workers-best-practices

disable-model-invocation	true
name	workers-best-practices
description	Reviews and authors Cloudflare Workers code against production best practices. Load when writing new Workers, reviewing Worker code, configuring wrangler.jsonc, or checking for common Workers anti-patterns (streaming, floating promises, global state, secrets, bindings, observability). Biases towards retrieval from Cloudflare docs over pre-trained knowledge.

Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated. Prefer retrieval over pre-training for any Workers code task — writing or reviewing.

Retrieval Sources

Fetch the latest versions before writing or reviewing Workers code. Do not rely on baked-in knowledge for API signatures, config fields, or binding shapes.

Source	How to retrieve	Use for
Workers best practices	Fetch https://developers.cloudflare.com/workers/best-practices/workers-best-practices/	Canonical rules, patterns, anti-patterns
Workers types	See references/review.md for retrieval steps	API signatures, handler types, binding types
Wrangler config schema	node_modules/wrangler/config-schema.json	Config fields, binding shapes, allowed values
Cloudflare docs	Search tool or https://developers.cloudflare.com/workers/	API reference, compatibility dates/flags

FIRST: Fetch Latest References

Before reviewing or writing Workers code, retrieve the current best practices page and relevant type definitions. If the project's node_modules has an older version, prefer the latest published version.

# Fetch latest workers types
mkdir -p /tmp/workers-types-latest && \
  npm pack @cloudflare/workers-types --pack-destination /tmp/workers-types-latest && \
  tar -xzf /tmp/workers-types-latest/cloudflare-workers-types-*.tgz -C /tmp/workers-types-latest
# Types at /tmp/workers-types-latest/package/index.d.ts

Reference Documentation

• references/rules.md — all best practice rules with code examples and anti-patterns
• references/review.md — type validation, config validation, binding access patterns, review process

Rules Quick Reference

Configuration

Rule	Summary
Compatibility date	Set compatibility_date to today on new projects; update periodically on existing ones
nodejs_compat	Enable the nodejs_compat flag — many libraries depend on Node.js built-ins
wrangler types	Run wrangler types to generate Env — never hand-write binding interfaces
Secrets	Use wrangler secret put, never hardcode secrets in config or source
wrangler.jsonc	Use JSONC config for non-secret settings — newer features are JSON-only

Request & Response Handling

Rule	Summary
Streaming	Stream large/unknown payloads — never await response.text() on unbounded data
waitUntil	Use ctx.waitUntil() for post-response work; do not destructure ctx

Architecture

Rule	Summary
Bindings over REST	Use in-process bindings (KV, R2, D1, Queues) — not the Cloudflare REST API
Queues & Workflows	Move async/background work off the critical path
Service bindings	Use service bindings for Worker-to-Worker calls — not public HTTP
Hyperdrive	Always use Hyperdrive for external PostgreSQL/MySQL connections

Observability

Rule	Summary
Logs & Traces	Enable observability in config with head_sampling_rate; use structured JSON logging

Code Patterns

Rule	Summary
No global request state	Never store request-scoped data in module-level variables
Floating promises	Every Promise must be awaited, returned, voided, or passed to ctx.waitUntil()

Security

Rule	Summary
Web Crypto	Use crypto.randomUUID() / crypto.getRandomValues() — never Math.random() for security
No passThroughOnException	Use explicit try/catch with structured error responses

Anti-Patterns to Flag

Anti-pattern	Why it matters
await response.text() on unbounded data	Memory exhaustion — 128 MB limit
Hardcoded secrets in source or config	Credential leak via version control
Math.random() for tokens/IDs	Predictable, not cryptographically secure
Bare fetch() without await or waitUntil	Floating promise — dropped result, swallowed error
Module-level mutable variables for request state	Cross-request data leaks, stale state, I/O errors
Cloudflare REST API from inside a Worker	Unnecessary network hop, auth overhead, added latency
ctx.passThroughOnException() as error handling	Hides bugs, makes debugging impossible
Hand-written Env interface	Drifts from actual wrangler config bindings
Direct string comparison for secret values	Timing side-channel — use crypto.subtle.timingSafeEqual
Destructuring ctx (const { waitUntil } = ctx)	Loses this binding — throws "Illegal invocation" at runtime
any on Env or handler params	Defeats type safety for all binding access
as unknown as T double-cast	Hides real type incompatibilities — fix the design
implements on platform base classes (instead of extends)	Legacy — loses this.ctx, this.env. Applies to DurableObject, WorkerEntrypoint, Workflow
env.X inside platform base class	Should be this.env.X in classes extending DurableObject, WorkerEntrypoint, etc.

Review Workflow

1. Retrieve — fetch latest best practices page, workers types, and wrangler schema
2. Read full files — not just diffs; context matters for binding access patterns
3. Check types — binding access, handler signatures, no any, no unsafe casts (see references/review.md)
4. Check config — compatibility_date, nodejs_compat, observability, secrets, binding-code consistency
5. Check patterns — streaming, floating promises, global state, serialization boundaries
6. Check security — crypto usage, secret handling, timing-safe comparisons, error handling
7. Validate with tools — npx tsc --noEmit, lint for no-floating-promises
8. Reference rules — see references/rules.md for each rule's correct pattern

Scope

This skill covers Workers-specific best practices and code review. For related topics:

• Durable Objects: load the durable-objects skill
• Workflows: see Rules of Workflows
• Wrangler CLI commands: load the wrangler skill

Principles

• Be certain. Retrieve before flagging. If unsure about an API, config field, or pattern, fetch the docs first.
• Provide evidence. Reference line numbers, tool output, or docs links.
• Focus on what developers will copy. Workers code in examples and docs gets pasted into production.
• Correctness over completeness. A concise example that works beats a comprehensive one with errors.