diff --git a/src/modules/ai-runtime/user-ai-quota.service.ts b/src/modules/ai-runtime/user-ai-quota.service.ts
index f2287df..acaf25c 100644
--- a/src/modules/ai-runtime/user-ai-quota.service.ts
+++ b/src/modules/ai-runtime/user-ai-quota.service.ts
@@ -11,8 +11,8 @@ export class UserAiQuotaService {
     return new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
   }
 
-  /** Check if user can create a new AI job, throw if over limit */
-  async checkAndReserve(userId: string, apiKeyMode: string): Promise<void> {
+  /** Read-only check: throws if user is over daily job or token limit. Actual reservation (upsert) is done by incrementJobCount. */
+  async checkQuota(userId: string, apiKeyMode: string): Promise<void> {
     const settings = await this.prisma.userAiSettings.findUnique({ where: { userId } });
     const maxJobs = settings?.maxDailyAiJobs ?? 20;
     const maxTokens = settings?.maxDailyTokenBudget ?? 100_000;
diff --git a/src/modules/ai-runtime/user-ai.service.spec.ts b/src/modules/ai-runtime/user-ai.service.spec.ts
index effb3eb..25db00a 100644
--- a/src/modules/ai-runtime/user-ai.service.spec.ts
+++ b/src/modules/ai-runtime/user-ai.service.spec.ts
@@ -29,7 +29,7 @@ describe('UserAiService.createAnalysisJob', () => {
     };
     snapshotBuilder = { buildSnapshot: jest.fn().mockResolvedValue(mockSnapshot) };
     priorityRules = { computeJobPriority: jest.fn().mockReturnValue(50) };
-    quota = { checkAndReserve: jest.fn().mockResolvedValue(undefined), incrementJobCount: jest.fn().mockResolvedValue(undefined) };
+    quota = { checkQuota: jest.fn().mockResolvedValue(undefined), incrementJobCount: jest.fn().mockResolvedValue(undefined) };
     budget = { checkPlatformBudget: jest.fn().mockResolvedValue(undefined) };
     const crypto = { encrypt: jest.fn(), decrypt: jest.fn(), hash: jest.fn(), mask: jest.fn() } as any;
 
@@ -132,7 +132,7 @@ describe('UserAiService.createAnalysisJob', () => {
 
     await service.createAnalysisJob('u1', validDto);
 
-    expect(quota.checkAndReserve).toHaveBeenCalledWith('u1', 'platform_key');
+    expect(quota.checkQuota).toHaveBeenCalledWith('u1', 'platform_key');
     expect(budget.checkPlatformBudget).toHaveBeenCalledWith('deepseek', 'deepseek-chat');
     expect(quota.incrementJobCount).toHaveBeenCalledWith('u1', 'platform_key');
   });
diff --git a/src/modules/ai-runtime/user-ai.service.ts b/src/modules/ai-runtime/user-ai.service.ts
index d101a99..afc8f61 100644
--- a/src/modules/ai-runtime/user-ai.service.ts
+++ b/src/modules/ai-runtime/user-ai.service.ts
@@ -230,6 +230,7 @@ export class UserAiService {
     }
 
     // 4. Resolve apiKeyMode / credentialId
+    // NOTE: apiKeyMode may be reassigned in step 6 fallback — steps 7–11 consume the final values
     let apiKeyMode = dto.apiKeyMode ?? settings.apiKeyMode;
     let credentialId: string | undefined = dto.credentialId ?? settings.defaultCredentialId ?? undefined;
     if (apiKeyMode === 'user_deepseek_key' && !credentialId) {
@@ -244,8 +245,8 @@ export class UserAiService {
       }
     }
 
-    // 5. Quota check
-    await this.quota.checkAndReserve(userId, apiKeyMode);
+    // 5. Quota check (read-only fast-fail; actual reservation at step 11)
+    await this.quota.checkQuota(userId, apiKeyMode);
 
     // 6. Platform budget check (platform_key only) — with fallback to user key
     if (apiKeyMode === 'platform_key') {
@@ -253,7 +254,7 @@ export class UserAiService {
         await this.budget.checkPlatformBudget('deepseek', 'deepseek-chat');
       } catch (err: any) {
         if (err?.response?.errorCode === 'PLATFORM_CIRCUIT_OPEN' && settings.fallbackToPlatformKey && settings.defaultCredentialId) {
-          // Fallback: switch to user key mode
+          // Fallback: apiKeyMode + credentialId reassigned — downstream steps use updated values
           apiKeyMode = 'user_deepseek_key';
           credentialId = settings.defaultCredentialId;
         } else {