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Custom enrichers

defineEnricher derives context from request headers, env, or anything else, and adds it to every wide event before drain — without touching call sites.

An enricher runs on every emitted event before it reaches drains. It's the right tool when you want a field on every event without touching every call site — geo, user agent, trace context, deploy id, tenant id…

Canonical guide

Full reference at Custom enrichers:

defineEnricher() API
Reading request headers
Async enrichers
Combining built-in + custom

This page exists in the build-on-top section as a pointer — same content, classified by axis.

When to reach for plugins instead

If your feature mixes enrichment with other hooks (e.g. enrich + tail-sample + side-effect on drain), use a plugin instead — one cohesive object covering several lifecycle points.

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Plugins

definePlugin is the canonical extension contract for evlog. One plugin can opt into any subset of lifecycle hooks — enrich, drain, tail sampling, request lifecycle, client log observation.

Tail sampling

Decide post-hoc whether to keep an event based on the request outcome — keep all errors, keep slow requests, keep specific paths, drop the rest.