diff --git a/src/features/member-work-sync/main/adapters/output/TeamInboxMemberWorkSyncNudgeSink.ts b/src/features/member-work-sync/main/adapters/output/TeamInboxMemberWorkSyncNudgeSink.ts
index 3415c22a..6f13335c 100644
--- a/src/features/member-work-sync/main/adapters/output/TeamInboxMemberWorkSyncNudgeSink.ts
+++ b/src/features/member-work-sync/main/adapters/output/TeamInboxMemberWorkSyncNudgeSink.ts
@@ -11,7 +11,11 @@ export class TeamInboxMemberWorkSyncNudgeSink implements MemberWorkSyncInboxNudg
 
   async insertIfAbsent(input: Parameters<MemberWorkSyncInboxNudgePort['insertIfAbsent']>[0]) {
     const existing = await this.inboxReader.getMessagesFor(input.teamName, input.memberName);
-    if (existing.some((message) => message.messageId === input.messageId)) {
+    const existingMessage = existing.find((message) => message.messageId === input.messageId);
+    if (existingMessage) {
+      if (existingMessage.workSyncPayloadHash !== input.payloadHash) {
+        return { inserted: false, messageId: input.messageId, conflict: true };
+      }
       return { inserted: false, messageId: input.messageId };
     }
 
@@ -30,6 +34,7 @@ export class TeamInboxMemberWorkSyncNudgeSink implements MemberWorkSyncInboxNudg
       workSyncIntent: input.payload.workSyncIntent,
       workSyncIntentKey: input.payload.workSyncIntentKey,
       workSyncReviewRequestEventIds: input.payload.workSyncReviewRequestEventIds,
+      workSyncPayloadHash: input.payloadHash,
     });
 
     return {
diff --git a/src/main/services/team/TeamInboxReader.ts b/src/main/services/team/TeamInboxReader.ts
index 2e952c96..1e60b846 100644
--- a/src/main/services/team/TeamInboxReader.ts
+++ b/src/main/services/team/TeamInboxReader.ts
@@ -159,6 +159,8 @@ export class TeamInboxReader {
               (id): id is string => typeof id === 'string' && id.length > 0
             )
           : undefined,
+        workSyncPayloadHash:
+          typeof row.workSyncPayloadHash === 'string' ? row.workSyncPayloadHash : undefined,
         slashCommand:
           row.slashCommand &&
           typeof row.slashCommand === 'object' &&
diff --git a/src/main/services/team/TeamInboxWriter.ts b/src/main/services/team/TeamInboxWriter.ts
index addae1b6..63326acf 100644
--- a/src/main/services/team/TeamInboxWriter.ts
+++ b/src/main/services/team/TeamInboxWriter.ts
@@ -77,6 +77,7 @@ export class TeamInboxWriter {
       ...(request.workSyncReviewRequestEventIds?.length
         ? { workSyncReviewRequestEventIds: request.workSyncReviewRequestEventIds }
         : {}),
+      ...(request.workSyncPayloadHash ? { workSyncPayloadHash: request.workSyncPayloadHash } : {}),
       ...(request.slashCommand && { slashCommand: request.slashCommand }),
       ...(request.commandOutput && { commandOutput: request.commandOutput }),
     };
diff --git a/src/shared/types/team.ts b/src/shared/types/team.ts
index 1abf4fd4..1feac3b9 100644
--- a/src/shared/types/team.ts
+++ b/src/shared/types/team.ts
@@ -670,6 +670,8 @@ export interface InboxMessage {
   workSyncIntentKey?: string;
   /** Concrete review_requested event IDs covered by this nudge. */
   workSyncReviewRequestEventIds?: string[];
+  /** Durable hash for idempotent hidden member-work-sync automation rows. */
+  workSyncPayloadHash?: string;
   /** Structured slash-command metadata for sent command rows. */
   slashCommand?: SlashCommandMeta;
   /** Structured command-output metadata for session-derived result rows. */
@@ -717,6 +719,7 @@ export interface SendMessageRequest {
   workSyncIntent?: InboxMessage['workSyncIntent'];
   workSyncIntentKey?: string;
   workSyncReviewRequestEventIds?: string[];
+  workSyncPayloadHash?: string;
   slashCommand?: SlashCommandMeta;
   commandOutput?: CommandOutputMeta;
 }
diff --git a/test/features/member-work-sync/main/TeamInboxMemberWorkSyncNudgeSink.test.ts b/test/features/member-work-sync/main/TeamInboxMemberWorkSyncNudgeSink.test.ts
index f250ed4b..021036c9 100644
--- a/test/features/member-work-sync/main/TeamInboxMemberWorkSyncNudgeSink.test.ts
+++ b/test/features/member-work-sync/main/TeamInboxMemberWorkSyncNudgeSink.test.ts
@@ -31,7 +31,9 @@ describe('TeamInboxMemberWorkSyncNudgeSink', () => {
   it('returns inserted=false when the inbox already contains the stable messageId', async () => {
     const input = makeInput();
     const inboxReader = {
-      getMessagesFor: vi.fn(async () => [{ messageId: input.messageId }]),
+      getMessagesFor: vi.fn(async () => [
+        { messageId: input.messageId, workSyncPayloadHash: input.payloadHash },
+      ]),
     };
     const inboxWriter = {
       sendMessage: vi.fn(),
@@ -47,6 +49,52 @@ describe('TeamInboxMemberWorkSyncNudgeSink', () => {
     expect(inboxWriter.sendMessage).not.toHaveBeenCalled();
   });
 
