feat(coding-agent): add configurable title system prompt for sessions

- add discovery of `TITLE_SYSTEM.md` and pass it through interactive startup context
- route custom title prompts to online and local tiny title generators via protocol
- update session-title docs and changelog with override behavior
- add tests for prompt discovery, forwarding, and fallback to bundled title prompts

docs/config-usage.md

@@ -243,6 +243,20 @@ Native provider (`id: native`) reads native config from:
 - `Settings.init()` loads global `config.yml` + discovered project settings capability items.
 - Only capability items with `level === "project"` are merged into project layer.
 
+### Session title prompt override
+
+Create `TITLE_SYSTEM.md` in the same config locations as `SYSTEM.md` / `APPEND_SYSTEM.md`:
+
+```text
+# ~/.omp/agent/TITLE_SYSTEM.md
+Generate a session name using lowercase `<type>:<primary-objective>`.
+```
+
+- Missing `TITLE_SYSTEM.md` keeps the bundled title prompts.
+- Discovery uses the same project-then-user config directory pattern as `SYSTEM.md`: project `.omp/TITLE_SYSTEM.md` first, then user `~/.omp/agent/TITLE_SYSTEM.md` and the other supported config bases.
+- The override replaces only the automatic session-title generation system prompt; normal `SYSTEM.md` / `APPEND_SYSTEM.md` prompt customization is unaffected.
+- The online path still forces the `set_title` tool call. The local tiny-title path keeps the `<title>...</title>` prefill/stop wrapper and uses this file as its system turn.
+
 ## Skills subsystem
 
 - `extensibility/skills.ts` loads via `loadCapability(skillCapability.id, { cwd })`.

docs/system-prompt-customization.md

@@ -124,6 +124,18 @@ The dynamic project/environment footer that remains after `SYSTEM.md` is only bl
 
 There is currently no supported CLI mode for "replace the stable default instructions but keep the generated skills/rules/tool guidance." If you need automatic skills loading, keep the default block and add your customization via `APPEND_SYSTEM.md`. If you fully replace with `SYSTEM.md`, you must hard-code any skill names/instructions you want the model to know about, and those will not track discovery automatically.
 
+### "Customize automatic session titles"
+
+`SYSTEM.md` and `APPEND_SYSTEM.md` do not affect the model call that names a new session. Create the title-specific prompt file instead:
+
+```text
+# ~/.omp/agent/TITLE_SYSTEM.md
+Generate a session name using lowercase `<type>:<primary-objective>`.
+If the message carries no concrete task, output exactly `none`.
+```
+
+`TITLE_SYSTEM.md` is discovered with the same project-then-user config-directory pattern as `SYSTEM.md` / `APPEND_SYSTEM.md`. When absent, OMP uses the bundled `title-system.md` / `tiny-title-system.md` prompts. When present, the online title path still forces the `set_title` tool call, and the local tiny-model path keeps the `<title>...</title>` wrapper while using this file as the system turn.
+
 ### "Replace everything, including project context" — SDK-only
 
 The normal CLI file/flag path intentionally preserves `defaultPrompt.slice(1)`. Code using `CreateAgentSessionOptions.systemPrompt` directly can return a full replacement array and omit the project footer, but that is not what `.omp/SYSTEM.md`, `~/.omp/agent/SYSTEM.md`, or `--system-prompt` do.
@@ -163,6 +175,7 @@ Net effect for CLI users: put `SYSTEM.md` / `APPEND_SYSTEM.md` directly under `<
 | Add an instruction on top of the full default prompt | `APPEND_SYSTEM.md` or `--append-system-prompt` |
 | Replace the stable default instructions but keep project/environment context | `SYSTEM.md` or `--system-prompt` |
 | Preserve generated skills/rules/tool guidance while customizing | `APPEND_SYSTEM.md`; `SYSTEM.md` replaces that generated block |
+| Customize automatic session titles | `TITLE_SYSTEM.md`; chat-turn `SYSTEM.md` / `APPEND_SYSTEM.md` do not affect title generation |
 | Use `{{cwd}}` / `{{date}}` / other internals in my file | Not supported. Files are inserted verbatim. |
 | Inherit specific sections from `system-prompt.md` | Not supported; use append, or copy what you need into `SYSTEM.md`. |
 | Override at a per-repo level | Project `.omp/SYSTEM.md` under the cwd you launch `omp` from |

