diff --git a/libs/ag-ui/src/lib/internal/apply-patch.spec.ts b/libs/ag-ui/src/lib/internal/apply-patch.spec.ts
index 8cb30fca3..c06c2b9c5 100644
--- a/libs/ag-ui/src/lib/internal/apply-patch.spec.ts
+++ b/libs/ag-ui/src/lib/internal/apply-patch.spec.ts
@@ -158,3 +158,167 @@ describe('applyPatch — move/copy/test', () => {
     ).toThrowError(/'test' op failed/);
   });
 });
+
+describe('applyPatch — existing own paths', () => {
+  it.each([
+    { op: 'replace', path: '/missing', value: 2 },
+    { op: 'copy', from: '/missing', path: '/copy' },
+    { op: 'move', from: '/missing', path: '/moved' },
+    { op: 'test', path: '/missing', value: undefined },
+    { op: 'copy', from: '/toString', path: '/copy' },
+    { op: 'remove', path: '/constructor' },
+  ] satisfies JsonPatchOp[])('rejects absent or inherited members: $op $path', (op) => {
+    expect(() => applyPatch({ a: 1 }, [op])).toThrow();
+  });
+
+  it.each(['constructor', 'toString', '__proto__'])('never traverses inherited %s', (key) => {
+    const input = { nested: {} };
+    for (const op of [
+      { op: 'add', path: `/nested/${key}/field`, value: 1 },
+      { op: 'replace', path: `/nested/${key}`, value: 1 },
+      { op: 'remove', path: `/nested/${key}` },
+      { op: 'copy', from: `/nested/${key}`, path: '/copy' },
+    ] satisfies JsonPatchOp[]) {
+      expect(() => applyPatch(input, [op])).toThrow();
+    }
+  });
+
+  it('distinguishes an own undefined value from an absent member', () => {
+    const input = Object.freeze({ present: undefined });
+    expect(applyPatch(input, [{ op: 'test', path: '/present', value: undefined }])).toBe(input);
+    expect(applyPatch(input, [{ op: 'replace', path: '/present', value: 1 }])).toEqual({ present: 1 });
+    expect(applyPatch(input, [{ op: 'remove', path: '/present' }])).toEqual({});
+    const copied = applyPatch(input, [{ op: 'copy', from: '/present', path: '/copy' }]);
+    expect(Object.hasOwn(copied, 'copy')).toBe(true);
+    expect(copied).toEqual({ present: undefined, copy: undefined });
+    expect(applyPatch(input, [{ op: 'move', from: '/present', path: '/moved' }])).toEqual({ moved: undefined });
+  });
+
+  it.each(['missing', 'scalar', 'nil', 'present'])('rejects non-container or missing intermediate %s', (key) => {
+    const input = { scalar: 1, nil: null, present: undefined };
+    expect(() => applyPatch(input, [{ op: 'add', path: `/${key}/child`, value: 1 }])).toThrow();
+  });
+});
+
+describe('applyPatch — array path bounds', () => {
+  it.each(['1', '2', '0'])('rejects missing array source or target index %s', (index) => {
+    // Index 0 is a hole; 1 is the end; 2 exceeds the end.
