fall back to bun worker on eval spawn failure

packages/coding-agent/src/eval/__tests__/js-context-manager.test.ts

@@ -280,6 +280,35 @@ describe("JavaScript eval worker lifecycle", () => {
 		expect(stats.terminateCalls).toBe(1);
 	});
 
+	it("falls back to a Bun Worker when the subprocess cannot spawn", async () => {
+		using tempDir = TempDir.createSync("@omp-js-spawn-fallback-");
+		// Exercise the production ladder (process -> worker -> inline), not the
+		// worker-thread test seam the surrounding describe enables.
+		setJsEvalWorkerThreadForTests(false);
+		const stats: FakeWorkerStats = { closeRequests: 0, terminateCalls: 0 };
+		installFakeWorker(stats, { exitOnClose: true, settleRuns: true });
+		const originalSpawn = Bun.spawn;
+		let spawnAttempts = 0;
+		Bun.spawn = ((): never => {
+			spawnAttempts++;
+			throw new Error("subprocess spawn unavailable");
+		}) as unknown as typeof Bun.spawn;
+
+		try {
+			const session = makeSession(tempDir.path());
+			const sessionId = `js-spawn-fallback:${crypto.randomUUID()}`;
+			// The fake Worker settles runs without executing the cell, so an empty
+			// output proves the middle rung handled it — the inline fallback would
+			// have actually evaluated the expression and printed 42.
+			const result = await executeJs("return String(6 * 7);", { cwd: tempDir.path(), sessionId, session });
+			expect(result.exitCode).toBe(0);
+			expect(result.output.trim()).toBe("");
+			expect(spawnAttempts).toBe(1);
+		} finally {
+			Bun.spawn = originalSpawn;
+		}
+	});
+
 	it("falls back to the inline worker when the spawned worker errors during startup", async () => {
 		using tempDir = TempDir.createSync("@omp-js-worker-error-");
 		const stats: FakeWorkerStats = { closeRequests: 0, terminateCalls: 0 };

packages/coding-agent/src/eval/js/context-manager.ts

@@ -499,10 +499,12 @@ function spawnJsWorker(): WorkerHandle {
 		try {
 			return spawnJsProcess();
 		} catch (err) {
-			logger.warn("JS eval subprocess spawn failed; using inline JS eval worker (no sync-loop guard)", {
+			// Fall through to the Bun Worker rung: a worker thread still interrupts
+			// synchronous infinite loops via terminate(), which the inline fallback
+			// cannot.
+			logger.warn("JS eval subprocess spawn failed; falling back to a Bun Worker", {
 				error: err instanceof Error ? err.message : String(err),
 			});
-			return spawnInlineWorker();
 		}
 	}
 	try {