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 (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({ 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.

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
    },
  }),
]

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
`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_TOKEN`, `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_SOURCE_TOKEN`
Datadog	`evlog/datadog`	`DD_API_KEY` or `DATADOG_API_KEY`, optional `DD_SITE` / `DATADOG_LOGS_URL`
File System	`evlog/fs`	None (local file system)
HTTP (browser ingest)	`evlog/http`	None (configure `endpoint` in code). `evlog/browser` is deprecated; same API, removed next major

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
  },
})

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 >= 6.0.0 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),
  experimental_telemetry: {
    isEnabled: true,
    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 `experimental_telemetry.integrations`
Manual cost calculation	Use `cost` option in `createAILogger()`

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

review-logging-patterns

Review logging patterns

When to Use

Quick Reference

Installation

Framework Setup

Nuxt

Next.js

SvelteKit

Nitro v3

TanStack Start

Nitro v2

NestJS

Express

Hono

Fastify

Elysia

React Router

Cloudflare Workers

Vite Plugin (any Vite-based framework)

Standalone TypeScript

Configuration Options

Nitro Hooks (Nuxt, Nitro v2/v3)

Drain Adapters

Enrichers

Auto-Redaction (PII Protection)

AI SDK Integration

Basic setup (middleware)

Telemetry integration (deeper observability)

Embeddings

Cost estimation

Wide event ai field

Structured Errors

Anti-Patterns to Detect

Loading Reference Files

Wide event `ai` field