fix(billing): add idempotency to billing (#4157)

* fix(billing): add idempotency to billing

* Only release redis lock if billed

apps/sim/app/api/billing/update-cost/route.ts

@@ -6,6 +6,7 @@ import { recordUsage } from '@/lib/billing/core/usage-log'
 import { checkAndBillOverageThreshold } from '@/lib/billing/threshold-billing'
 import { checkInternalApiKey } from '@/lib/copilot/request/http'
 import { isBillingEnabled } from '@/lib/core/config/feature-flags'
+import { type AtomicClaimResult, billingIdempotency } from '@/lib/core/idempotency/service'
 import { generateRequestId } from '@/lib/core/utils/request'
 
 const logger = createLogger('BillingUpdateCostAPI')
@@ -19,6 +20,7 @@ const UpdateCostSchema = z.object({
   source: z
     .enum(['copilot', 'workspace-chat', 'mcp_copilot', 'mothership_block'])
     .default('copilot'),
+  idempotencyKey: z.string().min(1).optional(),
 })
 
 /**
@@ -28,6 +30,8 @@ const UpdateCostSchema = z.object({
 export async function POST(req: NextRequest) {
   const requestId = generateRequestId()
   const startTime = Date.now()
+  let claim: AtomicClaimResult | null = null
+  let usageCommitted = false
 
   try {
     logger.info(`[${requestId}] Update cost request started`)
@@ -75,9 +79,30 @@ export async function POST(req: NextRequest) {
       )
     }
 
-    const { userId, cost, model, inputTokens, outputTokens, source } = validation.data
+    const { userId, cost, model, inputTokens, outputTokens, source, idempotencyKey } =
+      validation.data
     const isMcp = source === 'mcp_copilot'
 
+    claim = idempotencyKey
+      ? await billingIdempotency.atomicallyClaim('update-cost', idempotencyKey)
+      : null
+
+    if (claim && !claim.claimed) {
+      logger.warn(`[${requestId}] Duplicate billing update rejected`, {
+        idempotencyKey,
+        userId,
+        source,
+      })
+      return NextResponse.json(
+        {
+          success: false,
+          error: 'Duplicate request: idempotency key already processed',
+          requestId,
+        },
+        { status: 409 }
+      )
+    }
+
     logger.info(`[${requestId}] Processing cost update`, {
       userId,
       cost,
@@ -113,6 +138,7 @@ export async function POST(req: NextRequest) {
       ],
       additionalStats,
     })
+    usageCommitted = true
 
     logger.info(`[${requestId}] Recorded usage`, {
       userId,
@@ -149,6 +175,22 @@ export async function POST(req: NextRequest) {
       duration,
     })
 
+    if (claim?.claimed && !usageCommitted) {
+      await billingIdempotency
+        .release(claim.normalizedKey, claim.storageMethod)
+        .catch((releaseErr) => {
+          logger.warn(`[${requestId}] Failed to release idempotency claim`, {
+            error: releaseErr instanceof Error ? releaseErr.message : String(releaseErr),
+            normalizedKey: claim?.normalizedKey,
+          })
+        })
+    } else if (claim?.claimed && usageCommitted) {
+      logger.warn(
+        `[${requestId}] Error occurred after usage committed; retaining idempotency claim to prevent double-billing`,
+        { normalizedKey: claim.normalizedKey }
+      )
+    }
+
     return NextResponse.json(
       {
         success: false,

apps/sim/lib/core/idempotency/service.ts

@@ -343,6 +343,10 @@ export class IdempotencyService {
     logger.debug(`Stored idempotency result in database: ${normalizedKey}`)
   }
 
+  async release(normalizedKey: string, storageMethod: 'redis' | 'database'): Promise<void> {
+    return this.deleteKey(normalizedKey, storageMethod)
+  }
+
   private async deleteKey(
     normalizedKey: string,
     storageMethod: 'redis' | 'database'
@@ -482,3 +486,8 @@ export const pollingIdempotency = new IdempotencyService({
   ttlSeconds: 60 * 60 * 24 * 3, // 3 days
   retryFailures: true,
 })
+
+export const billingIdempotency = new IdempotencyService({
+  namespace: 'billing',
+  ttlSeconds: 60 * 60, // 1 hour
+})