swagger-mcp

An MCP (Model Context Protocol) server that reads Swagger/OpenAPI specs and lets MCP clients explore API schemas and descriptions through natural language.

The core idea: register a project once, and the whole team can query it conversationally — no need to re-upload specs every time.

Supported Clients

Streamable HTTP mode is recommended for clients that only support URL-based connections (e.g., Gemini).

Quick Start

1. Install & Build

git clone https://github.com/yunhwane/swagger-mcp.git
cd swagger-mcp
npm install
npm run build

2. Configure your MCP client

Two transport modes are available: STDIO (default) and Streamable HTTP.

#### Option A: STDIO (default)

Claude Desktop — edit claude_desktop_config.json:

{
  "mcpServers": {
    "swagger-mcp": {
      "command": "node",
      "args": ["/absolute/path/to/swagger-mcp/dist/index.js"]
    }
  }
}

Claude Code — add .mcp.json in your project root:

{
  "mcpServers": {
    "swagger-mcp": {
      "command": "node",
      "args": ["/absolute/path/to/swagger-mcp/dist/index.js"]
    }
  }
}

#### Option B: Streamable HTTP

Start the HTTP server separately, then point your client to the URL. This mode is ideal for development — tsx watch auto-restarts on code changes without requiring manual MCP reconnection.

# Start the server (dev mode with hot reload)
npm run dev:http

# Or production mode
npm run build && npm run start:http

Claude Code — add .mcp.json in your project root:

{
  "mcpServers": {
    "swagger-mcp": {
      "url": "http://localhost:3000/mcp"
    }
  }
}

Gemini CLI — edit ~/.gemini/settings.json:

{
  "mcpServers": {
    "swagger-mcp": {
      "url": "http://localhost:3000/mcp"
    }
  }
}

The HTTP server listens on port 3000 by default (override with PORT env var).

3. Try it with the Petstore API

Once connected, just ask your MCP client:

"Register the Petstore API from https://petstore3.swagger.io/api/v3/openapi.json and explore its endpoints."

Or walk through the drill-down workflow:

1. add_project        →  Register "petstore" with the spec URL
2. list_services      →  See registered services and their API groups
3. list_apis          →  Browse all endpoints for "petstore"
4. describe_api       →  "GET /pet/{petId}" → see parameters, request/response schemas
5. describe_component →  "#/components/schemas/Pet" → drill into a specific schema

Example Conversations

• "Add the Petstore API from https://petstore3.swagger.io/api/v3/openapi.json."
• "What endpoints are available for managing pets?"
• "Describe the GET /pet/{petId} endpoint."
• "What fields does the Pet schema have?"
• "Compare the current spec against this new version URL."

Features

• OpenAPI 3.0.x / 3.1.x support (JSON & YAML, URL or local file)
• 4-step drill-down: services → APIs → endpoint detail → component schemas
• Shallow $ref resolution — keeps responses concise while letting the LLM decide which schemas to explore
• Spec diff with breaking change detection (responses, requestBody, parameters, schemas)
• Snapshot store — auto-saves normalized specs on registration and after diffs, enabling offline comparison (max 5 per project)
• LRU spec cache (max 20 entries, 5 min TTL)
• Project registry persisted to ~/.swagger-mcp/registry.json
• Built-in help tool for discoverability

Tools (8)

4-Step Drill-Down Pattern

The center tools (list_services → list_apis → describe_api → describe_component) use shallow resolution: endpoint schemas are expanded one level, but component $refs are preserved. This lets the LLM decide which schemas to drill into, keeping responses concise and navigable.

Core Concepts

Project

A reusable unit representing an API spec source. Each project has a projectId, name, and source URL. Project metadata is persisted to ~/.swagger-mcp/registry.json.

Example: "petstore" project pointing to https://petstore3.swagger.io/api/v3/openapi.json

Spec Cache

Parsed OpenAPI documents are cached in-memory (LRU, max 20 entries, 5-minute TTL) to avoid re-fetching on every query.

Snapshot Store

When a project is registered via add_project, the spec is automatically normalized and saved as a snapshot. Each call to diff_apis that detects changes also saves a new snapshot. Snapshots are stored in ~/.swagger-mcp/snapshots/<projectId>/ (max 5 per project, deduplicated by content hash).

