Syrin

A linter + test runner for MCP servers.

The Problem

MCP (Model Context Protocol) is how AI agents call external tools — read files, query databases, hit APIs. If you are building or using an MCP server, your AI agent depends on those tool definitions being correct.

They usually are not.

• Tool descriptions too vague for the LLM to pick the right one
• Two tools look so similar the model picks one at random
• Parameter schemas missing or wrong — LLM hallucinates values
• A tool returns 12MB of JSON and blows the context window
• Another tool silently writes to disk when it should not
• Your logs look fine. The agent is broken.

Syrin catches all of this before production.

$ syrin analyse --transport http --url http://localhost:8000/mcp

 E110  Tool Ambiguity           get_user ↔ fetch_user
 E101  Missing Tool Description process_data has no description
 E102  Underspecified Input     user_id: no format, no example, no enum
 E105  Free Text Propagation    get_status → update_user (unconstrained string)
 W104  Generic Description      "Get data" — too vague for tool selection

 5 issues found (4 errors, 1 warning)

See It In Action

Try It Right Now

One command. No install, no config, no API keys:

npx @syrin/cli analyse --transport http --url https://docs.syrin.dev/mcp

Have your own MCP server running? Point Syrin at it:

npx @syrin/cli analyse --transport http --url http://localhost:8000/mcp

If your server uses stdio instead of HTTP:

npx @syrin/cli analyse --transport stdio --script "python server.py"

Want to try more commands against a local example server?

git clone https://github.com/Syrin-Labs/cli.git
cd cli/examples/demo-mcp-py
python3 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt
python server.py --mode http --port 8000 &

npx @syrin/cli list tools --transport http --url http://localhost:8000/mcp
npx @syrin/cli analyse --transport http --url http://localhost:8000/mcp

Requirements: Node.js >= 20.12, npm >= 9

What Syrin Catches

| Code | Issue | What Happens Without Syrin | | ---- | --------------------- | ------------------------------------------- | | E110 | Tool Ambiguity | LLM picks the wrong tool at random | | E101 | Missing Description | LLM has no idea what the tool does | | E102 | Underspecified Input | LLM hallucinates parameter values | | E105 | Free Text Propagation | LLM passes sentences where data is expected | | E103 | Type Mismatch | Tool chains break silently | | E107 | Circular Dependency | Agent loops forever, burns tokens | | E301 | Output Explosion | 12MB response blows the context window | | E500 | Side Effect Detected | Tool writes to disk when it should not |

See the full list: Error Reference · Warning Reference

Commands

| Command | What It Does | | --------------- | ------------------------------------------------------------------------ | | syrin list | Show tools, resources, and prompts a server exposes | | syrin analyse | Static analysis — catch contract issues without executing tools | | syrin test | Run tools in a sandbox and validate behavior against contracts | | syrin dev | Interactive session — watch an LLM interact with your tools in real time | | syrin doctor | Validate your config, environment, and connections |

Zero-config commands: list, analyse, and test --connection work with just --url or --script. No config file needed.

Config required: dev mode needs LLM API keys. Run syrin init --global to set up once.

All Demos

Install

# Run without installing
npx @syrin/cli analyse --transport http --url http://localhost:8000/mcp

# Or install globally
npm install -g @syrin/cli
syrin --version

Set Up for a Project

syrin init                 # Creates syrin.yaml + tools/ directory
syrin doctor               # Validates config and connections
syrin analyse              # Analyse your MCP server
syrin test                 # Run contract tests
syrin dev --exec           # Interactive LLM-MCP session

Tool Contracts

Define behavioral guarantees for your tools in tools/<tool-name>.yaml:

version: 1
tool: fetch_user

contract:
  input_schema: FetchUserRequest
  output_schema: User

guarantees:
  side_effects: none
  max_output_size: 10kb

tests:
  - name: 'valid user'
    input:
      user_id: '123'
    expect:
      output_schema: User

  - name: 'invalid input'
    input:
      user_id: 123
    expect:
      error:
        type: input_validation

Run tests: syrin test or syrin test --tool fetch_user

Documentation: Writing Test Cases · Test Your MCP Tools

CI Integration

# .github/workflows/syrin.yml
name: MCP Validation
on: [push, pull_request]
jobs:
  validate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
      - run: npm install -g @syrin/cli
      - run: syrin analyse --ci
      - run: syrin test --ci --strict

See full CI docs: Add Syrin to CI

Documentation

Full docs at docs.syrin.dev

| Topic | Link | | --------------- | ---------------------------------------------------------------------------------------- | | Getting Started | docs.syrin.dev/getting-started | | Setup Guide | docs.syrin.dev/setup | | Configuration | docs.syrin.dev/configuration | | All Commands | docs.syrin.dev/commands | | Error Reference | docs.syrin.dev/testing/error-reference |

Community

• Discord — Ask questions, share feedback
• GitHub Discussions — Feature ideas, show & tell
• Issues — Bug reports, feature requests

Contributing

Contributions welcome. See Contributing Guide and Code of Conduct.

For security issues: Security Policy.

License

ISC License. See LICENSE.

Made by Syrin Labs.