diff --git a/test/scripts/upgrade-survivor-probe-gateway.test.ts b/test/scripts/upgrade-survivor-probe-gateway.test.ts
index 7f5f7d669187..024270779f1a 100644
--- a/test/scripts/upgrade-survivor-probe-gateway.test.ts
+++ b/test/scripts/upgrade-survivor-probe-gateway.test.ts
@@ -1,11 +1,11 @@
 // Upgrade Survivor Probe Gateway tests cover upgrade survivor probe gateway script behavior.
-import { spawn } from "node:child_process";
+import childProcess from "node:child_process";
 import fs from "node:fs";
 import { createServer as createHttpServer } from "node:http";
 import { createServer as createTcpServer, type Server, type Socket } from "node:net";
 import path from "node:path";
 import { pathToFileURL } from "node:url";
-import { afterEach, describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import { createBoundedChildOutput } from "../helpers/bounded-child-output.js";
 import { useAutoCleanupTempDirTracker } from "../helpers/temp-dir.js";
 
@@ -35,7 +35,7 @@ function runProbe(
   nodeArgs: string[] = [],
 ): Promise<ProbeResult> {
   return new Promise((resolve) => {
-    const child = spawn(process.execPath, [...nodeArgs, probePath, ...args], {
+    const child = childProcess.spawn(process.execPath, [...nodeArgs, probePath, ...args], {
       env: { ...process.env, ...env },
       stdio: ["ignore", "pipe", "pipe"],
     });
@@ -58,7 +58,7 @@ function runProbe(
       clearTimeout(timer);
       resolve({ error, signal: null, status: null, stderr: stderr.text(), stdout: stdout.text() });
     });
-    child.on("exit", (status, signal) => {
+    child.on("close", (status, signal) => {
       clearTimeout(timer);
       resolve({
         error: timedOut ? new Error(`probe timed out after ${timeout}ms`) : undefined,
@@ -87,6 +87,33 @@ async function listen(server: Server): Promise<string> {
 }
 
 describe("scripts/e2e/lib/upgrade-survivor/probe-gateway.mjs", () => {
+  it("drains stderr after process exit before publishing the probe result", async () => {
+    const spawn = childProcess.spawn;
+    let exitObserved:
+      | { status: number | null; signal: NodeJS.Signals | null; stderrPaused: boolean }
+      | undefined;
+    const spawnSpy = vi
+      .spyOn(childProcess, "spawn")
+      .mockImplementationOnce((command, args, options) => {
+        const child = spawn(command, args, options);
+        child.stderr?.pause();
+        child.once("exit", (status, signal) => {
+          exitObserved = { status, signal, stderrPaused: child.stderr?.isPaused() === true };
+          child.stderr?.resume();
+        });
+        return child;
+      });
+    try {
+      const result = await runProbe(["--timeout-ms", "1e3"]);
+
+      expect(exitObserved).toEqual({ status: 1, signal: null, stderrPaused: true });
+      expect(result).toMatchObject({ status: 1, signal: null, error: undefined });
+      expect(result.stderr).toContain("invalid --timeout-ms: 1e3");
+    } finally {
+      spawnSpy.mockRestore();
+    }
+  });
+
   it("does not hard-code degraded ready allowlists into Docker survivor probes", () => {
     const script = fs.readFileSync(dockerSurvivorPath, "utf8");
 
@@ -108,7 +135,7 @@ describe("scripts/e2e/lib/upgrade-survivor/probe-gateway.mjs", () => {
       "1e3",
     ]);
 
-    expect(timeoutResult.status).not.toBe(0);
+    expect(timeoutResult).toMatchObject({ status: 1, signal: null, error: undefined });
     expect(timeoutResult.stderr).toContain("invalid --timeout-ms: 1e3");
 
     const bodyLimitResult = await runProbe(
@@ -119,7 +146,7 @@ describe("scripts/e2e/lib/upgrade-survivor/probe-gateway.mjs", () => {
       },
     );
 
-    expect(bodyLimitResult.status).not.toBe(0);
+    expect(bodyLimitResult).toMatchObject({ status: 1, signal: null, error: undefined });
     expect(bodyLimitResult.stderr).toContain(
       "invalid OPENCLAW_UPGRADE_SURVIVOR_PROBE_MAX_BODY_BYTES: 64bytes",
     );