packages/coding-agent/CHANGELOG.md

@@ -133,6 +133,10 @@
 - Fixed Windows stdio MCP servers launched through PATH shims such as `codegraph.cmd` so bare commands like `codegraph` resolve via `PATHEXT` before spawn ([#2174](https://github.com/can1357/oh-my-pi/issues/2174)).
 - Fixed compiled-binary extensions failing to load `@oh-my-pi/pi-*` packages when `bun --compile` quietly dropped one of the extra entrypoints (observed on macOS arm64 release builds): the legacy-pi compat shim's package-root override branch returned the bunfs path without checking the target was present, so the rewrite emitted a `file://` URL to a missing module and the #1216 fallback (scoped to the throwing `getResolvedSpecifier` path) never ran. Override targets are now validated against the on-disk filesystem at module init, missing entries are dropped, and resolution falls through to canonical lookup so Bun resolves the import from the extension's own `node_modules` ([#2168](https://github.com/can1357/oh-my-pi/issues/2168)).
 
+### Added
+
+- Added `TITLE_SYSTEM.md` discovery so users can override the automatic session-title generation prompt for online and local tiny title models without patching installed prompt files.
+
 ## [15.10.8] - 2026-06-09
 
 ### Added

packages/coding-agent/src/main.ts

@@ -370,6 +370,7 @@ async function runInteractiveMode(
 	eventBus?: EventBus,
 	initialMessage?: string,
 	initialImages?: ImageContent[],
+	titleSystemPrompt?: string,
 ): Promise<void> {
 	const mode = new InteractiveMode(
 		session,
@@ -379,6 +380,7 @@ async function runInteractiveMode(
 		lspServers,
 		mcpManager,
 		eventBus,
+		titleSystemPrompt,
 	);
 
 	// Cold-launch gate: the full setup wizard (every scene + the overlay and
@@ -718,13 +720,26 @@ function discoverAppendSystemPromptFile(): string | undefined {
 	return undefined;
 }
 
+/** Discover TITLE_SYSTEM.md file for automatic session-title prompt overrides */
+export function discoverTitleSystemPromptFile(cwd?: string): string | undefined {
+	const projectPath = findConfigFile("TITLE_SYSTEM.md", { user: false, cwd });
+	if (projectPath) {
+		return projectPath;
+	}
+	const globalPath = findConfigFile("TITLE_SYSTEM.md", { user: true, cwd });
+	if (globalPath) {
+		return globalPath;
+	}
+	return undefined;
+}
+
 async function buildSessionOptions(
 	parsed: Args,
 	scopedModels: ScopedModel[],
 	sessionManager: SessionManager | undefined,
 	modelRegistry: ModelRegistry,
 	activeSettings: Settings,
-): Promise<{ options: CreateAgentSessionOptions }> {
+): Promise<{ options: CreateAgentSessionOptions; titleSystemPrompt?: string }> {
 	const options: CreateAgentSessionOptions = {
 		cwd: parsed.cwd ?? getProjectDir(),
 		autoApprove: parsed.autoApprove ?? false,
@@ -735,6 +750,8 @@ async function buildSessionOptions(
 	const resolvedSystemPrompt = await resolvePromptInput(systemPromptSource, "system prompt");
 	const appendPromptSource = parsed.appendSystemPrompt ?? discoverAppendSystemPromptFile();
 	const resolvedAppendPrompt = await resolvePromptInput(appendPromptSource, "append system prompt");
+	const titleSystemPromptSource = discoverTitleSystemPromptFile();
+	const titleSystemPrompt = await resolvePromptInput(titleSystemPromptSource, "title system prompt");
 
 	if (sessionManager) {
 		options.sessionManager = sessionManager;
@@ -880,7 +897,7 @@ async function buildSessionOptions(
 		options.additionalExtensionPaths = [];
 	}
 
