Review logging patterns Skill

Review logging patterns

Review and improve logging patterns in TypeScript/JavaScript codebases. Transform scattered console.logs into structured wide events and convert generic errors into self-documenting structured errors.

When to Use

Setting up evlog in a new or existing project (any supported framework)
Reviewing code for logging best practices
Converting console.log statements to structured logging
Improving error handling with better context
Configuring log draining, sampling, or enrichment

Quick Reference

| Working on... | Resource | | ----------------------- | ------------------------------------------------------------------ | | Setup (CLI) | evlog init — wire evlog into the project | | Project conventions (CLI) | evlog agents — write the evlog block into the project's AGENTS.md | | Coverage map (CLI) | evlog map — score dark entry points | | CI gating (CLI) | evlog map --min-score / --baseline — gate regressions | | Wide events patterns | references/wide-events.md | | Error handling | references/structured-errors.md | | Code review checklist | references/code-review.md | | Drain pipeline | references/drain-pipeline.md | | Audit logs | build-audit-logs skill + docs |

Audit logs

For security-sensitive actions (auth, billing, admin, data export), use evlog's audit layer — a typed audit field on wide events, not a parallel logger. See the build-audit-logs skill for end-to-end setup (log.audit, withAudit, denials, auditEnricher, auditOnly, signed, mockAudit).

log.audit({
  action: 'invoice.refund',
  actor: { type: 'user', id: user.id },
  target: { type: 'invoice', id: invoice.id },
  outcome: 'success',
})

Docs: https://www.evlog.dev/use-cases/audit/overview

Installation

npm install evlog

Use the CLI (recommended on Nuxt, Nitro, Next.js, TanStack Start)

@evlog/cli is a separate package from evlog — early, but worth trying. It reads the project on disk (no traffic, no config). On the four supported frameworks it covers the whole loop: wire evlog in (init), score coverage (map), lock the score in CI (--min-score, --baseline). If the CLI is unavailable, the framework has no adapter yet, or the user declines — continue with the manual sections below; the skill does not depend on it. Ask before installing anything; prefer npx / pnpm dlx for one-shots.

1. Setup: `evlog init`

On a project that doesn't use evlog yet, prefer init over hand-writing the setup — it detects the framework, reads what the project already has, and generates config, drains, enrichers, and extras in one pass. It is fully scriptable for agents:

# preview everything without writing (always start here)
npx @evlog/cli init --dry-run --yes

# then apply — flags instead of prompts
npx @evlog/cli init --yes \
  --service my-app \
  --drain fs \
  --prodDrain axiom \
  --extras enrichers,pipeline,sampling \
  --sampling medium

Useful flags: --framework (override detection: nuxt, nitro, next, tanstack-start), --prodDrain (comma-separated: axiom, otlp, posthog, sentry, better-stack, datadog, hyperdx), --extras (enrichers, pipeline, sampling, vite, error-catalog, audit-catalog, ai, better-auth), --enrichers, --sampling (traffic tier: all, low, medium, high, very-high), --apps (monorepo: which workspace packages), --no-install. Review the --dry-run output with the user before applying. Docs: https://www.evlog.dev/cli/init

2. Score: `evlog map`

npx @evlog/cli map --no-write
# agents: npx @evlog/cli map --json --no-write

What you get:

A project score and which entry points are still dark
FIX FIRST — the three most valuable places to fix
GOING FURTHER — opportunities (catalogs, audit coverage, AI logging, auth identity) that never cost points
Per-file inspect: npx @evlog/cli map <file> --no-write shows the shape the handler could take
Re-run after fixes and watch the score move

Work FIX FIRST in order, keep changes minimal (useLogger(), log.set(), log.audit(), createError({ why, fix })), then re-run with --no-write. Omit --no-write only when the user wants evlog.map.json written.

3. Lock it in CI: `--min-score` and `--baseline`

After fixing, propose making the score durable — this is where the CLI earns its keep:

# in CI, after pnpm add -D @evlog/cli (project-local, pinned by the lockfile)
pnpm exec evlog map --min-score 80   # absolute gate: exits 1 below the threshold
pnpm exec evlog map --baseline       # ratchet: exits 1 if this PR made things worse

--baseline compares the fresh scan against the committed evlog.map.json, per entry point and per requirement — a refactor that instruments one route and breaks another fails even if the total score is unchanged. Disabling a passing check with a comment counts as a regression too. New uninstrumented routes are listed as NEW AND DARK without failing. Workflow: commit evlog.map.json once, add the --baseline run to CI (pnpm add -D @evlog/cli for a pinned version — ask first), and re-run map without --baseline to accept an intentional change. Docs: https://www.evlog.dev/cli/ci

Early days: adapters and rules are still evolving; expect scores to move between releases. Docs: https://www.evlog.dev/cli/map · Rules: https://www.evlog.dev/cli/rules

Framework Setup

Nuxt

// nuxt.config.ts
export default defineNuxtConfig({
  modules: ['evlog/nuxt'],
  evlog: {
    env: { service: 'my-app' },
    include: ['/api/**'],
  },
})

All evlog functions (useLogger, createError, parseError, log) are auto-imported — no import statements needed.

