof text !== "string") return undefined;
     index ??= indexed(
       rules.flatMap((rule) =>
         SOURCES.has(rule.rule)
           ? candidatesOf(
               rule.field,
-              entity[rule.field],
+              ownMember(entity, rule.field),
               // A generated value is short and still the call's own, as a template's is.
               GENERATED.has(rule.rule) ? 1 : COPY_MIN_LENGTH,
             )
@@ -246,8 +251,8 @@ export const composedChange = (
   },
 ): FieldRule | undefined => {
   const { before, after, sent, idField } = entity;
-  const text = after[field];
-  if (typeof text !== "string" || typeof before[field] !== "string")
+  const text = ownMember(after, field);
+  if (typeof text !== "string" || typeof ownMember(before, field) !== "string")
     return undefined;
   const known = Object.keys(after).filter(
     (other) =>
@@ -260,14 +265,14 @@ export const composedChange = (
     known.flatMap((other) =>
       candidatesOf(
         other,
-        after[other],
+        ownMember(after, other),
         other === idField ? 1 : COPY_MIN_LENGTH,
       ),
     ),
   );
   const parts = segmented(text, index, samePart);
   return parts === undefined ||
-    composedValue(parts, before) !== before[field] ||
+    composedValue(parts, before) !== ownMember(before, field) ||
     heldSpellings(parts, field, entity.others) === undefined
     ? undefined
     : { field, rule: "composed", parts };
diff --git a/src/domain/analysisView/environmentStateWriteCopies.ts b/src/domain/analysisView/environmentStateWriteCopies.ts
index f4a736c9f1..6059c467d1 100644
--- a/src/domain/analysisView/environmentStateWriteCopies.ts
+++ b/src/domain/analysisView/environmentStateWriteCopies.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   pointerTokens,
   scalarText,
   type JsonObject,
@@ -18,10 +19,10 @@ export const templateRule = (
   field: string,
   sources: readonly FieldRule[],
 ): FieldRule | undefined => {
-  const value = entity[field];
+  const value = ownMember(entity, field);
   if (typeof value !== "string") return undefined;
   for (const rule of sources) {
-    const token = scalarText(entity[rule.field]);
+    const token = scalarText(ownMember(entity, rule.field));
     const generated = GENERATED.has(rule.rule);
     if (
       rule.field !== field &&
@@ -95,7 +96,7 @@ export const strandedCopies = (
     ),
   );
   const sources = rules.flatMap((rule) => {
-    const token = scalarText(entity[rule.field]);
+    const token = scalarText(ownMember(entity, rule.field));
     return (rule.rule === "copy" || rule.rule === "echo") &&
       token !== undefined &&
       token.length >= COPY_MIN_LENGTH
@@ -105,7 +106,7 @@ export const strandedCopies = (
   const holders = holdersOf(
     new Map(
       texts.flatMap((field) => {
-        const value = entity[field];
+        const value = ownMember(entity, field);
         return typeof value === "string" ? [[field, value] as const] : [];
       }),
     ),
@@ -187,7 +188,7 @@ export const referencedMembers = (
     if (referenced.length === 0) continue;
     // A reference repeated under another reference's name is the same entity named twice, not its text.
     const repeats = (field: string): boolean => {
-      const value = entity[field];
+      const value = ownMember(entity, field);
       return (
         typeof value === "string" &&
         value.length >= COPY_MIN_LENGTH &&
@@ -226,7 +227,7 @@ export const instantMembers = (
   constants: readonly string[],
 ): Set<string> => {
   const instants = constants.flatMap((field) => {
-    const at = instantOf(entity[field]);
+    const at = instantOf(ownMember(entity, field));
     return at === undefined ? [] : [{ field, at }];
   });
   return new Set(
diff --git a/src/domain/analysisView/environmentStateWriteCreate.ts b/src/domain/analysisView/environmentStateWriteCreate.ts
index 18992de54f..2b77b8df67 100644
--- a/src/domain/analysisView/environmentStateWriteCreate.ts
+++ b/src/domain/analysisView/environmentStateWriteCreate.ts
@@ -1,6 +1,7 @@
 import { jsonTextCodeUnits } from "../jsonSerialization.js";
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   collectionPathOf,
   isObject,
   isParam,
@@ -223,7 +224,7 @@ const strandedInteger = (
   rules.some((rule) => {
     const digits =
       rule.rule === "unique"
-        ? generatedIntegerDigits(entity[rule.field])
+        ? generatedIntegerDigits(ownMember(entity, rule.field))
         : undefined;
     return (
       digits !== undefined &&
@@ -346,7 +347,7 @@ const buildCreate = (
   // Identity and clock orders first, then any other scalar field the collection may be sorted by.
   const scalars = Object.keys(members[0] ?? {}).filter((field) =>
     members.every((member) =>
-      ["string", "number"].includes(typeof member[field]),
+      ["string", "number"].includes(typeof ownMember(member, field)),
     ),
   );
   const generated = explained.rules.some(
@@ -437,12 +438,16 @@ const followsSequences = (
         rule.scope === "type" || entity.collection === created.collection,
     );
     const next = nextSequence(
-      nextInteger(fieldsOf(scope), rule.field, draft.observed?.[rule.field]),
+      nextInteger(
+        fieldsOf(scope),
+        rule.field,
+        ownMember(draft.observed, rule.field),
+      ),
       draft.sequences.get(key),
     );
     return (
-      created.fields[rule.field] ===
-      spelledLike(draft.observed?.[rule.field] ?? "", next)
+      ownMember(created.fields, rule.field) ===
+      spelledLike(ownMember(draft.observed, rule.field) ?? "", next)
     );
   });
 
@@ -467,7 +472,7 @@ const contradictsDefault = (
       rule.rule === "echo" &&
       !Object.hasOwn(call.body, rule.field) &&
       Object.hasOwn(call.observed, rule.field) &&
-      !same(call.observed[rule.field], written[rule.field]),
+      !same(call.observed[rule.field], ownMember(written, rule.field)),
   );
 