+    const input = { items: Object.freeze(new Array(1)) };
+    for (const op of [
+      { op: 'copy', from: `/items/${index}`, path: '/copy' },
+      { op: 'move', from: `/items/${index}`, path: '/moved' },
+      { op: 'test', path: `/items/${index}`, value: undefined },
+      { op: 'replace', path: `/items/${index}`, value: 2 },
+      { op: 'remove', path: `/items/${index}` },
+      { op: 'add', path: `/items/${index}/child`, value: 2 },
+    ] satisfies JsonPatchOp[]) {
+      expect(() => applyPatch(input, [op])).toThrow();
+    }
+    expect(Object.hasOwn(input.items, 0)).toBe(false);
+  });
+
+  it.each(['-', '-1', '01', '1.0', '1e0'])('rejects non-element index %s for reads', (index) => {
+    expect(() => applyPatch([1, 2], [{ op: 'copy', from: `/${index}`, path: '/0' }])).toThrow();
+  });
+
+  it('accepts own undefined array elements', () => {
+    expect(applyPatch([undefined], [{ op: 'test', path: '/0', value: undefined }])).toEqual([undefined]);
+    expect(applyPatch([undefined], [{ op: 'replace', path: '/0', value: 2 }])).toEqual([2]);
+    expect(applyPatch([undefined], [{ op: 'remove', path: '/0' }])).toEqual([]);
+  });
+
+  it.each(['2', '-'])('inserts at the array end using %s', (index) => {
+    expect(applyPatch([1, 2], [{ op: 'add', path: `/${index}`, value: 3 }])).toEqual([1, 2, 3]);
+  });
+
+  it('rejects add beyond the array end', () => {
+    expect(() => applyPatch([1, 2], [{ op: 'add', path: '/3', value: 3 }])).toThrow();
+  });
+
+  it('preserves array move shifts and nested traversal', () => {
+    expect(applyPatch(['a', 'b', 'c'], [{ op: 'move', from: '/0', path: '/2' }])).toEqual(['b', 'c', 'a']);
+    const input = [{ value: 1 }];
+    expect(applyPatch(input, [{ op: 'replace', path: '/0/value', value: 2 }])).toEqual([{ value: 2 }]);
+    expect(applyPatch(input, [{ op: 'copy', from: '/0', path: '/1' }])).toEqual([{ value: 1 }, { value: 1 }]);
+  });
+});
+
+describe('applyPatch — safe own data keys', () => {
+  it.each(['__proto__', 'constructor', 'prototype'])('adds and operates on own %s data safely', (key) => {
+    const globalDescriptors = Object.getOwnPropertyDescriptors(Object.prototype);
+    const input = Object.freeze({});
+    const added = applyPatch<Record<string, unknown>>(input, [{ op: 'add', path: `/${key}`, value: { value: 1 } }]);
+    expect(Object.hasOwn(added, key)).toBe(true);
+    expect(Object.getPrototypeOf(added)).toBe(Object.prototype);
+    expect(added[key]).toEqual({ value: 1 });
+    expect(Object.getPrototypeOf(input)).toBe(Object.prototype);
+    expect(Object.hasOwn(input, key)).toBe(false);
+    const updated = applyPatch(added, [
+      { op: 'test', path: `/${key}/value`, value: 1 },
+      { op: 'replace', path: `/${key}/value`, value: 2 },
+      { op: 'copy', from: `/${key}`, path: '/copied' },
+      { op: 'remove', path: `/${key}/value` },
+    ]);
+    expect(updated[key]).toEqual({});
+    expect(updated['copied']).toEqual({ value: 2 });
+    expect(Object.getPrototypeOf(updated)).toBe(Object.prototype);
+    expect(applyPatch(updated, [{ op: 'replace', path: `/${key}`, value: 3 }])[key]).toBe(3);
+    expect(Object.hasOwn(applyPatch(updated, [{ op: 'remove', path: `/${key}` }]), key)).toBe(false);
+    expect(Object.getOwnPropertyDescriptors(Object.prototype)).toEqual(globalDescriptors);
+  });
+