-	return { options };
+	return { options, titleSystemPrompt };
 }
 
 interface RunRootCommandDependencies {
@@ -1133,7 +1150,7 @@ export async function runRootCommand(
 		clearPluginRootsCache: clearPluginRootsAndCaches,
 	});
 
-	const { options: sessionOptions } = await logger.time(
+	const { options: sessionOptions, titleSystemPrompt } = await logger.time(
 		"buildSessionOptions",
 		buildSessionOptions,
 		parsedArgs,
@@ -1338,6 +1355,7 @@ export async function runRootCommand(
 				eventBus,
 				initialMessage,
 				initialImages,
+				titleSystemPrompt,
 			);
 		} else {
 			// Branch-only single-shot runner: keep print-mode code out of normal interactive startup.

packages/coding-agent/src/modes/controllers/input-controller.ts

@@ -467,6 +467,7 @@ export class InputController {
 					this.ctx.session.sessionId,
 					this.ctx.session.model,
 					provider => this.ctx.session.agent.metadataForProvider(provider),
+					this.ctx.titleSystemPrompt,
 				)
 					.then(async title => {
 						// Re-check: a concurrent attempt for an earlier message may have

packages/coding-agent/src/modes/interactive-mode.ts

@@ -260,6 +260,7 @@ export class InteractiveMode implements InteractiveModeContext {
 	keybindings: KeybindingsManager;
 	agent: Agent;
 	historyStorage?: HistoryStorage;
+	titleSystemPrompt?: string;
 
 	ui: TUI;
 	chatContainer: TranscriptContainer;
@@ -382,6 +383,7 @@ export class InteractiveMode implements InteractiveModeContext {
 		lspServers: LspStartupServerInfo[] | undefined = undefined,
 		mcpManager?: import("../mcp").MCPManager,
 		eventBus?: EventBus,
+		titleSystemPrompt?: string,
 	) {
 		this.session = session;
 		this.sessionManager = session.sessionManager;
@@ -394,6 +396,7 @@ export class InteractiveMode implements InteractiveModeContext {
 		this.lspServers = lspServers;
 		this.mcpManager = mcpManager;
 		this.#eventBus = eventBus;
+		this.titleSystemPrompt = titleSystemPrompt;
 		if (eventBus) {
 			this.#eventBusUnsubscribers.push(
 				eventBus.on(LSP_STARTUP_EVENT_CHANNEL, data => {

packages/coding-agent/src/modes/types.ts

@@ -99,6 +99,7 @@ export interface InteractiveModeContext {
 	historyStorage?: HistoryStorage;
 	mcpManager?: MCPManager;
 	lspServers?: LspStartupServerInfo[];
+	titleSystemPrompt?: string;
 
 	// State
 	isInitialized: boolean;

packages/coding-agent/src/tiny/title-client.ts

@@ -39,6 +39,11 @@ export interface TinyTitleDownloadOptions {
 	onProgress?: (event: TinyTitleProgressEvent) => void;
 }
 
+export interface TinyTitleGenerateOptions {
+	signal?: AbortSignal;
+	systemPrompt?: string;
+}
+
 // Cold-starting the worker subprocess from a compiled binary (decompress + module
 // graph load) is slow on contended CI runners — the macos-15-intel release smoke
 // blew past 5s while arm64/linux/win passed. The probe only needs to prove the
@@ -46,6 +51,14 @@ export interface TinyTitleDownloadOptions {
 // generous bound removes the flake without weakening the check.
 const SMOKE_TEST_TIMEOUT_MS = 30_000;
 