// server/api/checkout.post.ts — no imports needed
export default defineEventHandler(async (event) => {
  const log = useLogger(event)
  log.set({ user: { id: user.id, plan: user.plan } })
  return { success: true }
})

Drain, enrich, and tail sampling use Nitro hooks in server plugins:

// server/plugins/evlog-drain.ts
import { createAxiomDrain } from 'evlog/axiom'

export default defineNitroPlugin((nitroApp) => {
  nitroApp.hooks.hook('evlog:drain', createAxiomDrain())
})

Client transport (auto-configured Vue plugin):

// nuxt.config.ts
evlog: {
  transport: { enabled: true },  // logs sent to /api/_evlog/ingest
}

Client-side: log, setIdentity, clearIdentity are auto-imported in components.

Next.js

Step 1: Create central config — all exports come from here:

// lib/evlog.ts
import type { DrainContext } from 'evlog'
import { createEvlog } from 'evlog/next'
import { createUserAgentEnricher, createRequestSizeEnricher } from 'evlog/enrichers'
import { createDrainPipeline } from 'evlog/pipeline'

const enrichers = [createUserAgentEnricher(), createRequestSizeEnricher()]
const pipeline = createDrainPipeline<DrainContext>({ batch: { size: 50, intervalMs: 5000 } })
const drain = pipeline(createAxiomDrain({ dataset: 'logs', apiKey: process.env.AXIOM_API_KEY! }))

export const { withEvlog, useLogger, log, createError } = createEvlog({
  service: 'my-app',
  sampling: {
    rates: { info: 10 },
    keep: [{ status: 400 }, { duration: 1000 }],
  },
  routes: {
    '/api/auth/**': { service: 'auth-service' },
    '/api/checkout/**': { service: 'checkout-service' },
  },
  keep: (ctx) => {
    const user = ctx.context.user as { premium?: boolean } | undefined
    if (user?.premium) ctx.shouldKeep = true
  },
  enrich: (ctx) => {
    for (const enricher of enrichers) enricher(ctx)
  },
  drain,
})

Step 2: Wrap route handlers with withEvlog():

// app/api/checkout/route.ts
import { withEvlog, useLogger } from '@/lib/evlog'

export const POST = withEvlog(async (request: Request) => {
  const log = useLogger()  // Zero arguments — uses AsyncLocalStorage
  log.set({ user: { id: 'user_123', plan: 'enterprise' } })
  log.set({ cart: { items: 3, total: 14999 } })
  return Response.json({ success: true })
})

Step 3: Server Actions — same withEvlog() wrapper:

// app/actions.ts
'use server'
import { withEvlog, useLogger } from '@/lib/evlog'

export const checkout = withEvlog(async (formData: FormData) => {
  const log = useLogger()
  log.set({ action: 'checkout', source: 'server-action' })
  return { success: true }
})

Step 4: Middleware (optional — sets x-request-id + timing headers):

// proxy.ts
import { evlogMiddleware } from 'evlog/next'
export const proxy = evlogMiddleware()
export const config = { matcher: ['/api/:path*'] }

Step 5: Client Provider — wrap root layout:

// app/layout.tsx
import { EvlogProvider } from 'evlog/next/client'

export default function Layout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en">
      <body>
        <EvlogProvider service="my-app" transport={{ enabled: true, endpoint: '/api/evlog/ingest' }}>
          {children}
        </EvlogProvider>
      </body>
    </html>
  )
}

Step 6: Client logging — in any client component:

'use client'
import { log, setIdentity, clearIdentity } from 'evlog/next/client'

setIdentity({ userId: 'usr_123' })
log.info({ action: 'checkout_click' })
clearIdentity()

Step 7 (optional): Instrumentation — startup + global onRequestError (SSR/RSC errors outside withEvlog). Use defineNodeInstrumentation(() => import('./lib/evlog')) in root instrumentation.ts to gate Node + cache the import, or write register/onRequestError manually — both are valid. For custom logic, wrap evlog’s register/onRequestError inside lib/evlog.ts (compose with your own init or metrics), then re-export.

