diff --git a/scripts/e2e/lib/plugin-update/probe.mjs b/scripts/e2e/lib/plugin-update/probe.mjs
index 1e2b86638b63..4198ab392dbe 100644
--- a/scripts/e2e/lib/plugin-update/probe.mjs
+++ b/scripts/e2e/lib/plugin-update/probe.mjs
@@ -10,6 +10,9 @@ import {
 } from "../plugin-index-sqlite.mjs";
 
 const home = os.homedir();
+const OUTPUT_TAIL_BYTES = 64 * 1024;
+const OUTPUT_TAIL_LINES = 120;
+const OUTPUT_SCAN_WINDOW_BYTES = 8 * 1024;
 
 const readJson = (file) => {
   try {
@@ -106,15 +109,51 @@ function assertSnapshot(beforePath) {
   }
 }
 
-function assertOutput(logPath) {
-  const output = fs.readFileSync(logPath, "utf8");
-  const failure = output.includes("Downloading @example/lossless-claw")
+function appendBufferTail(tail, chunk, maxBytes) {
+  if (chunk.length >= maxBytes) {
+    return chunk.subarray(chunk.length - maxBytes);
+  }
+  if (tail.length + chunk.length <= maxBytes) {
+    return Buffer.concat([tail, chunk]);
+  }
+  return Buffer.concat([tail, chunk]).subarray(tail.length + chunk.length - maxBytes);
+}
+
+async function readOutputEvidence(logPath) {
+  let outputTail = Buffer.alloc(0);
+  let scanWindow = "";
+  let sawDownload = false;
+  let sawUpToDate = false;
+  for await (const chunk of fs.createReadStream(logPath)) {
+    const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+    const text = buffer.toString("utf8");
+    const searchable = `${scanWindow}${text}`;
+    outputTail = appendBufferTail(outputTail, buffer, OUTPUT_TAIL_BYTES);
+    sawDownload ||= searchable.includes("Downloading @example/lossless-claw");
+    sawUpToDate ||= searchable.includes("lossless-claw is up to date (0.9.0).");
+    scanWindow = searchable.slice(-OUTPUT_SCAN_WINDOW_BYTES);
+  }
+  return {
+    outputTail: outputTail
+      .toString("utf8")
+      .split(/\r?\n/u)
+      .slice(-OUTPUT_TAIL_LINES)
+      .join("\n")
+      .trimEnd(),
+    sawDownload,
+    sawUpToDate,
+  };
+}
+
+async function assertOutput(logPath) {
+  const evidence = await readOutputEvidence(logPath);
+  const failure = evidence.sawDownload
     ? "Unexpected npm download/reinstall path"
-    : !output.includes("lossless-claw is up to date (0.9.0).")
+    : !evidence.sawUpToDate
       ? "Expected up-to-date output missing"
       : "";
   if (failure) {
-    throw new Error(`${failure}\n${output}`);
+    throw new Error(`${failure}\nOutput tail:\n${evidence.outputTail}`);
   }
 }
 
diff --git a/scripts/e2e/lib/plugin-update/unchanged-scenario.sh b/scripts/e2e/lib/plugin-update/unchanged-scenario.sh
index cb6d6ed53127..bb613011e170 100644
--- a/scripts/e2e/lib/plugin-update/unchanged-scenario.sh
+++ b/scripts/e2e/lib/plugin-update/unchanged-scenario.sh
@@ -23,7 +23,7 @@ trap 'openclaw_e2e_stop_process "${registry_pid:-}"' EXIT
 
 if ! node "$probe" wait-registry; then
   echo "Local npm metadata registry failed to start"
-  cat /tmp/openclaw-e2e-registry.log || true
+  openclaw_e2e_print_log /tmp/openclaw-e2e-registry.log
   exit 1
 fi
 
@@ -42,9 +42,9 @@ set -e
 if [ "$plugin_update_status" -ne 0 ]; then
   echo "Plugin update command failed or timed out after ${plugin_update_timeout_seconds}s (status ${plugin_update_status})"
   echo "--- plugin update output ---"
-  cat /tmp/plugin-update-output.log || true
+  openclaw_e2e_print_log /tmp/plugin-update-output.log
   echo "--- local registry output ---"
-  cat /tmp/openclaw-e2e-registry.log || true
+  openclaw_e2e_print_log /tmp/openclaw-e2e-registry.log
   exit "$plugin_update_status"
 fi
 
