Imported from Noot-Studio/analytics (.agents/skills/review-logging-patterns/SKILL.md). Install upstream with npx skills add Noot-Studio/analytics --skill review-logging-patterns. Copyright stays with the author (MIT).

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

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", 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. 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;
    },
  }),
];

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, 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()).

Nitro Hooks (Nuxt, Nitro v2/v3)

Drain Adapters

In Nuxt/Nitro, use the NUXT_ prefix (e.g., NUXT_AXIOM_API_KEY) 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:

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:

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

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