---

name: review-logging-patterns description: Review code for logging patterns and suggest evlog adoption. Guides setup on Nuxt, Next.js, SvelteKit, Nitro, TanStack Start, NestJS, Express, Hono, Fastify, Elysia, Cloudflare Workers, and standalone TypeScript. Detects console.log spam, unstructured errors, and missing context. Covers wide events, structured errors, drain adapters (Axiom, OTLP, PostHog, Sentry, Better Stack), sampling, enrichers, and AI SDK integration (token usage, tool calls, streaming metrics). license: MIT metadata: author: HugoRCD version: "0.5"

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 | | ----------------------- | ------------------------------------------------------------------ | | 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 |

Installation

npm install evlog

---

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', token: process.env.AXIOM_TOKEN! }))

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: 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({ token: config.get('AXIOM_TOKEN') }),
  }),
})

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. No useLogger() — use c.get('log') and pass it down explicitly, or use Express/Fastify/Elysia if you need useLogger() across async boundaries.

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
  },
}))

Cloudflare Workers

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

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

export default {
  async fetch(request: Request) {
    const log = createWorkersLogger(request)
    try {
      log.set({ route: 'health' })
      const response = new Response('ok', { status: 200 })
      log.emit({ status: response.status })
      return response
    } catch (error) {
      log.error(error as Error)
      log.emit({ status: 500 })
      throw error
    }
  },
}

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 | | 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) |

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_TOKEN, AXIOM_DATASET | | OTLP | evlog/otlp | OTLP_ENDPOINT (or OTEL_EXPORTER_OTLP_ENDPOINT) | | PostHog | evlog/posthog | POSTHOG_API_KEY, POSTHOG_HOST | | Sentry | evlog/sentry | SENTRY_DSN | | Better Stack | evlog/better-stack | BETTER_STACK_SOURCE_TOKEN | | File System | evlog/fs | None (local file system) |

In Nuxt/Nitro, use the NUXT_ prefix (e.g., NUXT_AXIOM_TOKEN) so values are available via useRuntimeConfig(). All adapters also read unprefixed variables as fallback.

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.

// 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
  },
})

---

AI SDK Integration

Capture token usage, tool calls, model info, and streaming metrics from the Vercel AI SDK into wide events. Import from evlog/ai. Requires ai >= 6.0.0 as a peer dependency.

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,
  onFinish: ({ text }) => {
    // User callbacks remain free — no conflict
  },
})

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

For embeddings (different model type):

const { embedding, usage } = await embed({ model: embeddingModel, value: query })
ai.captureEmbed({ usage })

Wide event ai field includes: calls, model, provider, inputTokens, outputTokens, totalTokens, cacheReadTokens, reasoningTokens, finishReason, toolCalls, steps, msToFirstChunk, msToFinish, tokensPerSecond, error.

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() |

---

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,
})

Frontend — extract all fields with parseError():

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 } } |

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