diff --git a/src/domain/javascript/javascriptMemberMutationCoverage.test.ts b/src/domain/javascript/javascriptMemberMutationCoverage.test.ts
new file mode 100644
index 0000000000..2311416559
--- /dev/null
+++ b/src/domain/javascript/javascriptMemberMutationCoverage.test.ts
@@ -0,0 +1,173 @@
+import { expect, it } from "vitest";
+import {
+  analyzeJavaScriptSemantics,
+  indexJavaScriptBindingsWithReceiversSteps,
+} from "./javascriptSemanticAnalysis.js";
+import { onlyCallable } from "./javascriptSemanticAnalysis.fixture.js";
+import { semanticResourceLimitReason } from "./javascriptSemanticResourceLimits.js";
+import { analyzeJavaScriptStaticSource } from "./javascriptStaticAnalysis.js";
+import { parseJavaScriptSource } from "./javascriptSourceParser.js";
+import { completeSemanticSteps } from "./javascriptSemanticTraversal.js";
+
+import {
+  joinSemanticPropertyPaths,
+  semanticPropertyPathKeyMatches,
+} from "./javascriptSemanticPropertyPaths.js";
+
+const resultValue = (body: string) =>
+  onlyCallable(
+    analyzeJavaScriptSemantics(`export function result() { ${body} }`),
+    "result",
+  ).returnSites[0]?.value;
+
+it("retains common copy exclusions and the union of array prefixes", () => {
+  const path = joinSemanticPropertyPaths(
+    [{ excludedKeys: ["1", "2"], startIndex: 1 }],
+    [{ excludedKeys: ["2", "3"], startIndex: 2 }],
+  );
+  const key = path?.[0];
+  if (key === undefined) throw new Error("Expected joined projection");
+  expect(
+    ["0", "1", "2", "3", "4", "named"].filter((name) =>
+      semanticPropertyPathKeyMatches(key, name),
+    ),
+  ).toEqual(["1", "3", "4"]);
+  const withScalar = joinSemanticPropertyPaths(path ?? [], [0]);
+  const scalarKey = withScalar?.[0];
+  if (scalarKey === undefined) throw new Error("Expected joined scalar");
+  expect(
+    ["0", "1", "2", "3", "named"].filter((name) =>
+      semanticPropertyPathKeyMatches(scalarKey, name),
+    ),
+  ).toEqual(["0", "1", "3"]);
+  const exact = [1] as const;
+  expect(joinSemanticPropertyPaths(exact, ["1"])).toBe(exact);
+});
+
+const memberAliasSource = (count: number) => `
+  import { Worker } from "node:worker_threads";
+  const alias0 = globalThis;
+  ${Array.from(
+    { length: count },
+    (_, index) =>
+      `const alias${index + 1} = flag${index} ? alias${index}.globalThis : alias${index}.window;`,
+  ).join("\n")}
+  alias${count}.URL = replacement;
+  new Worker(new URL("./worker.js", import.meta.url));
+`;
+
+it("joins overflowing member-alias histories into disclosed possible effects", () => {
+  const source = memberAliasSource(16);
+  const ir = analyzeJavaScriptSemantics(source);
+  expect(ir.coverage).toMatchObject({
+    status: "partial",
+    omittedCount: null,
+    resourceLimits: ["alias-selections"],
+  });
+  expect(ir.limitations).toContain(
+    semanticResourceLimitReason("alias-selections"),
+  );
+  const facts = analyzeJavaScriptStaticSource(source);
+  expect(facts.limitations).toContain(
+    semanticResourceLimitReason("alias-selections"),
+  );
+  expect(
+    facts.references.filter(({ kind }) => kind === "worker"),
+  ).toMatchObject([{ specifier: null }]);
+});
+
+it("never records a joined selector as a definite receiver replacement", () => {
+  const parsed = parseJavaScriptSource(memberAliasSource(16));
+  if (parsed === null) throw new Error("Expected parsed source");
+  const index = completeSemanticSteps(
+    indexJavaScriptBindingsWithReceiversSteps(parsed.program),
+  );
+  completeSemanticSteps(index.indexMemberMutationFactsSteps());
+  const joined = [...index.bindings.bindingsById.values()]
+    .flatMap(({ memberWrites }) => memberWrites)
+    .filter(({ path }) => path.includes(null));
+  expect(joined.length).toBeGreaterThan(0);
+  expect(
+    joined.every(({ receiverIdentity }) => receiverIdentity === "possible"),
+  ).toBe(true);
+});
+
+it("keeps small selector alternatives exact and preserves an untouched third sibling", () => {