 /** Whether a later create preserves generation shapes and its derived fields reproduce the captured observation. */
@@ -479,13 +484,14 @@ const followsCreateRules = (
   },
 ): boolean =>
   draft.plan.rules.every((rule) => {
-    const value = created.fields[rule.field];
-    const exemplar = draft.observed?.[rule.field];
+    const value = ownMember(created.fields, rule.field);
+    const exemplar = ownMember(draft.observed, rule.field);
     if (rule.rule === "composed" || rule.rule === "reference")
       return (
         !Object.hasOwn(created.observed, rule.field) ||
-        value === created.observed[rule.field] ||
-        (rule.rule === "reference" && same(value, created.observed[rule.field]))
+        value === ownMember(created.observed, rule.field) ||
+        (rule.rule === "reference" &&
+          same(value, ownMember(created.observed, rule.field)))
       );
     if (rule.rule === "clock")
       return (
@@ -504,7 +510,7 @@ const followsCreateRules = (
             : isOpaqueToken(value, exemplar)) &&
         !context.state
           .ofType(created.type)
-          .some((held) => held.fields[rule.field] === value)
+          .some((held) => ownMember(held.fields, rule.field) === value)
       );
     return (
       rule.rule !== "identity" || rule.generator !== "uuid" || isUuid(value)
@@ -517,7 +523,7 @@ const fitClocks = (draft: WriteDraft, fields: JsonObject): void => {
     if (rule.rule !== "clock") return rule;
     const format = fittedClockFormat(
       rule.format,
-      fields[rule.field],
+      ownMember(fields, rule.field),
       clockEvidence(draft, rule.field),
     );
     return format === undefined || format === rule.format
@@ -540,8 +546,8 @@ const takeSequences = (
 ): void => {
   for (const { rule, key } of sequencesOf(draft, created.collection)) {
     const value = sequenceNumber(
-      created.fields[rule.field],
-      draft.observed?.[rule.field],
+      ownMember(created.fields, rule.field),
+      ownMember(draft.observed, rule.field),
     );
     if (value !== undefined) draft.sequences.set(key, value);
   }
@@ -736,8 +742,8 @@ export const applyCreate = (
       (rule) =>
         recordedGeneratedValue(
           rule,
-          draft.observed![rule.field],
-          observed.object[rule.field] ?? null,
+          ownMember(draft.observed!, rule.field),
+          ownMember(observed.object, rule.field) ?? null,
         ),
       referencedIn(context),
     );
@@ -751,7 +757,9 @@ export const applyCreate = (
       envelope: request?.envelope,
     });
   const fields = fieldsOfCall();
-  const id = fields.ok ? scalarText(fields.value[idField]) : undefined;
+  const id = fields.ok
+    ? scalarText(ownMember(fields.value, idField))
+    : undefined;
   if (
     !fields.ok ||
     id === undefined ||
diff --git a/src/domain/analysisView/environmentStateWriteCreateCalls.ts b/src/domain/analysisView/environmentStateWriteCreateCalls.ts
index 2146c5b2e9..168545e645 100644
--- a/src/domain/analysisView/environmentStateWriteCreateCalls.ts
+++ b/src/domain/analysisView/environmentStateWriteCreateCalls.ts
@@ -1,5 +1,5 @@
 import type { JsonValue } from "../jsonValue.js";
-import { same, type JsonObject } from "./environmentStateRules.js";
+import { ownMember, same, type JsonObject } from "./environmentStateRules.js";
 import { storedKinds } from "./environmentStateWriteApply.js";
 import { fittedClockFormat } from "./environmentStateWriteClockFormats.js";
 import {
@@ -127,9 +127,12 @@ const mergedField = (
   // This call echoes a member the plan's calls left out: what they stored there is its default.
   if (after !== undefined)
     return after.rule === "echo" && !calls.required.includes(field)
-      ? { rule: after, default: { member: field, value: exemplar[field]! } }
+      ? {
+          rule: after,
+          default: { member: field, value: ownMember(exemplar, field)! },
+        }
       : undefined;
-  return same(exemplar[field], later.entity[field]) ? {} : undefined;
+  return same(ownMember(exemplar, field), later.entity[field]) ? {} : undefined;
 };
 
 /**
@@ -151,7 +154,7 @@ export const mergedCreatePlan = (
   },
   earlier: EarlierEvidence = {
     kept: new Set(),
-    clockOf: (field) => exemplar[field],
+    clockOf: (field) => ownMember(exemplar, field),
   },
 ): { readonly rules: FieldRule[]; readonly body: WriteBody } | undefined => {
   const held = new Map(plan.rules.map((rule) => [rule.field, rule]));
diff --git a/src/domain/analysisView/environmentStateWriteEffects.ts b/src/domain/analysisView/environmentStateWriteEffects.ts
index 12db6e04bf..a5ca517de0 100644
--- a/src/domain/analysisView/environmentStateWriteEffects.ts
+++ b/src/domain/analysisView/environmentStateWriteEffects.ts
@@ -1,6 +1,7 @@
 import { jsonTextCodeUnits } from "../jsonSerialization.js";
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   isObject,
   locateItem,
   at,
@@ -103,7 +104,7 @@ export const clockEvidence = (
   draft: WriteDraft,
   field: string,
 ): JsonValue | undefined =>
-  draft.fixedWidth.get(field) ?? draft.observed?.[field];
+  draft.fixedWidth.get(field) ?? ownMember(draft.observed, field);
 
