Logjar

Unified frontend + backend log capture for dev environments. A single rolling log file that coding agents (Claude, Cursor, Copilot, etc.) can read to understand what's happening in your app.

The Problem

When a coding agent is helping you debug, it can't see your terminal output or browser console. You end up copy-pasting error messages back and forth. Logjar fixes this by writing all logs (backend stdout/stderr and frontend console output) into one small, rolling file the agent can read directly.

Quick Start

npm install logjar --save-dev

1. Wrap your dev command

# Before
turbo run dev --parallel

# After
npx logjar -- turbo run dev --parallel

That's it for the backend. All stdout/stderr from your command is captured.

2. Add frontend capture

Choose one of these options:

Option A: ES module import (React, Vue, Svelte, etc.)

// Add to your app's entry point (e.g., main.ts, _app.tsx)
import "logjar/client";

// Or initialize with custom options
import { initLogjar } from "logjar/client";
initLogjar({ app: "web", levels: ["log", "info", "warn", "error", "debug"] });

Option B: Script tag

<script src="./node_modules/logjar/src/client/snippet.js"></script>

Option C: Conditional (only in dev)

if (process.env.NODE_ENV === "development") {
  import("logjar/client");
}

Option D: Chrome/Chromium extension (no app code changes)

If you do not want to import logjar/client into the app, you can use the optional browser extension instead. Download the latest asset from GitHub Releases, extract it, then load it locally:

1. Open chrome://extensions.
2. Turn on Developer mode.
3. Click Load unpacked.
4. Select the extracted extension folder.

Then start Logjar locally, open your app in the browser, click the Logjar extension, confirm the port matches your CLI process (default 8797), and enable Capture this tab.

The extension captures browser logs for the current tab and forwards them to your local Logjar server. This is useful when you want frontend capture without changing application code.

3. Tell your coding agent

Add the following to your AGENTS.md / CLAUDE.md / .cursorrules:

Read .logjar/logs.txt to check for frontend and backend errors.

4. Update your .gitignore

Update your .gitignore to exclude the logjar directory:

.logjar

CLI Options

logjar [options] -- <command>

Options:
  --port <n>       HTTP port for frontend logs  (default: 8797)
  --out <path>     Log file path                (default: .logjar/logs.txt)
  --max-age <s>    Rolling window in seconds    (default: 900)
  --max-size <kb>  Max log file size in KB      (default: 200)

Examples

# Custom port and output
logjar --port 9000 --out logs/dev.txt -- npm run dev

# Longer window, bigger cap
logjar --max-age 600 --max-size 100 -- next dev

# Add to package.json scripts
{
  "scripts": {
    "dev": "logjar -- turbo run dev --parallel"
  }
}

License

MIT