diff --git a/src/agents/sessions/auth-storage.test.ts b/src/agents/sessions/auth-storage.test.ts
new file mode 100644
index 00000000000..f91cb2467b5
--- /dev/null
+++ b/src/agents/sessions/auth-storage.test.ts
@@ -0,0 +1,115 @@
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+// auth-storage.ts persists via the named import `writeFileSync` from node:fs,
+// and replaceFileAtomicSync (in @openclaw/fs-safe) writes its temp file via the
+// default import `syncFs.writeFileSync`. A namespace `vi.spyOn(fs, ...)` cannot
+// rebind an already-captured named import, so we mock node:fs and route every
+// writeFileSync (named + default) through a single controllable write-failure hook.
+const writeFailHook = vi.hoisted(() => ({
+  fn: undefined as ((file: unknown, data: unknown, options: unknown) => void) | undefined,
+  // The unwrapped writeFileSync, so the hook can mutate disk state
+  // (e.g. truncate the destination) without re-entering itself.
+  raw: undefined as ((...args: unknown[]) => unknown) | undefined,
+}));
+
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs")>();
+  writeFailHook.raw = actual.writeFileSync as (...args: unknown[]) => unknown;
+  const writeFileSync: typeof actual.writeFileSync = ((
+    file: unknown,
+    data: unknown,
+    options: unknown,
+  ) => {
+    writeFailHook.fn?.(file, data, options);
+    return (actual.writeFileSync as (...a: unknown[]) => unknown)(file, data, options);
+  }) as typeof actual.writeFileSync;
+  return {
+    ...actual,
+    writeFileSync,
+    default: { ...actual, writeFileSync },
+  };
+});
+
+const fs = await import("node:fs");
+const { AuthStorage } = await import("./auth-storage.js");
+
+describe("auth-storage survives an interrupted write during persist (atomic write)", () => {
+  let tmpDir: string | undefined;
+
+  afterEach(() => {
+    writeFailHook.fn = undefined;
+    if (tmpDir) {
+      fs.rmSync(tmpDir, { recursive: true, force: true });
+      tmpDir = undefined;
+    }
+  });
+
+  it("does not lock out credentials when a write fails mid-flush", async () => {
+    tmpDir = fs.mkdtempSync(join(tmpdir(), "auth-writefail-"));
+    const authPath = join(tmpDir, "auth.json");
+
+    // Seed a valid credential that round-trips cleanly.
+    const seed = AuthStorage.create(authPath);
+    seed.set("anthropic", { type: "api_key", key: "sk-test-SEED-12345" });
+    expect(await AuthStorage.create(authPath).getApiKey("anthropic")).toBe("sk-test-SEED-12345");
+
+    // Model a write that fails partway during the next persist (a full disk, an
+    // over-quota volume, or a power loss before the flush). Only a *direct* write
+    // to auth.json leaves it corrupt:
+    //   - raw writeFileSync (RED): the persist writes straight to auth.json, so
+    //     the OS has already O_TRUNC-opened the destination. We model the partial
+    //     on-disk state by truncating auth.json, then failing the write.
+    //   - replaceFileAtomicSync (GREEN): the persist writes to a sibling temp
+    //     file (via fs-safe) and renames it into place. It never writes auth.json
+    //     directly, so this hook never fires and the persist completes cleanly --
+    //     which is exactly the write-atomicity we are asserting (auth.json is
+    //     never left partial).
+    writeFailHook.fn = (file) => {
+      if (typeof file === "number") {
+        return;
+      }
+      if (String(file) === authPath) {
+        writeFailHook.raw?.(authPath, "", "utf-8");
+        throw Object.assign(new Error("simulated write failure mid-flush"), {
+          code: "ENOSPC",
+        });
+      }
+    };
+
+    const persisting = AuthStorage.create(authPath);
+    // Raw writeFileSync targets auth.json directly, so the hook truncates it and
+    // throws (RED). The atomic write targets a temp file + rename, so the hook
+    // never fires and set() completes cleanly (GREEN).
