diff --git a/src/services/chat/cache.ts b/src/services/chat/cache.ts
index d1902f1..bf735d7 100644
--- a/src/services/chat/cache.ts
+++ b/src/services/chat/cache.ts
@@ -25,6 +25,15 @@ export class ChatCache {
   private cooldown: InvitationCooldown | null = null;
 
   constructor(private logger: Logger, filePath?: string) {
+    if (!filePath && process.env.NODE_ENV === 'test') {
+      // Without this guard a test that constructs a ChatCache (or a
+      // ChatService that builds one) would silently write to the prod
+      // ./data/chat-cache.json — concurrent test runs have already torn
+      // that file once. Force every test to opt into a tmp path.
+      throw new Error(
+        'ChatCache: refusing to fall back to ./data/chat-cache.json under NODE_ENV=test — pass an explicit filePath',
+      );
+    }
     this.filePath = filePath ?? path.join(process.cwd(), 'data', 'chat-cache.json');
   }
 
@@ -127,8 +136,7 @@ export class ChatCache {
           cooldown: this.cooldown,
         };
     try {
-      await fs.mkdir(path.dirname(this.filePath), { recursive: true });
-      await fs.writeFile(this.filePath, JSON.stringify(payload, null, 2), 'utf-8');
+      await this.atomicWrite(payload);
     } catch (error) {
       this.logger.warn('Failed to save chat cache cooldown:', error);
     }
@@ -145,10 +153,30 @@ export class ChatCache {
       cooldown: this.cooldown,
     };
     try {
-      await fs.mkdir(path.dirname(this.filePath), { recursive: true });
-      await fs.writeFile(this.filePath, JSON.stringify(payload, null, 2), 'utf-8');
+      await this.atomicWrite(payload);
     } catch (error) {
       this.logger.warn('Failed to save chat cache:', error);
     }
   }
+
+  /**
+   * Write the cache file via a temp+rename so a crashed/interrupted/concurrent
+   * writer can never leave the on-disk file half-written. rename(2) is atomic
+   * on the same filesystem, so any reader sees either the prior complete file
+   * or the new complete file — never NULs or truncated JSON.
+   *
+   * Serialises against itself so two concurrent saves don't fight over the
+   * same temp path or trample each other's rename.
+   */
+  private writeQueue: Promise<unknown> = Promise.resolve();
+  private async atomicWrite(payload: ChatCacheFile): Promise<void> {
+    const task = this.writeQueue.then(async () => {
+      await fs.mkdir(path.dirname(this.filePath), { recursive: true });
+      const tmp = `${this.filePath}.${process.pid}.${Date.now()}.${Math.random().toString(36).slice(2, 8)}.tmp`;
+      await fs.writeFile(tmp, JSON.stringify(payload, null, 2), 'utf-8');
+      await fs.rename(tmp, this.filePath);
+    });
+    this.writeQueue = task.catch(() => undefined);
+    return task;
+  }
 }
diff --git a/tests/chat-service-cooldown.test.ts b/tests/chat-service-cooldown.test.ts
index 3b3b84e..26d6c2b 100644
--- a/tests/chat-service-cooldown.test.ts
+++ b/tests/chat-service-cooldown.test.ts
@@ -23,7 +23,13 @@ function buildService() {
       new Response('{}', { status: 200, headers: { 'content-type': 'application/json' } }),
     )) as typeof fetch;
   const matrix = new MatrixClient(creds, logger, { minRequestIntervalMs: 0, fetchImpl });
-  const svc = new ChatService(matrix, logger);
+  // Point the cache at a tmp path; without this, the default falls back to
+  // ./data/chat-cache.json and the test would clobber the production cache.
+  const cache = new ChatCache(
+    logger,
+    path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+  );
+  const svc = new ChatService(matrix, logger, undefined, undefined, undefined, undefined, cache);
   return { svc, logger };
 }
 
@@ -168,7 +174,11 @@ describe('ChatService invitation cooldown', () => {
     matrix.sendTextMessage = async () => {
       throw new MatrixRateLimitError('429', 90_000, 'M_LIMIT_EXCEEDED', null);
     };
-    const svc = new ChatService(matrix, logger);
+    const cache = new ChatCache(
+      logger,
+      path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+    );
+    const svc = new ChatService(matrix, logger, undefined, undefined, undefined, undefined, cache);
 
     const result = await svc.sendChatMessage('!room:example.test', 'hi');
     expect(result).toBe(false);
diff --git a/tests/chat-service-delta-media.test.ts b/tests/chat-service-delta-media.test.ts
index 255890b..00d2506 100644
--- a/tests/chat-service-delta-media.test.ts
+++ b/tests/chat-service-delta-media.test.ts
@@ -172,7 +172,11 @@ describe('ChatService delta sync — media attachment', () => {
       },
     })) as typeof matrix.sync;
 
-    const svc = new ChatService(matrix, logger);
+    const cache = new ChatCache(
+      logger,
+      path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+    );
+    const svc = new ChatService(matrix, logger, undefined, undefined, undefined, undefined, cache);
     await expect(svc.getActiveChats()).resolves.toBeDefined();
   });
 });
diff --git a/tests/chat-service-username-persist.test.ts b/tests/chat-service-username-persist.test.ts
index 2bca8b8..a504b5a 100644
--- a/tests/chat-service-username-persist.test.ts
+++ b/tests/chat-service-username-persist.test.ts
@@ -26,6 +26,13 @@ function buildMatrix() {
 }
 
 describe('ChatService username persistence', () => {
+  function tmpCache(logger: Parameters<typeof ChatCache>[0]) {
+    return new ChatCache(
+      logger,
+      path.join(os.tmpdir(), `chat-cache-${Date.now()}-${Math.random()}.json`),
+    );
+  }
+
   it('seeds the in-memory cache from RedditUsersRepository on construction', () => {
     const db = new Database(':memory:');
     runMigrations(db);
@@ -36,7 +43,9 @@ describe('ChatService username persistence', () => {
     const svc = new ChatService(
       buildMatrix(),
       logger,
-      undefined, undefined, undefined, undefined, undefined, undefined,
+      undefined, undefined, undefined, undefined,
+      tmpCache(logger),
+      undefined,
       repo,
     );
 
@@ -47,7 +56,12 @@ describe('ChatService username persistence', () => {
 
   it('falls back to t2 form when no mapping is known', () => {
     const logger = createMockLogger();
-    const svc = new ChatService(buildMatrix(), logger);
+    const svc = new ChatService(
+      buildMatrix(),
+      logger,
+      undefined, undefined, undefined, undefined,
+      tmpCache(logger),
+    );
     expect(svc.resolveSenderDisplay('@t2_unknown:reddit.com')).toBe('t2_unknown');
   });