@@ -100,7 +100,7 @@ export const make = Effect.gen(function* () {
 			newSession: ({ cwd }) =>
 				run(
 					Effect.gen(function* () {
-						const handle = yield* Session.create({ directory: cwd, hostDir: cwd, title: "ACP" });
+						const handle = yield* Session.create({ directory: cwd, hostDir: cwd });
 						const current = yield* selection(handle.id, cwd);
 						return { sessionId: handle.id, configOptions: yield* Config.options(current) };
 					}),
@@ -208,9 +208,18 @@ export const make = Effect.gen(function* () {
 							Stream.runHead,
 							Effect.forkScoped({ startImmediately: true }),
 						);
+						const before = (yield* handle.info).title;
 						yield* handle.run(text);
 						const end = yield* Fiber.join(settled);
 
+						// The harness titles a session from its first prompt; tell the editor.
+						const { title } = yield* handle.info;
+						if (title !== before) {
+							yield* client
+								.sessionUpdate({ sessionId: id, update: { sessionUpdate: "session_info_update", title } })
+								.pipe(Effect.ignore);
+						}
+
 						if (Option.isNone(end) || isInterrupted(end.value)) return { stopReason: "cancelled" as const };
 						if (isFailed(end.value)) return yield* AcpRequestError.internalError(end.value.data.error.message);
 						return { stopReason: reason === "length" ? ("max_tokens" as const) : ("end_turn" as const) };
 
@@ -145,7 +145,6 @@ export default Runtime.handler(
 			const info = Option.isSome(session)
 				? yield* rpc["session.configure"]({ sessionId: Session.SessionSchema.ID.make(session.value), runtime })
 				: yield* rpc["session.create"]({
-						title: "CLI",
 						runtime,
 						...(Option.isNone(cwd) ? {} : { directory: cwd.value }),
 						hostDir,
@@ -176,7 +175,6 @@ export default Runtime.handler(
 			} else {
 				const selected = yield* selectSandbox(selection);
 				handle = yield* Session.create({
-					title: "CLI",
 					...runtime,
 					...(selected === undefined ? {} : { sandbox: selected }),
 					...(Option.isNone(cwd) ? {} : { directory: cwd.value }),
 
@@ -1,5 +1,8 @@
 {
 	"pong": "end_turn",
+	"titled": [
+		"Reply with exactly: PONG"
+	],
 	"notices": [
 		1,
 		1
@@ -28,6 +31,7 @@
 	],
 	"failed": {
 		"code": -32603,
-		"hasMessage": true
+		"hasMessage": true,
+		"title": "Summarise how sessions, plugins and sandboxes…"
 	}
 }
@@ -168,7 +168,11 @@ describe("codework acp", () => {
 								sessionId,
 								prompt: [{ type: "text", text: "Reply with exactly: PONG" }],
 							});
-							const pongText = text(yield* Queue.clear(updates), "agent_message_chunk");
+							const pongUpdates = yield* Queue.clear(updates);
+							const pongText = text(pongUpdates, "agent_message_chunk");
+							const titled = pongUpdates.flatMap((update) =>
+								update.sessionUpdate === "session_info_update" ? [update.title] : [],
+							);
 							expect(pongText).toContain("PONG");
 
 							const tool = yield* acp.agent.prompt({
@@ -206,6 +210,8 @@ describe("codework acp", () => {
 								sessionId,
 								record: {
 									pong: pong.stopReason,
+									// The first prompt names the untitled session, and the editor is told once.
+									titled,
 									// Once per prompt, however many exchanges the prompt takes.
 									notices: [pongText, toolText].map(
 										(answer) =>
@@ -252,9 +258,22 @@ describe("codework acp", () => {
 								const { sessionId } = yield* acp.agent.createSession({ cwd: project, mcpServers: [] });
 								yield* acp.agent.setSessionConfigOption({ sessionId, configId: "model", value: LIVE_MODEL });
 								const result = yield* settle(
-									acp.agent.prompt({ sessionId, prompt: [{ type: "text", text: "hi" }] }),
+									acp.agent.prompt({
+										sessionId,
+										prompt: [
+											{
+												type: "text",
+												text: "Summarise how sessions, plugins and sandboxes fit together in the codework harness.\nKeep it short.",
+											},
+										],
+									}),
 								);
-								return "code" in result ? { code: result.code, hasMessage: Boolean(result.message) } : result;
+								// Titled at promotion, before the provider call fails: first line, cut to 48 characters.
+								const { sessions } = yield* acp.agent.listSessions({ cwd: project });
+								return {
+									...("code" in result ? { code: result.code, hasMessage: Boolean(result.message) } : result),
+									title: sessions.find((session) => session.sessionId === sessionId)?.title,
+								};
 							}),
 						{ OPENROUTER_API_KEY: "sk-or-invalid" },
 					);
Original file line number	Diff line number	Diff line change
`@@ -1,5 +1,8 @@`
`1`	`1`	`{`
`2`	`2`	`"pong": "end_turn",`
	`3`	`+ "titled": [`
	`4`	`+ "Reply with exactly: PONG"`
	`5`	`+ ],`
`3`	`6`	`"notices": [`
`4`	`7`	`1,`
`5`	`8`	`1`
`@@ -28,6 +31,7 @@`
`28`	`31`	`],`
`29`	`32`	`"failed": {`
`30`	`33`	`"code": -32603,`
`31`		`- "hasMessage": true`
	`34`	`+ "hasMessage": true,`
	`35`	`+ "title": "Summarise how sessions, plugins and sandboxes…"`
`32`	`36`	`}`
`33`	`37`	`}`