MCP Contract Diff — GitHub Action

Diff MCP server tool schemas against a baseline contract. Detects breaking changes, warnings, and safe additions in your MCP server's API surface.

Usage

name: MCP Contract Check
on:
  pull_request:
    paths:
      - "src/**"
      - "package.json"

jobs:
  contract-check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: "22"
      - run: npm install
      - name: Check MCP contract
        uses: mcp-contracts/github-action@main
        with:
          baseline: contracts/baseline.mcpc.json
          command: node
          args: dist/index.js
          fail-on: breaking

Inputs

Input	Required	Default	Description
baseline	Yes		Path to baseline snapshot (.mcpc.json)
command	No*		Server command to run via stdio (e.g., node dist/index.js)
args	No		Arguments for the server command (space-separated)
url	No*		Server URL for streamable-http transport
fail-on	No	breaking	Severity threshold for failure: safe, warning, breaking
comment-on-pr	No	true	Post diff as a PR comment
github-token	No	github.token	GitHub token for PR comments

* Either command or url is required.

Outputs

Output	Description
has-changes	Whether any changes were detected (true/false)
has-breaking	Whether breaking changes were detected
summary	JSON summary object with change counts
exit-code	0 = pass, 1 = fail

How It Works

1. Reads the baseline snapshot from the file path you provide
2. Connects to your MCP server (via stdio command or HTTP URL)
3. Captures the current tool/resource/prompt schemas
4. Diffs the current state against the baseline
5. Reports changes as a step summary and optional PR comment
6. Fails the check if changes meet the fail-on severity threshold

Creating a Baseline

Use the mcpdiff CLI to capture a baseline snapshot:

npx mcpdiff snapshot --command node --args server.js -o contracts/baseline.mcpc.json

Commit this file to your repository and reference it in the baseline input.