Export createInstrumentation() from lib/evlog.ts alongside createEvlog(). See framework docs for coexistence with lockLogger.

Step 8: Client ingest endpoint — receives client logs:

// app/api/evlog/ingest/route.ts
import { NextRequest } from 'next/server'

const VALID_LEVELS = ['info', 'error', 'warn', 'debug'] as const

export async function POST(request: NextRequest) {
  const origin = request.headers.get('origin')
  const host = request.headers.get('host')
  if (origin && new URL(origin).host !== host) {
    return Response.json({ error: 'Invalid origin' }, { status: 403 })
  }
  const body = await request.json()
  if (!body?.timestamp || !body?.level || !VALID_LEVELS.includes(body.level)) {
    return Response.json({ error: 'Invalid payload' }, { status: 400 })
  }
  const { service: _, ...sanitized } = body
  console.log('[CLIENT LOG]', JSON.stringify({ ...sanitized, service: 'my-app', source: 'client' }))
  return new Response(null, { status: 204 })
}

SvelteKit

// src/hooks.server.ts
import { initLogger } from 'evlog'
import { createEvlogHooks } from 'evlog/sveltekit'

initLogger({ env: { service: 'my-app' } })

export const { handle, handleError } = createEvlogHooks()

Access the logger via event.locals.log in route handlers or useLogger() from anywhere in the call stack:

// src/routes/api/users/[id]/+server.ts
import { json } from '@sveltejs/kit'

export const GET = ({ locals, params }) => {
  locals.log.set({ user: { id: params.id } })
  return json({ id: params.id })
}

import { useLogger } from 'evlog/sveltekit'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

export const { handle, handleError } = createEvlogHooks({
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
})

Nitro v3

// nitro.config.ts
import { defineConfig } from 'nitro'
import evlog from 'evlog/nitro/v3'

export default defineConfig({
  modules: [evlog({ env: { service: 'my-api' } })],
})

// routes/api/checkout.post.ts
import { defineHandler } from 'nitro/h3'
import { useLogger } from 'evlog/nitro/v3'

export default defineHandler(async (event) => {
  const log = useLogger(event)
  log.set({ action: 'checkout' })
  return { ok: true }
})

TanStack Start

TanStack Start uses Nitro v3. Install evlog and add a nitro.config.ts:

// nitro.config.ts
import { defineConfig } from 'nitro'
import evlog from 'evlog/nitro/v3'

export default defineConfig({
  experimental: { asyncContext: true },
  modules: [evlog({ env: { service: 'my-app' } })],
})

Add the error handling middleware to __root.tsx:

// src/routes/__root.tsx
import { createMiddleware } from '@tanstack/react-start'
import { evlogErrorHandler } from 'evlog/nitro/v3'

export const Route = createRootRoute({
  server: {
    middleware: [createMiddleware().server(evlogErrorHandler)],
  },
})

Use useRequest() from nitro/context to access the logger:

import { useRequest } from 'nitro/context'
import type { RequestLogger } from 'evlog'

const req = useRequest()
const log = req.context.log as RequestLogger
log.set({ user: { id: 'user_123' } })

Nitro v2

// nitro.config.ts
import { defineNitroConfig } from 'nitropack/config'
import evlog from 'evlog/nitro'

export default defineNitroConfig({
  modules: [evlog({ env: { service: 'my-api' } })],
})

Import useLogger from evlog/nitro in routes.

NestJS

// src/app.module.ts
import { Module } from '@nestjs/common'
import { EvlogModule } from 'evlog/nestjs'

@Module({
  imports: [EvlogModule.forRoot()],
})
export class AppModule {}

EvlogModule.forRoot() registers a global middleware. Use useLogger() to access the request-scoped logger from any controller or service:

import { useLogger } from 'evlog/nestjs'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

EvlogModule.forRoot({
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
})

For async configuration with NestJS DI, use forRootAsync():

EvlogModule.forRootAsync({
  imports: [ConfigModule],
  inject: [ConfigService],
  useFactory: (config) => ({
    drain: createAxiomDrain({ apiKey: config.get('AXIOM_API_KEY') }),
  }),
})

Express

import express from 'express'
import { initLogger } from 'evlog'
import { evlog, useLogger } from 'evlog/express'

initLogger({ env: { service: 'my-api' } })

const app = express()
app.use(evlog())

app.get('/api/users', (req, res) => {
  req.log.set({ users: { count: 42 } })
  res.json({ users: [] })
})