+function normalizeTinyTitleGenerateOptions(
+	options: AbortSignal | TinyTitleGenerateOptions | undefined,
+): TinyTitleGenerateOptions {
+	if (!options) return {};
+	if ("aborted" in options && "addEventListener" in options) return { signal: options };
+	return options;
+}
+
 /**
  * Hidden subcommand on the main CLI that boots the tiny-model worker in the
  * spawned subprocess. Kept in sync with the dispatch in `cli.ts`.
@@ -295,9 +308,16 @@ export class TinyTitleClient {
 		return () => this.#progressListeners.delete(listener);
 	}
 
-	async generate(modelKey: string, message: string, signal?: AbortSignal): Promise<string | null> {
+	async generate(modelKey: string, message: string, signal?: AbortSignal): Promise<string | null>;
+	async generate(modelKey: string, message: string, options?: TinyTitleGenerateOptions): Promise<string | null>;
+	async generate(
+		modelKey: string,
+		message: string,
+		optionsOrSignal?: AbortSignal | TinyTitleGenerateOptions,
+	): Promise<string | null> {
+		const options = normalizeTinyTitleGenerateOptions(optionsOrSignal);
 		if (!isTinyTitleLocalModelKey(modelKey)) return null;
-		if (signal?.aborted) return null;
+		if (options.signal?.aborted) return null;
 
 		try {
 			const worker = this.#ensureWorker();
@@ -310,12 +330,15 @@ export class TinyTitleClient {
 				this.#pending.delete(id);
 				pending.resolve(null);
 			};
-			signal?.addEventListener("abort", abort, { once: true });
+			options.signal?.addEventListener("abort", abort, { once: true });
 			try {
-				worker.send({ type: "generate", id, modelKey, message });
+				const request: TinyTitleWorkerInbound = options.systemPrompt
+					? { type: "generate", id, modelKey, message, systemPrompt: options.systemPrompt }
+					: { type: "generate", id, modelKey, message };
+				worker.send(request);
 				return await promise;
 			} finally {
-				signal?.removeEventListener("abort", abort);
+				options.signal?.removeEventListener("abort", abort);
 				this.#pending.delete(id);
 			}
 		} catch (error) {

packages/coding-agent/src/tiny/title-protocol.ts

@@ -29,7 +29,7 @@ export interface TinyTitleProgressEvent {
 
 export type TinyTitleWorkerInbound =
 	| { type: "ping"; id: string }
-	| { type: "generate"; id: string; modelKey: TinyTitleLocalModelKey; message: string }
+	| { type: "generate"; id: string; modelKey: TinyTitleLocalModelKey; message: string; systemPrompt?: string }
 	| { type: "complete"; id: string; modelKey: TinyLocalModelKey; prompt: string; maxTokens?: number }
 	| { type: "download"; id: string; modelKey: TinyLocalModelKey };
 

