fix: wait for worker stderr drain

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: yuranich

packages/sdk/client/rpc/node-rpc-client.ts

@@ -406,7 +406,20 @@ async function ensureRPC(): Promise<RPC> {
               );
               return;
             }
-            // Worker died before handshake — reject the init promise.
+            // Pre-handshake failures are rejected from "close" so stderr has
+            // drained before we assemble the startup error cause.
+          },
+        );
+
+        bareWorkerProc.on(
+          "close",
+          (...args: unknown[]) => {
+            if (settled) return;
+            const code = typeof args[0] === "number" ? args[0] : null;
+            const exitSignal = typeof args[1] === "string" ? args[1] : null;
+
+            // Worker died before handshake. Use close, not exit, so piped
+            // stderr has drained before we build the error cause.
             settled = true;
             clearTimeout(timer);
             const serverToClose = ipcServer;
@@ -422,7 +435,7 @@ async function ensureRPC(): Promise<RPC> {
               new RPCInitTimeoutError(
                 RPC_INIT_TIMEOUT_MS,
                 createWorkerStartupError(
-                  `Worker process exited with code ${code} before IPC connection was established`,
+                  `Worker process exited with code ${code}, signal ${exitSignal} before IPC connection was established`,
                   workerStderrTail,
                 ),
               ),