diff --git a/src/__tests__/api-clients.test.ts b/src/__tests__/api-clients.test.ts
index 2390736..4afd5ef 100644
--- a/src/__tests__/api-clients.test.ts
+++ b/src/__tests__/api-clients.test.ts
@@ -712,6 +712,64 @@ describe('Auxiliary API clients', () => {
     );
   });
 
+  it('RemoteConversation.start sends the optional observability user ID', async () => {
+    global.fetch = jest.fn().mockResolvedValue(
+      new Response(JSON.stringify({ id: 'conv-123' }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      })
+    ) as typeof fetch;
+
+    const agent = new Agent({ llm: { model: 'gpt-4o', api_key: 'k' } });
+    const workspace = new RemoteWorkspace({ host: 'http://example.com', workingDir: '/tmp' });
+    const conversation = new RemoteConversation(agent, workspace, {
+      userId: 'user-42',
+    });
+
+    await conversation.start({ initialMessage: 'hello' });
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      'http://example.com/api/conversations',
+      expect.objectContaining({
+        method: 'POST',
+      })
+    );
+    const [, init] = (global.fetch as jest.Mock).mock.calls[0];
+    expect(JSON.parse(init.body)).toMatchObject({
+      user_id: 'user-42',
+      initial_message: {
+        role: 'user',
+        content: [{ type: 'text', text: 'hello' }],
+      },
+    });
+  });
+
+  it('ConversationManager.createACPConversation sends the optional observability user ID', async () => {
+    global.fetch = jest.fn().mockResolvedValue(
+      new Response(JSON.stringify({ id: 'acp-123' }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      })
+    ) as typeof fetch;
+
+    const manager = new ConversationManager({ host: 'http://example.com' });
+    await manager.createACPConversation(
+      { kind: 'ACPAgent', llm: { model: 'gpt-4o' } },
+      { userId: 'user-42' }
+    );
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      'http://example.com/api/acp/conversations',
+      expect.objectContaining({
+        method: 'POST',
+      })
+    );
+    const [, init] = (global.fetch as jest.Mock).mock.calls[0];
+    expect(JSON.parse(init.body)).toMatchObject({
+      user_id: 'user-42',
+    });
+  });
+
   it('HttpClient can parse blob responses when requested', async () => {
     global.fetch = jest.fn().mockResolvedValue(
       new Response(new Blob(['zip-data']), {
diff --git a/src/conversation/conversation-manager.ts b/src/conversation/conversation-manager.ts
index b1dac5b..6d40871 100644
--- a/src/conversation/conversation-manager.ts
+++ b/src/conversation/conversation-manager.ts
@@ -215,6 +215,7 @@ export class ConversationManager {
       maxIterations?: number;
       stuckDetection?: boolean;
       workingDir?: string;
+      userId?: string;
     } = {}
   ): Promise<RemoteConversation> {
     const workspace = new RemoteWorkspace({
@@ -226,10 +227,12 @@ export class ConversationManager {
     const conversation = new RemoteConversation(agent, workspace, {
       maxIterations: options.maxIterations,
       stuckDetection: options.stuckDetection,
+      userId: options.userId,
     });
 
     await conversation.start({
       initialMessage: options.initialMessage,
+      userId: options.userId,
     });
 
     return conversation;
@@ -340,6 +343,7 @@ export class ConversationManager {
       maxIterations?: number;
       stuckDetection?: boolean;
       workingDir?: string;
+      userId?: string;
     } = {}
   ): Promise<ACPConversationInfo> {
     let initialMessage: CreateACPConversationRequest['initial_message'];
@@ -356,6 +360,7 @@ export class ConversationManager {
       max_iterations: options.maxIterations || 500,
       stuck_detection: options.stuckDetection ?? true,
       workspace: { type: 'local', working_dir: options.workingDir || '/tmp' },
+      user_id: options.userId ?? null,
     };
 
     const response = await this.client.post<ACPConversationInfo>('/api/acp/conversations', request);
diff --git a/src/conversation/remote-conversation.ts b/src/conversation/remote-conversation.ts
index 6468bc3..8e221d6 100644
--- a/src/conversation/remote-conversation.ts
+++ b/src/conversation/remote-conversation.ts
@@ -42,6 +42,10 @@ import type { HookConfig } from '../hooks';
  * Options for creating a RemoteConversation instance.
  */
 export interface RemoteConversationOptions extends BaseConversationOptions {
+  /**
+   * Optional user ID to associate with server-side observability traces.
+   */
+  userId?: string;
   /**
    * Optional hook configuration for this conversation.
    * Hooks are shell scripts that run server-side at key lifecycle events
@@ -87,6 +91,7 @@ export class RemoteConversation implements IConversation {
   private callback?: ConversationCallbackType;
   private onError?: ErrorCallbackType;
   private hookConfig?: HookConfig;
+  private userId?: string;
 
   constructor(
     agent: AgentBase,
@@ -99,6 +104,7 @@ export class RemoteConversation implements IConversation {
     this.onError = options.onError;
     this._conversationId = options.conversationId;
     this.hookConfig = options.hookConfig;
+    this.userId = options.userId;
 
     this.client = new HttpClient({
       baseUrl: workspace.host,
@@ -132,6 +138,7 @@ export class RemoteConversation implements IConversation {
       maxIterations?: number;
       stuckDetection?: boolean;
       hookConfig?: HookConfig;
+      userId?: string;
     } = {}
   ): Promise<void> {
     if (this._conversationId) {
@@ -151,6 +158,7 @@ export class RemoteConversation implements IConversation {
 
     // Use hook config from start options, falling back to constructor option
     const hookConfig = options.hookConfig ?? this.hookConfig ?? undefined;
+    const userId = options.userId ?? this.userId ?? undefined;
 
     const request: CreateConversationRequest = {
       agent: this.agent,
@@ -159,6 +167,7 @@ export class RemoteConversation implements IConversation {
       stuck_detection: options.stuckDetection ?? true,
       workspace: { type: 'local', working_dir: this.workspace.workingDir },
       hook_config: hookConfig ?? null,
+      user_id: userId ?? null,
     };
 
     const response = await this.client.post<ConversationInfo>('/api/conversations', request);
diff --git a/src/models/conversation.ts b/src/models/conversation.ts
index d201111..3fce699 100644
--- a/src/models/conversation.ts
+++ b/src/models/conversation.ts
@@ -89,6 +89,7 @@ export interface CreateConversationRequest {
   stuck_detection: boolean;
   workspace: Record<string, unknown>;
   hook_config?: HookConfig | null;
+  user_id?: string | null;
 }
 
 export interface CreateACPConversationRequest {
@@ -98,6 +99,7 @@ export interface CreateACPConversationRequest {
   stuck_detection: boolean;
   workspace: Record<string, unknown>;
   hook_config?: HookConfig | null;
+  user_id?: string | null;
 }
 
 export interface GenerateTitleRequest {