+  it('accepts explicit own special keys from null-prototype input', () => {
+    const input = Object.freeze(Object.assign(Object.create(null), JSON.parse('{"__proto__":{"value":1}}')));
+    const out = applyPatch(input, [{ op: 'replace', path: '/__proto__/value', value: 2 }]);
+    expect(Object.getPrototypeOf(input)).toBe(null);
+    expect(Object.hasOwn(out, '__proto__')).toBe(true);
+    expect(Object.getPrototypeOf(out)).toBe(Object.prototype);
+    expect(out['__proto__']).toEqual({ value: 2 });
+    expect(input['__proto__']).toEqual({ value: 1 });
+  });
+});
+
+describe('applyPatch — ownership and atomic failure', () => {
+  it.each([
+    { op: 'replace', path: '/missing', value: 2 },
+    { op: 'copy', from: '/missing', path: '/copy' },
+    { op: 'move', from: '/missing', path: '/moved' },
+  ] satisfies JsonPatchOp[])('leaves frozen input and inserted payload unchanged after failing $op', (invalid) => {
+    const items = Object.freeze([1, 2]);
+    const branch = Object.freeze({ value: 1 });
+    const input = Object.freeze({ items, branch });
+    const payload = Object.freeze({ nested: Object.freeze({ value: 1 }) });
+    expect(() => applyPatch(input, [
+      { op: 'replace', path: '/items/0', value: 9 },
+      { op: 'add', path: '/added', value: payload },
+      { op: 'replace', path: '/added/nested/value', value: 9 },
+      invalid,
+    ])).toThrow();
+    expect(input).toEqual({ items: [1, 2], branch: { value: 1 } });
+    expect(input.items).toBe(items);
+    expect(input.branch).toBe(branch);
+    expect(payload.nested.value).toBe(1);
+  });
+
+  it('shares untouched branches and detaches inserted mutable values', () => {
+    const input = Object.freeze({ changed: Object.freeze({ value: 1 }), untouched: Object.freeze({ value: 2 }) });
+    const payload = { nested: { value: 3 } };
+    const out = applyPatch<Record<string, unknown>>(input, [
+      { op: 'replace', path: '/changed/value', value: 4 },
+      { op: 'add', path: '/added', value: payload },
+    ]);
+    expect(out['untouched']).toBe(input.untouched);
+    expect(out['changed']).not.toBe(input.changed);
+    expect(out['added']).not.toBe(payload);
+    payload.nested.value = 5;
+    expect(out['added']).toEqual({ nested: { value: 3 } });
+  });
+
+  it('preserves root replacement and rejects root removal', () => {
+    const value = { nested: { value: 2 } };
+    const out = applyPatch({ old: 1 }, [{ op: 'replace', path: '', value }]);
+    expect(out).toEqual(value);
+    expect(out).not.toBe(value);
+    expect(() => applyPatch(out, [{ op: 'remove', path: '' }])).toThrow('Cannot remove root');
+  });
+});
diff --git a/libs/ag-ui/src/lib/internal/apply-patch.ts b/libs/ag-ui/src/lib/internal/apply-patch.ts
index ee337209e..bf56fe2a3 100644
--- a/libs/ag-ui/src/lib/internal/apply-patch.ts
+++ b/libs/ag-ui/src/lib/internal/apply-patch.ts
@@ -15,8 +15,9 @@ export interface JsonPatchOp {
  * new document. The input is not mutated.
  *
  * Operations apply in order; if any operation fails (invalid path, failed
- * test, etc.) the whole patch throws — matching `fast-json-patch`'s
- * `validate: false` behavior used by the reducer.
+ * test, etc.) the whole patch throws without changing the input. Paths must
+ * resolve through existing own members; only a final add may create a member
+ * or insert at the array end. Root add/replace is supported, root remove is not.