Spec Diff

diff_apis compares the latest saved snapshot against a new spec source. If no snapshot exists, it falls back to fetching from the registered URL. The diff engine detects:

• Endpoint additions, removals, and modifications
• Parameter changes (type, required, location)
• Response status code and media type changes (with breaking change flags)
• RequestBody additions, removals, and schema changes
• Schema property and $ref changes

Architecture

┌─────────────────────────────────────────────┐
│                MCP Client                    │
│   (Claude Desktop / Code / Cursor / Gemini)  │
└──────────────────┬──────────────────────────┘
                   │ MCP Protocol
                   │ (STDIO or Streamable HTTP)
┌──────────────────▼──────────────────────────┐
│              swagger-mcp Server              │
│                                              │
│  ┌────────────┐ ┌────────────┐ ┌─────────┐  │
│  │  Project   │ │   Center   │ │  Diff   │  │
│  │  Tools (2) │ │  Tools (4) │ │ Tool (1)│  │
│  └─────┬──────┘ └─────┬──────┘ └────┬────┘  │
│        │              │             │        │
│  ┌─────▼──────┐ ┌─────▼─────────────▼────┐  │
│  │  Registry  │ │      Spec Cache        │  │
│  │ (~/.swagger│ │    (in-memory LRU)     │  │
│  │  -mcp/)    │ │                        │  │
│  └────────────┘ └─────────┬──────────────┘  │
│                           │                  │
│                 ┌─────────▼──────────────┐   │
│                 │  Loader + Normalizer   │   │
│                 │  (fetch, parse,        │   │
│                 │   resolve $refs)       │   │
│                 └────────────────────────┘   │
└──────────────────────────────────────────────┘

1. Registry — stores project metadata, persists to disk
2. Loader — fetches OpenAPI specs from URLs or local files, parses JSON/YAML
3. Normalizer — resolves $ref references recursively with circular ref detection
4. Differ — computes structural diff between two normalized specs (endpoints, parameters, responses, requestBody, schemas)
5. Spec Cache — LRU in-memory cache for parsed OpenAPI documents
6. Snapshot Store — persists normalized specs to disk for reliable diff comparisons

Tech Stack

• Runtime: Node.js 20+
• Language: TypeScript (strict mode, noUncheckedIndexedAccess)
• MCP SDK: @modelcontextprotocol/sdk
• Validation: zod
• Build: tsup (ESM-only, target node20)
• Test: vitest

Development

npm run dev        # Run STDIO mode with tsx
npm run dev:http   # Run HTTP mode with tsx watch (auto-reload)
npm run build      # Build with tsup → dist/
npm run check      # TypeScript type check
npm run start:http # Run HTTP mode in production
npm test           # Run all tests (vitest)

# Test with MCP Inspector
npx @modelcontextprotocol/inspector node dist/index.js

TDD Workflow

This project follows the Red-Green-Refactor cycle:

1. RED — Write a failing test first (tests/ mirrors src/ structure)
2. GREEN — Write the minimum implementation to pass the test
3. REFACTOR — Clean up while keeping tests green

Always run npm run check && npm test before finishing a change.

Project Structure

src/
├── index.ts           # STDIO entry point
├── http.ts            # Streamable HTTP entry point
├── http-handler.ts    # HTTP request handler (session management, DNS rebinding protection)
├── server.ts          # Shared McpServer creation (tool registration)
├── registry.ts        # Project registry state management
├── loader.ts          # OpenAPI spec fetcher (URL/file, JSON/YAML)
├── normalizer.ts      # $ref resolution and spec normalization
├── differ.ts          # Spec diff engine (endpoints, responses, requestBody, schemas)
├── snapshot-store.ts  # Persistent snapshot storage for diff comparisons
├── spec-cache.ts      # In-memory LRU cache for parsed specs
├── types.ts           # TypeScript type definitions
└── tools/
    ├── project.ts     # add_project, list_projects
    ├── center.ts      # list_services, list_apis, describe_api, describe_component
    ├── diff.ts        # diff_apis
    └── help.ts        # help
tests/                 # Mirrors src/ structure (vitest)
  ├── tools/           # Tool unit tests
  ├── fixtures/        # Test OpenAPI specs (petstore variants)
  └── *.test.ts        # Unit tests for loader, normalizer, registry, etc.

License

MIT