MCP API Guardian

MCP API Guardian

Stop your AI agent from breaking your API: OpenAPI contract checks and breaking-change diffs over MCP, fully local.

Local-first guardrails for AI coding agents working with OpenAPI specs: catch contract errors (missing operation IDs, unresolved references, public auth), detect breaking changes (removed operations, new required parameters, changed authentication), and keep API consumers working before you ship.

Zero configuration. No API keys. No network. Runs entirely on your machine via the Model Context Protocol.

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Table of Contents

• Quick Install
• Why API Guardian
• Tools
• Use Cases
• Comparison with Alternatives
• Limitations
• Run from Source
• Roadmap
• Hosted Path (Control Plane)
• Contributing
• License
• More MCP Servers by the Author

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Quick Install

One-Click Install

Cursor:

Or use the direct protocol link: cursor://anysphere.cursor-deeplink/mcp/install?name=api-guardian&config=eyJjb21tYW5kIjoibnB4IiwiYXJncyI6WyIteSIsIm1jcC1hcGktZ3VhcmRpYW4iXX0=

Claude Code:

claude mcp add api-guardian -- npx -y mcp-api-guardian

Or if installed in the petrovicistefan/mcp-marketplace:

/plugin marketplace add petrovicistefan/mcp-marketplace

Manual Install

Claude Desktop:

Add to claude_desktop_config.json:

{

"mcpServers": { "api-guardian": { "command": "npx", "args": ["-y", "mcp-api-guardian"] } } }

Cursor:

Add to ~/.cursor/mcp.json:

{

"mcpServers": { "api-guardian": { "command": "npx", "args": ["-y", "mcp-api-guardian"] } } }

VS Code / GitHub Copilot:

Or manually create/edit .vscode/mcp.json:

{

"servers": { "api-guardian": { "type": "stdio", "command": "npx", "args": ["-y", "mcp-api-guardian"] } } }

Windsurf:

Add to ~/.codeium/windsurf/mcp_config.json:

{

"mcpServers": { "api-guardian": { "command": "npx", "args": ["-y", "mcp-api-guardian"] } } }

Zed:

Add to your Zed settings.json:

{

"context_servers": { "api-guardian": { "command": "npx", "args": ["-y", "mcp-api-guardian"] } } }

Direct npx (one-line test):

npx -y mcp-api-guardian

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Why API Guardian

AI coding agents increasingly modify OpenAPI specs as they work on API features, documentation, and integrations. A single agent iteration can:

• Remove an operation a client depends on
• Add a required request parameter without a default
• Change an operationId that generated SDKs rely on
• Accidentally publish a public endpoint with no authentication
• Break external references or introduce duplicate IDs

API Guardian catches these before they break production. It provides two MCP tools your AI agent can call:

1. `audit_openapi` — lints a single OpenAPI spec for contract errors, missing metadata, auth risks
2. `compare_openapi` — diffs two spec versions and detects breaking changes

Fully local, read-only, zero dependencies. No telemetry, no API keys, no network calls.

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Tools

API Guardian provides 2 MCP tools for validating OpenAPI 3.0.x and 3.1.x documents:

| `compare_openapi` | Find selected breaking changes between two OpenAPI documents; not exhaustive schema compatibility. | before (object): Original OpenAPI document after (object): Updated OpenAPI document |

audit_openapi({spec})

Static contract checks on a single OpenAPI spec:

• Contract errors: Unresolved $ref references, missing info.title/info.version, invalid paths
• Operation metadata: Missing or duplicate operationId, missing response definitions
• Path parameters: All {param} placeholders must be declared as required path parameters
• Security warnings: Public operations (no auth required), basic auth over HTTP, API keys in query strings
• External references: Flags unchecked external $ref URLs (not fetched)

Reports errors and warnings with severity, location, and actionable messages. Explicitly states coverage limits.

compare_openapi({before, after})

Breaking-change detection between two OpenAPI spec versions:

• Removed operations: Previously available endpoints deleted
• Changed operation IDs: Generated clients and agents may depend on stable IDs
• New required parameters: Added inline required parameters (clients can't provide them)
• Request body requirement: Optional body became required
• Removed response codes: Status codes clients may handle are gone
• Security changes: Authentication requirements modified (warning-level)

Reports definite breaks (error) and potential compatibility issues (warning). Schema variance and composed schemas are not yet analyzed (see Limitations).

