diff --git a/src/artifacts/apple/NibArchive.ts b/src/artifacts/apple/NibArchive.ts
index bd8ef12caf..c673184f46 100644
--- a/src/artifacts/apple/NibArchive.ts
+++ b/src/artifacts/apple/NibArchive.ts
@@ -134,7 +134,14 @@ export const decodeNibArchive = (
       const value = values.get(index);
       if (value === undefined) continue;
       const key = keys[value.key_index];
-      if (key !== undefined) fields[key] = value.value;
+      if (key !== undefined)
+        // Archive keys are data, including the legacy __proto__ setter name.
+        Object.defineProperty(fields, key, {
+          value: value.value,
+          enumerable: true,
+          configurable: true,
+          writable: true,
+        });
     }
     return [{ id, class_name: className, values: fields }];
   });
@@ -519,7 +526,8 @@ const decodeUtf8 = (
   label: string,
 ): string => {
   try {
-    return new TextDecoder("utf-8", { fatal: true }).decode(
+    // These are length-prefixed names, not standalone text documents.
+    return new TextDecoder("utf-8", { fatal: true, ignoreBOM: true }).decode(
       bytes.subarray(start, end),
     );
   } catch (cause: unknown) {
diff --git a/tests/acceptance/applications/nibFieldIdentity.test.ts b/tests/acceptance/applications/nibFieldIdentity.test.ts
new file mode 100644
index 0000000000..5af16f890f
--- /dev/null
+++ b/tests/acceptance/applications/nibFieldIdentity.test.ts
@@ -0,0 +1,110 @@
+import { Client } from "@modelcontextprotocol/client";
+import { StdioClientTransport } from "@modelcontextprotocol/client/stdio";
+import { createHash } from "node:crypto";
+import { mkdir, readFile, writeFile } from "node:fs/promises";
+import { join, resolve } from "node:path";
+import { expect } from "vitest";
+
+import { encodeNibArchiveFixture } from "../../../src/artifacts/apple/NibArchive.fixture.js";
+import { toolContract } from "../../../src/contracts/toolContracts.js";
+import { interfaceBuilderAnalysisSchema } from "../../../src/domain/apple/interfaceBuilderGraph.js";
+import { thinMach } from "../../../src/domain/binaryTarget.fixture.js";
+import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
+import { cliTest } from "../../support/cli/cliFixture.js";
+
+cliTest(
+  "preserves literal NIB fields and UTF-8 names through CLI and stdio MCP",
+  async ({ cli }) => {
+    const root = await createTestTempDirectory("rea-nib-field-identity-");
+    const app = join(root, "Fixture.app");
+    const contents = join(app, "Contents");
+    await mkdir(join(contents, "Resources"), { recursive: true });
+    await mkdir(join(contents, "MacOS"));
+    await writeFile(
+      join(contents, "MacOS/App"),
+      thinMach(0xfeedfacf, 0x0100000c),
+    );
+    await writeFile(
+      join(contents, "Info.plist"),
+      "<plist><dict><key>CFBundleExecutable</key><string>App</string><key>CFBundleIdentifier</key><string>dev.rea.nib-fields</string></dict></plist>",
+    );
+    const archive = encodeNibArchiveFixture({
+      classes: ["UIView", "\uFEFFCustomView"],
+      objects: [
+        {
+          classIndex: 0,
+          values: {
+            ["__proto__"]: { ref: 1 },
+            constructor: 7,
+            enabled: true,
+            "\uFEFFenabled": false,
+          },
+        },
+        { classIndex: 1, values: { ["__proto__"]: 23 } },
+      ],
+    });
+    const path = join(contents, "Resources/Panel.nib");
+    await writeFile(path, archive);
+    const result = await cli.run({
+      arguments: ["decode-interface-builder", app, "--json"],
+      environment: { REA_LOG_LEVEL: "silent", REA_ANALYSIS_PROVIDER: "auto" },
+    });
+    expect(result.exitCode, result.stdout + result.stderr).toBe(0);
+    const contract = toolContract("decode_interface_builder");
+    const evidence = contract.outputSchema.parse(result.json);
+    const transport = new StdioClientTransport({
+      command: process.execPath,
+      args: [resolve("scripts/rea.mjs"), "mcp"],
+      env: {
+        PATH: process.env.PATH ?? "",
+        REA_LOG_LEVEL: "silent",
+        REA_ANALYSIS_PROVIDER: "auto",
+      },
+      stderr: "pipe",
+    });
+    const client = new Client({ name: "nib-field-identity", version: "1" });
+    try {
+      await client.connect(transport);
+      const opened = await client.callTool({
+        name: "open_binary",
+        arguments: { path: app },
+      });
+      expect(opened.isError, JSON.stringify(opened)).not.toBe(true);
+      const response = await client.callTool({
+        name: contract.name,
+        arguments: {},
+      });
+      expect(response.isError, JSON.stringify(response)).not.toBe(true);
+      const mcp = contract.outputSchema.parse(response.structuredContent);
+      expect(mcp.normalized_result).toEqual(evidence.normalized_result);
+    } finally {
+      try {
+        await client.close();
+      } finally {
+        await transport.close();
+      }
+    }
+    const report = interfaceBuilderAnalysisSchema.parse(
+      evidence.normalized_result,
+    );
+    expect(report.documents[0]?.archive_sha256).toBe(
+      createHash("sha256").update(archive).digest("hex"),
+    );
+    const objects = report.graph.nodes.filter(
+      ({ attributes }) => attributes.interface_builder_object_id !== undefined,
+    );
+    expect(objects).toHaveLength(2);
+    expect(objects[0]?.attributes.nibValues).toEqual({
+      ["__proto__"]: { objectID: "1" },
+      constructor: 7,
+      enabled: true,
+      "\uFEFFenabled": false,
+    });
+    expect(objects[1]?.attributes).toMatchObject({
+      class_name: "\uFEFFCustomView",
+      nibValues: { ["__proto__"]: 23 },
+    });
+    expect(await readFile(path)).toEqual(archive);
+  },
+  60_000,
+);