diff --git a/scripts/e2e/lib/agent-turn-output.mjs b/scripts/e2e/lib/agent-turn-output.mjs
index d3a6b14231ec..14d6bc70a358 100644
--- a/scripts/e2e/lib/agent-turn-output.mjs
+++ b/scripts/e2e/lib/agent-turn-output.mjs
@@ -2,16 +2,13 @@ import fs from "node:fs";
 import { readTextFileTail, tailText } from "./text-file-utils.mjs";
 
 const ERROR_DETAIL_TAIL_BYTES = 64 * 1024;
+const OUTPUT_SCAN_TAIL_BYTES = 2 * 1024 * 1024;
 const REPLY_TEXT_PREVIEW_BYTES = 8 * 1024;
 const REPLY_TEXT_PREVIEW_COUNT = 5;
 const REQUEST_LOG_SCAN_CHUNK_BYTES = 64 * 1024;
 const REQUEST_LOG_SCAN_CARRY_CHARS = 256;
 const OPENAI_REQUEST_PATH_PATTERN = /\/v1\/(responses|chat\/completions)/u;
 
-function readTextFile(file) {
-  return fs.readFileSync(file, "utf8");
-}
-
 function textByteLength(text) {
   return Buffer.byteLength(text, "utf8");
 }
@@ -164,7 +161,7 @@ export function extractAgentReplyTexts(text) {
 }
 
 export function assertAgentReplyContainsMarker(marker, outputPath) {
-  const output = readTextFile(outputPath);
+  const output = readTextFileTail(outputPath, OUTPUT_SCAN_TAIL_BYTES);
   const replyTexts = extractAgentReplyTexts(output);
   if (replyTexts.some((text) => text.includes(marker))) {
     return;
diff --git a/test/scripts/e2e-agent-turn-output.test.ts b/test/scripts/e2e-agent-turn-output.test.ts
index 42608307da5a..868205983ce6 100644
--- a/test/scripts/e2e-agent-turn-output.test.ts
+++ b/test/scripts/e2e-agent-turn-output.test.ts
@@ -79,6 +79,27 @@ describe("scripts/e2e/lib/agent-turn-output", () => {
     }
   });
 
+  it("ignores stale reply markers outside the recent output tail", () => {
+    const dir = mkdtempSync(join(tmpdir(), "openclaw-e2e-agent-output-"));
+    try {
+      const outputPath = join(dir, "agent.log");
+      writeFileSync(
+        outputPath,
+        [
+          JSON.stringify({ payloads: [{ text: "OPENCLAW_E2E_OK_STALE" }] }),
+          "x".repeat(2_200_000),
+          JSON.stringify({ payloads: [{ text: "current reply without marker" }] }),
+        ].join("\n"),
+      );
+
+      expect(() => assertAgentReplyContainsMarker("OPENCLAW_E2E_OK_STALE", outputPath)).toThrow(
+        /agent reply payload did not contain marker/u,
+      );
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
   it("bounds missing marker diagnostics to the recent output tail", () => {
     const dir = mkdtempSync(join(tmpdir(), "openclaw-e2e-agent-output-"));
     try {