diff --git a/.changeset/job-sidecar-0600.md b/.changeset/job-sidecar-0600.md
new file mode 100644
index 00000000000..3cb7d0f8cdc
--- /dev/null
+++ b/.changeset/job-sidecar-0600.md
@@ -0,0 +1,5 @@
+---
+'nexus-agents': patch
+---
+
+security(jobs): write async job sidecar files (`<NEXUS_DATA_DIR>/jobs/result-<jobId>.json`) with `0600` permissions (#3753, defense-in-depth). The payload may carry job-result data; restricting to the owner matters if `NEXUS_DATA_DIR` is ever shared across users. Extracted a `persistJobRecord` helper (DRY over the four writers) that sets the mode on write and `chmod`s after, so the permission holds even when a terminal status overwrites a pre-existing pending file. Not exploitable today (per-user stdio MCP, randomUUID jobIds) — pure hardening.
diff --git a/packages/nexus-agents/src/mcp/jobs/job-result-store.test.ts b/packages/nexus-agents/src/mcp/jobs/job-result-store.test.ts
index 2c748fae2ec..07c12aa94fd 100644
--- a/packages/nexus-agents/src/mcp/jobs/job-result-store.test.ts
+++ b/packages/nexus-agents/src/mcp/jobs/job-result-store.test.ts
@@ -3,7 +3,7 @@
  */
 
 import { describe, it, expect, beforeEach, afterEach } from 'vitest';
-import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { mkdtempSync, rmSync, writeFileSync, statSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 
@@ -11,6 +11,7 @@ import {
   writeJobPending,
   writeJobComplete,
   writeJobFailed,
+  writeJobCancelled,
   readJobResult,
 } from './job-result-store.js';
 import { resetNexusDataDirCache, nexusDataPath } from '../../config/nexus-data-dir.js';
@@ -32,6 +33,31 @@ describe('job-result-store', () => {
     rmSync(tmpDir, { recursive: true, force: true });
   });
 
+  describe('sidecar file permissions (#3753 defense-in-depth)', () => {
+    const mode600 = (jobId: string): number =>
+      statSync(nexusDataPath('jobs', `result-${jobId}.json`)).mode & 0o777;
+
+    it('writes each terminal record with 0600 mode', () => {
+      writeJobComplete('mode-complete', 'orchestrate', { ok: true });
+      writeJobFailed('mode-failed', 'orchestrate', 'boom');
+      writeJobCancelled('mode-cancelled', 'orchestrate', 'stop');
+      expect(mode600('mode-complete')).toBe(0o600);
+      expect(mode600('mode-failed')).toBe(0o600);
+      expect(mode600('mode-cancelled')).toBe(0o600);
+    });
+
+    it('writeJobPending creates the file 0600', () => {
+      writeJobPending('mode-pending', 'orchestrate');
+      expect(mode600('mode-pending')).toBe(0o600);
+    });
+
+    it('a complete that OVERWRITES a pending stays 0600', () => {
+      writeJobPending('mode-overwrite', 'orchestrate');
+      writeJobComplete('mode-overwrite', 'orchestrate', { ok: true });
+      expect(mode600('mode-overwrite')).toBe(0o600);
+    });
+  });
+
   it('writeJobPending creates a pending record', () => {
     writeJobPending('job-test-1', 'orchestrate');
     const record = readJobResult('job-test-1');
diff --git a/packages/nexus-agents/src/mcp/jobs/job-result-store.ts b/packages/nexus-agents/src/mcp/jobs/job-result-store.ts
index f3258b880f9..055a2370055 100644
--- a/packages/nexus-agents/src/mcp/jobs/job-result-store.ts
+++ b/packages/nexus-agents/src/mcp/jobs/job-result-store.ts
@@ -27,7 +27,7 @@
  * @module mcp/jobs/job-result-store
  */
 
-import { existsSync, readFileSync, readdirSync, writeFileSync } from 'node:fs';
+import { existsSync, readFileSync, readdirSync, writeFileSync, chmodSync } from 'node:fs';
 
 import { z } from 'zod';
 
@@ -76,6 +76,17 @@ function jobResultPath(jobId: string): string {
   return nexusDataPathEnsure('jobs', `result-${jobId}.json`);
 }
 
+/**
+ * Write a job sidecar record with 0600 perms (#3753 defense-in-depth — the
+ * payload may carry job result data; restrict to the owner if NEXUS_DATA_DIR is
+ * ever shared). `chmodSync` after write guarantees the mode even when overwriting
+ * a pre-existing (default-umask) file, which the `writeFileSync` mode option skips.
+ */
+function persistJobRecord(path: string, record: JobResult): void {
+  writeFileSync(path, JSON.stringify(record, null, 2), { mode: 0o600 });
+  chmodSync(path, 0o600);
+}
+
 /**
  * Write the initial `pending` record for a new job. Idempotent: if a
  * record for `jobId` already exists (e.g. operator restart re-runs the
@@ -97,7 +108,7 @@ export function writeJobPending(jobId: string, toolName: string): void {
     status: 'pending',
     createdAt: new Date().toISOString(),
   };
-  writeFileSync(path, JSON.stringify(record, null, 2));
+  persistJobRecord(path, record);
   logger.debug('Wrote pending job record', { jobId, toolName });
 }
 
@@ -116,7 +127,7 @@ export function writeJobComplete(jobId: string, toolName: string, result: unknow
     completedAt: new Date().toISOString(),
     result,
   };
-  writeFileSync(jobResultPath(jobId), JSON.stringify(record, null, 2));
+  persistJobRecord(jobResultPath(jobId), record);
   logger.debug('Wrote complete job record', { jobId, toolName });
 }
 
@@ -131,7 +142,7 @@ export function writeJobFailed(jobId: string, toolName: string, error: string):
     completedAt: new Date().toISOString(),
     error,
   };
-  writeFileSync(jobResultPath(jobId), JSON.stringify(record, null, 2));
+  persistJobRecord(jobResultPath(jobId), record);
   logger.debug('Wrote failed job record', { jobId, toolName, error });
 }
 
@@ -159,7 +170,7 @@ export function writeJobCancelled(jobId: string, toolName: string, reason?: stri
     completedAt: new Date().toISOString(),
     ...(reason !== undefined ? { error: reason } : {}),
   };
-  writeFileSync(jobResultPath(jobId), JSON.stringify(record, null, 2));
+  persistJobRecord(jobResultPath(jobId), record);
   logger.debug('Wrote cancelled job record', { jobId, toolName, reason });
 }