diff --git a/scripts/e2e/lib/release-user-journey/assertions.mjs b/scripts/e2e/lib/release-user-journey/assertions.mjs
index 1c75648890e5..85143bdc6e84 100644
--- a/scripts/e2e/lib/release-user-journey/assertions.mjs
+++ b/scripts/e2e/lib/release-user-journey/assertions.mjs
@@ -8,6 +8,11 @@ import {
 import { readBoundedResponseText as readBoundedResponseTextWithLimit } from "../bounded-response-text.mjs";
 import { applyMockOpenAiModelConfig } from "../fixtures/mock-openai-config.mjs";
 import { readPluginInstallRecords } from "../plugin-index-sqlite.mjs";
+import { readTextFileTail } from "../text-file-utils.mjs";
+
+const SCAN_CHUNK_BYTES = 64 * 1024;
+const SCAN_CARRY_CHARS = 256;
+const ERROR_DETAIL_TAIL_BYTES = 16 * 1024;
 
 function clickClackHttpTimeoutMs() {
   return readPositiveInt(process.env.OPENCLAW_RELEASE_USER_JOURNEY_HTTP_TIMEOUT_MS, 5000);
@@ -33,6 +38,41 @@ function readPositiveInt(raw, fallback) {
   return Number.isInteger(parsed) && parsed > 0 ? parsed : fallback;
 }
 
+function fileContainsText(file, needle) {
+  let stat;
+  try {
+    stat = fs.statSync(file);
+  } catch {
+    return false;
+  }
+  if (!stat.isFile() || stat.size <= 0) {
+    return false;
+  }
+
+  const fd = fs.openSync(file, "r");
+  try {
+    const buffer = Buffer.alloc(Math.min(SCAN_CHUNK_BYTES, stat.size));
+    let carry = "";
+    let offset = 0;
+    while (offset < stat.size) {
+      const bytesToRead = Math.min(buffer.length, stat.size - offset);
+      const bytesRead = fs.readSync(fd, buffer, 0, bytesToRead, offset);
+      if (bytesRead <= 0) {
+        break;
+      }
+      offset += bytesRead;
+      const text = carry + buffer.subarray(0, bytesRead).toString("utf8");
+      if (text.includes(needle)) {
+        return true;
+      }
+      carry = text.slice(-Math.max(SCAN_CARRY_CHARS, needle.length - 1));
+    }
+    return false;
+  } finally {
+    fs.closeSync(fd);
+  }
+}
+
 async function withClickClackFixtureResponse(url, init, consume, options = {}) {
   const timeoutMs = options.timeoutMs ?? clickClackHttpTimeoutMs();
   const controller = new AbortController();
@@ -134,8 +174,10 @@ function assertAgentTurn() {
 function assertFileContains() {
   const file = process.argv[3];
   const needle = process.argv[4];
-  const raw = fs.readFileSync(file, "utf8");
-  assert(raw.includes(needle), `${file} did not contain ${needle}. Output: ${raw}`);
+  assert(
+    fileContainsText(file, needle),
+    `${file} did not contain ${needle}. Output tail: ${readTextFileTail(file, ERROR_DETAIL_TAIL_BYTES)}`,
+  );
 }
 
 function rememberPluginInstallPath() {
diff --git a/test/scripts/release-user-journey-assertions.test.ts b/test/scripts/release-user-journey-assertions.test.ts
index a31f552ad8cc..4cce0df528fa 100644
--- a/test/scripts/release-user-journey-assertions.test.ts
+++ b/test/scripts/release-user-journey-assertions.test.ts
@@ -87,6 +87,55 @@ async function startTcpFixtureServer(handler: (socket: Socket) => void): Promise
 }
 
 describe("release user journey assertions", () => {
+  it("scans large files when checking release user journey output text", () => {
+    const root = mkdtempSync(path.join(tmpdir(), "openclaw-release-user-assertions-"));
+    const home = path.join(root, "home");
+    const outputPath = path.join(root, "output.log");
+
+    try {
+      const needlePrefix = "journey-plugin";
+      writeFileSync(
+        outputPath,
+        `${"x".repeat(64 * 1024 - needlePrefix.length)}${needlePrefix}-a:pong\n`,
+        "utf8",
+      );
+
+      const result = runAssertion(home, [
+        "assert-file-contains",
+        outputPath,
+        "journey-plugin-a:pong",
+      ]);
+
+      expect(result.status).toBe(0);
+      expect(result.stderr).toBe("");
+    } finally {
+      rmSync(root, { force: true, recursive: true });
+    }
+  });
+
+  it("bounds release user journey output assertion diagnostics", () => {
+    const root = mkdtempSync(path.join(tmpdir(), "openclaw-release-user-assertions-"));
+    const home = path.join(root, "home");
+    const outputPath = path.join(root, "output.log");
+
+    try {
+      writeFileSync(
+        outputPath,
+        `DO_NOT_DUMP_OLD_OUTPUT${"x".repeat(70 * 1024)}\nrecent output tail\n`,
+        "utf8",
+      );
+
+      const result = runAssertion(home, ["assert-file-contains", outputPath, "missing"]);
+
+      expect(result.status).not.toBe(0);
+      expect(result.stderr).toContain("Output tail:");
+      expect(result.stderr).toContain("recent output tail");
+      expect(result.stderr).not.toContain("DO_NOT_DUMP_OLD_OUTPUT");
+    } finally {
+      rmSync(root, { force: true, recursive: true });
+    }
+  });
+
   it("fails when uninstall leaves the managed plugin directory behind", () => {
     const root = mkdtempSync(path.join(tmpdir(), "openclaw-release-user-assertions-"));
     const home = path.join(root, "home");