packages/coding-agent/src/tiny/worker.ts

@@ -436,9 +436,10 @@ async function loadPipeline(
 	return loaded;
 }
 
-function buildPrompt(generator: TextGenerationPipeline, message: string): string {
+function buildPrompt(generator: TextGenerationPipeline, message: string, systemPrompt?: string): string {
+	const selectedSystemPrompt = systemPrompt?.trim() || TINY_TITLE_SYSTEM_PROMPT;
 	const chat = [
-		{ role: "system", content: TINY_TITLE_SYSTEM_PROMPT },
+		{ role: "system", content: selectedSystemPrompt },
 		{ role: "user", content: formatTitleUserMessage(message) },
 	];
 	const chatTemplateOptions = {
@@ -464,9 +465,10 @@ async function generateTitle(
 	requestId: string,
 	modelKey: TinyTitleLocalModelKey,
 	message: string,
+	systemPrompt?: string,
 ): Promise<string | null> {
 	const generator = await loadPipeline(modelKey, transport, requestId);
-	const promptText = buildPrompt(generator, message);
+	const promptText = buildPrompt(generator, message, systemPrompt);
 	const transformers = await loadTransformers(transport, requestId, modelKey);
 	const output = (await generator(promptText, {
 		max_new_tokens: TITLE_MAX_NEW_TOKENS,
@@ -548,7 +550,7 @@ async function handleQueuedRequest(
 			transport.send({ type: "completion", id: request.id, text });
 			return;
 		}
-		const title = await generateTitle(transport, request.id, request.modelKey, request.message);
+		const title = await generateTitle(transport, request.id, request.modelKey, request.message, request.systemPrompt);
 		transport.send({ type: "title", id: request.id, title });
 	} catch (error) {
 		transport.send({ type: "error", id: request.id, error: errorText(error) });

packages/coding-agent/src/utils/title-generator.ts

@@ -33,7 +33,7 @@ const setTitleTool: Tool = {
 			title: {
 				type: "string",
 				description:
-					'A concise, sentence-case 3-7 word title for the session (capitalize only the first word and proper nouns), or exactly "none" when the message carries no concrete task yet (greeting, small talk, vague).',
+					'The generated session title, or exactly "none" when the message carries no concrete task yet.',
 			},
 		},
 		required: ["title"],
@@ -137,6 +137,7 @@ export async function raceFirstNonNull<T>(
  *   to produce request metadata (e.g. user_id for session attribution). Using a
  *   resolver instead of a pre-evaluated value ensures the metadata's account_uuid
  *   reflects the credential actually selected for this request.
+ * @param customSystemPrompt Optional title-specific system prompt override
  */
 export async function generateSessionTitle(
 	firstMessage: string,
@@ -145,6 +146,7 @@ export async function generateSessionTitle(
 	sessionId?: string,
 	currentModel?: Model<Api>,
 	metadataResolver?: (provider: string) => Record<string, unknown> | undefined,
+	customSystemPrompt?: string,
 ): Promise<string | null> {
 	// Defer titling for greetings / acknowledgements / empty input. The default
 	// tiny title model can't reliably decline trivial input, so this happens
@@ -155,13 +157,26 @@ export async function generateSessionTitle(
 		return null;
 	}
 
+	const titleSystemPrompt = customSystemPrompt?.trim() || undefined;
 	const tinyModel = settings.get("providers.tinyModel");
 	if (tinyModel === ONLINE_TINY_TITLE_MODEL_KEY) {
-		return generateTitleOnline(firstMessage, registry, settings, sessionId, currentModel, metadataResolver);
+		return generateTitleOnline(
+			firstMessage,
+			registry,
+			settings,
+			sessionId,
+			currentModel,
+			metadataResolver,
+			undefined,
+			titleSystemPrompt,
+		);
 	}
 
 	const onlineAbortController = new AbortController();
-	const localTitle = tinyTitleClient.generate(tinyModel, firstMessage).then(
+	const localTitlePromise = titleSystemPrompt
+		? tinyTitleClient.generate(tinyModel, firstMessage, { systemPrompt: titleSystemPrompt })
+		: tinyTitleClient.generate(tinyModel, firstMessage);
+	const localTitle = localTitlePromise.then(
 		title => title || null,
 		err => {
 			logger.warn("title-generator: local model error", {
@@ -181,6 +196,7 @@ export async function generateSessionTitle(
 			currentModel,
 			metadataResolver,
 			onlineAbortController.signal,
+			titleSystemPrompt,
 		);
 
 	return raceFirstNonNull(localTitle, startOnline, TITLE_LOCAL_FALLBACK_DELAY_MS, () => {
@@ -196,6 +212,7 @@ export async function generateTitleOnline(
 	currentModel?: Model<Api>,
 	metadataResolver?: (provider: string) => Record<string, unknown> | undefined,
 	signal?: AbortSignal,
+	customSystemPrompt?: string,
 ): Promise<string | null> {
 	const model = getTitleModel(registry, settings, currentModel);
 	if (!model) {
@@ -203,6 +220,8 @@ export async function generateTitleOnline(
 		return null;
 	}
 