+  const ir = analyzeJavaScriptSemantics(`
+    export function result() {
+      const source = { left: { value: 1 }, right: { value: 3 }, third: { value: 2 } };
+      const selected = flag ? source.left : source.right;
+      selected.value = replacement;
+      return source.third.value;
+    }
+  `);
+  expect(onlyCallable(ir, "result").returnSites[0]?.value).toEqual({
+    status: "literal",
+    value: 2,
+  });
+  expect(ir.coverage.resourceLimits ?? []).not.toContain("alias-selections");
+});
+
+it.each([
+  "const left={value:1}; const right={value:2}; const window={left,right}; const selected=flag?window.left:window.right; selected.value=9; return left.value;",
+  "const left={value:1}; const right={value:2}; const globalThis={window:left,globalThis:right}; const selected=flag?globalThis.window:globalThis.globalThis; selected.value=9; return left.value;",
+  "const local={value:1}; const selected=flag?local:external; selected.value=9; return local.value;",
+])("retains local and unresolved mutation targets: %s", (body) => {
+  expect(resultValue(body)?.status).toBe("unknown");
+});
+
+it.each(["first", "second"])(
+  "retains both captured targets across replacement: %s",
+  (name) => {
+    expect(
+      resultValue(`
+      const first = { value: 1 };
+      const second = { value: 2 };
+      let current = first;
+      const before = current;
+      current = second;
+      const after = current;
+      const selected = flag ? before : after;
+      selected.value = 9;
+      return ${name}.value;
+    `)?.status,
+    ).toBe("unknown");
+  },
+);
+
+it.each([
+  "const first={value:1}; const second={value:2}; const captured=first; let selected=first; selected=second; const alias=flag?captured:captured; alias.value=9; return second.value;",
+  "const first={value:2}; const second={value:1}; let selected=first; selected=second; const captured=selected; const alias=flag?captured:captured; alias.value=9; return first.value;",
+  "const source={left:{value:1},right:{value:2}}; source.left.value=9; return source.right.value;",
+])("preserves captured ownership and unjoined siblings: %s", (body) => {
+  expect(resultValue(body)).toEqual({ status: "literal", value: 2 });
+});
+
+it("preserves exclusions shared by shallow-copy alternatives", () => {
+  expect(
+    resultValue(`
+    const excluded = { value: 2 };
+    const first = { value: 1 };
+    const second = { value: 3 };
+    const source = { excluded, first, second };
+    const { excluded: leftExcluded, first: leftFirst, ...left } = source;
+    const { excluded: rightExcluded, second: rightSecond, ...right } = source;
+    const selected = flag ? left : right;
+    mutate(selected);
+    return excluded.value;
+  `),
+  ).toEqual({ status: "literal", value: 2 });
+});
+
+it("keeps a preceding global self-property replacement unresolved", () => {
+  const facts = analyzeJavaScriptStaticSource(`
+    import { Worker } from "node:worker_threads";
+    globalThis.window = replacement;
+    const selected = flag ? globalThis.window : globalThis;
+    selected.URL = replacement;
+    new Worker(new URL("./worker.js", import.meta.url));
+  `);
+  expect(
+    facts.references.filter(({ kind }) => kind === "worker"),
+  ).toMatchObject([{ specifier: null }]);
+});
diff --git a/src/domain/javascript/javascriptSemanticAmbientWrites.ts b/src/domain/javascript/javascriptSemanticAmbientWrites.ts
index 1ccce96da6..9853f2bea1 100644
--- a/src/domain/javascript/javascriptSemanticAmbientWrites.ts
+++ b/src/domain/javascript/javascriptSemanticAmbientWrites.ts
@@ -21,6 +21,7 @@ import {
   semanticIterationSources,
   type SemanticIterationSource,
 } from "./javascriptSemanticIterationSources.js";
+import { joinSemanticPropertyPaths } from "./javascriptSemanticPropertyPaths.js";
 import { createSemanticReturnedReferences } from "./javascriptSemanticReturnedReferences.js";
 import {
   semanticCopiedReferencePath,
@@ -189,10 +190,8 @@ export const createAmbientPathResolver = (