  */
 export function applyPatch<T>(target: T, ops: readonly JsonPatchOp[]): T {
   let current: unknown = target;
@@ -28,8 +29,8 @@ export function applyPatch<T>(target: T, ops: readonly JsonPatchOp[]): T {
 
 function applyOne(doc: unknown, op: JsonPatchOp): unknown {
   switch (op.op) {
-    case 'add':     return setAt(doc, parsePointer(op.path), op.value, /*replaceArrayDash*/ true);
-    case 'replace': return setAt(doc, parsePointer(op.path), op.value, /*replaceArrayDash*/ false);
+    case 'add':     return setAt(doc, parsePointer(op.path), op.value, /*add*/ true);
+    case 'replace': return setAt(doc, parsePointer(op.path), op.value, /*add*/ false);
     case 'remove':  return removeAt(doc, parsePointer(op.path));
     case 'move': {
       if (op.from == null) throw new Error("'move' op requires 'from'");
@@ -85,10 +86,11 @@ function getAt(doc: unknown, tokens: readonly string[]): unknown {
 
 function stepInto(node: unknown, token: string): unknown {
   if (Array.isArray(node)) {
-    const i = parseArrayIndex(token, node.length);
+    const i = existingArrayIndex(node, token);
     return node[i];
   }
   if (node !== null && typeof node === 'object') {
+    if (!Object.hasOwn(node, token)) throw new Error(`Cannot read non-existent key "${token}"`);
     return (node as Record<string, unknown>)[token];
   }
   throw new Error(`Cannot traverse non-container at token "${token}"`);
@@ -98,7 +100,7 @@ function setAt(
   doc: unknown,
   tokens: readonly string[],
   value: unknown,
-  allowArrayAppend: boolean,
+  add: boolean,
 ): unknown {
   if (tokens.length === 0) {
     // Replace root.
@@ -106,36 +108,35 @@ function setAt(
   }
   const [head, ...rest] = tokens;
   if (Array.isArray(doc)) {
+    const i = add && rest.length === 0
+      ? head === '-' ? doc.length : parseArrayIndex(head!, doc.length)
+      : existingArrayIndex(doc, head!);
     const arr = doc.slice();
-    const i = head === '-' && allowArrayAppend ? arr.length : parseArrayIndex(head!, arr.length + (allowArrayAppend ? 1 : 0));
     if (rest.length === 0) {
-      if (allowArrayAppend) {
+      if (add) {
         // RFC-6902 add: insert at index, shifting elements right
         arr.splice(i, 0, structuredCloneSafe(value));
       } else {
         // replace: overwrite at index
-        if (i >= arr.length) throw new Error(`Cannot replace beyond array length at "/${tokens.join('/')}"`);
         arr[i] = structuredCloneSafe(value);
       }
     } else {
-      if (i >= arr.length) throw new Error(`Cannot descend into non-existent array index ${i}`);
-      arr[i] = setAt(arr[i], rest, value, allowArrayAppend);
+      arr[i] = setAt(arr[i], rest, value, add);
     }
     return arr;
   }
   if (doc === null || typeof doc !== 'object') {
     throw new Error(`Cannot descend into non-container at "/${tokens.join('/')}"`);
   }
-  const obj = { ...(doc as Record<string, unknown>) };
+  const obj = doc as Record<string, unknown>;
+  if ((!add || rest.length > 0) && !Object.hasOwn(obj, head!)) {
+    throw new Error(`Cannot update missing path "/${tokens.join('/')}"`);
+  }
   if (rest.length === 0) {
-    obj[head!] = structuredCloneSafe(value);
-  } else {
-    if (!(head! in obj)) {
-      throw new Error(`Cannot descend into missing path "/${tokens.join('/')}"`);
-    }
-    obj[head!] = setAt(obj[head!], rest, value, allowArrayAppend);
+    // Computed properties create own data even for the key "__proto__".
