workers-best-practices

by Cloudflare

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. npx skills add https://github.com/cloudflare/skills --skill workers-best-practices Download ZIPGitHub2k Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated.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

• 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.