             continue;
           }
           if (suffix !== undefined && previous.length === suffix.length) {
-            const joined: AmbientPath = previous.map((key, index) =>
-              key === suffix?.[index] ? key : null,
-            );
-            if (joined.every((key, index) => key === previous[index])) {
+            const joined = joinSemanticPropertyPaths(previous, suffix);
+            if (joined === previous) {
               resolvedIdentity = true;
               continue;
             }
diff --git a/src/domain/javascript/javascriptSemanticMemberMutations.ts b/src/domain/javascript/javascriptSemanticMemberMutations.ts
index f010c52852..cc4b186a1a 100644
--- a/src/domain/javascript/javascriptSemanticMemberMutations.ts
+++ b/src/domain/javascript/javascriptSemanticMemberMutations.ts
@@ -73,6 +73,7 @@ import {
 } from "./javascriptSemanticReferenceSources.js";
 import {
   semanticArrayIndex,
+  joinSemanticPropertyPaths,
   SemanticPropertyPathCoverage,
   semanticPropertyPathKeyMatches,
   type JavaScriptSemanticPropertyPath,
@@ -661,31 +662,46 @@ export function* collectSemanticMemberMutationsSteps(
     definiteReceiver = true,
     selection?: MutationSelectionContext,
   ): void => {
-    const pending: (ReferencedValue &
+    type JoinedProjection = {
+      readonly path: PropertyPath;
+      readonly definiteReceiver: boolean;
+      scheduled: boolean;
+    };
+    type PendingValue = ReferencedValue &
       MutationSelectionContext & {
         readonly bindings: ReadonlySet<string>;
         readonly originAt: t.Node | undefined;
         readonly definiteReceiver: boolean;
-      })[] = [
-      {
-        node,
-        path,
-        bindings,
-        originAt,
-        definiteReceiver,
-        memberReadAt: selection?.memberReadAt ?? mutation ?? originAt,
-        capturedSelections: selection?.capturedSelections,
-        ...(ownerInitializer === undefined ? {} : { ownerInitializer }),
-      },
-    ];
-    // Visit shallower alias traversals first so their exact prefixes suppress
-    // longer loop paths before those paths enumerate every assignment order.
-    for (let cursor = 0; cursor < pending.length; cursor++) {
-      const candidate = pending[cursor];
-      if (candidate === undefined) break;
-      // A capture deeper than the selected path is no longer applicable. Keeping
-      // it would manufacture new contexts on a repeated source alias cycle.
-      const current = candidate.capturedSelections?.length
+        readonly exactKey?: string;
+        readonly joined?: {
+          readonly key: string;
+          readonly value: JoinedProjection;
+        };
+      };
+    const pending: PendingValue[] = [];
+    // The effect, mutation and assigned sources belong to this invocation.
+    // Alias histories do not select a value: the binding/capture/projection do.
+    // Keep small alternatives exact. Bound additional states across this whole
+    // query before scheduling/projecting or copying capture-history sets, then
+    // visit the complete same-depth may-projection closure with monotone joins.
+    const joinedProjections = new Map<string, JoinedProjection>();
+    const exactProjections = new Map<string, Set<string>>();
+    let additionalAlternatives = 0;
+    let projectionsWidened = false;
+    const nodeIdentities = new WeakMap<t.Node, number>();
+    let nextNodeIdentity = 0;
+    const nodeIdentity = (node: t.Node | undefined): number | null => {
+      if (node === undefined) return null;
+      const prior = nodeIdentities.get(node);
+      if (prior !== undefined) return prior;
+      const identity = nextNodeIdentity++;
+      nodeIdentities.set(node, identity);
+      return identity;
+    };
+    const enqueue = (candidate: PendingValue): void => {
+      // A capture deeper than this projection is no longer applicable. Keep
+      // the canonical context before indexing or scheduling alias histories.