+	const titleSystemPrompt = customSystemPrompt?.trim() || undefined;
+	const systemPrompt = titleSystemPrompt ?? TITLE_SYSTEM_PROMPT;
 	const userMessage = formatTitleUserMessage(firstMessage);
 	const modelName = `${model.provider}/${model.id}`;
 	const modelContext = {
@@ -234,7 +253,7 @@ export async function generateTitleOnline(
 		const response = await completeSimple(
 			model,
 			{
-				systemPrompt: [TITLE_SYSTEM_PROMPT],
+				systemPrompt: [systemPrompt],
 				messages: [{ role: "user", content: userMessage, timestamp: Date.now() }],
 				tools: [setTitleTool],
 			},

packages/coding-agent/test/issue-1940-repro.test.ts

@@ -33,6 +33,35 @@ class FakeTinyWorker {
 	}
 }
 
+describe("tiny title client prompt options", () => {
+	it("forwards a custom system prompt on local title requests", async () => {
+		let sent: TinyTitleWorkerInbound | undefined;
+		const worker = new FakeTinyWorker((message, worker) => {
+			sent = message;
+			if (message.type === "generate") {
+				worker.emit({ type: "title", id: message.id, title: "custom title" });
+			}
+		});
+		const client = new TinyTitleClient(() => worker);
+
+		try {
+			const title = await client.generate("lfm2-350m", "Investigate routing", {
+				systemPrompt: "Custom title prompt",
+			});
+
+			expect(title).toBe("custom title");
+			expect(sent).toMatchObject({
+				type: "generate",
+				modelKey: "lfm2-350m",
+				message: "Investigate routing",
+				systemPrompt: "Custom title prompt",
+			});
+		} finally {
+			await client.terminate();
+		}
+	});
+});
+
 describe("issue #1940 — local model failures release the worker process", () => {
 	it("recycles the tiny-model worker after model execution returns an error", async () => {
 		const first = new FakeTinyWorker((message, worker) => {

packages/coding-agent/test/main-interactive-input.test.ts

@@ -1,7 +1,16 @@
-import { describe, expect, it, vi } from "bun:test";
-import { submitInteractiveInput } from "@oh-my-pi/pi-coding-agent/main";
+import { afterEach, describe, expect, it, vi } from "bun:test";
+import * as fs from "node:fs/promises";
+import * as os from "node:os";
+import * as path from "node:path";
+import { discoverTitleSystemPromptFile, submitInteractiveInput } from "@oh-my-pi/pi-coding-agent/main";
 import type { SubmittedUserInput } from "@oh-my-pi/pi-coding-agent/modes/types";
 
+const cleanupDirs: string[] = [];
+
+afterEach(async () => {
+	await Promise.all(cleanupDirs.splice(0).map(dir => fs.rm(dir, { recursive: true, force: true })));
+});
+
 function createInput(overrides: Partial<SubmittedUserInput> = {}): SubmittedUserInput {
 	return {
 		text: "hello",
@@ -12,6 +21,19 @@ function createInput(overrides: Partial<SubmittedUserInput> = {}): SubmittedUser
 	};
 }
 
