diff --git a/src/server/__tests__/sessions.test.ts b/src/server/__tests__/sessions.test.ts
index 040009ba..38a56d4c 100644
--- a/src/server/__tests__/sessions.test.ts
+++ b/src/server/__tests__/sessions.test.ts
@@ -8,6 +8,8 @@ import { execFileSync } from 'node:child_process'
 import * as path from 'node:path'
 import * as os from 'node:os'
 import { SessionService } from '../services/sessionService.js'
+import { sessionService } from '../services/sessionService.js'
+import { conversationService } from '../services/conversationService.js'
 import { clearCommandsCache } from '../../commands.js'
 import { sanitizePath } from '../../utils/sessionStoragePortable.js'
 
@@ -1161,6 +1163,31 @@ describe('Sessions API', () => {
     expect(res2.status).toBe(404)
   })
 
+  it('DELETE /api/sessions/:id should roll back the deleted marker when file deletion fails', async () => {
+    const sessionId = 'aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee'
+    await writeSessionFile('-tmp-api-test', sessionId, [makeSnapshotEntry()])
+
+    const originalDeleteSession = sessionService.deleteSession.bind(sessionService)
+    sessionService.deleteSession = (async (targetSessionId: string) => {
+      if (targetSessionId === sessionId) {
+        throw new Error('simulated unlink failure')
+      }
+      return originalDeleteSession(targetSessionId)
+    }) as typeof sessionService.deleteSession
+
+    try {
+      const res = await fetch(`${baseUrl}/api/sessions/${sessionId}`, { method: 'DELETE' })
+      expect(res.status).toBe(500)
+      expect((conversationService as any).deletedSessions.has(sessionId)).toBe(false)
+
+      const detailRes = await fetch(`${baseUrl}/api/sessions/${sessionId}`)
+      expect(detailRes.status).toBe(200)
+    } finally {
+      sessionService.deleteSession = originalDeleteSession as typeof sessionService.deleteSession
+      conversationService.unmarkSessionDeleted(sessionId)
+    }
+  })
+
   it('PATCH /api/sessions/:id should rename the session', async () => {
     const sessionId = 'aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee'
     await writeSessionFile('-tmp-api-test', sessionId, [
diff --git a/src/server/api/sessions.ts b/src/server/api/sessions.ts
index 0c940c04..3b2fed64 100644
--- a/src/server/api/sessions.ts
+++ b/src/server/api/sessions.ts
@@ -316,7 +316,12 @@ function isSessionNotFoundError(error: unknown): error is Error {
 
 async function deleteSession(sessionId: string): Promise<Response> {
   conversationService.markSessionDeleted(sessionId)
-  await sessionService.deleteSession(sessionId)
+  try {
+    await sessionService.deleteSession(sessionId)
+  } catch (error) {
+    conversationService.unmarkSessionDeleted(sessionId)
+    throw error
+  }
   return Response.json({ ok: true })
 }
 
diff --git a/src/server/services/conversationService.ts b/src/server/services/conversationService.ts
index f7a20640..8457d508 100644
--- a/src/server/services/conversationService.ts
+++ b/src/server/services/conversationService.ts
@@ -545,6 +545,10 @@ export class ConversationService {
     this.stopSession(sessionId)
   }
 
+  unmarkSessionDeleted(sessionId: string): void {
+    this.deletedSessions.delete(sessionId)
+  }
+
   getActiveSessions(): string[] {
     return Array.from(this.sessions.keys())
   }