New in version: 2.0.0

FastMCP can automatically generate an MCP server from an OpenAPI specification or FastAPI app. Instead of manually creating tools and resources, you provide an OpenAPI spec and FastMCP intelligently converts your API endpoints into the appropriate MCP components.

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Quick Start

To convert an OpenAPI specification to an MCP server, you can use the FastMCP.from_openapi class method. This method takes an OpenAPI specification and an async HTTPX client that can be used to make requests to the API, and returns an MCP server.

Here’s an example:

import httpx
from fastmcp import FastMCP

# Create an HTTP client for your API
client = httpx.AsyncClient(base_url="https://api.example.com")

# Load your OpenAPI spec 
openapi_spec = httpx.get("https://api.example.com/openapi.json").json()

# Create the MCP server
mcp = FastMCP.from_openapi(
    openapi_spec=openapi_spec,
    client=client,
    name="My API Server"
)

if __name__ == "__main__":
    mcp.run()

That’s it! Your entire API is now available as an MCP server. Clients can discover and interact with your API endpoints through the MCP protocol, with full schema validation and type safety.

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Route Mapping

FastMCP analyzes your API specification and automatically creates MCP components based on HTTP semantics and REST conventions. By default, the following rules are used to determine what MCP component to create for each route:

OpenAPI RouteExampleMCP Component
GET with path paramsGET /users/{id}Resource Template
GET without path paramsGET /statsResource
POST, PUT, PATCH, DELETE, etc.POST /usersTool

Interally, FastMCP uses an ordered list of RouteMap objects to determine how to map OpenAPI routes to various MCP component types.

Each RouteMap specifies a combination of methods, patterns, and tags, as well as a corresponding MCP component type. Each OpenAPI route is checked against each RouteMap in order, and the first one that matches every criteria is used to determine its converted MCP type. A special type, EXCLUDE, can be used to exclude routes from the MCP server entirely.

  • Methods: HTTP methods to match (e.g. ["GET", "POST"] or "*" for all)
  • Pattern: Regex pattern to match the route path (e.g. r"^/users/.*" or r".*" for all)
  • Tags: A set of OpenAPI tags that must all be present. An empty set ({}) means no tag filtering, so the route matches regardless of its tags.
  • MCP type: What MCP component type to create (TOOL, RESOURCE, RESOURCE_TEMPLATE, or EXCLUDE)

To illustrate this in practice, here are FastMCP’s default rules as a list of RouteMap objects:

from fastmcp.server.openapi import RouteMap, MCPType

DEFAULT_ROUTE_MAPPINGS = [

    # GET with path parameters → ResourceTemplate
    RouteMap(
        methods=["GET"], 
        pattern=r".*\{.*\}.*", 
        mcp_type=MCPType.RESOURCE_TEMPLATE
    ),

    # GET without path parameters → Resource
    RouteMap(
        methods=["GET"], 
        pattern=r".*", 
        mcp_type=MCPType.RESOURCE
    ),

    # All other methods → Tool
    RouteMap(
        methods=["*"], 
        pattern=r".*", 
        mcp_type=MCPType.TOOL
    ),
]

​
Custom Route Maps

When creating your FastMCP server, you can customize routing behavior by providing your own list of RouteMap objects. Your custom maps are processed before the default route maps, and routes will be assigned to the first matching custom map.

For example, the following simple rule will treat every OpenAPI route as a tool:

from fastmcp import FastMCP
from fastmcp.server.openapi import RouteMap, MCPType

mcp = FastMCP.from_openapi(
    ...,
    route_maps=[
        RouteMap(mcp_type=MCPType.TOOL),
    ],
)

Here is a more complete example that uses custom route maps to convert all GET endpoints under /analytics/ to tools while excluding all admin endpoints and all routes tagged “internal”. All other routes will be handled by the default rules:

from fastmcp import FastMCP
from fastmcp.server.openapi import RouteMap, MCPType

mcp = FastMCP.from_openapi(
    ...,
    route_maps=[

        # Analytics `GET` endpoints are tools
        RouteMap(
            methods=["GET"], 
            pattern=r"^/analytics/.*", 
            mcp_type=MCPType.TOOL,
        ),

        # Exclude all admin endpoints
        RouteMap(
            pattern=r"^/admin/.*", 
            mcp_type=MCPType.EXCLUDE,
        ),

        # Exclude all routes tagged "internal"
        RouteMap(
            tags={"internal"},
            mcp_type=MCPType.EXCLUDE,
        ),
    ],
)

The default route maps are always applied after your custom maps, so you do not have to create route maps for every possible route.