Use useLogger() to access the logger from anywhere in the call stack without passing req:

import { useLogger } from 'evlog/express'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

app.use(evlog({
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
}))

Hono

import { Hono } from 'hono'
import { initLogger } from 'evlog'
import { evlog, type EvlogVariables } from 'evlog/hono'

initLogger({ env: { service: 'my-api' } })

const app = new Hono<EvlogVariables>()
app.use(evlog())

app.get('/api/users', (c) => {
  const log = c.get('log')
  log.set({ users: { count: 42 } })
  return c.json({ users: [] })
})

Access the logger via c.get('log') in handlers. Use useLogger() from evlog/hono in the layers underneath (services, repositories) where c is not in hand — both return the same logger:

import { useLogger } from 'evlog/hono'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

On Cloudflare Workers, useLogger() needs the nodejs_compat (or nodejs_als) compatibility flag; c.get('log') works with or without it.

Structured errors: throw createError(), then in app.onError use parseError() and pass parsed.status as ContentfulStatusCode to c.json() (Hono types the status argument as ContentfulStatusCode, not number).

import { createError, parseError } from 'evlog'
import type { ContentfulStatusCode } from 'hono/utils/http-status'

app.onError((error, c) => {
  c.get('log').error(error)
  const parsed = parseError(error)
  return c.json(
    { message: parsed.message, why: parsed.why, fix: parsed.fix, link: parsed.link },
    parsed.status as ContentfulStatusCode,
  )
})

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

app.use(evlog({
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
}))

Fastify

import Fastify from 'fastify'
import { initLogger } from 'evlog'
import { evlog, useLogger } from 'evlog/fastify'

initLogger({ env: { service: 'my-api' } })

const app = Fastify({ logger: false })
await app.register(evlog)

app.get('/api/users', async (request) => {
  request.log.set({ users: { count: 42 } })
  return { users: [] }
})

request.log is the evlog wide-event logger (shadows Fastify's built-in pino logger on the request). Fastify's pino logger remains accessible via fastify.log.

Use useLogger() to access the logger from anywhere in the call stack without passing request:

import { useLogger } from 'evlog/fastify'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

await app.register(evlog, {
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
})

Elysia

import { Elysia } from 'elysia'
import { initLogger } from 'evlog'
import { evlog, useLogger } from 'evlog/elysia'

initLogger({ env: { service: 'my-api' } })

const app = new Elysia()
  .use(evlog())
  .get('/api/users', ({ log }) => {
    log.set({ users: { count: 42 } })
    return { users: [] }
  })
  .listen(3000)

Use useLogger() to access the logger from anywhere in the call stack:

import { useLogger } from 'evlog/elysia'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

app.use(evlog({
  include: ['/api/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
}))

React Router

// react-router.config.ts
import type { Config } from '@react-router/dev/config'

export default {
  future: {
    v8_middleware: true,
  },
} satisfies Config

// app/root.tsx
import { initLogger } from 'evlog'
import { evlog } from 'evlog/react-router'

initLogger({ env: { service: 'my-api' } })

export const middleware: Route.MiddlewareFunction[] = [
  evlog(),
]

Access the logger via context.get(loggerContext) in loaders and actions:

// app/routes/api.users.$id.tsx
import { loggerContext } from 'evlog/react-router'

export async function loader({ params, context }: Route.LoaderArgs) {
  const log = context.get(loggerContext)
  log.set({ user: { id: params.id } })
  return { users: [] }
}

Use useLogger() to access the logger from anywhere in the call stack without passing context:

import { useLogger } from 'evlog/react-router'

async function findUsers() {
  const log = useLogger()
  log.set({ db: { query: 'SELECT * FROM users' } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

export const middleware: Route.MiddlewareFunction[] = [
  evlog({
    include: ['/api/**'],
    drain: createAxiomDrain(),
    enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
    keep: (ctx) => {
      if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
    },
  }),
]

oRPC

import { os } from '@orpc/server'
import { RPCHandler } from '@orpc/server/fetch'
import { initLogger } from 'evlog'
import { evlog, withEvlog, type EvlogOrpcContext } from 'evlog/orpc'

initLogger({ env: { service: 'my-rpc' } })

const base = os.$context<EvlogOrpcContext>().use(evlog())

const router = {
  ping: base.handler(({ context }) => {
    context.log.set({ pinged: true })
    return { ok: true }
  }),
}

const handler = withEvlog(new RPCHandler(router))

export default async function fetch(request: Request) {
  const { matched, response } = await handler.handle(request, { prefix: '/rpc' })
  return matched ? response : new Response('Not Found', { status: 404 })
}

withEvlog() wraps the handler so each matched request emits one wide event; os.use(evlog()) exposes context.log on every procedure that descends from base and tags the wide event with operation (the procedure path joined with .).