 /** A write observation plus the fields its selected read view actually covers, if any. */
 type UpdateObservation = Observed & {
@@ -599,8 +600,8 @@ export const updateEntity = (
     (rule) =>
       recordedGeneratedValue(
         rule,
-        entity.fields[rule.field],
-        after.object[rule.field],
+        ownMember(entity.fields, rule.field),
+        ownMember(after.object, rule.field),
       ),
     referencedIn(context),
   );
@@ -663,7 +664,7 @@ const fitUpdateClocks = (
       return rule;
     const format = fittedClockFormat(
       rule.format,
-      after[rule.field],
+      ownMember(after, rule.field),
       clockEvidence(draft, rule.field),
     );
     return format === undefined ? rule : { ...rule, format };
@@ -682,7 +683,10 @@ const fitUpdateClocks = (
         const held = planned.get(rule.field);
         return rule.rule === "clock" &&
           held?.rule === "clock" &&
-          matchesClockFormat(held.format, readClock(after[rule.field]))
+          matchesClockFormat(
+            held.format,
+            readClock(ownMember(after, rule.field)),
+          )
           ? { ...rule, format: held.format }
           : rule;
       }),
@@ -710,7 +714,7 @@ const contradictsGeneratedClock = (
       )
     )
       return false;
-    const previous = readClock(before[rule.field]);
+    const previous = readClock(ownMember(before, rule.field));
     if (previous === undefined) return false;
     const next = readClock(after[rule.field]);
     return (
diff --git a/src/domain/analysisView/environmentStateWriteExplain.ts b/src/domain/analysisView/environmentStateWriteExplain.ts
index 8aef269f26..644d04bec1 100644
--- a/src/domain/analysisView/environmentStateWriteExplain.ts
+++ b/src/domain/analysisView/environmentStateWriteExplain.ts
@@ -1,6 +1,7 @@
 import type { JsonValue } from "../jsonValue.js";
 import { compareUnicodeCodePoints as compare } from "../unicodeCodePointOrder.js";
 import {
+  ownMember,
   isObject,
   locateItem,
   pointerTo,
@@ -125,7 +126,7 @@ export const nextInteger = (
 ): number | undefined => {
   let largest: number | undefined;
   for (const member of members) {
-    const value = sequenceNumber(member[field], exemplar);
+    const value = sequenceNumber(ownMember(member, field), exemplar);
     if (value !== undefined && (largest === undefined || value > largest))
       largest = value;
   }
@@ -179,10 +180,12 @@ const learnedClockFormat = (
 ): ClockFormat => {
   let learned = format;
   for (const member of type) {
-    const fitted = fittedClockFormat(learned, member[field], value);
+    const fitted = fittedClockFormat(learned, ownMember(member, field), value);
     if (fitted?.kind === "iso" && fitted.trimmed === true) learned = fitted;
   }
-  return type.some((member) => showsFixedWidth(format, member[field]))
+  return type.some((member) =>
+    showsFixedWidth(format, ownMember(member, field)),
+  )
     ? format
     : learned;
 };
@@ -193,7 +196,7 @@ export const createdInteger = (
   entity: JsonObject,
   type: readonly JsonObject[],
 ): "independent" | "linked" | undefined => {
-  const value = entity[field];
+  const value = ownMember(entity, field);
   const digits = generatedIntegerDigits(value);
   if (
     digits === undefined ||
@@ -203,10 +206,10 @@ export const createdInteger = (
     return undefined;
   const held = new Set([digits]);
   const distinct = type.every((member) => {
-    const other = generatedIntegerDigits(member[field]);
+    const other = generatedIntegerDigits(ownMember(member, field));
     if (
       other === undefined ||
-      typeof member[field] !== typeof value ||
+      typeof ownMember(member, field) !== typeof value ||
       other.length !== digits.length ||
       held.has(other)
     )
@@ -252,7 +255,7 @@ const generatedRule = (
   if (
     (isUuid(value) &&
       evidence.type.length > 0 &&
-      evidence.type.every((member) => member[field] !== value)) ||
+      evidence.type.every((member) => ownMember(member, field) !== value)) ||
     createdInteger(field, evidence.entity, evidence.type) === "independent"
   )
     return { field, rule: "unique" };
@@ -311,7 +314,9 @@ export const explainCreate = (
   // Generated values first, then copies: an echoed value may recur in unrelated text, a copied one is the call's own unless also echoed.
   const echoed = new Set(
     rules.flatMap((rule) =>
-      rule.rule === "echo" ? [scalarText(evidence.entity[rule.field])] : [],
+      rule.rule === "echo"
+        ? [scalarText(ownMember(evidence.entity, rule.field))]
+        : [],
     ),
   );
   const sources = [
@@ -319,7 +324,7 @@ export const explainCreate = (
     ...rules.filter(
       (rule) =>
         rule.rule === "copy" &&
-        !echoed.has(scalarText(evidence.entity[rule.field])),
+        !echoed.has(scalarText(ownMember(evidence.entity, rule.field))),
     ),
   ];
   const constants: string[] = [];
@@ -375,7 +380,9 @@ export const explainCreate = (
     ),
     // A template rebuilds only its generated value, so other text it keeps ties it like a constant.
     tied: [
-      ...[...constants, ...templates].map((field) => evidence.entity[field]!),
+      ...[...constants, ...templates].map((field) =>
+        ownMember(evidence.entity, field)!,
+      ),
       ...literals,
     ].some((text) => tokens.some((token) => containsDelimited(text, token))),
   };
@@ -388,7 +395,7 @@ const echoedSlugs = (
 ): Map<string, string> => {
   const slugs = new Map<string, string>();
   for (const member of echoed) {
-    const sent = body[member];
+    const sent = ownMember(body, member);
     if (typeof sent === "string" && !slugs.has(slugOf(sent)))
       slugs.set(slugOf(sent), member);
   }
@@ -404,7 +411,7 @@ const keptReference = (
   if (known === undefined || evidence.named === undefined) return undefined;
   const shown = referenceRule(
     field,
-    evidence.after[field],
+    ownMember(evidence.after, field),
     evidence.call.body,
     evidence.named,
   );
@@ -439,7 +446,12 @@ export const explainUpdate = (
     // The identity is the entity's key: the path's id repeated is the identity member; another value is one the application ignored, or used to re-key the entity, which no rule models.
     if (member === evidence.idField) {
       if (scalarText(sent) === evidence.id) identity = member;
-      else if (!same(evidence.after[member], evidence.before[member]))
+      else if (
+        !same(
+          ownMember(evidence.after, member),
+          ownMember(evidence.before, member),
+        )
+      )
         unexplained.push(member);
       continue;
     }
@@ -465,7 +477,9 @@ export const explainUpdate = (
         !(evidence.shown ?? []).includes(field))
     )
       continue;
-    if (same(evidence.before[field], evidence.after[field])) {
+    if (
+      same(ownMember(evidence.before, field), ownMember(evidence.after, field))
+    ) {
       const kept = keptReference(evidence, field);
       if (kept !== undefined) rules.push(kept);
       continue;
@@ -553,7 +567,10 @@ const orderedBy = (
 ): boolean =>
   members.every((member, index) => {
     if (index === 0) return true;
-    const order = compareValues(members[index - 1]![field], member[field]);
+    const order = compareValues(
+      ownMember(members[index - 1]!, field),
+      ownMember(member, field),
+    );
     return (
       order !== undefined && (direction === "ascending" ? order < 0 : order > 0)
     );
@@ -572,17 +589,23 @@ export const randomIdentities = (
   idField: string,
   clocks: readonly string[],
 ): boolean => {
-  const ids = members.map((member) => scalarText(member[idField]) ?? "");
+  const ids = members.map(
+    (member) => scalarText(ownMember(member, idField)) ?? "",
+  );
   if (ids.length > 0 && ids.every((id) => UUID_V4.test(id))) return true;
   if (members.length < 2 || clocks.length === 0) return false;
   const byId = members.toSorted(
-    (left, right) => compareValues(left[idField], right[idField]) ?? 0,
+    (left, right) =>
+      compareValues(ownMember(left, idField), ownMember(right, idField)) ?? 0,
   );
   return clocks.every((clock) =>
     byId.some(
       (member, index) =>
         index > 0 &&
-        (compareValues(byId[index - 1]![clock], member[clock]) ?? 1) > 0,
+        (compareValues(
+          ownMember(byId[index - 1]!, clock),
+          ownMember(member, clock),
+        ) ?? 1) > 0,
     ),
   );
 };
@@ -628,7 +651,10 @@ export const insertIndex = (
 ): number => {
   if (rule.rule !== "sorted") return rule.rule === "first" ? 0 : members.length;
   const index = members.findIndex((member) => {
-    const order = compareValues(fields[rule.field], member[rule.field]);
+    const order = compareValues(
+      ownMember(fields, rule.field),
+      ownMember(member, rule.field),
+    );
     return (
       order !== undefined &&
       (rule.direction === "ascending" ? order < 0 : order > 0)
diff --git a/src/domain/analysisView/environmentStateWriteGenerated.ts b/src/domain/analysisView/environmentStateWriteGenerated.ts
index c5881019df..e52999cfe0 100644
--- a/src/domain/analysisView/environmentStateWriteGenerated.ts
+++ b/src/domain/analysisView/environmentStateWriteGenerated.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   isObject,
   pointerTo,
   type JsonObject,
@@ -27,8 +28,8 @@ export const freshUuid = (
   evidence: GeneratedUpdateEvidence,
   field: string,
 ): boolean => {
-  const before = evidence.before[field];
-  const after = evidence.after[field];
+  const before = ownMember(evidence.before, field);
+  const after = ownMember(evidence.after, field);
   if (!isUuid(before) || !isUuid(after)) return false;
   const fresh = after.toLowerCase();
   // Scalar generation and nested renewal have separate owners. A shared output UUID
@@ -97,7 +98,11 @@ const pairUuids = (before: JsonValue, after: JsonValue): Paired | undefined => {
       if (keys.length !== Object.keys(now).length) return undefined;
       for (const key of keys) {
         if (!Object.hasOwn(now, key)) return undefined;
-        pending.push([was[key]!, now[key]!, childPointer(pointer, key)]);
+        pending.push([
+          ownMember(was, key)!,
+          now[key]!,
+          childPointer(pointer, key),
+        ]);
       }
     } else if (was !== now) return undefined;
   }
@@ -146,7 +151,7 @@ export const renewedRule = (
   others: readonly JsonObject[],
 ): FieldRule | undefined => {
   const renewal = renewedUuids(change.before, change.after);
-  const held = uuidsIn(others.map((other) => other[field] ?? null));
+  const held = uuidsIn(others.map((other) => ownMember(other, field) ?? null));
   return renewal !== undefined &&
     [...renewal.uuids.values()].every((uuid) => !held.has(uuid.toLowerCase()))
     ? { field, rule: "renewed", pointers: renewal.pointers }
@@ -178,7 +183,10 @@ export const unexplainedRenewals = (
   const fields = new Set<string>();
   for (const rule of rules) {
     if (rule.rule !== "renewed") continue;
-    const renewal = renewedUuids(before[rule.field]!, after[rule.field]!);
+    const renewal = renewedUuids(
+      ownMember(before, rule.field)!,
+      ownMember(after, rule.field)!,
+    );
     for (const [was, now] of renewal?.uuids ?? []) {
       if (
         held.has(now) ||
diff --git a/src/domain/analysisView/environmentStateWriteLinks.ts b/src/domain/analysisView/environmentStateWriteLinks.ts
index ef4493fb5f..e7c7c340ff 100644
--- a/src/domain/analysisView/environmentStateWriteLinks.ts
+++ b/src/domain/analysisView/environmentStateWriteLinks.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   collectionPathOf,
   IDENTITY_NAMES,
   identityCandidates,
@@ -37,7 +38,7 @@ const succeeded = (record: StateRecordFacts): boolean =>
 const identityValues = (object: JsonValue | undefined): string[] =>
   isObject(object)
     ? IDENTITY_NAMES.flatMap((name) => {
-        const value = scalarText(object[name]);
+        const value = scalarText(ownMember(object, name));
         return value === undefined ? [] : [value];
       })
     : [];
@@ -101,7 +102,7 @@ const listedIds = (body: JsonValue | undefined): Set<string> => {
     ...items.flatMap(identityValues),
     ...(suffixed === undefined
       ? []
-      : items.flatMap((item) => scalarText(item[suffixed]) ?? [])),
+      : items.flatMap((item) => scalarText(ownMember(item, suffixed)) ?? [])),
   ]);
 };
 