+      const value = candidate.capturedSelections?.length
         ? {
             ...candidate,
             capturedSelections: semanticNormalizeCapturedSelections(
@@ -694,6 +710,101 @@ export function* collectSemanticMemberMutationsSteps(
             ),
           }
         : candidate;
+      const expression = unwrapJavaScriptExpression(value.node).node;
+      const binding = t.isIdentifier(expression)
+        ? resolveBinding(expression, expression.name)
+        : undefined;
+      if (binding === undefined) {
+        pending.push(value);
+        return;
+      }
+      const context = JSON.stringify([
+        binding.bindingId,
+        nodeIdentity(value.originAt),
+        nodeIdentity(value.ownerInitializer),
+        nodeIdentity(value.memberReadAt),
+        value.definiteReceiver,
+        value.path.length,
+        value.widenedGetterResult === true,
+      ]);
+      const captures = value.capturedSelections?.map(
+        ({ at, depth, ownEnumerable }) => [
+          nodeIdentity(at),
+          depth,
+          ownEnumerable === true,
+        ],
+      );
+      // Capture vectors are additional precise states, but distinct captured
+      // reads keep separate may-projection joins after the budget is reached.
+      const key = JSON.stringify([context, captures]);
+      if (!projectionsWidened) {
+        const exact = exactProjections.get(context) ?? new Set<string>();
+        const selection = JSON.stringify([captures, value.path]);
+        if (exact.has(selection)) return;
+        if (
+          exact.size > 0 &&
+          ++additionalAlternatives > SEMANTIC_ALIAS_SELECTION_LIMIT
+        ) {
+          projectionsWidened = true;
+          state.valueResourceLimits.add("alias-selections");
+          exactProjections.clear();
+        } else {
+          exact.add(selection);
+          exactProjections.set(context, exact);
+        }
+      }
+      const prior = joinedProjections.get(key);
+      const path =
+        prior === undefined
+          ? value.path
+          : joinSemanticPropertyPaths(prior.path, value.path);
+      if (path === undefined) throw new Error("Projection depth key mismatch");
+      const definiteReceiver =
+        value.definiteReceiver &&
+        (prior?.definiteReceiver ?? true) &&
+        path.every((key) => key !== null && typeof key !== "object");
+      const joined =
+        prior !== undefined &&
+        path === prior.path &&
+        definiteReceiver === prior.definiteReceiver
+          ? prior
+          : { path, definiteReceiver, scheduled: false };
+      joinedProjections.set(key, joined);
+      if (!projectionsWidened) {
+        pending.push({ ...value, exactKey: key });
+        return;
+      }
+      if (joined.scheduled) return;
+      joined.scheduled = true;
+      pending.push({
+        ...value,
+        path,
+        definiteReceiver,
+        joined: { key, value: joined },
+      });
+    };
+    enqueue({
+      node,
+      path,
+      bindings,
+      originAt,
+      definiteReceiver,
+      memberReadAt: selection?.memberReadAt ?? mutation ?? originAt,
+      capturedSelections: selection?.capturedSelections,
+      ...(ownerInitializer === undefined ? {} : { ownerInitializer }),
+    });
+    // Visit shallower alias traversals first so their exact prefixes suppress
+    // longer loop paths before those paths enumerate every assignment order.
+    for (let cursor = 0; cursor < pending.length; cursor++) {
+      const current = pending[cursor];
+      if (current === undefined) break;
+      if (
+        (current.exactKey !== undefined &&
+          joinedProjections.get(current.exactKey)?.scheduled === true) ||
+        (current.joined !== undefined &&
+          joinedProjections.get(current.joined.key) !== current.joined.value)
+      )
+        continue;
       const expression = unwrapJavaScriptExpression(current.node).node;
       const required = semanticRequireOrigin(expression, state);
       if (required !== undefined) {
@@ -1005,7 +1116,7 @@ export function* collectSemanticMemberMutationsSteps(
           // and captures; a local container does not isolate its children.