Use useLogger() to access the logger from utility modules:

import { useLogger } from 'evlog/orpc'

async function chargeCard(amount: number) {
  const log = useLogger()
  log.set({ payment: { amount } })
}

Full pipeline with drain, enrich, and tail sampling:

import { createAxiomDrain } from 'evlog/axiom'

const handler = withEvlog(new RPCHandler(router), {
  include: ['/rpc/**'],
  drain: createAxiomDrain(),
  enrich: (ctx) => { ctx.event.region = process.env.FLY_REGION },
  keep: (ctx) => {
    if (ctx.duration && ctx.duration > 2000) ctx.shouldKeep = true
  },
})

Cloudflare Workers

import { initWorkersLogger, withEvlog } from 'evlog/workers'

initWorkersLogger({ env: { service: 'edge-api' } })

export default withEvlog(async (request, _env, _ctx, log) => {
  log.set({ action: 'handle_request' })
  return Response.json({ ok: true })
})

withEvlog emits one wide event per request when the handler returns — no manual log.emit(). Async drains are registered with waitUntil so they survive the response; streaming responses defer the emit until the body completes. requestId comes from x-request-id (fallback cf-ray); method, path, cf-ray, traceparent, and the safe subset of request.cf are captured automatically. It accepts the same options (drain, enrich, keep, include, exclude, routes) as every other integration. For manual control (scheduled handlers, queues), createWorkersLogger(request) + log.emit() remains available. No ALS-based useLogger() on Workers — pass log explicitly.

AWS Lambda

Lambda has no HTTP middleware lifecycle, so evlog behaves like standalone TypeScript — with one critical rule: one logger per invocation, never a shared module-level logger (Lambda reuses execution environments, so a shared instance leaks fields between invocations).

import { initLogger, createLogger } from 'evlog'

initLogger({ env: { service: 'my-fn' } })  // once at module load (cold start)

export async function handler(event: SQSEvent) {
  for (const record of event.Records) {
    const log = createLogger({ messageId: record.messageId })
    try {
      log.set({ queue: { source: record.eventSourceARN } })
      await processMessage(record)
    } catch (error) {
      log.error(error as Error)
      throw error
    } finally {
      log.emit()
    }
  }
}

Astro

// src/middleware.ts
import { defineMiddleware } from 'astro:middleware'
import { initLogger, createRequestLogger } from 'evlog'

initLogger({ env: { service: 'my-astro-app' } })

export const onRequest = defineMiddleware(async ({ request, locals }, next) => {
  const url = new URL(request.url)
  const log = createRequestLogger({ method: request.method, path: url.pathname })
  locals.log = log

  try {
    const response = await next()
    log.emit()
    return response
  } catch (error) {
    log.error(error instanceof Error ? error : new Error(String(error)))
    log.emit()
    throw error
  }
})

Type locals.log in src/env.d.ts (interface Locals { log: RequestLogger }). Pair with the Vite plugin (below) for auto-imports and build-time DX.

Vite Plugin (any Vite-based framework)

For any Vite-based project (SvelteKit, Astro, SolidStart, React+Vite, etc.), use the Vite plugin for auto-init, auto-imports, and build-time features:

// vite.config.ts
import evlog from 'evlog/vite'

export default defineConfig({
  plugins: [
    evlog({
      service: 'my-app',
      autoImports: true,           // auto-import log, createEvlogError, parseError
      strip: ['debug'],            // remove log.debug() in production
      sourceLocation: true,        // inject file:line in dev + prod
      client: {                    // client-side logging
        transport: { endpoint: '/api/logs' },
      },
    }),
  ],
})

Server-side middleware (drain, enrich, keep, routes) is still configured in the framework integration (e.g., evlog() middleware for Hono/Express/SvelteKit). The Vite plugin handles build-time DX only.