diff --git a/src/domain/analysisView/environmentStateWriteObserve.ts b/src/domain/analysisView/environmentStateWriteObserve.ts
index 30956c85c0..1b228a0d9c 100644
--- a/src/domain/analysisView/environmentStateWriteObserve.ts
+++ b/src/domain/analysisView/environmentStateWriteObserve.ts
@@ -1,6 +1,7 @@
 import type { JsonValue } from "../jsonValue.js";
 import { overlapsWrite } from "./environmentStateWriteAttribution.js";
 import {
+  ownMember,
   at,
   collectionPathOf,
   isObject,
@@ -86,7 +87,9 @@ export const newEntityIn = (
   if (body === undefined) return undefined;
   for (const tier of entitySlots(body)) {
     const found = tier.flatMap(([pointer, object]) => {
-      const id = isObject(object) ? scalarText(object[idField]) : undefined;
+      const id = isObject(object)
+        ? scalarText(ownMember(object, idField))
+        : undefined;
       // An object holding only the identity names the entity without showing it.
       return isObject(object) &&
         id !== undefined &&
@@ -229,7 +232,8 @@ const itemObservation = (
         : entitySlots(body)
             .flat()
             .find(([at]) => at === pointer)?.[1];
-    return isObject(object) && scalarText(object[target.idField]) === id
+    return isObject(object) &&
+      scalarText(ownMember(object, target.idField)) === id
       ? { ordinal: record.ordinal, pointer, object, id }
       : undefined;
   });
@@ -248,7 +252,7 @@ const listObservation = (
       .map((object, index) => ({
         object,
         index,
-        id: scalarText(object[target.idField]),
+        id: scalarText(ownMember(object, target.idField)),
       }))
       .filter(
         (entry): entry is { object: JsonObject; index: number; id: string } =>
diff --git a/src/domain/analysisView/environmentStateWriteOmitted.ts b/src/domain/analysisView/environmentStateWriteOmitted.ts
index 10dc019bde..6a076d6e41 100644
--- a/src/domain/analysisView/environmentStateWriteOmitted.ts
+++ b/src/domain/analysisView/environmentStateWriteOmitted.ts
@@ -1,4 +1,5 @@
 import {
+  ownMember,
   at,
   isObject,
   listItems,
@@ -120,11 +121,11 @@ const heldEntities = (
       ? (listItems(read.body!, view.pointer) ?? [])
       : isObject(object) &&
           read.itemId !== undefined &&
-          scalarText(object[read.idField]) === read.itemId
+          scalarText(ownMember(object, read.idField)) === read.itemId
         ? [object]
         : [];
   return shown.filter((entity) => {
-    const id = scalarText(entity[read.idField]);
+    const id = scalarText(ownMember(entity, read.idField));
     return (
       id !== undefined &&
       state.entity(view.type, read.collection, id) !== undefined
diff --git a/src/domain/analysisView/environmentStateWriteRules.ts b/src/domain/analysisView/environmentStateWriteRules.ts
index 8af5ab58a3..041b9d212a 100644
--- a/src/domain/analysisView/environmentStateWriteRules.ts
+++ b/src/domain/analysisView/environmentStateWriteRules.ts
@@ -1,5 +1,6 @@
 import type { JsonValue } from "../jsonValue.js";
 import {
+  ownMember,
   decodedSegment,
   isParam,
   normalizedPath,
@@ -148,10 +149,10 @@ export const siblingTrimmedClocks = (
     const peer = trimmed.get(`${rule.format.separator}${rule.format.zone}`);
     if (peer === undefined) return rule;
     const format = later.some((fields) =>
-      showsFixedWidth(rule.format, fields[rule.field]),
+      showsFixedWidth(rule.format, ownMember(fields, rule.field)),
     )
       ? rule.format
-      : trimmedLike(rule.format, peer, values?.[rule.field]);
+      : trimmedLike(rule.format, peer, ownMember(values, rule.field));
     return format === rule.format ? rule : { ...rule, format };
   });
 };
@@ -178,7 +179,7 @@ export const keptOrders = (body: JsonObject, stored: JsonObject): string[] =>
   Object.keys(body).filter(
     (member) =>
       sortedStrings(body[member]) !== undefined &&
-      same(stored[member], body[member]),
+      same(ownMember(stored, member), body[member]),
   );
 