@@ -52,11 +52,11 @@ if [ -n "$before_config_hash" ]; then
   after_config_hash="$(sha256sum "$OPENCLAW_CONFIG_PATH" | awk '{print $1}')"
   if [ "$before_config_hash" != "$after_config_hash" ]; then
     echo "Config changed unexpectedly for modern package $package_version"
-    cat /tmp/plugin-update-output.log
+    openclaw_e2e_print_log /tmp/plugin-update-output.log
     exit 1
   fi
 fi
 
 node "$probe" assert-snapshot /tmp/plugin-update-before.json
 node "$probe" assert-output /tmp/plugin-update-output.log
-cat /tmp/plugin-update-output.log
+openclaw_e2e_print_log /tmp/plugin-update-output.log
diff --git a/test/scripts/plugin-update-unchanged-docker.test.ts b/test/scripts/plugin-update-unchanged-docker.test.ts
index 35e7dcc7164f..62e29adbaf58 100644
--- a/test/scripts/plugin-update-unchanged-docker.test.ts
+++ b/test/scripts/plugin-update-unchanged-docker.test.ts
@@ -47,6 +47,17 @@ function runProbeStatus(
   }
 }
 
+function runProbeFileStatus(
+  command: string,
+  filePath: string,
+): { status: number | null; stderr: string } {
+  const result = spawnSync("node", [PLUGIN_UPDATE_PROBE_SCRIPT, command, filePath], {
+    encoding: "utf8",
+    stdio: "pipe",
+  });
+  return { status: result.status, stderr: result.stderr };
+}
+
 describe("plugin update unchanged Docker E2E", () => {
   it("seeds current plugin install ledger state before checking config stability", () => {
     const runner = readFileSync(PLUGIN_UPDATE_DOCKER_SCRIPT, "utf8");
@@ -75,6 +86,56 @@ describe("plugin update unchanged Docker E2E", () => {
     );
     expect(script).toContain('"--- plugin update output ---"');
     expect(script).toContain('"--- local registry output ---"');
+    expect(script).toContain("openclaw_e2e_print_log /tmp/plugin-update-output.log");
+    expect(script).toContain("openclaw_e2e_print_log /tmp/openclaw-e2e-registry.log");
+    expect(script).not.toContain("cat /tmp/plugin-update-output.log");
+    expect(script).not.toContain("cat /tmp/openclaw-e2e-registry.log");
+  });
+
+  it("bounds assert-output diagnostics to the saved command log tail", () => {
+    const root = mkdtempSync(path.join(tmpdir(), "openclaw-plugin-update-probe-"));
+    const logPath = path.join(root, "plugin-update-output.log");
+    try {
+      writeFileSync(
+        logPath,
+        `DO_NOT_PRINT_OLD_PLUGIN_UPDATE_LOG\n${"filler line\n".repeat(12 * 1024)}missing marker tail`,
+        "utf8",
+      );
+
+      const result = runProbeFileStatus("assert-output", logPath);
+
+      expect(result.status).toBe(1);
+      expect(result.stderr).toContain("Expected up-to-date output missing");
+      expect(result.stderr).toContain("Output tail:");
+      expect(result.stderr).toContain("missing marker tail");
+      expect(result.stderr).not.toContain("DO_NOT_PRINT_OLD_PLUGIN_UPDATE_LOG");
+      expect(result.stderr.length).toBeLessThan(80 * 1024);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it("detects unexpected download output before a large log tail", () => {
+    const root = mkdtempSync(path.join(tmpdir(), "openclaw-plugin-update-probe-"));
+    const logPath = path.join(root, "plugin-update-output.log");
+    try {
+      writeFileSync(
+        logPath,
+        [
+          "Downloading @example/lossless-claw",
+          "filler line\n".repeat(12 * 1024),
+          "lossless-claw is up to date (0.9.0).",
+        ].join("\n"),
+        "utf8",
+      );
+
+      const result = runProbeFileStatus("assert-output", logPath);
+
+      expect(result.status).toBe(1);
+      expect(result.stderr).toContain("Unexpected npm download/reinstall path");
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
   });
 
   it("waits for the local registry process during cleanup", () => {