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Excluding Routes

To exclude routes from the MCP server, use a route map to assign them to MCPType.EXCLUDE.

You can use this to remove sensitive or internal routes by targeting them specifically:

from fastmcp import FastMCP
from fastmcp.server.openapi import RouteMap, MCPType

mcp = FastMCP.from_openapi(
    ...,
    route_maps=[
        RouteMap(pattern=r"^/admin/.*", mcp_type=MCPType.EXCLUDE),
        RouteMap(tags={"internal"}, mcp_type=MCPType.EXCLUDE),
    ],
)

Or you can use a catch-all rule to exclude everything that your maps don’t handle explicitly:

from fastmcp import FastMCP
from fastmcp.server.openapi import RouteMap, MCPType

mcp = FastMCP.from_openapi(
    ...,
    route_maps=[
        # custom mapping logic goes here
        ...,
        # exclude all remaining routes
        RouteMap(mcp_type=MCPType.EXCLUDE),
    ],
)

Using a catch-all exclusion rule will prevent the default route mappings from being applied, since it will match every remaining route. This is useful if you want to explicitly allow-list certain routes.

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Advanced Route Mapping

New in version: 2.5.0

For advanced use cases that require more complex logic, you can provide a route_map_fn callable. After the route map logic is applied, this function is called on each matched route and its assigned MCP component type. It can optionally return a different component type to override the mapped assignment. If it returns None, the assigned type is used.

In addition to more precise targeting of methods, patterns, and tags, this function can access any additional OpenAPI metadata about the route.

The route_map_fn is called on routes that matched MCPType.EXCLUDE in your custom maps, giving you an opportunity to override the exclusion.

from fastmcp import FastMCP
from fastmcp.server.openapi import RouteMap, MCPType, HTTPRoute

def custom_route_mapper(route: HTTPRoute, mcp_type: MCPType) -> MCPType | None:
    """Advanced route type mapping."""
    # Convert all admin routes to tools regardless of HTTP method
    if "/admin/" in route.path:
        return MCPType.TOOL

    elif "internal" in route.tags:
        return MCPType.EXCLUDE
    
    # Convert user detail routes to templates even if they're POST
    elif route.path.startswith("/users/") and route.method == "POST":
        return MCPType.RESOURCE_TEMPLATE
    
    # Use defaults for all other routes
    return None

mcp = FastMCP.from_openapi(
    ...,
    route_map_fn=custom_route_mapper,
)

​
Customizing MCP Components

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Component Names

New in version: 2.5.0

FastMCP automatically generates names for MCP components based on the OpenAPI specification. By default, it uses the operationId from your OpenAPI spec, up to the first double underscore (__).

All component names are automatically:

  • Slugified: Spaces and special characters are converted to underscores or removed
  • Truncated: Limited to 56 characters maximum to ensure compatibility
  • Unique: If multiple components have the same name, a number is automatically appended to make them unique

For more control over component names, you can provide an mcp_names dictionary that maps operationId values to your desired names. The operationId must be exactly as it appears in the OpenAPI spec. The provided name will always be slugified and truncated.

from fastmcp import FastMCP

mcp = FastMCP.from_openapi(
    ...
    mcp_names={
        "list_users__with_pagination": "user_list",
        "create_user__admin_required": "create_user", 
        "get_user_details__admin_required": "user_detail",
    }
)

Any operationId not found in mcp_names will use the default strategy (operationId up to the first __).

​
Advanced Customization

New in version: 2.5.0

By default, FastMCP creates MCP components using a variety of metadata from the OpenAPI spec, such as incorporating the OpenAPI description into the MCP component description.

At times you may want to modify those MCP components in a variety of ways, such as adding LLM-specific instructions or tags. For fine-grained customization, you can provide a mcp_component_fn when creating the MCP server. After each MCP component has been created, this function is called on it and has the opportunity to modify it in-place.

Your mcp_component_fn is expected to modify the component in-place, not to return a new component. The result of the function is ignored.

from fastmcp import FastMCP
from fastmcp.server.openapi import (
    HTTPRoute, 
    OpenAPITool, 
    OpenAPIResource, 
    OpenAPIResourceTemplate,
)

def customize_components(
    route: HTTPRoute, 
    component: OpenAPITool | OpenAPIResource | OpenAPIResourceTemplate,
) -> None:
    
    # Add custom tags to all components
    component.tags.add("openapi")
    
    # Customize based on component type
    if isinstance(component, OpenAPITool):
        component.description = f"🔧 {component.description} (via API)"
    
    if isinstance(component, OpenAPIResource):
        component.description = f"📊 {component.description}"
        component.tags.add("data")

mcp = FastMCP.from_openapi(
    ...,
    mcp_component_fn=customize_components,
)

​
Request Parameter Handling

FastMCP intelligently handles different types of parameters in OpenAPI requests:

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Query Parameters

By default, FastMCP only includes query parameters that have non-empty values. Parameters with None values or empty strings are automatically filtered out.