 /** One part of a composed string: literal text, or another written field as written, slugged, or as a clock's calendar digits. */
diff --git a/src/domain/analysisView/environmentStateWriteSorted.ts b/src/domain/analysisView/environmentStateWriteSorted.ts
index d361efbfd2..73a5aab75a 100644
--- a/src/domain/analysisView/environmentStateWriteSorted.ts
+++ b/src/domain/analysisView/environmentStateWriteSorted.ts
@@ -1,6 +1,8 @@
 import type { JsonValue } from "../jsonValue.js";
 import { compareUnicodeCodePoints as compare } from "../unicodeCodePointOrder.js";
 import {
+  ownMember,
+  setOwnMember,
   at,
   isObject,
   isStringArray,
@@ -35,7 +37,7 @@ const ordered = (value: JsonValue | undefined): value is string[] =>
 /** Whether no object a read shows lists the field as a string array out of order. */
 const listedSorted = (shown: readonly JsonObject[], field: string): boolean =>
   shown.every((object) => {
-    const value = object[field];
+    const value = ownMember(object, field);
     return !ordered(value) || inOrder(value);
   });
 
@@ -50,7 +52,7 @@ const shownBy = (
   )?.fields;
   return own !== undefined && Object.hasOwn(own, field)
     ? own[field]
-    : entity.fields[field];
+    : ownMember(entity.fields, field);
 };
 