+    return { ...obj, [head!]: structuredCloneSafe(value) };
   }
-  return obj;
+  return { ...obj, [head!]: setAt(obj[head!], rest, value, add) };
 }
 
 function removeAt(doc: unknown, tokens: readonly string[]): unknown {
@@ -144,9 +145,8 @@ function removeAt(doc: unknown, tokens: readonly string[]): unknown {
   }
   const [head, ...rest] = tokens;
   if (Array.isArray(doc)) {
+    const i = existingArrayIndex(doc, head!);
     const arr = doc.slice();
-    const i = parseArrayIndex(head!, arr.length);
-    if (i >= arr.length) throw new Error(`Cannot remove non-existent array index ${i}`);
     if (rest.length === 0) {
       arr.splice(i, 1);
     } else {
@@ -157,15 +157,20 @@ function removeAt(doc: unknown, tokens: readonly string[]): unknown {
   if (doc === null || typeof doc !== 'object') {
     throw new Error(`Cannot remove from non-container at token "${head}"`);
   }
-  const obj = { ...(doc as Record<string, unknown>) };
+  if (!Object.hasOwn(doc, head!)) throw new Error(`Cannot remove non-existent key "${head}"`);
+  const obj = doc as Record<string, unknown>;
   if (rest.length === 0) {
-    if (!(head! in obj)) throw new Error(`Cannot remove non-existent key "${head}"`);
-    delete obj[head!];
-  } else {
-    if (!(head! in obj)) throw new Error(`Cannot descend into missing path "${head}"`);
-    obj[head!] = removeAt(obj[head!], rest);
+    const out = { ...obj };
+    delete out[head!];
+    return out;
   }
-  return obj;
+  return { ...obj, [head!]: removeAt(obj[head!], rest) };
+}
+
+function existingArrayIndex(array: readonly unknown[], token: string): number {
+  const i = parseArrayIndex(token, array.length - 1);
+  if (!Object.hasOwn(array, i)) throw new Error(`Cannot access non-existent array index ${i}`);
+  return i;
 }
 
 function parseArrayIndex(token: string, lengthBound: number): number {
diff --git a/libs/ag-ui/src/lib/reducer.spec.ts b/libs/ag-ui/src/lib/reducer.spec.ts
index 0a665e2da..e589a6928 100644
--- a/libs/ag-ui/src/lib/reducer.spec.ts
+++ b/libs/ag-ui/src/lib/reducer.spec.ts
@@ -494,6 +494,39 @@ describe('reduceEvent', () => {
     expect(store.state()).toEqual({ a: 2, b: 3 });
   });
 
+  it.each([
+    { op: 'replace', path: '/missing', value: 2 },
+    { op: 'copy', from: '/missing', path: '/copy' },
+  ])('STATE_DELTA rejects a failed $op batch before publishing state or citations', (invalid) => {
+    const store = makeStore();
+    store.messages.set([{ id: 'm1', role: 'assistant', content: 'answer', delivery: staticDelivery('m1') }]);
+    reduceEvent({ type: 'STATE_SNAPSHOT', snapshot: {
+      citations: { m1: [{ id: 'prior', title: 'Prior citation' }] },
+    } } as never, store);
+    const state = store.state();
+    const messages = store.messages();
+    const citations = messages[0].citations;
+    expect(() => reduceEvent({ type: 'STATE_DELTA', delta: [
+      { op: 'replace', path: '/citations/m1', value: [{ id: 'next', title: 'Unpublished citation' }] },
+      invalid,
+    ] } as never, store)).toThrow();
+    expect(store.state()).toBe(state);
+    expect(store.messages()).toBe(messages);
+    expect(store.messages()[0].citations).toBe(citations);
+    expect(citations?.[0].id).toBe('prior');
+    expect(state['citations']).toEqual({ m1: [{ id: 'prior', title: 'Prior citation' }] });
+  });
+
+  it('STATE_DELTA publishes an own __proto__ data member with a safe prototype', () => {
+    const store = makeStore();
+    reduceEvent({ type: 'STATE_DELTA', delta: [
+      { op: 'add', path: '/__proto__', value: { label: 'data' } },
+    ] } as never, store);
+    expect(Object.hasOwn(store.state(), '__proto__')).toBe(true);