+  it('fails closed when the existing stable messageId has a different payload hash', async () => {
+    const input = makeInput();
+    const inboxReader = {
+      getMessagesFor: vi.fn(async () => [
+        { messageId: input.messageId, workSyncPayloadHash: 'different-payload-hash' },
+      ]),
+    };
+    const inboxWriter = {
+      sendMessage: vi.fn(),
+    };
+    const sink = new TeamInboxMemberWorkSyncNudgeSink(inboxReader as never, inboxWriter as never);
+
+    await expect(sink.insertIfAbsent(input)).resolves.toEqual({
+      inserted: false,
+      messageId: input.messageId,
+      conflict: true,
+    });
+
+    expect(inboxWriter.sendMessage).not.toHaveBeenCalled();
+  });
+
+  it('treats legacy work-sync rows without payload hash as conflicts', async () => {
+    const input = makeInput();
+    const inboxReader = {
+      getMessagesFor: vi.fn(async () => [
+        {
+          messageId: input.messageId,
+          messageKind: 'member_work_sync_nudge',
+          workSyncIntent: input.payload.workSyncIntent,
+        },
+      ]),
+    };
+    const inboxWriter = {
+      sendMessage: vi.fn(),
+    };
+    const sink = new TeamInboxMemberWorkSyncNudgeSink(inboxReader as never, inboxWriter as never);
+
+    await expect(sink.insertIfAbsent(input)).resolves.toEqual({
+      inserted: false,
+      messageId: input.messageId,
+      conflict: true,
+    });
+
+    expect(inboxWriter.sendMessage).not.toHaveBeenCalled();
+  });
+
   it('writes a system notification inbox message for a new nudge', async () => {
     const input = makeInput();
     const inboxReader = {
@@ -77,6 +125,7 @@ describe('TeamInboxMemberWorkSyncNudgeSink', () => {
       workSyncIntent: 'agenda_sync',
       workSyncIntentKey: undefined,
       workSyncReviewRequestEventIds: undefined,
+      workSyncPayloadHash: input.payloadHash,
     });
   });
 
diff --git a/test/main/services/team/TeamInboxReader.test.ts b/test/main/services/team/TeamInboxReader.test.ts
index c4a5ef7f..61622e54 100644
--- a/test/main/services/team/TeamInboxReader.test.ts
+++ b/test/main/services/team/TeamInboxReader.test.ts
@@ -177,6 +177,35 @@ describe('TeamInboxReader', () => {
     });
   });
 
+  it('preserves member-work-sync payload hash without changing visible message fields', async () => {
+    hoisted.files.set(
+      '/mock/teams/my-team/inboxes/alice.json',
+      JSON.stringify([
+        {
+          from: 'system',
+          to: 'alice',
+          text: 'Please reconcile current work.',
+          timestamp: '2026-01-01T02:30:00.000Z',
+          read: false,
+          messageId: 'member-work-sync:my-team:alice:agenda',
+          source: 'system_notification',
+          messageKind: 'member_work_sync_nudge',
+          workSyncIntent: 'agenda_sync',
+          workSyncPayloadHash: 'sha256:work-sync',
+        },
+      ])
+    );
+
+    const messages = await reader.getMessagesFor('my-team', 'alice');
+    expect(messages).toHaveLength(1);
+    expect(messages[0]).toMatchObject({
+      messageId: 'member-work-sync:my-team:alice:agenda',
+      messageKind: 'member_work_sync_nudge',
+      workSyncIntent: 'agenda_sync',
+      workSyncPayloadHash: 'sha256:work-sync',
+    });
+  });
+
   it('preserves agent error semantic kind from the team lead inbox', async () => {
     hoisted.files.set(
       '/mock/teams/my-team/inboxes/team-lead.json',
diff --git a/test/main/services/team/TeamInboxWriter.test.ts b/test/main/services/team/TeamInboxWriter.test.ts
index 08bf1468..6d24dccb 100644
--- a/test/main/services/team/TeamInboxWriter.test.ts
+++ b/test/main/services/team/TeamInboxWriter.test.ts
@@ -157,6 +157,25 @@ describe('TeamInboxWriter', () => {
     expect(persisted[0].source).toBe('system_notification');
   });
 
+  it('persists member-work-sync payload hash when provided', async () => {
+    await writer.sendMessage('my-team', {
+      member: 'alice',
+      text: 'sync your work state',
+      source: 'system_notification',
+      messageKind: 'member_work_sync_nudge',
+      workSyncIntent: 'agenda_sync',
+      workSyncPayloadHash: 'sha256:work-sync',
+    });
+
+    const persisted = JSON.parse(hoisted.files.get(inboxPath) ?? '[]') as Record<string, unknown>[];
+    expect(persisted).toHaveLength(1);
+    expect(persisted[0]).toMatchObject({
+      messageKind: 'member_work_sync_nudge',
+      workSyncIntent: 'agenda_sync',
+      workSyncPayloadHash: 'sha256:work-sync',
+    });
+  });
+
   it('preserves provided message identity fields for dedup across live and persisted rows', async () => {
     const result = await writer.sendMessage('my-team', {
       member: 'alice',