 /** Fields every initial entity shows, as the view renders it, as a sorted string array or null, one with two distinct strings. */
@@ -231,9 +233,9 @@ export class SortedArrays {
       if (!isObject(value)) return value;
       const object = { ...value };
       for (const field of fields) {
-        const array = object[field];
+        const array = ownMember(object, field);
         if (array !== undefined && isStringArray(array))
-          object[field] = array.toSorted(compare);
+          setOwnMember(object, field, array.toSorted(compare));
       }
       return object;
     };
@@ -290,11 +292,11 @@ export class SortedArrays {
     const idField = this.facts.idField(view.type) ?? "";
     const existing = this.#existing.get(`${view.type}\u0000${field}`)!;
     return shown.some((object) => {
-      const value = object[field];
-      const id = scalarText(object[idField]);
+      const value = ownMember(object, field);
+      const id = scalarText(ownMember(object, idField));
       if (!ordered(value) || id === undefined) return false;
       return this.facts.state.named(id).some((entity) => {
-        const written = entity.fields[field];
+        const written = ownMember(entity.fields, field);
         return (
           targets.has(
             entityTarget(entity.type, entity.collection, entity.id),
@@ -332,8 +334,8 @@ export class SortedArrays {
     if (values === undefined) this.#existing.set(key, (values = new Set()));
     const held = entities.flatMap((entity) =>
       [
-        entity.fields[field],
-        ...entity.view_fields.map((view) => view.fields[field]),
+        ownMember(entity.fields, field),
+        ...entity.view_fields.map((view) => ownMember(view.fields, field)),
       ].flatMap((value) =>
         value !== undefined && value !== null && isStringArray(value)
           ? value
diff --git a/src/domain/analysisView/environmentStateWriteStamps.test.ts b/src/domain/analysisView/environmentStateWriteStamps.test.ts
index 92e90cd27a..8e460f3ba4 100644
--- a/src/domain/analysisView/environmentStateWriteStamps.test.ts
+++ b/src/domain/analysisView/environmentStateWriteStamps.test.ts
@@ -630,3 +630,20 @@ it("does not explain nested opaque-token references as independent generation",
     expect(update.unexplained).toContain("version");
   }
 });
+
+it.each(["__proto__", "constructor", "toString"])(
+  "treats an absent %s stamp field as missing JSON data",
+  (field) => {
+    const rule: FieldRule = {
+      rule: "derived",
+      field,
+      member: "change",
+      from: null,
+      value: 1,
+    };
+    expect(stampProblem([rule], {}, { change: true })).toBeUndefined();
+    expect(
+      stampProblem([rule], Object.fromEntries([[field, 7]]), { change: true }),
+    ).toContain("accepted only");
+  },
+);
diff --git a/src/domain/analysisView/environmentStateWriteStamps.ts b/src/domain/analysisView/environmentStateWriteStamps.ts
index 09a1b50f9d..000b72583d 100644
--- a/src/domain/analysisView/environmentStateWriteStamps.ts
+++ b/src/domain/analysisView/environmentStateWriteStamps.ts
@@ -1,6 +1,8 @@
 import type { JsonValue } from "../jsonValue.js";
 import { err, ok, type Result } from "../result.js";
 import {
+  ownMember,
+  setOwnMember,
   IDENTITY_NAMES,
   isObject,
   same,
@@ -81,7 +83,8 @@ export const envelopeOf = (
 ): { readonly member: string; readonly object: JsonObject } | undefined => {
   if (!isObject(body)) return undefined;
   const members = Object.keys(body);
-  const wrapped = members.length === 1 ? body[members[0]!] : undefined;
+  const wrapped =
+    members.length === 1 ? ownMember(body, members[0]!) : undefined;
   return Array.isArray(wrapped) && wrapped.length === 1 && isObject(wrapped[0]!)
     ? { member: members[0]!, object: wrapped[0] }
     : undefined;
@@ -154,7 +157,7 @@ const sharedBesides = (
   others: readonly JsonObject[],
 ): boolean => {
   const held = others.flatMap((other) => {
-    const value = other[field];
+    const value = ownMember(other, field);
     return value !== undefined && isObject(value) ? [value] : [];
   });
   return (
@@ -163,9 +166,9 @@ const sharedBesides = (
       [...new Set([...Object.keys(other), ...Object.keys(object)])].every(
         (key) =>
           members.includes(key)
-            ? readClock(other[key]) !== undefined &&
-              !same(other[key], object[key])
-            : same(other[key], object[key]),
+            ? readClock(ownMember(other, key)) !== undefined &&
+              !same(ownMember(other, key), ownMember(object, key))
+            : same(ownMember(other, key), ownMember(object, key)),
       ),
     )
   );
@@ -179,7 +182,9 @@ const nestedRule = (
   others: readonly JsonObject[],
 ): FieldRule | undefined => {
   const formats = new Set(
-    members.map((key) => JSON.stringify(readClock(object[key])?.format)),
+    members.map((key) =>
+      JSON.stringify(readClock(ownMember(object, key))?.format),
+    ),
   );
   return members.length > 0 &&
     formats.size === 1 &&
@@ -188,7 +193,7 @@ const nestedRule = (
         field,
         rule: "nested-clock",
         members: [...members],
-        format: readClock(object[members[0]!])!.format,
+        format: readClock(ownMember(object, members[0]!))!.format,
       }
     : undefined;
 };
@@ -207,7 +212,7 @@ export const nestedClockRule = (
     return undefined;
   const changed = [
     ...new Set([...Object.keys(before), ...Object.keys(after)]),
-  ].filter((key) => !same(before[key], after[key]));
+  ].filter((key) => !same(ownMember(before, key), ownMember(after, key)));
   return changed.every(
     (key) =>
       Object.hasOwn(after, key) && isClockNear(after[key]!, evidence.startMs),
@@ -232,7 +237,7 @@ export const createdNestedClock = (
   );
   // A clock read before the request, past a whole second's truncation, belongs to an entity made earlier.
   const stamped = members.every((key) => {
-    const ms = readClock(value[key])!.ms;
+    const ms = readClock(ownMember(value, key))!.ms;
     return ms >= startMs - TRUNCATION_MS && ms <= startMs + NESTED_WINDOW_MS;
   });
   return stamped ? nestedRule(field, value, members, evidence.type) : undefined;
@@ -257,13 +262,14 @@ export const createdToken = (
   entity: JsonObject,
   type: readonly JsonObject[],
 ): "independent" | "linked" | undefined => {
-  const value = entity[field];
+  const value = ownMember(entity, field);
   if (
     !isOpaqueToken(value) ||
     type.length === 0 ||
     !type.every(
       (member) =>
-        member[field] !== value && isOpaqueToken(member[field], value),
+        ownMember(member, field) !== value &&
+        isOpaqueToken(ownMember(member, field), value),
     )
   )
     return undefined;
@@ -294,8 +300,8 @@ export const freshToken = (
   evidence: ChangeEvidence,
   field: string,
 ): boolean => {
-  const before = evidence.before[field];
-  const after = evidence.after[field];
+  const before = ownMember(evidence.before, field);
+  const after = ownMember(evidence.after, field);
   return (
     isOpaqueToken(after, before) &&
     isOpaqueToken(before) &&
@@ -337,7 +343,7 @@ export const stampRule = (
   evidence: ChangeEvidence,
   field: string,
 ): FieldRule | undefined => {
-  const after = evidence.after[field];
+  const after = ownMember(evidence.after, field);
   const creator = creatorOf(field);
   if (typeof after !== "string" || after.length === 0 || creator === undefined)
     return undefined;
@@ -395,7 +401,7 @@ export const derivedRule = (
   evidence: ChangeEvidence,
   field: string,
 ): FieldRule | undefined => {
-  const after = evidence.after[field];
+  const after = ownMember(evidence.after, field);
   if (after === undefined || readClock(after) !== undefined) return undefined;
   const { body } = evidence.call;
   const unstored = Object.keys(body).filter(
@@ -403,7 +409,8 @@ export const derivedRule = (
       !Object.hasOwn(evidence.after, member) &&
       !Object.hasOwn(evidence.before, member),
   );
-  const text = unstored.length === 1 ? textOf(body[unstored[0]!]) : undefined;
+  const text =
+    unstored.length === 1 ? textOf(ownMember(body, unstored[0]!)) : undefined;
   const shown = typeof after === "string" ? after : JSON.stringify(after);
   return text !== undefined &&
     text.length >= DERIVED_TEXT_MIN &&
@@ -412,7 +419,7 @@ export const derivedRule = (
         field,
         rule: "derived",
         value: after,
-        from: evidence.before[field] ?? null,
+        from: ownMember(evidence.before, field) ?? null,
         member: unstored[0]!,
       }
     : undefined;
@@ -428,14 +435,14 @@ export const stampProblem = (
     if (
       rule.rule === "derived" &&
       Object.hasOwn(sent, rule.member) &&
-      !same(before[rule.field] ?? null, rule.from)
+      !same(ownMember(before, rule.field) ?? null, rule.from)
     )
       return `Member ${rule.member} is accepted only on an entity whose ${rule.field} is as the recorded call found it, since the application derives ${rule.field} from it.`;
     if (rule.rule === "held-clock" && !Object.hasOwn(sent, rule.field))
       return `Member ${rule.field} must be sent with the value the entity holds, as the recorded call did.`;
     if (
       rule.rule === "held-clock" &&
-      !same(sent[rule.field], before[rule.field])
+      !same(ownMember(sent, rule.field), ownMember(before, rule.field))
     )
       return `Member ${rule.field} must repeat the value the entity holds, as the recorded call did.`;
   }
@@ -448,7 +455,7 @@ const nestedIdentities = (entity: JsonObject): Map<string, Set<string>> => {
   for (const [field, nested] of Object.entries(entity)) {
     if (!isObject(nested)) continue;
     const texts = IDENTITY_NAMES.flatMap((name) => {
-      const text = scalarText(nested[name]);
+      const text = scalarText(ownMember(nested, name));
       return text === undefined ? [] : [text];
     });
     if (texts.length === 0) continue;
@@ -480,7 +487,10 @@ const stateOf = (
     Object.keys(entity).some(
       (field) =>
         field !== member &&
-        others.every((other) => unset(other[field]) !== unset(entity[field])),
+        others.every(
+          (other) =>
+            unset(ownMember(other, field)) !== unset(ownMember(entity, field)),
+        ),
     )
   );
 };
@@ -527,11 +537,11 @@ export const restampedFields = (
   const fields = { ...shown };
   for (const rule of nested) {
     const read = storedRestamp(
-      shown[rule.field],
-      later?.object[rule.field],
+      ownMember(shown, rule.field),
+      ownMember(later?.object, rule.field),
       rule.members,
     );
-    if (read !== undefined) fields[rule.field] = read;
+    if (read !== undefined) setOwnMember(fields, rule.field, read);
   }
   return fields;
 };
@@ -546,10 +556,13 @@ export const storedRestamp = (
   if (!isObject(own) || !isObject(read)) return undefined;
   for (const key of new Set([...Object.keys(own), ...Object.keys(read)])) {
     if (!members.includes(key)) {
-      if (!same(own[key], read[key])) return undefined;
+      if (!same(ownMember(own, key), ownMember(read, key))) return undefined;
       continue;
     }
-    const [written, stored] = [readClock(own[key]), readClock(read[key])];
+    const [written, stored] = [
+      readClock(ownMember(own, key)),
+      readClock(ownMember(read, key)),
+    ];
     if (
       written === undefined ||
       stored === undefined ||
diff --git a/src/domain/analysisView/environmentStateWrites.test.ts b/src/domain/analysisView/environmentStateWrites.test.ts
index 002a10484e..2ff3fd6a4a 100644
--- a/src/domain/analysisView/environmentStateWrites.test.ts
+++ b/src/domain/analysisView/environmentStateWrites.test.ts
@@ -729,7 +729,6 @@ describe("unserved write plans", () => {
       stateOf(sending()),
       stateOf(sending([{ name: "Third Book" }])),
       recorded("application/x-www-form-urlencoded", "name=Third+Book"),
-      recorded("application/json", '{"__proto__":{},"name":"Third Book"}'),
       recorded(
         "application/json",
         '{"name":"Third Book","copies":12345678901234567890}',
@@ -2602,3 +2601,26 @@ describe("recorded calls serving would refuse", () => {
     }
   });
 });
+
+it.each(["__proto__", "constructor"])(
+  "learns a confirmed update of an own %s field",
+  (field) => {
+    const initial = {
+      id: 1,
+      name: "One",
+      ...Object.fromEntries([[field, { label: "old" }]]),
+    };
+    const body = Object.fromEntries([[field, { label: "stored" }]]);
+    const updated = { ...initial, ...body };
+    const state = stateOf([
+      ["GET", "/api/items", [initial, { id: 2, name: "Two" }]],
+      ["GET", "/api/items/1", initial],
+      ["PATCH", "/api/items/1", updated, 200, body],
+      ["GET", "/api/items/1", updated],
+    ]);
+    expect(planOf(state, "patch_items_by_id")).toMatchObject({
+      served: true,
+      rules: expect.arrayContaining([{ rule: "echo", field }]),
+    });
+  },
+);
diff --git a/src/domain/jsonValue.test.ts b/src/domain/jsonValue.test.ts
index cc6050ea56..a23a8ee949 100644
--- a/src/domain/jsonValue.test.ts
+++ b/src/domain/jsonValue.test.ts
@@ -4,6 +4,7 @@ import { z } from "zod";
 
 import {
   jsonObjectSchema,
+  ownedJsonObjectSchema,
   jsonValueSchema,
   jsonValueValidationIssue,
   jsonValueValidationSteps,
@@ -42,6 +43,23 @@ it("still parses only JSON values at runtime", () => {
     expect(jsonValueSchema.safeParse(value).success).toBe(false);
 });
 