+    try {
+      persisting.set("openai", { type: "api_key", key: "sk-test-NEW-67890" });
+    } catch {
+      // raw path throws after truncating auth.json; atomic path does not throw.
+    }
+    writeFailHook.fn = undefined;
+
+    // Next boot: the original credential must still load.
+    // RED (raw writeFileSync): auth.json is now empty -> JSON.parse throws on
+    //   reload -> loadError -> getApiKey returns undefined -> lockout.
+    // GREEN (replaceFileAtomicSync): auth.json was never written directly, so it
+    //   holds the atomically-renamed new content (seed still present).
+    const reopened = AuthStorage.create(authPath);
+    expect(reopened.drainErrors()).toHaveLength(0);
+    expect(await reopened.getApiKey("anthropic")).toBe("sk-test-SEED-12345");
+    expect(fs.existsSync(authPath)).toBe(true);
+    expect(fs.readFileSync(authPath, "utf-8").length).toBeGreaterThan(0);
+  });
+
+  it("preserves existing auth directory permissions while replacing the file", () => {
+    tmpDir = fs.mkdtempSync(join(tmpdir(), "auth-dir-mode-"));
+    fs.chmodSync(tmpDir, 0o755);
+    const authPath = join(tmpDir, "auth.json");
+
+    const storage = AuthStorage.create(authPath);
+    storage.set("anthropic", { type: "api_key", key: "sk-test-SEED-12345" });
+
+    expect(fs.statSync(tmpDir).mode & 0o777).toBe(0o755);
+    expect(fs.statSync(authPath).mode & 0o777).toBe(0o600);
+  });
+});
diff --git a/src/agents/sessions/auth-storage.ts b/src/agents/sessions/auth-storage.ts
index 445256de6a9..faf86e96ba0 100644
--- a/src/agents/sessions/auth-storage.ts
+++ b/src/agents/sessions/auth-storage.ts
@@ -6,9 +6,10 @@
  * try to refresh tokens simultaneously.
  */
 
-import { chmodSync, existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { chmodSync, existsSync, mkdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
 import { dirname, join } from "node:path";
 import lockfile from "proper-lockfile";
+import { replaceFileAtomicSync } from "../../infra/replace-file.js";
 import { findEnvKeys, getEnvApiKey } from "../../llm/env-api-keys.js";
 import {
   getOAuthApiKey,
@@ -79,6 +80,19 @@ export class FileAuthStorageBackend implements AuthStorageBackend {
     }
   }
 
+  private replaceAuthFileAtomic(content: string): void {
+    const dirMode = statSync(dirname(this.authPath)).mode & 0o7777;
+    replaceFileAtomicSync({
+      filePath: this.authPath,
+      content,
+      dirMode,
+      mode: 0o600,
+      tempPrefix: "auth.json",
+      syncTempFile: true,
+      syncParentDir: true,
+    });
+  }
+
   private acquireLockSyncWithRetry(path: string): () => void {
     const maxAttempts = 10;
     const delayMs = 20;
@@ -116,8 +130,7 @@ export class FileAuthStorageBackend implements AuthStorageBackend {
       const current = existsSync(this.authPath) ? readFileSync(this.authPath, "utf-8") : undefined;
       const { result, next } = fn(current);
       if (next !== undefined) {
-        writeFileSync(this.authPath, next, "utf-8");
-        chmodSync(this.authPath, 0o600);
+        this.replaceAuthFileAtomic(next);
       }
       return result;
     } finally {
@@ -161,8 +174,7 @@ export class FileAuthStorageBackend implements AuthStorageBackend {
       const { result, next } = await fn(current);
       throwIfCompromised();
       if (next !== undefined) {
-        writeFileSync(this.authPath, next, "utf-8");
-        chmodSync(this.authPath, 0o600);
+        this.replaceAuthFileAtomic(next);
       }
       throwIfCompromised();
       return result;