Marketplace @knack workers-best-practices

Workers Best Practices

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, observabil

v0.1.0 by @knack (Knack) workers-best-practices

Install with the knack CLI: knack pull @knack/workers-best-practices — then it runs in Claude Code, Cursor, Codex, or any agent that reads the open Anthropic Skills format.

Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated.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.