           if (receiver !== undefined) {
             for (const source of receiver.sources)
-              pending.push({
+              enqueue({
                 node: source.node,
                 path: [...source.path, current.path.at(-1)!],
                 bindings: nested,
@@ -1024,7 +1135,7 @@ export function* collectSemanticMemberMutationsSteps(
           for (const initializer of retainsInitializers
             ? origins.initializers
             : [])
-            pending.push({
+            enqueue({
               node: initializer.node,
               path: [...initializer.projection, ...path],
               bindings: nested,
@@ -1082,7 +1193,7 @@ export function* collectSemanticMemberMutationsSteps(
           expression.property,
           expression.computed,
         );
-        pending.push({
+        enqueue({
           node: expression.object,
           path: [key, ...current.path],
           bindings: current.bindings,
@@ -1155,7 +1266,7 @@ export function* collectSemanticMemberMutationsSteps(
               ...(mutation === undefined ? {} : { mutation }),
             });
           else
-            pending.push({
+            enqueue({
               ...value,
               bindings: current.bindings,
               definiteReceiver:
diff --git a/src/domain/javascript/javascriptSemanticPropertyPaths.ts b/src/domain/javascript/javascriptSemanticPropertyPaths.ts
index 6d1fe61486..1018bbed84 100644
--- a/src/domain/javascript/javascriptSemanticPropertyPaths.ts
+++ b/src/domain/javascript/javascriptSemanticPropertyPaths.ts
@@ -12,6 +12,57 @@ export type JavaScriptSemanticPropertyPath = readonly (
   | JavaScriptSemanticPropertySelection
 )[];
 
+/** Join equal-depth may-effect projections without claiming an exact selector. */
+export function joinSemanticPropertyPaths(
+  previous: readonly (string | null)[],
+  incoming: readonly (string | null)[],
+): readonly (string | null)[] | undefined;
+export function joinSemanticPropertyPaths(
+  previous: JavaScriptSemanticPropertyPath,
+  incoming: JavaScriptSemanticPropertyPath,
+): JavaScriptSemanticPropertyPath | undefined;
+export function joinSemanticPropertyPaths(
+  previous: JavaScriptSemanticPropertyPath,
+  incoming: JavaScriptSemanticPropertyPath,
+): JavaScriptSemanticPropertyPath | undefined {
+  if (previous.length !== incoming.length) return undefined;
+  const joined = previous.map((key, index) =>
+    joinPropertySelections(key, incoming[index]!),
+  );
+  return joined.every((key, index) => key === previous[index])
+    ? previous
+    : joined;
+}
+
+const joinPropertySelections = (
+  previous: JavaScriptSemanticPropertyPath[number],
+  incoming: JavaScriptSemanticPropertyPath[number],
+): JavaScriptSemanticPropertyPath[number] => {
+  if (keyCovers(previous, incoming)) return previous;
+  if (keyCovers(incoming, previous)) return incoming;
+  if (previous === null || incoming === null) return null;
+  if (typeof previous !== "object" && typeof incoming !== "object") return null;
+  if (typeof previous === "object" && typeof incoming === "object")
+    return {
+      excludedKeys: previous.excludedKeys.filter((key) =>
+        incoming.excludedKeys.includes(key),
+      ),
+      ...(previous.startIndex === undefined || incoming.startIndex === undefined
+        ? {}
+        : { startIndex: Math.min(previous.startIndex, incoming.startIndex) }),
+    };
+  const selected = typeof previous === "object" ? previous : incoming;
+  const scalar = typeof previous === "object" ? incoming : previous;
+  if (typeof selected !== "object" || typeof scalar === "object") return null;
+  const index = semanticArrayIndex(scalar);
+  return {
+    excludedKeys: selected.excludedKeys.filter((key) => key !== String(scalar)),
+    ...(selected.startIndex === undefined || index === null
+      ? {}
+      : { startIndex: Math.min(selected.startIndex, index) }),
+  };
+};
+
 /** Read only canonical JavaScript array indices, excluding named properties. */
 export const semanticArrayIndex = (key: string | number): number | null => {
   const index = Number(key);