feat: stop generation button with abort signal plumbing

- Add POST /chat/stop endpoint on API
- Thread abortSignal from agent-manager through Agent.run() to streamText
- Thread abortSignal option through the Agent.run() signature
- Emit status:idle on stopTab() so frontend WS gets the update
- Add stopGeneration() store method on frontend tabStore
- Add stop button in ChatInput (btn-sm lg:btn-xs for mobile tap target)
- Add tests for /chat/stop endpoint
- Refactor processMessage to pass abortSignal to agent.run

3 files changed, 36 insertions, 4 deletions

diff --git a/packages/api/src/agent-manager.ts b/packages/api/src/agent-manager.ts
index 69c071d..c09a607 100644
--- a/packages/api/src/agent-manager.ts
+++ b/packages/api/src/agent-manager.ts
@@ -885,6 +885,7 @@ export class AgentManager {
 			}
 			tabAgent.abortController?.abort();
 			tabAgent.status = "idle";
+			this.emit({ type: "status", status: "idle" }, tabId);
 			tabAgent.agent = null;
 			// Resolve any pending completion promise so retrieve doesn't hang
 			tabAgent.completionResolve?.({ status: "error", error: "Agent was stopped." });
@@ -1193,10 +1194,10 @@ export class AgentManager {
 					// Best-effort — if this fails, appendMessage will throw and we'll catch it below
 				}
 
-				for await (const event of agent.run(
-					message,
-					reasoningEffort ? { reasoningEffort } : undefined,
-				)) {
+				for await (const event of agent.run(message, {
+					...(reasoningEffort ? { reasoningEffort } : {}),
+					abortSignal: tabAgent.abortController?.signal,
+				})) {
 					// Stop processing if the tab was aborted (closed/stopped).
 					// stopTab() already injected a `cancelled` system chunk into
 					// `chunks` before flipping the abort flag, so we just need
diff --git a/packages/api/src/app.ts b/packages/api/src/app.ts
index ba5dabd..73d3de5 100644
--- a/packages/api/src/app.ts
+++ b/packages/api/src/app.ts
@@ -94,6 +94,15 @@ app.post("/chat/cancel", async (c) => {
 	return c.json({ success: cancelled });
 });
 
+app.post("/chat/stop", async (c) => {
+	const body = await c.req.json();
+	if (typeof body.tabId !== "string") {
+		return c.json({ error: "tabId is required" }, 400);
+	}
+	agentManager.stopTab(body.tabId);
+	return c.json({ success: true });
+});
+
 app.route("/skills", skillsRoutes);
 app.route("/models", modelsRoutes);
 app.route("/tabs", tabsRoutes);
diff --git a/packages/api/tests/routes.test.ts b/packages/api/tests/routes.test.ts
index 49bae49..eba2226 100644
--- a/packages/api/tests/routes.test.ts
+++ b/packages/api/tests/routes.test.ts
@@ -318,3 +318,25 @@ describe("POST /chat", () => {
 		expect(typeof body.messageId).toBe("string");
 	});
 });
+
+describe("POST /chat/stop", () => {
+	it("returns 200 with success: true for valid tabId", async () => {
+		const res = await app.request("/chat/stop", {
+			method: "POST",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({ tabId: "tab-stop-1" }),
+		});
+		expect(res.status).toBe(200);
+		const body = await res.json();
+		expect(body).toEqual({ success: true });
+	});
+
+	it("returns 400 when tabId is missing", async () => {
+		const res = await app.request("/chat/stop", {
+			method: "POST",
+			headers: { "Content-Type": "application/json" },
+			body: JSON.stringify({}),
+		});
+		expect(res.status).toBe(400);
+	});
+});