feat: stop generation mid-turn (POST /conversations/:id/stop)

Add stopTurn to the orchestrator: aborts the in-flight turn's
AbortController without changing conversation status. The turn
seals normally (finishReason: 'aborted'), partial messages are
persisted, and the conversation transitions active → idle via the
normal settle path.

Distinct from closeConversation which marks the conversation closed.

- POST /conversations/:id/stop endpoint
- dispatch stop <id> CLI command
- FE handoff: frontend-stop-generation-handoff.md

4 files changed, 20 insertions, 0 deletions

diff --git a/packages/transport-http/src/app.test.ts b/packages/transport-http/src/app.test.ts
index b265f5e..674fd94 100644
--- a/packages/transport-http/src/app.test.ts
+++ b/packages/transport-http/src/app.test.ts
@@ -165,6 +165,9 @@ function createFakeOrchestrator(events: AgentEvent[]): SessionOrchestrator {
 		closeConversation() {
 			return { abortedTurn: false };
 		},
+		stopTurn() {
+			return { abortedTurn: false };
+		},
 		async handleMessage(input) {
 			for (const event of events) {
 				input.onEvent(event);
@@ -198,6 +201,9 @@ function createCapturingOrchestrator(): SessionOrchestrator & {
 		closeConversation() {
 			return { abortedTurn: false };
 		},
+		stopTurn() {
+			return { abortedTurn: false };
+		},
 		async handleMessage(input) {
 			state.received = input;
 		},
@@ -221,6 +227,9 @@ function createThrowingOrchestrator(error: Error): SessionOrchestrator {
 		closeConversation() {
 			return { abortedTurn: false };
 		},
+		stopTurn() {
+			return { abortedTurn: false };
+		},
 		async handleMessage() {
 			throw error;
 		},
diff --git a/packages/transport-http/src/app.ts b/packages/transport-http/src/app.ts
index 64e46fd..a2ee18c 100644
--- a/packages/transport-http/src/app.ts
+++ b/packages/transport-http/src/app.ts
@@ -396,6 +396,13 @@ export function createApp(opts: CreateServerOptions): Hono {
 		return c.json(body, 200);
 	});
 
+	app.post("/conversations/:id/stop", (c) => {
+		const conversationId = c.req.param("id");
+		const { abortedTurn } = opts.orchestrator.stopTurn(conversationId);
+		log.info("conversations: stop", { conversationId, abortedTurn });
+		return c.json({ conversationId, abortedTurn }, 200);
+	});
+
 	app.post("/conversations/:id/queue", async (c) => {
 		const conversationId = c.req.param("id");
 
diff --git a/packages/transport-http/src/extension.ts b/packages/transport-http/src/extension.ts
index 351dd4a..f7af615 100644
--- a/packages/transport-http/src/extension.ts
+++ b/packages/transport-http/src/extension.ts
@@ -39,6 +39,7 @@ export const manifest: Manifest = {
 			"/conversations/:id/open",
 			"/conversations/:id/queue",
 			"/conversations/:id/reasoning-effort",
+			"/conversations/:id/stop",
 			"/conversations/:id/title",
 			"/health",
 			"/models",
diff --git a/packages/transport-http/src/server.bun.test.ts b/packages/transport-http/src/server.bun.test.ts
index 3b7700e..fa519af 100644
--- a/packages/transport-http/src/server.bun.test.ts
+++ b/packages/transport-http/src/server.bun.test.ts
@@ -92,6 +92,9 @@ function fakeOrchestrator(): SessionOrchestrator {
 		closeConversation() {
 			return { abortedTurn: false };
 		},
+		stopTurn() {
+			return { abortedTurn: false };
+		},
 		async handleMessage() {},
 	};
 }