# When calling this tool...
await client.call_tool("search_products", {
    "category": "electronics",  # ✅ Included
    "min_price": 100,           # ✅ Included  
    "max_price": None,          # ❌ Excluded
    "brand": "",                # ❌ Excluded
})

# The HTTP request will be: GET /products?category=electronics&min_price=100

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Path Parameters

Path parameters are typically required by REST APIs. FastMCP:

  • Filters out None values
  • Validates that all required path parameters are provided
  • Raises clear errors for missing required parameters
# ✅ This works
await client.call_tool("get_user", {"user_id": 123})

# ❌ This raises: "Missing required path parameters: {'user_id'}"
await client.call_tool("get_user", {"user_id": None})

​
Array Parameters

FastMCP handles array parameters according to OpenAPI specifications:

  • Query arrays: Serialized based on the explode parameter (default: True)
  • Path arrays: Serialized as comma-separated values (OpenAPI ‘simple’ style)
# Query array with explode=true (default)
# ?tags=red&tags=blue&tags=green

# Query array with explode=false  
# ?tags=red,blue,green

# Path array (always comma-separated)
# /items/red,blue,green

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Headers

Header parameters are automatically converted to strings and included in the HTTP request.

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Auth

If your API requires authentication, configure it on the HTTP client before creating the MCP server:

import httpx
from fastmcp import FastMCP

# Bearer token authentication
api_client = httpx.AsyncClient(
    base_url="https://api.example.com",
    headers={"Authorization": "Bearer YOUR_TOKEN"}
)

# Create MCP server with authenticated client
mcp = FastMCP.from_openapi(..., client=api_client)

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Timeouts

Set a timeout for all API requests:

mcp = FastMCP.from_openapi(
    openapi_spec=spec, 
    client=api_client,
    timeout=30.0  # 30 second timeout for all requests
)

​
FastAPI Integration

New in version: 2.0.0

FastMCP can directly convert FastAPI applications into MCP servers by extracting their OpenAPI specifications:

FastMCP does not include FastAPI as a dependency; you must install it separately to use this integration.

from fastapi import FastAPI
from fastmcp import FastMCP

# Your FastAPI app
app = FastAPI(title="My API", version="1.0.0")

@app.get("/items", tags=["items"], operation_id="list_items")
def list_items():
    return [{"id": 1, "name": "Item 1"}, {"id": 2, "name": "Item 2"}]

@app.get("/items/{item_id}", tags=["items", "detail"], operation_id="get_item")
def get_item(item_id: int):
    return {"id": item_id, "name": f"Item {item_id}"}

@app.post("/items", tags=["items", "create"], operation_id="create_item")
def create_item(name: str):
    return {"id": 3, "name": name}

# Convert FastAPI app to MCP server
mcp = FastMCP.from_fastapi(app=app)

if __name__ == "__main__":
    mcp.run()  # Run as MCP server

Note that operation ids are optional, but are used to create component names. You can also provide custom names, just like with OpenAPI specs.

FastMCP servers are not FastAPI apps, even when created from one. To learn how to deploy them as an ASGI app, see the ASGI Integration documentation.

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FastAPI Configuration

All OpenAPI integration features work with FastAPI apps:

from fastmcp.server.openapi import RouteMap, MCPType

# Custom route mapping with FastAPI
mcp = FastMCP.from_fastapi(
    app=app,
    name="My Custom Server",
    timeout=5.0,
    mcp_names={"operationId": "friendly_name"},  # Custom component names
    route_maps=[
        # Admin endpoints become tools
        RouteMap(methods="*", pattern=r"^/admin/.*", mcp_type=MCPType.TOOL),
        # Internal endpoints are excluded
        RouteMap(methods="*", pattern=r".*", mcp_type=MCPType.EXCLUDE, tags={"internal"}),
    ],
    route_map_fn=my_route_mapper,
    mcp_component_fn=my_component_customizer,
    mcp_names={
        "get_user_details_users__user_id__get": "get_user_details",
    }
)

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FastAPI Benefits

  • Zero code duplication: Reuse existing FastAPI endpoints
  • Schema inheritance: Pydantic models and validation are preserved
  • ASGI transport: Direct in-memory communication (no HTTP overhead)
  • Full FastAPI features: Dependencies, middleware, authentication all work