diff --git a/src/collaboration/awareness.ts b/src/collaboration/awareness.ts
index df3a414a..677927f4 100644
--- a/src/collaboration/awareness.ts
+++ b/src/collaboration/awareness.ts
@@ -30,6 +30,7 @@ const FALLBACK_CURSOR_COLOR = '#475569';
 const MAX_CURSOR_LABEL_LENGTH = 80;
 const MAX_PUBLIC_IDENTIFIER_LENGTH = 80;
 const MAX_LOCAL_FIELD_SOURCE_LENGTH = 1_024;
+const MAX_REMOTE_FIELD_SOURCE_LENGTH = 1_024;
 type CollaborationUserField = 'userId' | 'displayName' | 'cursorColor';
 
 /** Reject malformed or oversized local identity fields before normalization. */
@@ -85,6 +86,23 @@ function exceedsPublicIdentifierLength(value: string): boolean {
   return false;
 }
 
+/** Read one own enumerable data field without invoking caller-defined accessors. */
+function ownEnumerableDataValue(
+  value: unknown,
+  property: string,
+): unknown {
+  if (typeof value !== 'object' || value === null) return undefined;
+  try {
+    const descriptor = Object.getOwnPropertyDescriptor(value, property);
+    if (!descriptor) return undefined;
+    if (!descriptor.enumerable) return undefined;
+    if (!('value' in descriptor)) return undefined;
+    return descriptor.value;
+  } catch {
+    return undefined;
+  }
+}
+
 /** Read and validate the host-owned awareness capability without leaking failures. */
 function readCompatibleCollaborationAwareness(
   provider: CollaborationProviderLike,
@@ -248,7 +266,7 @@ export function createScopedCollaborationProvider(
   };
 }
 