+it("retains owned object identity while caller objects remain isolated", () => {
+  const source = JSON.parse('{"__proto__":{"name":"recorded"},"items":[1]}');
+  const owned = ownedJsonObjectSchema.parse(source);
+  const copied = jsonObjectSchema.parse(source);
+  expect(owned).toBe(source);
+  expect(copied).not.toBe(source);
+  expect(copied["__proto__"]).not.toBe(source["__proto__"]);
+  expect(Object.hasOwn(owned, "__proto__")).toBe(true);
+  expect(JSON.stringify(owned)).toBe(JSON.stringify(copied));
+  for (const input of [null, [], { value: undefined }, { value: Infinity }])
+    expect(ownedJsonObjectSchema.safeParse(input).success).toBe(false);
+  for (const io of ["input", "output"] as const)
+    expect(z.toJSONSchema(ownedJsonObjectSchema, { io })).toEqual(
+      z.toJSONSchema(jsonObjectSchema, { io }),
+    );
+});
+
 it("rejects sparse arrays and enumerable symbol-keyed object members", () => {
   const sparse = new Array(1);
   const symbolKeyed = { [Symbol("caller data")]: "retain or reject" };
diff --git a/src/domain/jsonValue.ts b/src/domain/jsonValue.ts
index c1629837ab..9a54dd1bf8 100644
--- a/src/domain/jsonValue.ts
+++ b/src/domain/jsonValue.ts
@@ -204,3 +204,8 @@ objectValidation._zod.toJSONSchema = objectProjection;
 /** JSON object boundary for caller-visible parameter maps. */
 export const jsonObjectSchema = jsonValueSchema.pipe(objectValidation);
 jsonObjectSchema._zod.toJSONSchema = objectProjection;
+
+/** Validate an exclusively owned JSON object while retaining its existing tree. */
+export const ownedJsonObjectSchema =
+  ownedJsonValueSchema.pipe(objectValidation);
+ownedJsonObjectSchema._zod.toJSONSchema = objectProjection;
diff --git a/src/server/environment/serveEnvironmentState.test.ts b/src/server/environment/serveEnvironmentState.test.ts
index 7ce720bdef..027330a32b 100644
--- a/src/server/environment/serveEnvironmentState.test.ts
+++ b/src/server/environment/serveEnvironmentState.test.ts
@@ -245,7 +245,7 @@ describe("state data plane", () => {
       },
     });
     expect(loginOf(plane.store)).toBe("probe");
-    // Values parsing would alter are refused where they sit, never rounded or turned into a prototype.
+    // Numbers parsing or serialization would alter are refused where they sit.
     const body = JSON.stringify({
       jsonrpc: "2.0",
       id: 2,
@@ -256,7 +256,6 @@ describe("state data plane", () => {
       '"login":9007199254740993',
       // Exact in this spelling, but data/get would write it as digits it could not load again.
       '"login":1e20',
-      '"__proto__":{"polluted":true},"login":"renamed"',
     ]) {
       const reply = await plane.post(
         body.replace('"login":"renamed"', altered),
@@ -267,12 +266,45 @@ describe("state data plane", () => {
           data: { path: expect.stringMatching(/\/fields$/u) },
         },
       });
-      expect(JSON.stringify(reply.json)).not.toContain("polluted");
     }
     expect(loginOf(plane.store)).toBe("probe");
   });
 });
 
+it("preserves arbitrary own data keys in inline and encoded file parts", async () => {
+  const plane = dataPlane();
+  const initial = renamedDocument();
+  const original = initial.rea_state.entities[0]!;
+  const members = JSON.parse(
+    '{"__proto__":{"constructor":0},"toString":false}',
+  );
+  const entity = { ...original, fields: { ...original.fields, ...members } };
+  const document = {
+    rea_state: {
+      ...initial.rea_state,
+      entities: [entity, ...initial.rea_state.entities.slice(1)],
+    },
+  };
+  for (const part of [
+    { kind: "data", data: document },
+    {
+      kind: "file",
+      file: {
+        bytes: Buffer.from(JSON.stringify(document)).toString("base64"),
+      },
+    },
+  ]) {
+    await expect(plane.add(part)).resolves.toMatchObject({ result: {} });
+    const retained = plane.store.entity(
+      entity.type,
+      entity.collection,
+      entity.id,
+    )!;
+    expect(Object.hasOwn(retained.fields, "__proto__")).toBe(true);
+    expect(retained.fields).toEqual(entity.fields);
+  }
+});
+
 describe("state data plane intake", () => {
   it("loads file parts from bytes and from a file uri", async () => {
     const plane = dataPlane();
diff --git a/src/server/environment/stateIntake.ts b/src/server/environment/stateIntake.ts
index 86112345c7..c323fdf780 100644
--- a/src/server/environment/stateIntake.ts
+++ b/src/server/environment/stateIntake.ts
@@ -73,17 +73,16 @@ export const DEFAULT_STATE_INTAKE: StateIntake = {
   timeoutMs: STATE_FETCH_TIMEOUT_MS,
 };
 
-/** Stands in for a value that JSON parsing would change: a number without an exact native value, or a `__proto__` member. */
+/** Stands in for a number that native JSON parsing or serialization would change. */
 const NOT_EXACT_JSON = Symbol("not exact JSON");
 
 /** Mark values native parsing would change, so validation refuses them rather than storing an altered value. */
 export const exactJsonReviver = (
-  key: string,
+  _key: string,
   value: unknown,
   context?: { readonly source?: string },
 ): unknown =>
-  key === "__proto__" ||
-  (typeof value === "number" && !isExactJsonNumber(value, context?.source))
+  typeof value === "number" && !isExactJsonNumber(value, context?.source)
     ? NOT_EXACT_JSON
     : value;
 
diff --git a/tests/boundary/mcp/environmentServeMcp.test.ts b/tests/boundary/mcp/environmentServeMcp.test.ts
index 4215346a7a..83831a6496 100644
--- a/tests/boundary/mcp/environmentServeMcp.test.ts
+++ b/tests/boundary/mcp/environmentServeMcp.test.ts
@@ -1,5 +1,6 @@
 import { createHash } from "node:crypto";
 import { timedCaptureEvidence } from "../../fixtures/analysisView.js";
+import { jsonObjectSchema } from "../../../src/domain/jsonValue.js";
 
 import {
   Client,
@@ -57,6 +58,36 @@ const connect = async (url: URL) => {
   return client;
 };
 
