fix(coding-agent): tighten btw branchability

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

@@ -53,6 +53,7 @@ export class BtwController {
 			this.#lastQuestion !== undefined &&
 			this.#lastReplyText !== undefined &&
 			this.#lastAssistantMessage !== undefined &&
+			this.#lastLeafId !== null &&
 			this.#lastLeafId === this.ctx.sessionManager.getLeafId()
 		);
 	}

packages/coding-agent/test/agent-session-btw-branch.test.ts

@@ -226,7 +226,7 @@ describe("AgentSession.branchFromBtw", () => {
 		activeSession.sessionManager.appendMessage({ role: "user", content: "seed", timestamp: Date.now() });
 		await activeSession.sessionManager.flush();
 		const abortController = new AbortController();
-		const execution = new Promise<void>(() => {});
+		const execution = Promise.withResolvers<void>().promise;
 		activeSession.trackEvalExecution(execution, abortController).catch(() => undefined);
 		expect(activeSession.isEvalRunning).toBe(true);
 

packages/coding-agent/test/modes/controllers/btw-controller.test.ts

@@ -112,10 +112,8 @@ describe("BtwController", () => {
 
 	it("replaces a previous request by aborting it before issuing the next runEphemeralTurn", async () => {
 		const signals: AbortSignal[] = [];
-		let firstRelease!: () => void;
-		const firstPromise = new Promise<RunEphemeralTurnResult>(resolve => {
-			firstRelease = () => resolve({ replyText: "first", assistantMessage: createAssistantMessage("first") });
-		});
+		const first = Promise.withResolvers<RunEphemeralTurnResult>();
+		const firstPromise = first.promise;
 		const runEphemeralTurn = vi
 			.fn<(args: RunEphemeralTurnArgs) => Promise<RunEphemeralTurnResult>>()
 			.mockImplementationOnce(async args => {
@@ -141,11 +139,11 @@ describe("BtwController", () => {
 		expect(signals[1]?.aborted).toBe(false);
 		expect(btwContainer.children).toHaveLength(1);
 		// Allow the orphaned first request to finish to keep the test clean.
-		firstRelease();
+		first.resolve({ replyText: "first", assistantMessage: createAssistantMessage("first") });
 	});
 
 	it("clears the panel when the active request is dismissed via Escape", async () => {
-		const runEphemeralTurn = vi.fn(async () => new Promise<RunEphemeralTurnResult>(() => {}));
+		const runEphemeralTurn = vi.fn(async () => Promise.withResolvers<RunEphemeralTurnResult>().promise);
 		const btwContainer = new Container();
 		const ctx = makeCtx(makeFakeSession(runEphemeralTurn), btwContainer);
 		const controller = new BtwController(ctx);
@@ -185,7 +183,7 @@ describe("BtwController", () => {
 	});
 
 	it("does not allow branch while /btw is still running", async () => {
-		const runEphemeralTurn = vi.fn(async () => new Promise<RunEphemeralTurnResult>(() => {}));
+		const runEphemeralTurn = vi.fn(async () => Promise.withResolvers<RunEphemeralTurnResult>().promise);
 		const ctx = makeCtx(makeFakeSession(runEphemeralTurn));
 		const controller = new BtwController(ctx);
 
@@ -194,6 +192,21 @@ describe("BtwController", () => {
 		expect(controller.canBranch()).toBe(false);
 	});
 
+	it("does not allow branch when the completed answer has no originating leaf", async () => {
+		const assistantMessage = createAssistantMessage("Answer");
+		const runEphemeralTurn = vi.fn(async () => ({ replyText: "Answer", assistantMessage }));
+		const ctx = makeCtx(makeFakeSession(runEphemeralTurn)) as InteractiveModeContext & {
+			setTestLeafId(nextLeafId: string | null): void;
+		};
+		ctx.setTestLeafId(null);
+		const controller = new BtwController(ctx);
+
+		await controller.start("Question?");
+		await drainBtwRequest();
+
+		expect(controller.canBranch()).toBe(false);
+	});
+
 	it("allows branch after a complete non-empty reply", async () => {
 		const assistantMessage = createAssistantMessage("Answer");
 		const runEphemeralTurn = vi.fn(async () => ({ replyText: "Answer", assistantMessage }));
@@ -221,7 +234,7 @@ describe("BtwController", () => {
 	});
 
 	it("does not allow branch after aborted or errored requests", async () => {
-		const abortedRun = vi.fn(async () => new Promise<RunEphemeralTurnResult>(() => {}));
+		const abortedRun = vi.fn(async () => Promise.withResolvers<RunEphemeralTurnResult>().promise);
 		const abortedController = new BtwController(makeCtx(makeFakeSession(abortedRun)));
 		await abortedController.start("Question?");
 		expect(abortedController.handleEscape()).toBe(true);