Standalone TypeScript

import { initLogger, createRequestLogger } from 'evlog'

initLogger({ env: { service: 'my-worker', environment: 'production' } })

const log = createRequestLogger({ jobId: job.id })
log.set({ source: job.source, recordsSynced: 150 })
log.emit()  // Manual emit required in standalone

Configuration Options

All options work in Nuxt (evlog key), Nitro (passed to evlog()), Next.js (createEvlog()), and standalone (initLogger()).

| Option | Type | Default | Description | |--------|------|---------|-------------| | env.service / service | string | 'app' | Service name in logs | | enabled | boolean | true | Global toggle (no-ops when false) | | pretty | boolean | true in dev | Pretty tree format vs JSON | | silent | boolean | false | Suppress console output. Events still go to drains | | include | string[] | All routes | Route glob patterns to log | | exclude | string[] | None | Route patterns to exclude (takes precedence) | | routes | Record<string, { service }> | -- | Route-specific service names | | minLevel | 'debug' \| 'info' \| 'warn' \| 'error' | 'debug' | Hard threshold for the global log API and client log (not request wide events). Use sampling.rates for probabilistic volume on requests | | sampling.rates | object | -- | Head sampling: { info: 10, warn: 50 } (0-100%) | | sampling.keep | array | -- | Tail sampling: [{ status: 400 }, { duration: 1000 }] | | drain | (ctx) => void | -- | Drain callback (Next.js, standalone) | | enrich | (ctx) => void | -- | Enrich callback (Next.js) | | keep | (ctx) => void | -- | Custom tail sampling callback (Next.js) | | redact | boolean \| RedactConfig | true in production | Enabled by default in production. false to disable. Object for fine-grained control |

Nitro Hooks (Nuxt, Nitro v2/v3)

| Hook | When | Use | |------|------|-----| | evlog:drain | After enrichment | Send events to external services | | evlog:enrich | After emit, before drain | Add derived context | | evlog:emit:keep | During emit | Custom tail sampling logic | | close | Server shutdown | Flush drain pipeline buffers |

Drain Adapters

| Adapter | Import | Env Vars | |---------|--------|----------| | Axiom | evlog/axiom | AXIOM_API_KEY, AXIOM_DATASET | | OTLP | evlog/otlp | OTLP_ENDPOINT (or OTEL_EXPORTER_OTLP_ENDPOINT) | | HyperDX | evlog/hyperdx | HYPERDX_API_KEY (optional HYPERDX_OTLP_ENDPOINT; defaults to https://in-otel.hyperdx.io) | | PostHog | evlog/posthog | POSTHOG_API_KEY, POSTHOG_HOST | | Sentry | evlog/sentry | SENTRY_DSN | | Better Stack | evlog/better-stack | BETTER_STACK_API_KEY | | Datadog | evlog/datadog | DD_API_KEY or DATADOG_API_KEY, optional DD_SITE / DATADOG_LOGS_URL | | Grafana Loki | evlog/loki | LOKI_ENDPOINT, optional LOKI_API_KEY + LOKI_USER (Grafana Cloud) or LOKI_TENANT_ID (multi-tenant) | | ClickHouse | evlog/clickhouse | CLICKHOUSE_ENDPOINT, optional CLICKHOUSE_USER / CLICKHOUSE_PASSWORD / CLICKHOUSE_DATABASE / CLICKHOUSE_TABLE | | File System | evlog/fs | None (local file system) | | Memory | evlog/memory | None (in-process ring buffer; optional EVLOG_MEMORY_STORE, EVLOG_MEMORY_MAX_EVENTS). Read back with readMemoryLogs() — ideal for dev-only log endpoints agents can query | | NuxtHub | @evlog/nuxthub (separate package, Nuxt module) | None — stores wide events in the NuxtHub database with retention-based cleanup (evlog.nuxthub: { retentionDays, batchSize }) | | HTTP (browser ingest) | evlog/http | None (configure endpoint in code). evlog/browser is deprecated; same API, removed next major |

Use canonical env var names (e.g. AXIOM_API_KEY, BETTER_STACK_API_KEY) — the same names work in every framework.

Setup pattern per framework:

// Nuxt/Nitro: server/plugins/evlog-drain.ts
import { createAxiomDrain } from 'evlog/axiom'
export default defineNitroPlugin((nitroApp) => {
  nitroApp.hooks.hook('evlog:drain', createAxiomDrain())
})

// Hono / Express / Elysia: pass drain in middleware options
import { createAxiomDrain } from 'evlog/axiom'
app.use(evlog({ drain: createAxiomDrain() }))

// Fastify: pass drain in plugin options
import { createAxiomDrain } from 'evlog/axiom'
await app.register(evlog, { drain: createAxiomDrain() })

// NestJS: pass drain in module options
import { createAxiomDrain } from 'evlog/axiom'
EvlogModule.forRoot({ drain: createAxiomDrain() })

// Next.js: pass drain to createEvlog()
import { createAxiomDrain } from 'evlog/axiom'
import { createDrainPipeline } from 'evlog/pipeline'
const pipeline = createDrainPipeline<DrainContext>({ batch: { size: 50 } })
const drain = pipeline(createAxiomDrain())
// then: createEvlog({ ..., drain })

// Standalone: pass drain to initLogger()
initLogger({ env: { service: 'my-app' }, drain: createAxiomDrain() })

See references/drain-pipeline.md for batching, retry, and buffer overflow config.

