fix(gateway): route watch trace spam to artifacts

src/infra/gateway-watch-tmux.test.ts

@@ -121,6 +121,13 @@ describe("gateway-watch tmux wrapper", () => {
     expect(code).toBe(0);
     const command = spawnSync.mock.calls[1]?.[1]?.[6] as string;
     expect(command).toContain("'OPENCLAW_TRACE_SYNC_IO=1'");
+    expect(command).toContain(
+      "'OPENCLAW_RUN_NODE_OUTPUT_LOG=.artifacts/gateway-watch-profiles/gateway-watch-output.log'",
+    );
+    expect(command).toContain("'OPENCLAW_RUN_NODE_FILTER_SYNC_IO_STDERR=1'");
+    expect(stderr.chunks.join("")).toContain(
+      "gateway:watch benchmark trace output: .artifacts/gateway-watch-profiles/gateway-watch-output.log",
+    );
   });
 
   it("can remove --force from benchmarked watch runs", () => {

src/infra/run-node.test.ts

@@ -448,6 +448,59 @@ describe("run-node script", () => {
     });
   });
 
+  it("routes sync I/O trace stderr blocks to the output log without flooding stderr", async () => {
+    await withTempDir({ prefix: "openclaw-run-node-" }, async (tmp) => {
+      await setupTrackedProject(tmp);
+      const outputPath = path.join(tmp, ".artifacts", "gateway-watch-profiles", "output.log");
+      const childStderr = [
+        "normal before\n",
+        "(node:12345) WARNING: Detected use of sync API\n",
+        "    at statSync (node:fs:1739:25)\n",
+        "    at loadConfig (/repo/src/config.ts:1:1)\n",
+        "\n",
+        "normal after\n",
+      ].join("");
+      const spawn = (_cmd: string, args: string[]) =>
+        createPipedExitedProcess({
+          stderr: args[0] === "openclaw.mjs" ? childStderr : "",
+        });
+      const stderrChunks: string[] = [];
+      const stderr = {
+        write: (chunk: string | Buffer) => {
+          stderrChunks.push(String(chunk));
+          return true;
+        },
+      } as unknown as NodeJS.WriteStream;
+      const stdout = {
+        write: () => true,
+      } as unknown as NodeJS.WriteStream;
+
+      const exitCode = await runNodeMain({
+        cwd: tmp,
+        args: ["status"],
+        env: {
+          ...process.env,
+          OPENCLAW_RUNNER_LOG: "0",
+          OPENCLAW_RUN_NODE_FILTER_SYNC_IO_STDERR: "1",
+          OPENCLAW_RUN_NODE_OUTPUT_LOG: outputPath,
+        },
+        spawn,
+        stderr,
+        stdout,
+        execPath: process.execPath,
+        platform: process.platform,
+      } as Parameters<typeof runNodeMain>[0] & { stdout: NodeJS.WriteStream });
+
+      expect(exitCode).toBe(0);
+      const terminalStderr = stderrChunks.join("");
+      expect(terminalStderr).toContain("normal before\n");
+      expect(terminalStderr).toContain("normal after\n");
+      expect(terminalStderr).not.toContain("Detected use of sync API");
+      expect(terminalStderr).not.toContain("statSync");
+      await expect(fs.readFile(outputPath, "utf-8")).resolves.toContain(childStderr);
+    });
+  });
+
   it("adds Node CPU profiling flags to the launched OpenClaw child when requested", async () => {
     await withTempDir({ prefix: "openclaw-run-node-" }, async (tmp) => {
       await setupTrackedProject(tmp, {