diff --git a/scripts/test-group-report.mjs b/scripts/test-group-report.mjs
index b86fc8231e5..8c44c0c484d 100644
--- a/scripts/test-group-report.mjs
+++ b/scripts/test-group-report.mjs
@@ -254,7 +254,7 @@ export function spawnText(command, args, options) {
       env: options.env,
       stdio: ["ignore", "pipe", "pipe"],
     });
-    let output = "";
+    let outputBytes = 0;
     let outputTail = Buffer.alloc(0);
     let stderrTail = Buffer.alloc(0);
     let streamedLogBytes = 0;
@@ -383,7 +383,7 @@ export function spawnText(command, args, options) {
         appendTail(buffer);
         return;
       }
-      output += message;
+      appendTail(buffer);
     }
     const appendOutput = (chunk, streamName) => {
       if (logFd !== null) {
@@ -417,10 +417,15 @@ export function spawnText(command, args, options) {
         return;
       }
 
-      output += chunk.toString("utf8");
-      if (Buffer.byteLength(output) > maxBuffer) {
+      const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(String(chunk), "utf8");
+      outputBytes += buffer.byteLength;
+      appendTail(buffer);
+      if (streamName === "stderr") {
+        appendTail(buffer, "stderr");
+      }
+      if (outputBytes > maxBuffer) {
         outputExceeded = true;
-        output += `\n[test-group-report] output exceeded ${String(maxBuffer)} bytes\n`;
+        appendDiagnostic(`\n[test-group-report] output exceeded ${String(maxBuffer)} bytes\n`);
         signalChild("SIGTERM");
         scheduleKill(
           "[test-group-report] command did not exit after output limit; sending SIGKILL\n",
@@ -444,7 +449,7 @@ export function spawnText(command, args, options) {
       const result = {
         status: outputExceeded || timedOut ? 1 : (code ?? 1),
         signal,
-        output: logFd === null ? output : streamedOutput(),
+        output: streamedOutput(),
         timedOut,
       };
       if (waitingForKillGrace && processGroupIsAlive()) {
diff --git a/test/scripts/test-group-report.test.ts b/test/scripts/test-group-report.test.ts
index f37ed313e58..a5d12f3dac8 100644
--- a/test/scripts/test-group-report.test.ts
+++ b/test/scripts/test-group-report.test.ts
@@ -668,6 +668,32 @@ describe("scripts/test-group-report child process guard", () => {
     }
   });
 
+  it("keeps no-log child output bounded to a tail", async () => {
+    const result = await spawnText(
+      process.execPath,
+      [
+        "--input-type=module",
+        "--eval",
+        [
+          "const chunk = Buffer.alloc(1024 * 1024, 120);",
+          "for (let index = 0; index < 3; index += 1) process.stdout.write(chunk);",
+        ].join("\n"),
+      ],
+      {
+        cwd: process.cwd(),
+        env: process.env,
+        killGraceMs: 50,
+        maxBufferBytes: 1024 * 1024,
+        outputTailBytes: 4096,
+        timeoutMs: 10_000,
+      },
+    );
+
+    expect(result.status).toBe(1);
+    expect(result.output.length).toBeLessThan(8 * 1024);
+    expect(result.output).toContain("output exceeded 1048576 bytes");
+  });
+
   it("stops streamed child output after the configured log cap", async () => {
     const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "openclaw-test-group-report-log-cap-"));
     const logPath = path.join(tempDir, "child.log");