workers-best-practices

Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated.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 `await`ed, `return`ed, `void`ed, 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

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