/vercel-functions

--- name: vercel-functions description: Vercel Functions expert guidance — Serverless Functions, Edge Functions, Fluid Compute, streaming, Cron Jobs, and runtime configuration. Use when configuring, debugging, or optimizing server-side code running on Vercel. metadata: priority: 8 docs:

• "https://vercel.com/docs/functions"
• "https://vercel.com/docs/functions/runtimes"
• "https://vercel.com/docs/functions/websockets"

sitemap: "https://vercel.com/sitemap/docs.xml" pathPatterns:

• 'api/**/*.*'
• 'pages/api/**'
• 'src/pages/api/**'
• 'app/**/route.*'
• 'src/app/**/route.*'
• 'apps/*/api/**/*.*'
• 'apps/*/app/**/route.*'
• 'apps/*/src/app/**/route.*'
• 'apps/*/pages/api/**'
• 'vercel.json'
• 'apps/*/vercel.json'

bashPatterns:

• '\bvercel\s+dev\b'
• '\bvercel\s+logs\b'

importPatterns:

• 'ws'
• 'socket.io'
• 'socket.io-client'

promptSignals: phrases:

• "websocket"
• "websockets"
• "web socket"
• "socket.io"

Polling is the classic technique people reach for when they think Vercel

lacks websockets. We intentionally do NOT trigger on named third-party

services (Pusher, PubNub, Ably) — those are deliberate choices, not a

signal that someone is working around a missing feature.

• "long polling"
• "long-polling"

allOf: [] anyOf:

• "realtime"
• "bidirectional"
• "ws server"
• "polling"
• "server-sent events"

noneOf: [] minScore: 6 validate: - pattern: export\s+default\s+function message: 'Use named exports (GET, POST, PUT, DELETE) instead of default export for route handlers' severity: error

Skip on App Router page / layout / loading / error / not-found / sitemap / template / default files,

which require a default export by Next.js convention. Detected via the 'use client' directive,

an App Router config export (metadata, dynamic, revalidate, fetchCache, runtime), an `export default

function` whose name matches an App Router file (Page / Layout / Loading / etc.), or any JSX

element with a capitalised component tag — all signals that the file is a page-style file rather

than a route handler. See anthropics/claude-code#54989.

skipIfFileContains: "(?:^|\\n)\\s*['\"]use\\s+client['\"]|export\\s+const\\s+(?:metadata|dynamic|revalidate|fetchCache|runtime)\\b|export\\s+default\\s+(?:async\\s+)?function\\s+\\w*(?:Page|Layout|Loading|Error|NotFound|Sitemap|Template|Default|sitemap|robots|opengraph|manifest)\\b|<[A-Z][A-Za-z0-9]*|\\{\\s*children\\s*[,}:]|MetadataRoute\\.|from\\s+['\"]next/(?:font|image|link|navigation|headers|cookies)['\"]" - pattern: NextApiRequest|NextApiResponse message: 'NextApiRequest/NextApiResponse are Pages Router types — use Web API Request/Response' severity: error - pattern: 'from\s+[''"](openai|@anthropic-ai/sdk|anthropic)[''"]|new\s+(OpenAI|Anthropic)\(' message: 'Direct AI provider SDK detected in route handler. Use the Vercel AI SDK for streaming, tools, and provider abstraction.' severity: recommended upgradeToSkill: ai-sdk upgradeWhy: 'Replace vendor-locked provider SDKs with @ai-sdk/openai or @ai-sdk/anthropic for unified streaming and tool support.' skipIfFileContains: '@ai-sdk/|from\s+[''"](ai)[''"]|import.*from\s+[''"](ai)[''"]|streamText|generateText' - pattern: 'setTimeout\s*\(|setInterval\s*\(|await\s+new\s+Promise\s*\([^)]*setTimeout' message: 'Long-running or polling logic detected in a serverless handler. Functions have execution time limits.' severity: recommended upgradeToSkill: workflow upgradeWhy: 'Move delayed/polling logic to Vercel Workflow for durable execution with pause, resume, retries, and crash safety.' skipIfFileContains: 'use workflow|use step|@vercel/workflow' - pattern: 'writeFile(Sync)?\(|createWriteStream\(|from\s+[''"](multer|formidable)[''"]|fs\.writeFile' message: 'Local filesystem write detected. Serverless functions have ephemeral, read-only filesystems.' severity: error upgradeToSkill: vercel-storage upgradeWhy: 'Replace local filesystem writes with Vercel Blob, Neon, or Upstash for persistent, platform-native storage.' skipIfFileContains: '@vercel/blob|@upstash/|@neondatabase/' - pattern: 'export\s+(async\s+)?function\s+(GET|POST|PUT|PATCH|DELETE)\b' message: 'Route handler has no observability instrumentation. Add logging and error tracking for production debugging.' severity: warn skipIfFileContains: 'console\.error|logger\.|captureException|Sentry|@vercel/otel|withTracing' - pattern: 'from\s+[''""](lru-cache|node-cache|memory-cache)[''""]|new\s+(LRUCache|NodeCache|Map)\(\s*\).*cache' message: 'In-process memory cache detected in serverless function. Process memory is not shared across invocations.' severity: recommended upgradeToSkill: runtime-cache upgradeWhy: 'Replace in-process caches with Vercel Runtime Cache (getCache from @vercel/functions) for region-aware caching that persists across invocations.' skipIfFileContains: 'getCache|from\s+[''""]\@vercel/functions[''""]' - pattern: 'maxRetries\s*[=:]|retryCount\s*[=:]|retry\s*\(\s*|for\s*\([^)]*retry|while\s*\([^)]*retry' message: 'Manual retry logic detected. Use Vercel Workflow SDK for automatic retries with durable execution.' severity: recommended upgradeToSkill: workflow upgradeWhy: 'Replace manual retry loops with Workflow SDK steps that provide automatic retries, crash safety, and observability.' skipIfFileContains: 'use workflow|use step|@vercel/workflow|from\s+[''""](workflow)[''""]' - pattern: 'from\s+[''"](express)[''""]|require\s*\(\s*[''"](express)[''""\)]' message: 'Express.js detected in a Vercel project. Vercel Functions use the Web Request/Response API — Express middleware, req/res, and app.listen() do not work in serverless.' severity: re