fix(cursor): a closed-stream write in the Cursor SDK no longer stops the server

packages/provider-cursor/src/server/CursorSdk.test.ts

@@ -0,0 +1,47 @@
+import { assert, describe, it } from "@effect/vitest";
+import * as Effect from "effect/Effect";
+import * as Layer from "effect/Layer";
+import { vi } from "vite-plus/test";
+
+import * as CursorSdk from "./CursorSdk.ts";
+
+vi.mock("./sdk.ts", () => ({ Agent: {}, createAgentPlatform: {}, SqliteLocalAgentStore: {} }));
+
+// Shaped like the SDK's minified class: the constructor is not named after the error.
+class o extends Error {
+  override name = "WriteIterableClosedError";
+}
+
+// Calls the guard directly, with Vitest's own listener detached so the guard does not defer to it.
+const rejectThroughGuard = (reason: unknown) =>
+  Effect.gen(function* () {
+    const others = process.listeners("unhandledRejection");
+    yield* Layer.build(CursorSdk.layer);
+    const guard = process.listeners("unhandledRejection").find((l) => !others.includes(l));
+    assert.isDefined(guard);
+    using exit = vi.spyOn(process, "exit").mockImplementation(() => undefined as never);
+    using _log = vi.spyOn(console, "error").mockImplementation(() => {});
+    for (const listener of others) process.off("unhandledRejection", listener);
+    try {
+      guard(reason, Promise.resolve());
+    } finally {
+      for (const listener of others) process.on("unhandledRejection", listener);
+    }
+    return exit.mock.calls;
+  }).pipe(Effect.scoped);
+
+describe("CursorSdk unhandled rejection guard", () => {
+  it.effect("keeps the server running when the SDK writes to a closed stream", () =>
+    Effect.gen(function* () {
+      assert.deepStrictEqual(yield* rejectThroughGuard(new o("WritableIterable is closed")), []);
+    }),
+  );
+
+  it.effect("still exits on other rejections", () =>
+    Effect.gen(function* () {
+      assert.deepStrictEqual(yield* rejectThroughGuard(new Error("WritableIterable is closed")), [
+        [1],
+      ]);
+    }),
+  );
+});

packages/provider-cursor/src/server/CursorSdk.ts

@@ -34,11 +34,24 @@ function isCursorShellSpawnFailure(reason: unknown): boolean {
   );
 }
 
+/**
+ * After a local exec fails, the SDK writes a final frame to the exec's stream from
+ * a promise nothing awaits. If that stream has already closed, the write rejects.
+ * The SDK's class is minified, so only its `name` identifies it.
+ */
+function isCursorClosedStreamWrite(reason: unknown): boolean {
+  return reason instanceof Error && reason.name === "WriteIterableClosedError";
+}
+
 function onUnhandledRejection(reason: unknown): void {
   if (isCursorShellSpawnFailure(reason)) {
     console.error("Cursor shell spawn failed. The server will keep running.", reason);
     return;
   }
+  if (isCursorClosedStreamWrite(reason)) {
+    console.error("Cursor SDK wrote to a closed exec stream. The server will keep running.");
+    return;
+  }
   if (process.listenerCount("unhandledRejection") > 1) {
     return;
   }