Enrichers

Built-in: createUserAgentEnricher(), createGeoEnricher(), createRequestSizeEnricher(), createTraceContextEnricher() — all from evlog/enrichers. Each accepts { overwrite?: boolean } (default false). Use createDefaultEnrichers() to compose all four in one call:

import { createDefaultEnrichers } from 'evlog/enrichers'
app.use(evlog({ enrich: createDefaultEnrichers() }))

// Nuxt/Nitro: server/plugins/evlog-enrich.ts
import { createUserAgentEnricher, createGeoEnricher } from 'evlog/enrichers'
export default defineNitroPlugin((nitroApp) => {
  const enrichers = [createUserAgentEnricher(), createGeoEnricher()]
  nitroApp.hooks.hook('evlog:enrich', (ctx) => {
    for (const enricher of enrichers) enricher(ctx)
  })
})

// Next.js: in lib/evlog.ts
createEvlog({
  enrich: (ctx) => {
    for (const enricher of enrichers) enricher(ctx)
    ctx.event.region = process.env.VERCEL_REGION
  },
})

Auto-Redaction (PII Protection)

Built-in redaction scrubs sensitive data from wide events before console output and before any drain sees the data. Enabled by default in production (NODE_ENV === 'production'), disabled in development. Uses smart partial masking — preserving enough context for debugging.

// Disable in production (opt-out)
evlog: { redact: false }

// Add custom paths on top of built-ins
evlog: {
  redact: {
    paths: ['user.password', 'headers.authorization'],
  }
}

// Only specific built-ins
evlog: {
  redact: {
    builtins: ['email', 'creditCard'],
  }
}

// No built-ins, only custom (uses flat [REDACTED] replacement)
evlog: {
  redact: {
    builtins: false,
    paths: ['user.ssn'],
    patterns: [/SECRET_\w+/g],
  }
}

Built-in patterns with smart masking output:

| Pattern | Example Input | Masked Output | |---------|---------------|---------------| | creditCard | 4111111111111111 | ****1111 | | email | alice@example.com | a***@***.com | | ipv4 | 192.168.1.100 | ***.***.***.100 | | phone | +33 6 12 34 56 78 | +33 ****5678 | | jwt | eyJhbGciOi... | eyJ***.*** | | bearer | Bearer sk_live_abc... | Bearer *** | | iban | FR76 3000 6000 ...189 | FR76****189 |

Works in all frameworks: Nuxt (evlog config), Nitro (evlog() module options), Next.js (createEvlog()), standalone (initLogger()), and all middleware integrations (Hono, Express, Fastify, Elysia, NestJS).

AI SDK Integration

Capture token usage, tool calls, model info, streaming metrics, tool execution timing, cost estimation, and embedding metadata from the Vercel AI SDK into wide events. Import from evlog/ai. Requires ai >=6.0.168 <8.0.0 as a peer dependency.

Basic setup (middleware)

import { createAILogger } from 'evlog/ai'

const log = useLogger(event) // or any RequestLogger
const ai = createAILogger(log)

const result = streamText({
  model: ai.wrap('anthropic/claude-sonnet-4.6'),  // accepts string or model object
  messages,
})

ai.wrap() uses model middleware to transparently capture all LLM calls. Works with generateText, streamText, and ToolLoopAgent.

Telemetry integration (deeper observability)

For tool execution timing, success/failure tracking, and total generation wall time, add createEvlogIntegration():

import { createAILogger, createEvlogIntegration } from 'evlog/ai'

const ai = createAILogger(log)

const agent = new ToolLoopAgent({
  model: ai.wrap('anthropic/claude-sonnet-4.6'),
  tools: { searchWeb, queryDatabase },
  stopWhen: stepCountIs(5),
  telemetry: {
    integrations: [createEvlogIntegration(ai)],
  },
})

This adds ai.tools (per-tool { name, durationMs, success, error? }) and ai.totalDurationMs to the wide event.

