Prompt Lab MCP Server

Prompt optimization loops and regression test suites for Claude Code, with a companion web UI.

The agent runs inside your Claude Code session and owns all LLM work — scoring responses, proposing improved prompts, applying suggestions. The server holds workspace state and keeps the agent and the Prompt Lab UI in sync.

Quick start

Copy `mcp-connect.json` from this repo into your project as `.mcp.json`:

{
  "mcpServers": {
    "prompt-lab": {
      "type": "http",
      "url": "https://prompt-lab-mcp.up.railway.app/mcp"
    }
  }
}

Claude Code connects automatically on next start. Verify with `/mcp`.

Example session

# 1. Open a workspace — agent shares the UI URL
start_web_app()
→ "Open https://prompt-lab-mcp.vercel.app?s=abc123 to follow along."

# 2. Register an API key
register_api_key(workspaceId, "sk-ant-...")

# 3. Set a system prompt and a test case
set_system_prompt(workspaceId, "You are a concise customer support agent...")
add_test_cases(workspaceId, [{
  query: "How do I reset my password?",
  targetAnswer: "Click 'Forgot password' on the login page and follow the email link."
}])

# 4. Run the optimization loop
loop_optimization(workspaceId, threshold=85)
→ Iteration 1 — score 58: response too long, no mention of email link
→ Iteration 2 — score 74: better, but missing the exact step
→ Iteration 3 — score 91: SUCCESS — prompt updated to require step-by-step answers

The UI shows each iteration's score, the agent's reasoning, and the revised system prompt in real time.

How it works

Prompt Lab UI (github.com/jurek-f/prompt-lab)
  ↕  HTTP
Prompt Lab MCP Server (Railway)
  ↕  MCP
Claude Code (your machine)

API keys

API keys are never stored in the MCP server config. Instead, pass them to Claude Code as environment variables — the agent reads them and registers them with the server at the start of each session using `register_api_key`.

Set the key(s) for the provider(s) you want to use. The agent auto-detects the provider from the key prefix when calling `register_api_key`.

If they're already in your system environment, Claude Code inherits them automatically — nothing else to do. Otherwise add them to `~/.claude/env` or your shell profile:

ANTHROPIC_API_KEY=sk-ant-...
GEMINI_API_KEY=AIza...
OPENAI_API_KEY=sk-...

MCP tools

Setup

Templates

Templates are global and appear in the UI dropdowns as soon as they are pushed.

Workspace state

Optimization

Requires a workspace with at least one test case.

Regression

Archive

Self-hosting

Deploy to Railway and set these environment variables:

npm install
npm run dev    # starts on :3000

MCP endpoint: `http://localhost:3000/mcp`

License

MIT — see LICENSE.

© 2026 Jurek Föllmer