+describe("discoverTitleSystemPromptFile", () => {
+	it("discovers TITLE_SYSTEM.md from the project omp config directory", async () => {
+		const projectDir = await fs.mkdtemp(path.join(os.tmpdir(), "omp-title-system-"));
+		cleanupDirs.push(projectDir);
+		const configDir = path.join(projectDir, ".omp");
+		await fs.mkdir(configDir, { recursive: true });
+		const promptPath = path.join(configDir, "TITLE_SYSTEM.md");
+		await fs.writeFile(promptPath, "custom title prompt");
+
+		expect(discoverTitleSystemPromptFile(projectDir)).toBe(promptPath);
+	});
+});
+
 describe("submitInteractiveInput", () => {
 	it("routes already-started synthetic continue submissions to a hidden developer prompt", async () => {
 		const mode = {

packages/coding-agent/test/tiny-title-generator.test.ts

@@ -243,6 +243,27 @@ describe("tiny title generator routing", () => {
 		expect(online).not.toHaveBeenCalled();
 	});
 
+	it("passes the resolved TITLE_SYSTEM.md prompt to the local client", async () => {
+		const model = getModelOrThrow("claude-sonnet-4-5");
+		const customPrompt = "Generate lowercase colon-delimited session names.";
+		const local = vi.spyOn(tinyTitleClient, "generate").mockResolvedValue("Local Title");
+		const online = mockOnlineTitle("Online Title");
+
+		const title = await generateSessionTitle(
+			"Investigate routing",
+			createRegistry(model),
+			createSettings(model, "lfm2-350m"),
+			undefined,
+			undefined,
+			undefined,
+			customPrompt,
+		);
+
+		expect(title).toBe("Local Title");
+		expect(local).toHaveBeenCalledWith("lfm2-350m", "Investigate routing", { systemPrompt: customPrompt });
+		expect(online).not.toHaveBeenCalled();
+	});
+
 	it("starts online fallback immediately when local returns null", async () => {
 		const model = getModelOrThrow("claude-sonnet-4-5");
 		vi.spyOn(tinyTitleClient, "generate").mockResolvedValue(null);

packages/coding-agent/test/title-generator.test.ts

@@ -71,6 +71,49 @@ describe("title generator", () => {
 		});
 	});
 
+	it("uses the bundled default prompt when no title prompt file is resolved", async () => {
+		const model = getModelOrThrow("claude-sonnet-4-5");
+		const completeSimpleMock = vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "toolCall", id: "call-title", name: "set_title", arguments: { title: "Default Prompt" } }],
+		} as never);
+
+		await generateSessionTitle("Investigate the resolver", createRegistry(model), createSettings(model));
+
+		const request = completeSimpleMock.mock.calls[0]?.[1] as { systemPrompt?: string[] } | undefined;
+		expect(request?.systemPrompt).toHaveLength(1);
+		expect(request?.systemPrompt?.[0]).toContain("Generate a concise, sentence-case title");
+	});
+
+	it("uses the resolved TITLE_SYSTEM.md prompt for online title generation", async () => {
+		const model = getModelOrThrow("claude-sonnet-4-5");
+		const customPrompt = "Generate lowercase colon-delimited session names.";
+		const completeSimpleMock = vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "toolCall", id: "call-title", name: "set_title", arguments: { title: "fix:resolver" } }],
+		} as never);
+
+		await generateSessionTitle(
+			"Investigate the resolver",
+			createRegistry(model),
+			createSettings(model),
+			undefined,
+			undefined,
+			undefined,
+			customPrompt,
+		);
+
+		const request = completeSimpleMock.mock.calls[0]?.[1] as
+			| { systemPrompt?: string[]; tools?: Array<{ name?: string }> }
+			| undefined;
+		const options = completeSimpleMock.mock.calls[0]?.[2] as
+			| { toolChoice?: { type?: string; name?: string } }
+			| undefined;
+		expect(request?.systemPrompt).toEqual([customPrompt]);
+		expect(request?.tools?.[0]?.name).toBe("set_title");
+		expect(options?.toolChoice).toEqual({ type: "tool", name: "set_title" });
+	});
+
 	it("falls back to text content when no set_title tool call is returned", async () => {
 		const model = getModelOrThrow("claude-sonnet-4-5");
 		vi.spyOn(ai, "completeSimple").mockResolvedValue({