fix(mcp): suppress unhandled error on stderr pipe in stdio transport (#99803)

cxbAsDev · claude · vincentkoc · web-flow · commit 1b84316a91dc · 2026-07-03T22:42:12.000-07:00
* fix(mcp): suppress unhandled error on stderr pipe in stdio transport

When child.stderr is piped to stderrStream without an error
handler, a stream-level error (EPIPE, I/O failure) crashes the
process. Add a noop error handler before the pipe, consistent
with the error handlers already present on stdin and stdout.

Co-Authored-By: Claude &lt;noreply@anthropic.com&gt;

* test(mcp): add regression test for stderr pipe error suppression

Co-Authored-By: Claude &lt;noreply@anthropic.com&gt;

* fix(mcp): report stderr stream errors

* fix(mcp): report stderr stream errors

---------

Co-authored-by: Claude &lt;noreply@anthropic.com&gt;
Co-authored-by: Vincent Koc &lt;vincentkoc@ieee.org&gt;
diff --git a/src/agents/mcp-stdio-transport.test.ts b/src/agents/mcp-stdio-transport.test.ts
@@ -204,4 +204,23 @@ describe("OpenClawStdioClientTransport", () => {
       "write after end",
     );
   });
+
+  it("reports stderr pipe errors without an unhandled error crash", async () => {
+    const child = new MockChildProcess();
+    spawnMock.mockReturnValue(child);
+
+    const transport = new OpenClawStdioClientTransport({ command: "npx", stderr: "pipe" });
+    const onerror = vi.fn();
+    Object.assign(transport, { onerror });
+    const started = transport.start();
+    child.emit("spawn");
+    await started;
+
+    const error = new Error("simulated pipe error");
+    expect(() => child.stderr?.emit("error", error)).not.toThrow();
+    expect(onerror).toHaveBeenCalledWith(error);
+
+    child.stderr.write("server diagnostic");
+    expect(transport.stderr?.read()?.toString()).toBe("server diagnostic");
+  });
 });
diff --git a/src/agents/mcp-stdio-transport.ts b/src/agents/mcp-stdio-transport.ts
@@ -86,6 +86,7 @@ export class OpenClawStdioClientTransport implements Transport {
       });
       child.stdout?.on("error", (error: Error) => this.onerror?.(error));
       if (this.stderrStream && child.stderr) {
+        child.stderr.on("error", (error: Error) => this.onerror?.(error));
         child.stderr.pipe(this.stderrStream);
       }
     });

Original file line number	Diff line number	Diff line change
`@@ -86,6 +86,7 @@ export class OpenClawStdioClientTransport implements Transport {`
`86`	`86`	`});`
`87`	`87`	`child.stdout?.on("error", (error: Error) => this.onerror?.(error));`
`88`	`88`	`if (this.stderrStream && child.stderr) {`
	`89`	`+ child.stderr.on("error", (error: Error) => this.onerror?.(error));`
`89`	`90`	`child.stderr.pipe(this.stderrStream);`
`90`	`91`	`}`
`91`	`92`	`});`