+processTest(
+  "replays recorded own JSON keys without interpreting inherited members as fields",
+  async ({ processes }) => {
+    const body = JSON.parse(
+      '{"id":1,"__proto__":{"source":"recorded"},"nested":{"constructor":null,"__proto__":[false,0,""]}}',
+    );
+    const fixture = environmentStateServeFixture(
+      timedCaptureEvidence([["GET", "/api/books/1", body]]),
+    );
+    const started = await serveReplayOnly(processes, {}, fixture);
+    const client = await connect(started.url);
+    try {
+      const response = await client.callTool({
+        name: "get_books_by_id",
+        arguments: { id: "1" },
+      });
+      expect(response.isError).toBeFalsy();
+      expect(response.structuredContent).toMatchObject({ body });
+      const replayed = jsonObjectSchema.parse(
+        jsonObjectSchema.parse(response.structuredContent).body,
+      );
+      expect(Object.hasOwn(replayed, "__proto__")).toBe(true);
+      expect(Object.hasOwn(replayed, "constructor")).toBe(false);
+    } finally {
+      await client.close();
+    }
+  },
+  HTTP_BOUNDARY_TIMEOUT_MS,
+);
+
 processTest(
   "validates repeated query arrays through the advertised MCP contract",
   async ({ processes }) => {
diff --git a/tests/boundary/mcp/environmentServeWritesMcp.test.ts b/tests/boundary/mcp/environmentServeWritesMcp.test.ts
index 46e2973e37..c6c9fdfcb0 100644
--- a/tests/boundary/mcp/environmentServeWritesMcp.test.ts
+++ b/tests/boundary/mcp/environmentServeWritesMcp.test.ts
@@ -32,12 +32,137 @@ import { unsupportedInputLiteralSchemaError } from "../../fixtures/mcpClientSche
 import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
 import { startListeningProcess } from "../../support/mcp/streamableHttpProcess.js";
 import { processTest } from "../../support/process/processFixture.js";
+import {
+  jsonObjectSchema,
+  type JsonValue,
+} from "../../../src/domain/jsonValue.js";
 
 const HTTP_BOUNDARY_TIMEOUT_MS = 90_000;
 
 /** The book the first served create of "Boundary Book" makes: one second past the served clock's start. */
 const BOUNDARY_BOOK = book(3, "Boundary Book", bookClock(13));
 
+processTest(
+  "preserves own JSON keys through capture, state reload and computed writes",
+  async ({ processes }) => {
+    const nested = JSON.parse(
+      '{"__proto__":{"recorded":true},"constructor":null,"values":[false,0,""]}',
+    ) as JsonValue;
+    const captured = (id: number, name: string, updated = bookClock(id)) => {
+      const { slug, ...fields } = book(id, name, bookClock(id), updated);
+      if (typeof slug !== "string") throw new Error("Fixture book has no slug");
+      return { ...fields, ["__proto__"]: slug, nested };
+    };
+    const first = captured(1, "First Book");
+    const second = captured(2, "Second Book");
+    const third = captured(3, "Third Book", bookClock(3));
+    const renamed = captured(1, "Renamed Book", bookClock(5));
+    const rows: readonly TimedCaptureRow[] = [
+      ["GET", "/api/books", [first, second]],
+      ["GET", "/api/books/1", first],
+      ["GET", "/api/books/2", second],
+      ["POST", "/api/books", third, 201, { name: "Third Book" }],
+      ["GET", "/api/books/3", third],
+      ["PATCH", "/api/books/1", renamed, 200, { name: "Renamed Book" }],
+      ["GET", "/api/books/1", renamed],
+    ];
+    const fixture = environmentStateServeFixture(timedCaptureEvidence(rows));
+    expect(fixture.state.normalized_result).toMatchObject({
+      entities: expect.arrayContaining([
+        expect.objectContaining({ id: "1", fields: { ...first } }),
+      ]),
+    });
+    const root = await createTestTempDirectory("rea-own-keys-state-mcp-");
+    const files = await writeEnvironmentServeFiles(root, fixture);
+    const started = await startListeningProcess(processes, [
+      "environment-serve",
+      files.manifestPath,
+      "--evidence",
+      files.capturePath,
+      "--port",
+      "0",
+    ]);
+    type StateDocument = {
+      rea_state: {
+        entities: Array<{ id: string; fields: Record<string, JsonValue> }>;
+        [key: string]: unknown;
+      };
+    };
+    const rpc = async (method: string, params?: unknown) =>
+      (
+        await fetch(new URL("/agentenv", started.url.origin), {
+          method: "POST",
+          headers: { "content-type": "application/json" },
+          body: JSON.stringify({ jsonrpc: "2.0", id: 1, method, params }),
+        })
+      ).json() as Promise<{
+        result?: { parts?: Array<{ data: StateDocument }> };
+        error?: { code: number };
+      }>;
+    const document = (await rpc("data/get")).result?.parts?.[0]?.data;
+    if (document === undefined)
+      throw new Error("Missing served state document");
+    const held = document.rea_state.entities.find(({ id }) => id === "1");
+    if (held === undefined) throw new Error("Missing initial book");
+    expect(Object.hasOwn(held.fields, "__proto__")).toBe(true);
+    expect(held.fields.nested).toEqual(nested);
+    expect(Object.hasOwn(held.fields, "constructor")).toBe(false);
+    await expect(
+      rpc("data/add", { parts: [{ kind: "data", data: document }] }),
+    ).resolves.toMatchObject({ result: {} });
+    const reloaded = (await rpc("data/get")).result?.parts?.[0]?.data;
+    expect(reloaded).toEqual(document);
+    const invalid = {
+      rea_state: {
+        ...document.rea_state,
+        ["__proto__"]: "unexpected structural field",
+      },
+    };
+    await expect(
+      rpc("data/add", { parts: [{ kind: "data", data: invalid }] }),
+    ).resolves.toMatchObject({ error: { code: -32602 } });
+    delete held.fields["__proto__"];
+    await expect(
+      rpc("data/add", { parts: [{ kind: "data", data: document }] }),
+    ).resolves.toMatchObject({ result: {} });
+    const client = new Client({ name: "own-keys-writes", version: "1" });
+    await client.connect(new StreamableHTTPClientTransport(started.url));
+    try {
+      for (const [tool, args, name, id] of [
+        [
+          "patch_books_by_id",
+          { id: "1", body: { name: "Updated Book" } },
+          "Updated Book",
+          "1",
+        ],
+        ["post_books", { body: { name: "Created Book" } }, "Created Book", "3"],
+      ] as const) {
+        const written = await client.callTool({ name: tool, arguments: args });
+        expect(written.isError).toBeFalsy();
+        const expected = {
+          name,
+          ["__proto__"]: name.toLowerCase().replaceAll(" ", "-"),
+          nested,
+        };
+        expect(written.structuredContent).toMatchObject({ body: expected });
+        const read = await client.callTool({
+          name: "get_books_by_id",
+          arguments: { id },
+        });
+        expect(read.structuredContent).toMatchObject({ body: expected });
+        const body = jsonObjectSchema.parse(
+          jsonObjectSchema.parse(read.structuredContent).body,
+        );
+        expect(Object.hasOwn(body, "__proto__")).toBe(true);
+        expect(Object.hasOwn(body, "constructor")).toBe(false);
+      }
+    } finally {
+      await client.close();
+    }
+  },
+  HTTP_BOUNDARY_TIMEOUT_MS,
+);
+
 processTest(
   "keeps create response clocks separate from enriched item reads through the SDK",
   async ({ processes }) => {