-/** Count remote awareness clients without leaking host awareness failures. */
+/** Count valid remote collaborators without leaking host awareness failures. */
 export function countRemoteCollaborators(
   awareness: CollaborationAwareness | undefined,
 ): number {
@@ -258,15 +276,20 @@ export function countRemoteCollaborators(
     let count = 0;
     for (const [clientId, state] of awareness.getStates()) {
       if (clientId === localClientId) continue;
-      const user = state.user;
+      const user = ownEnumerableDataValue(state, 'user');
+      if (typeof user !== 'object' || user === null) continue;
+      const id = ownEnumerableDataValue(user, 'id');
+      if (typeof id !== 'string') continue;
+      if (id.length > MAX_REMOTE_FIELD_SOURCE_LENGTH) continue;
+      const normalizedId = id.trim();
       if (
-        typeof user === 'object' &&
-        user !== null &&
-        typeof (user as Record<string, unknown>).id === 'string' &&
-        (user as Record<string, unknown>).id !== ''
+        normalizedId === '' ||
+        NUMERIC_IDENTIFIER_PATTERN.test(normalizedId) ||
+        exceedsPublicIdentifierLength(normalizedId)
       ) {
-        count += 1;
+        continue;
       }
+      count += 1;
     }
     return count;
   } catch {
diff --git a/src/collaboration/awarenessIdentityCount.test.ts b/src/collaboration/awarenessIdentityCount.test.ts
new file mode 100644
index 00000000..8ad38703
--- /dev/null
+++ b/src/collaboration/awarenessIdentityCount.test.ts
@@ -0,0 +1,143 @@
+import { describe, expect, it, vi } from 'vitest';
+import { countRemoteCollaborators } from './awareness.js';
+import type { CollaborationAwareness } from './types.js';
+
+function awarenessWith(
+  states: Map<number, Record<string, unknown>>,
+): CollaborationAwareness {
+  return {
+    clientID: 11,
+    states,
+    getLocalState: () => states.get(11) ?? null,
+    getStates: () => states,
+    setLocalStateField: () => undefined,
+    on: () => undefined,
+    off: () => undefined,
+  };
+}
+
+describe('collaboration awareness identity counting', () => {
+  it('excludes remote identities that violate the public identifier contract', () => {
+    const states = new Map<number, Record<string, unknown>>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, { user: { id: '   ' } }],
+      [13, { user: { id: '12345' } }],
+      [14, { user: { id: `editor-${'a'.repeat(74)}` } }],
+      [15, { user: { id: 'editor-bob' } }],
+      [16, { user: { id: `${'a'.repeat(79)}😀` } }],
+    ]);
+
+    expect(countRemoteCollaborators(awarenessWith(states))).toBe(2);
+  });
+
+  it('rejects oversized remote identifiers before normalization', () => {
+    const oversizedId = `editor-${'a'.repeat(1_024)}`;
+    const states = new Map<number, Record<string, unknown>>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, { user: { id: oversizedId } }],
+      [13, { user: { id: ' editor-bob ' } }],
+    ]);
+    const trimSpy = vi.spyOn(String.prototype, 'trim');
+
+    try {
+      expect(countRemoteCollaborators(awarenessWith(states))).toBe(1);
+      expect(
+        trimSpy.mock.instances.some(
+          (receiver) => String(receiver) === oversizedId,
+        ),
+      ).toBe(false);
+    } finally {
+      trimSpy.mockRestore();
+    }
+  });
+
+  it('skips accessor-backed remote identity fields without executing caller code', () => {
+    let userGetterCalls = 0;
+    let idGetterCalls = 0;
+    const accessorBackedState: Record<string, unknown> = {};
+    Object.defineProperty(accessorBackedState, 'user', {
+      enumerable: true,
+      get() {
+        userGetterCalls += 1;
+        throw new Error('private remote user getter must not execute');
+      },
+    });
+    const accessorBackedUser: Record<string, unknown> = {};
+    Object.defineProperty(accessorBackedUser, 'id', {
+      enumerable: true,
+      get() {
+        idGetterCalls += 1;
+        throw new Error('private remote id getter must not execute');
+      },
+    });
+    const states = new Map<number, Record<string, unknown>>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, accessorBackedState],
+      [13, { user: accessorBackedUser }],
+      [14, { user: { id: 'editor-bob' } }],
+    ]);
+
+    expect(countRemoteCollaborators(awarenessWith(states))).toBe(1);
+    expect(userGetterCalls).toBe(0);
+    expect(idGetterCalls).toBe(0);
+  });
+
+  it('ignores inherited and non-enumerable remote identity fields', () => {
+    const inheritedState = Object.create({
+      user: { id: 'editor-inherited' },
+    }) as Record<string, unknown>;
+    const nonEnumerableState: Record<string, unknown> = {};
+    Object.defineProperty(nonEnumerableState, 'user', {
+      enumerable: false,
+      value: { id: 'editor-hidden' },
+    });
+    const nonEnumerableIdUser: Record<string, unknown> = {};
+    Object.defineProperty(nonEnumerableIdUser, 'id', {
+      enumerable: false,
+      value: 'editor-hidden-id',
+    });
+    const states = new Map<number, Record<string, unknown>>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, inheritedState],
+      [13, nonEnumerableState],
+      [14, { user: { name: 'missing-id' } }],
+      [15, { user: nonEnumerableIdUser }],
+      [16, { user: { id: 'editor-bob' } }],
+    ]);
+
+    expect(countRemoteCollaborators(awarenessWith(states))).toBe(1);
+  });
+
+  it('skips null and primitive remote states before descriptor reflection', () => {
+    const runtimeStates = new Map<number, unknown>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, null],
+      [13, 'not-an-awareness-state'],
+      [14, { user: { id: 'editor-bob' } }],
+    ]);
+    const states = runtimeStates as unknown as Map<
+      number,
+      Record<string, unknown>
+    >;
+
+    expect(countRemoteCollaborators(awarenessWith(states))).toBe(1);
+  });
+
+  it('skips reflection-hostile remote identity shapes without leaking failures', () => {
+    const hostileState = new Proxy<Record<string, unknown>>(
+      {},
+      {
+        getOwnPropertyDescriptor() {
+          throw new Error('private remote descriptor trap must not escape');
+        },
+      },
+    );
+    const states = new Map<number, Record<string, unknown>>([
+      [11, { user: { id: 'local-editor' } }],
+      [12, hostileState],
+      [13, { user: { id: 'editor-bob' } }],
+    ]);
+
+    expect(countRemoteCollaborators(awarenessWith(states))).toBe(1);
+  });
+});