diff --git a/src/features/member-work-sync/main/infrastructure/FileMemberWorkSyncAuditJournal.ts b/src/features/member-work-sync/main/infrastructure/FileMemberWorkSyncAuditJournal.ts
index 6dda5604..28042b6d 100644
--- a/src/features/member-work-sync/main/infrastructure/FileMemberWorkSyncAuditJournal.ts
+++ b/src/features/member-work-sync/main/infrastructure/FileMemberWorkSyncAuditJournal.ts
@@ -125,6 +125,7 @@ export class NoopMemberWorkSyncAuditJournal implements MemberWorkSyncAuditJourna
 export class FileMemberWorkSyncAuditJournal implements MemberWorkSyncAuditJournalPort {
   private readonly maxBytes: number;
   private readonly rotatedFileCount: number;
+  private readonly appendChains = new Map<string, Promise<void>>();
 
   constructor(
     private readonly paths: MemberWorkSyncStorePaths,
@@ -136,9 +137,24 @@ export class FileMemberWorkSyncAuditJournal implements MemberWorkSyncAuditJourna
   }
 
   async append(event: MemberWorkSyncAuditEvent): Promise<void> {
+    const filePath = this.paths.getMemberJournalPath(event.teamName, event.memberName);
+    const previous = this.appendChains.get(filePath) ?? Promise.resolve();
+    const next = previous.catch(() => undefined).then(() => this.appendToFile(filePath, event));
+
+    this.appendChains.set(filePath, next);
+
+    try {
+      await next;
+    } finally {
+      if (this.appendChains.get(filePath) === next) {
+        this.appendChains.delete(filePath);
+      }
+    }
+  }
+
+  private async appendToFile(filePath: string, event: MemberWorkSyncAuditEvent): Promise<void> {
     try {
       await this.paths.ensureMemberWorkSyncDir(event.teamName, event.memberName);
-      const filePath = this.paths.getMemberJournalPath(event.teamName, event.memberName);
       await mkdir(dirname(filePath), { recursive: true });
       await withFileLock(filePath, async () => {
         await rotateIfNeeded(filePath, this.maxBytes, this.rotatedFileCount);
diff --git a/test/features/member-work-sync/main/FileMemberWorkSyncAuditJournal.test.ts b/test/features/member-work-sync/main/FileMemberWorkSyncAuditJournal.test.ts
index 3f1c0611..a559e0aa 100644
--- a/test/features/member-work-sync/main/FileMemberWorkSyncAuditJournal.test.ts
+++ b/test/features/member-work-sync/main/FileMemberWorkSyncAuditJournal.test.ts
@@ -90,6 +90,26 @@ describe('FileMemberWorkSyncAuditJournal', () => {
     expect(latest.taskRefs[0].taskId).toHaveLength(243);
   });
 
+  it('serializes concurrent appends for the same member journal', async () => {
+    const journal = new FileMemberWorkSyncAuditJournal(new MemberWorkSyncStorePaths(root));
+    const events = Array.from({ length: 80 }, (_, index) => ({
+      timestamp: `2026-04-30T00:01:${String(index).padStart(2, '0')}.000Z`,
+      teamName: 'team-a',
+      memberName: 'bob',
+      event: 'queue_coalesced' as const,
+      source: 'test',
+      reason: `event-${index}`,
+    }));
+
+    await Promise.all(events.map((event) => journal.append(event)));
+
+    const lines = (await readFile(journalPath(root), 'utf8')).trim().split('\n');
+    expect(lines).toHaveLength(events.length);
+    expect(lines.map((line) => JSON.parse(line).reason)).toEqual(
+      events.map((event) => event.reason)
+    );
+  });
+
   it('logs and swallows append failures', async () => {
     const logger = { debug: vi.fn(), warn: vi.fn(), error: vi.fn() };
     const paths = new MemberWorkSyncStorePaths(root);