+    expect(store.state()['__proto__']).toEqual({ label: 'data' });
+    expect(Object.getPrototypeOf(store.state())).toBe(Object.prototype);
+  });
+
   it('MESSAGES_SNAPSHOT replaces messages wholesale', () => {
     const store = makeStore();
     store.deliveryRun = null;
@@ -910,6 +943,44 @@ describe('ACTIVITY events (F5 subagent activities)', () => {
     expect(store.activities().size).toBe(0);
   });
 
+  it.each([
+    { op: 'replace', path: '/missing', value: 2 },
+    { op: 'copy', from: '/missing', path: '/copy' },
+  ])('ACTIVITY_DELTA retains the original content and membership after a failed $op batch', (invalid) => {
+    const store = makeStore();
+    const content = Object.freeze({ text: 'prior', items: Object.freeze([1, 2]) });
+    reduceEvent({ type: 'ACTIVITY_SNAPSHOT', messageId: 'tc-1', activityType: 'subagent', content } as never, store);
+    const activities = store.activities();
+    const entry = activities.get('tc-1');
+    const warning = vi.spyOn(console, 'warn').mockImplementation(() => undefined);
+    try {
+      expect(() => reduceEvent({ type: 'ACTIVITY_DELTA', messageId: 'tc-1', patch: [
+        { op: 'replace', path: '/text', value: 'unpublished' },
+        { op: 'replace', path: '/items/0', value: 9 },
+        invalid,
+      ] } as never, store)).not.toThrow();
+      expect(entry?.content()).toBe(content);
+      expect(entry?.content()).toEqual({ text: 'prior', items: [1, 2] });
+      expect(store.activities()).toBe(activities);
+      expect(store.activities().get('tc-1')).toBe(entry);
+      expect(warning).toHaveBeenCalledExactlyOnceWith('[ag-ui] dropping malformed ACTIVITY_DELTA patch', expect.any(Error));
+    } finally {
+      warning.mockRestore();
+    }
+  });
+
+  it('ACTIVITY_DELTA publishes an own __proto__ data member with a safe prototype', () => {
+    const store = makeStore();
+    reduceEvent({ type: 'ACTIVITY_SNAPSHOT', messageId: 'tc-1', activityType: 'subagent', content: {} } as never, store);
+    reduceEvent({ type: 'ACTIVITY_DELTA', messageId: 'tc-1', patch: [
+      { op: 'add', path: '/__proto__', value: { label: 'data' } },
+    ] } as never, store);
+    const content = store.activities().get('tc-1')?.content() ?? {};
+    expect(Object.hasOwn(content, '__proto__')).toBe(true);
+    expect(content['__proto__']).toEqual({ label: 'data' });
+    expect(Object.getPrototypeOf(content)).toBe(Object.prototype);
+  });
+
   it('two concurrent subagents are keyed independently', () => {
     const store = makeStore();
     reduceEvent({ type: 'ACTIVITY_SNAPSHOT', messageId: 'a', activityType: 'subagent', content: { text: '' } } as any, store);
diff --git a/scripts/react-parity/baseline.json b/scripts/react-parity/baseline.json
index 49aa81384..cc9250174 100644
--- a/scripts/react-parity/baseline.json
+++ b/scripts/react-parity/baseline.json
@@ -1,11 +1,11 @@
 {
   "schemaVersion": 1,
-  "baselineHead": "3e7da956bd7e12927412a7a533045f69fcf5c27b",
+  "baselineHead": "9a87f53641064041cae89e1e83fd2da071440e5b",
   "sourceState": {
-    "modified": [],
-    "untracked": [
-      "libs/ag-ui/src/runtime/transcript.ts"
-    ]
+    "modified": [
+      "libs/ag-ui/src/lib/internal/apply-patch.ts"
+    ],
+    "untracked": []
   },
   "scope": {
     "libraries": [
@@ -10536,7 +10536,7 @@
       "id": "source:libs/ag-ui/src/lib/internal/apply-patch.ts",
       "kind": "source",
       "path": "libs/ag-ui/src/lib/internal/apply-patch.ts",
-      "sha256": "6a348d876814bc7f1ec10c698bcbca11eb5e224866f0df55546f4e3481d2af1f"
+      "sha256": "7d241f7d6cfbffbb4a562c5e73ff660c628e11e34aa59229dba771dc14c68427"
     },
     {
       "id": "source:libs/ag-ui/src/lib/interrupt-persistence.ts",