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Use Cases

For Individual Developers

Prompt: "Audit the OpenAPI spec at api/openapi.json before I commit"

Your AI agent calls audit_openapi, catches a missing operation ID, and suggests a fix — before the PR.

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For CI / PR Checks

Prompt: "Compare the OpenAPI spec on this branch to main and fail if there are breaking changes"

Your agent runs compare_openapi, detects a removed endpoint, and blocks the merge with an actionable report.

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For Teams and Enterprises

Prompt: "Check all OpenAPI specs in api/ for public endpoints without authentication"

Your agent audits each spec, flags public operations, and generates a security review checklist.

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Comparison with Alternatives

When to use API Guardian:

• ✅ You want your AI agent (Claude, Cursor, etc.) to natively check OpenAPI contracts without CLI wrappers
• ✅ You need local-first, zero-network validation (no external dependencies)
• ✅ You want targeted linting + breaking-change detection in one MCP server

When to use alternatives:

• Use oasdiff if you need the most comprehensive breaking-change ruleset (773 checks) and CI-native workflows
• Use pb33f/openapi-changes if you need beautiful HTML/Markdown diff reports for human review
• Both oasdiff and openapi-changes support full schema compatibility checks and git-native comparison, which API Guardian will add in future releases

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Limitations

API Guardian is a targeted linter, not a complete OpenAPI validator. Current limitations:

• ❌ YAML specs not supported: Only JSON OpenAPI documents are accepted. Pass parsed JSON objects to the MCP tools. YAML support is on the roadmap (see below).
• ❌ External `$ref` not fetched: External HTTP/HTTPS references are flagged as warnings but not resolved or validated
• ❌ Schema compatibility not checked: Request vs response variance, oneOf/anyOf/allOf composed schemas, and property-level breaking changes require a later release
• ❌ Referenced parameters not compared: Only inline parameters are checked in compare_openapi
• ❌ No live HTTP probes: Does not test actual API behavior or validate runtime responses
• ❌ No filesystem access: You must pass the spec as a parsed JSON object (no file paths)

Public endpoints may be intentional; warnings are not proof of a vulnerability. Reports explicitly state coverage limits.

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Run from Source

git clone https://github.com/petrovicistefan/mcp-api-guardian.git

cd mcp-api-guardian npm test npm start

Add to an MCP client's configuration, replacing the absolute path:

{

"mcpServers": { "api-guardian": { "command": "node", "args": ["/absolute/path/mcp-api-guardian/src/server.js"] } } }

Requirements: Node.js 22+

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Roadmap

Future releases will add:

1. YAML support: Parse and validate YAML OpenAPI specs (currently JSON only)
2. Full schema compatibility: Detect property-level breaking changes, composed schemas (oneOf/anyOf/allOf), request vs response variance
3. Local reference resolution: Fetch and validate external $ref references with configurable allowlists
4. Git-native comparison: Compare specs across branches/commits without manual file extraction
5. Live response validation: Opt-in HTTP probes with target allowlists, size limits, and secret redaction

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Hosted Path (Control Plane)

The local stdio MCP server above stays fully useful without an account or network. Quotas are enforced only by a separate hosted HTTP process that reserves units on mcp-control-plane before running the same analysis core.

# Terminal 1 — control plane (see its README for ADMIN_TOKEN)

node --env-file=.env src/server.js # in mcp-control-plane

Terminal 2 — hosted api-guardian

cp .env.example .env # set CONTROL_PLANE_URL npm run start:hosted

Bootstrap a customer key with the control-plane admin API (POST /v1/admin/accounts, POST /v1/admin/keys). Then: