Workers Best Practices skill

Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated.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

Use the project's installed versions, generated types, and Wrangler compatibility settings as the baseline for existing code. Retrieve relevant Cloudflare documentation to verify API, configuration, runtime behavior, and limit claims.

References

Read the sections relevant to the task:

Reference	When to use it
Configuration and observability	Compatibility dates, bindings, generated types, secrets, logs, and traces
Runtime patterns	Streaming, promise lifetime, request state, service calls, security, and runtime tests
Platform API checks	Handler signatures, platform classes, binding access, and serialization

For missing evidence, consult Workers best practices or find the affected product in the Cloudflare docs directory. Use the installed Wrangler schema for config fields. A newer type package does not supersede the project's configured target.

Keep Compatibility Dates Current

Use today's date for new Workers. Encourage periodic updates for existing Workers, reviewing compatibility changes and running relevant tests. Assess existing behavior against its configured date and flags; see compatibility guidance.

Enable Observability

Enable Workers Logs and Traces when creating or preparing a Worker for production. Set observability.enabled and observability.traces.enabled to true; the top-level setting alone does not enable traces. Use structured JSON logging and configure sampling for the workload. During reviews, flag missing logs or traces. See the configuration example.

Place Compute Near Regional Databases

For Workers that make multiple sequential queries to one regional database, configure explicit placement matching the database's provider and region before deployment. Ask for missing location information, preserve existing placement choices, and verify handler eligibility and deployed request duration. See database placement.

Anti-Patterns to Flag

Anti-pattern	Consequence and preferred pattern
await response.text() or similar buffering on unbounded data	Can exhaust Worker memory; stream large or unbounded bodies.
Hardcoded secrets in source or config	Leaks credentials through version control; use Wrangler secrets.
Math.random() for security-sensitive tokens or IDs	Predictable values; use crypto.randomUUID() or crypto.getRandomValues().
Async work started without awaiting, returning, or attaching it to ctx.waitUntil()	Work can be dropped and errors missed; tie it to the request or background-work lifetime.
Module-level mutable request state	Leaks data across requests and can cause I/O ownership errors; pass request state explicitly.
Cloudflare REST API calls for operations available through Worker bindings	Adds network and authentication overhead; use the available binding.
ctx.passThroughOnException() used as general error handling	Can conceal Worker failures by forwarding to the origin; use explicit error handling and structured error responses.
Hand-written Env that duplicates Wrangler bindings	Can drift from configuration; generate binding types with wrangler types.
Direct string comparison of secret values	Can expose timing differences; use the Web Crypto comparison pattern.
Destructuring ctx methods, such as const { waitUntil } = ctx	Loses the receiver; call ctx.waitUntil(...).
any on Env or handler parameters	Hides binding and handler contract errors; use the project's generated and platform types.
as unknown as T to force a platform type match	Hides incompatibilities; fix the underlying contract.
implements used in place of extending a platform base class	Does not inherit runtime behavior, this.ctx, or this.env; use the appropriate base class.
Unbound env.X in a platform class method	Bindings are available through this.env.X; see binding access patterns.
Applying one serialization rule across Queues, Workflow steps, storage, and WebSockets	Can reject valid payloads or accept unsupported ones; check the specific API and encoding.

Validation

Use the project's existing checks for affected Workers behavior: type-check binding or handler contract changes, and run relevant runtime tests for behavior changes. Preserve required repository checks; a narrow edit does not require a full Workers audit.

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
• CLI commands: load the cf-cli skill, or the wrangler skill in a project with a Wrangler configuration file

Skill feedback

If the user reports that this skill gave incorrect or outdated guidance, or is missing information, read references/skill-feedback.md.