fix(coding-agent): reactivate a live debug session when the active child terminates

When the active js-debug child exits or terminates while other tree
sessions remain alive, #activeSessionId pointed at the dead child and
every subsequent tool action failed. Reassign to a live tree session
(preferring stopped, then non-root) on exited/terminated/proc-exit.
Regression test proves threads route to the surviving session.

packages/coding-agent/src/dap/session.ts

@@ -1359,10 +1359,12 @@ export class DapSessionManager {
 		client.onEvent("exited", body => {
 			session.exitCode = (body as DapExitedEventBody | undefined)?.exitCode;
 			session.status = "terminated";
+			this.#reactivateAfterTermination(session);
 			this.#resolveTreeOutcome(session);
 		});
 		client.onEvent("terminated", () => {
 			session.status = "terminated";
+			this.#reactivateAfterTermination(session);
 			this.#resolveTreeOutcome(session);
 		});
 		this.#sessions.set(session.id, session);
@@ -1379,6 +1381,7 @@ export class DapSessionManager {
 		void client.proc.exited.finally(() => {
 			clearInterval(heartbeat);
 			session.status = "terminated";
+			this.#reactivateAfterTermination(session);
 			this.#resolveTreeOutcome(session);
 		});
 		return session;
@@ -1735,6 +1738,20 @@ export class DapSessionManager {
 		}
 	}
 
+	/** Point the active session at a live tree member when the active one terminates. */
+	#reactivateAfterTermination(session: DapSession): void {
+		if (this.#activeSessionId !== session.id) return;
+		const live = this.#getTreeSessions(session).filter(
+			candidate => candidate.status !== "terminated" && candidate.client.isAlive(),
+		);
+		if (live.length === 0) return;
+		const replacement =
+			live.find(candidate => candidate.status === "stopped") ??
+			live.find(candidate => candidate.parentSessionId !== undefined) ??
+			live[0];
+		this.#activeSessionId = replacement.id;
+	}
+
 	#resolveTreeOutcome(session: DapSession): void {
 		const rootId = this.#getRootSession(session).id;
 		for (const waiter of [...this.#treeOutcomeWaiters]) {

packages/coding-agent/test/debug/dap-multi-session.test.ts

@@ -123,6 +123,10 @@ class FakeDapClient {
 		for (const handler of this.#events.get(event) ?? []) void handler(body, message);
 	}
 
+	emit(event: string, body: unknown): void {
+		this.#emit(event, body);
+	}
+
 	async #emitReverse(command: string, args: unknown): Promise<void> {
 		const handler = this.#reverseHandlers.get(command);
 		if (!handler) throw new Error(`Missing reverse handler for ${command}`);
@@ -209,4 +213,37 @@ describe("DAP multi-session debugging", () => {
 
 		await manager.terminate(undefined, 100);
 	});
+
+	it("reactivates a live session when the active child terminates", async () => {
+		const root = new FakeDapClient({
+			name: "target.js",
+			type: "pwa-node",
+			__pendingTargetId: "child",
+		});
+		const child = new FakeDapClient();
+		spyOn(DapClient, "spawn").mockResolvedValue(root as unknown as DapClient);
+		spyOn(DapClient, "connect").mockResolvedValue(child as unknown as DapClient);
+		const manager = new DapSessionManager();
+
+		const launched = await manager.launch(
+			{ adapter: TEST_ADAPTER, program: "/tmp/target.js", cwd: "/tmp" },
+			undefined,
+			1_000,
+		);
+		expect(launched.parentSessionId).toBeDefined();
+
+		child.emit("terminated", {});
+		await child.dispose();
+
+		const active = manager.getActiveSession();
+		expect(active).not.toBeNull();
+		expect(active?.id).not.toBe(launched.id);
+		expect(active?.status).not.toBe("terminated");
+
+		const threads = await manager.threads(undefined, 100);
+		expect(threads.threads).toEqual([{ id: 7, name: "target.js" }]);
+		expect(root.requests.filter(request => request.command === "threads")).toHaveLength(1);
+
+		await manager.terminate(undefined, 100);
+	});
 });