diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 786eb8d..9cd53f0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -8,6 +8,10 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    timeout-minutes: 15
+    env:
+      npm_config_jobs: 2
+      GOMAXPROCS: 2
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
@@ -15,10 +19,17 @@ jobs:
           node-version: '20'
           cache: npm
       - run: npm ci
+      - name: Verify bounded Vitest options
+        run: |
+          ./node_modules/.bin/vitest --help > "$RUNNER_TEMP/vitest-help.txt"
+          cat "$RUNNER_TEMP/vitest-help.txt"
+          grep -q -- '--maxWorkers' "$RUNNER_TEMP/vitest-help.txt"
+          grep -q -- '--minWorkers' "$RUNNER_TEMP/vitest-help.txt"
+          grep -Eq -- '--(fileParallelism|file-parallelism)' "$RUNNER_TEMP/vitest-help.txt"
       - name: Typecheck
         run: npm run typecheck
       - name: Test
-        run: npm test
+        run: npm test -- --maxWorkers=2 --minWorkers=1 --no-file-parallelism
       - name: Build (emits dist/index.d.ts)
         run: npm run build
       # `npm run build` runs `check:dts`, which fails the job if the declaration
@@ -26,3 +37,13 @@ jobs:
       # this explicit assertion is what actually gates the type contract).
       - name: Assert declaration file shipped
         run: test -f dist/index.d.ts
+
+  canonicalize-regression:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+      - run: npm run test:canonicalize
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..f069dd6
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,20 @@
+# Changelog
+
+## Unreleased
+
+### Fixed
+
+- Canonicalization now preserves an own JSON `__proto__` key as an enumerable
+  data property without changing the result object's ordinary prototype.
+  Nested cloning, sorted object keys, array order and omission of undefined
+  object values are preserved. Consumers storing canonical strings or hashes
+  for objects with this key should regenerate those values: earlier versions
+  could omit the key from serialized output.
+
+### Tests
+
+- Added dependency-free canonicalization regression tests, including nested
+  prototype-like keys, descriptors, nonmutation and JSON compatibility.
+  Run `npm run test:canonicalize` with Node 24 or later (native TypeScript
+  loading). This focused command does not change the package's runtime support
+  policy or replace the existing Vitest suite.
diff --git a/package.json b/package.json
index 857bc1d..25ca615 100644
--- a/package.json
+++ b/package.json
@@ -30,7 +30,8 @@
     "test": "vitest run",
     "test:watch": "vitest watch",
     "test:coverage": "vitest run --coverage",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "test:canonicalize": "node --test --test-concurrency=1 tests/canonicalize.test.mjs"
   },
   "keywords": [
     "graph",
diff --git a/src/bundle/canonicalize.ts b/src/bundle/canonicalize.ts
index 078930d..9864176 100644
--- a/src/bundle/canonicalize.ts
+++ b/src/bundle/canonicalize.ts
@@ -46,7 +46,12 @@ export function canonicalize<T>(value: T): T {
       if (v === undefined) {
         continue;
       }
-      result[key] = canonicalize(v);
+      Object.defineProperty(result, key, {
+        value: canonicalize(v),
+        enumerable: true,
+        writable: true,
+        configurable: true,
+      });
     }
     return result as unknown as T;
   }
diff --git a/tests/canonicalize.test.mjs b/tests/canonicalize.test.mjs
new file mode 100644
index 0000000..228229a
--- /dev/null
+++ b/tests/canonicalize.test.mjs
@@ -0,0 +1,52 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { canonicalize, canonicalStringify } from '../src/bundle/canonicalize.ts';
+
+test('preserves own prototype-like JSON keys as ordinary data properties', () => {
+  const source = JSON.parse('{"__proto__":{"polluted":true},"constructor":3,"prototype":4,"a":1}');
+  const result = canonicalize(source);
+  assert.equal(Object.getPrototypeOf(result), Object.prototype);
+  assert.equal(Object.hasOwn(Object.prototype, 'polluted'), false);
+  assert.deepEqual(Object.keys(result), ['__proto__', 'a', 'constructor', 'prototype']);
+  assert.deepEqual(Object.getOwnPropertyDescriptor(result, '__proto__'), {
+    value: { polluted: true }, enumerable: true, writable: true, configurable: true,
+  });
+  assert.deepEqual(JSON.parse(canonicalStringify(source)), source);
+});
+
+test('clones nested objects and arrays without changing the input or array order', () => {
+  const source = JSON.parse('{"list":[{"__proto__":7,"z":3},2,1],"b":{"y":2,"x":1}}');
+  const before = JSON.stringify(source);
+  Object.freeze(source.list[0]);
+  Object.freeze(source.list);
+  Object.freeze(source.b);
+  Object.freeze(source);
+  const result = canonicalize(source);
+  assert.equal(JSON.stringify(source), before);
+  assert.notEqual(result, source);
+  assert.notEqual(result.list, source.list);
+  assert.notEqual(result.list[0], source.list[0]);
+  assert.notEqual(result.b, source.b);
+  assert.deepEqual(result.list, source.list);
+  assert.equal(Object.getPrototypeOf(result.list[0]), Object.prototype);
+  assert.equal(Object.hasOwn(result.list[0], '__proto__'), true);
+});
+
+test('sorts object keys deterministically including own __proto__', () => {
+  const a = JSON.parse('{"z":3,"__proto__":null,"a":{"y":2,"x":1}}');
+  const b = JSON.parse('{"a":{"x":1,"y":2},"__proto__":null,"z":3}');
+  const expected = '{"__proto__":null,"a":{"x":1,"y":2},"z":3}';
+  assert.equal(canonicalStringify(a), expected);
+  assert.equal(canonicalStringify(b), expected);
+});
+
+test('retains undefined omission, null, primitives and array JSON behavior', () => {
+  const source = { missing: undefined, present: null, list: [undefined, false, 0, ''] };
+  const result = canonicalize(source);
+  assert.equal(Object.hasOwn(result, 'missing'), false);
+  assert.deepEqual(result.list, [undefined, false, 0, '']);
+  assert.equal(canonicalStringify(source), '{"list":[null,false,0,""],"present":null}');
+  for (const value of [null, undefined, false, true, 0, 4, 'hello']) {
+    assert.equal(canonicalize(value), value);
+  }
+});