fix(coding-agent): ACP factory re-resolves TITLE_SYSTEM.md per session cwd

createAcpSessionFactory spreads baseOptions into every per-session
createAgentSession call. baseOptions.titleSystemPrompt was resolved
once at launch from main.ts, so opening session/new for a client-
supplied workspace different from the launch cwd would carry the
launch repo's TITLE_SYSTEM.md into the new session and bias replan
title refreshes against the wrong policy.

The factory now calls discoverTitleSystemPromptFile(cwd) +
resolvePromptInput per session and passes the result after the spread,
overriding the launch-time value. When the target workspace has no
override, undefined wins and the bundled prompt is used as expected.

Regression test asserts that a per-session cwd with a project-level
TITLE_SYSTEM.md beats the launch-cwd prompt threaded through
baseOptions.

packages/coding-agent/CHANGELOG.md

@@ -4,7 +4,7 @@
 
 ### Fixed
 
-- Fixed replan-driven session title refresh (`title.refreshOnReplan`, triggered after `todo init`) ignoring the user's `TITLE_SYSTEM.md` override and silently falling back to the bundled prompt — most visible in Plan Mode, where early todo init replans would overwrite the auto title against the configured policy. `AgentSession` now owns the resolved title prompt (threaded through `CreateAgentSessionOptions.titleSystemPrompt`) and both first-input titling and replan refresh read from the same source ([#3734](https://github.com/can1357/oh-my-pi/issues/3734)).
+- Fixed replan-driven session title refresh (`title.refreshOnReplan`, triggered after `todo init`) ignoring the user's `TITLE_SYSTEM.md` override and silently falling back to the bundled prompt — most visible in Plan Mode, where early todo init replans would overwrite the auto title against the configured policy. `AgentSession` now owns the resolved title prompt (threaded through `CreateAgentSessionOptions.titleSystemPrompt`), the ACP per-`session/new` factory re-resolves `TITLE_SYSTEM.md` against each client-supplied workspace cwd, and both first-input titling and replan refresh read from the same source ([#3734](https://github.com/can1357/oh-my-pi/issues/3734)).
 
 ## [16.2.3] - 2026-06-28
 

packages/coding-agent/src/main.ts

@@ -361,6 +361,13 @@ export function createAcpSessionFactory(args: AcpSessionFactoryOptions): AcpSess
 		const nextSettings = await args.settings.cloneForCwd(cwd);
 		const nextSessionManager = SessionManager.create(cwd, args.sessionDir);
 		const agentId = `acp:${nextSessionManager.getSessionId()}`;
+		// `baseOptions.titleSystemPrompt` is resolved from the launch cwd; an ACP
+		// host can open `session/new` for any client-supplied workspace, so
+		// re-discover `TITLE_SYSTEM.md` against THIS session's `cwd` to keep the
+		// replan-driven title refresh consistent with the target project's
+		// policy (PR #3736 follow-up).
+		const titleSystemPromptSource = discoverTitleSystemPromptFile(cwd);
+		const titleSystemPrompt = await resolvePromptInput(titleSystemPromptSource, "title system prompt");
 		const { session: nextSession } = await args.createSession({
 			...args.baseOptions,
 			cwd,
@@ -371,6 +378,7 @@ export function createAcpSessionFactory(args: AcpSessionFactoryOptions): AcpSess
 			agentId,
 			hasUI: false,
 			enableMCP: false,
+			titleSystemPrompt,
 		});
 		if (args.parsedArgs.apiKey && !args.baseOptions.model && nextSession.model) {
 			args.authStorage.setRuntimeApiKey(nextSession.model.provider, args.parsedArgs.apiKey);

packages/coding-agent/test/acp-mcp-isolation.test.ts

@@ -75,3 +75,65 @@ describe("createAcpSessionFactory MCP isolation (issue #1234)", () => {
 		}
 	});
 });
+
+describe("createAcpSessionFactory TITLE_SYSTEM.md per-cwd resolution (PR #3736)", () => {
+	it("re-resolves the title prompt for the per-session cwd instead of inheriting the launch cwd's override", async () => {
+		const tempDir = TempDir.createSync("@pi-acp-title-prompt-");
+		let authStorage: AuthStorage | undefined;
+		try {
+			authStorage = await AuthStorage.create(tempDir.join("auth.db"));
+			const modelRegistry = new ModelRegistry(authStorage);
+			const settings = Settings.isolated({});
+
+			const projectDir = tempDir.join("project");
+			await Bun.write(`${projectDir}/.omp/TITLE_SYSTEM.md`, "Project-specific title policy.");
+
+			const fakeSession = {} as AgentSession;
+			const captured: CreateAgentSessionOptions[] = [];
+			const createSession = async (options: CreateAgentSessionOptions): Promise<CreateAgentSessionResult> => {
+				captured.push(options);
+				return {
+					session: fakeSession,
+					extensionsResult: {
+						extensions: [],
+						errors: [],
+						runner: undefined,
+					} as unknown as CreateAgentSessionResult["extensionsResult"],
+					setToolUIContext: () => {},
+					eventBus: {
+						emit: () => {},
+						on: () => () => {},
+						off: () => {},
+					} as unknown as CreateAgentSessionResult["eventBus"],
+				};
+			};
+
+			// baseOptions carries the LAUNCH cwd's prompt; the factory must
+			// override it with the per-session cwd's `TITLE_SYSTEM.md`.
+			const factory = createAcpSessionFactory({
+				baseOptions: {
+					titleSystemPrompt: "Launch-cwd policy that must not leak.",
+				} as CreateAgentSessionOptions,
+				settings,
+				sessionDir: tempDir.join("sessions"),
+				authStorage,
+				modelRegistry,
+				parsedArgs: {},
+				rawArgs: [],
+				createSession,
+			});
+
+			await factory(projectDir);
+
+			expect(captured).toHaveLength(1);
+			expect(captured[0].titleSystemPrompt).toBe("Project-specific title policy.");
+		} finally {
+			try {
+				authStorage?.close();
+			} finally {
+				await Bun.sleep(0);
+				await tempDir.remove();
+			}
+		}
+	});
+});