Embeddings

const { embedding, usage } = await embed({ model: embeddingModel, value: query })
ai.captureEmbed({ usage, model: 'text-embedding-3-small', dimensions: 1536 })

For embedMany, pass the batch count:

ai.captureEmbed({ usage, model: 'text-embedding-3-small', count: documents.length })

Cost estimation

Pass a pricing map to get ai.estimatedCost in the wide event:

const ai = createAILogger(log, {
  cost: {
    'claude-sonnet-4.6': { input: 3, output: 15 },
    'gpt-4o': { input: 2.5, output: 10 },
  },
})

Wide event `ai` field

Includes: calls, model, provider, inputTokens, outputTokens, totalTokens, cacheReadTokens, reasoningTokens, finishReason, toolCalls, steps, msToFirstChunk, msToFinish, tokensPerSecond, error, tools (via telemetry integration), totalDurationMs (via telemetry integration), embedding (via captureEmbed), estimatedCost (via cost option).

Anti-patterns to detect:

| Anti-Pattern | Fix | |--------------|-----| | Manual token tracking in onFinish | ai.wrap() — middleware captures automatically | | console.log('tokens:', result.usage) | ai.wrap() — structured ai.* fields in wide event | | No AI observability | Add createAILogger(log) + ai.wrap() | | No tool execution timing | Add createEvlogIntegration(ai) to telemetry.integrations | | Manual cost calculation | Use cost option in createAILogger() |

Related Capabilities (pointers)

Recommend these when the review surfaces the matching need — each has full docs on evlog.dev:

| Need spotted | evlog answer | Docs | |--------------|--------------|------| | Auth'd user identity missing from events (Better Auth apps) | evlog/better-auth — identifyUser(), createAuthMiddleware(), client identity sync | https://www.evlog.dev/use-cases/better-auth/overview | | Ad-hoc field names drifting across the codebase | Typed fields + error/audit catalogs (evlog/catalog) | https://www.evlog.dev/learn/typed-fields · https://www.evlog.dev/learn/catalogs | | Cross-cutting hooks (request start/finish, client logs, logger extension) | Plugins — definePlugin | https://www.evlog.dev/extend/plugins | | Tail logs live during dev / build a log viewer | createStreamDrain (evlog/stream, SSE) + readFsLogs / tailFsLogs (evlog/fs) | https://www.evlog.dev/extend/stream | | Agents need to query logs over HTTP in dev | Memory adapter + readMemoryLogs() behind a dev-only endpoint | https://www.evlog.dev/integrate/adapters/self-hosted/memory |

Structured Errors

import { createError } from 'evlog'  // or auto-imported in Nuxt

// Minimal
throw createError({ message: 'Database connection failed', status: 500 })

// Standard
throw createError({ message: 'Payment failed', status: 402, why: 'Card declined by issuer' })

// Complete
throw createError({
  message: 'Payment failed',
  status: 402,
  why: 'Card declined by issuer - insufficient funds',
  fix: 'Please use a different payment method or contact your bank',
  link: 'https://docs.example.com/payments/declined',
  cause: originalError,
})

// Backend-only context (wide events / drains — never HTTP body or parseError())
throw createError({
  message: 'Not allowed',
  status: 403,
  why: 'Insufficient permissions',
  internal: { correlationId: 'req_abc', resourceId: 'proj_123' },
})

Frontend — extract user-facing fields with parseError() (internal is never returned to clients):

import { parseError } from 'evlog'

const error = parseError(err)
// error.message, error.status, error.why, error.fix, error.link

See references/structured-errors.md for common patterns and templates.

Anti-Patterns to Detect

| Anti-Pattern | Fix | |--------------|-----| | Multiple console.log in one function | Single wide event with log.set() | | throw new Error('...') | throw createError({ message, status, why, fix }) | | console.error(e); throw e | log.error(e); throw createError(...) | | No logging in request handlers | Add useLogger(event) / useLogger() / createRequestLogger() | | Flat log data { uid, n, t } | Grouped objects: { user: {...}, cart: {...} } | | Logging sensitive data log.set({ user: body }) | Explicit fields: { user: { id: body.id, plan: body.plan } } + enable redact: true | | Putting support-only IDs in why / message | Use createError({ ..., internal: { ... } }) for non-user-facing diagnostics |

See references/code-review.md for the full checklist.

Loading Reference Files

Load based on what you're working on — do not load all at once:

Designing wide events → references/wide-events.md
Improving errors → references/structured-errors.md
Full code review → references/code-review.md
Drain pipeline setup → references/drain-pipeline.md

Agent Skills: Review logging patterns

Install this agent skill to your local

Skill Files