OpenAPI 🤝 FastMCP

New in version: 2.0.0

New in 2.11: FastMCP is introducing a next-generation OpenAPI parser. The new parser has greatly improved performance and compatibility, and is also easier to maintain. To enable it, set the environment variable FASTMCP_EXPERIMENTAL_ENABLE_NEW_OPENAPI_PARSER=true.The new parser is largely API-compatible with the existing implementation and will become the default in a future version. We encourage all users to test it and report any issues before it becomes the default.

FastMCP can automatically generate an MCP server from any OpenAPI specification, allowing AI models to interact with existing APIs through the MCP protocol. Instead of manually creating tools and resources, you provide an OpenAPI spec and FastMCP intelligently converts API endpoints into the appropriate MCP components.

Generating MCP servers from OpenAPI is a great way to get started with FastMCP, but in practice LLMs achieve significantly better performance with well-designed and curated MCP servers than with auto-converted OpenAPI servers. This is especially true for complex APIs with many endpoints and parameters.We recommend using the FastAPI integration for bootstrapping and prototyping, not for mirroring your API to LLM clients. See the post Stop Converting Your REST APIs to MCP for more details.

Create a Server

To convert an OpenAPI specification to an MCP server, use the FastMCP.from_openapi() class method:

server.py

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()

Authentication

If your API requires authentication, configure it on the HTTP client:

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

Route Mapping

By default, FastMCP converts every endpoint in your OpenAPI specification into an MCP Tool. This provides a simple, predictable starting point that ensures all your API’s functionality is immediately available to the vast majority of LLM clients which only support MCP tools. While this is a pragmatic default for maximum compatibility, you can easily customize this behavior. Internally, 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)
MCP tags: A set of custom tags to add to components created from matching routes

Here is FastMCP’s default rule:

from fastmcp.server.openapi import RouteMap, MCPType

DEFAULT_ROUTE_MAPPINGS = [
    # All routes become tools
    RouteMap(mcp_type=MCPType.TOOL),
]

Experimental Parser: If you’re using the new parser (enabled via FASTMCP_EXPERIMENTAL_ENABLE_NEW_OPENAPI_PARSER=true), import from the experimental module instead:

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

The API is identical, but the implementation provides better performance and serverless compatibility.

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, prior to FastMCP 2.8.0, GET requests were automatically mapped to Resource and ResourceTemplate components based on whether they had path parameters. (This was changed solely for client compatibility reasons.) You can restore this behavior by providing custom route maps:

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

# Restore pre-2.8.0 semantic mapping
semantic_maps = [
    # GET requests with path parameters become ResourceTemplates
    RouteMap(methods=["GET"], pattern=r".*\{.*\}.*", mcp_type=MCPType.RESOURCE_TEMPLATE),
    # All other GET requests become Resources
    RouteMap(methods=["GET"], pattern=r".*", mcp_type=MCPType.RESOURCE),
]

mcp = FastMCP.from_openapi(
    openapi_spec=spec,
    client=client,
    route_maps=semantic_maps,
)

With these maps, GET requests are handled semantically, and all other methods (POST, PUT, etc.) will fall through to the default rule and become Tools. 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(
    openapi_spec=spec,
    client=client,
    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.

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(
    openapi_spec=spec,
    client=client,
    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(
    openapi_spec=spec,
    client=client,
    route_maps=[
        # custom mapping logic goes here
        # ... your specific route maps ...
        # 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.

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 all routes, even those that matched MCPType.EXCLUDE in your custom maps. This gives you an opportunity to customize the mapping or even override an 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(
    openapi_spec=spec,
    client=client,
    route_map_fn=custom_route_mapper,
)

Customization

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.

mcp = FastMCP.from_openapi(
    openapi_spec=spec,
    client=client,
    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.server.openapi import (
    HTTPRoute, 
    OpenAPITool, 
    OpenAPIResource, 
    OpenAPIResourceTemplate,
)

# If using experimental parser, import from experimental module:
# from fastmcp.experimental.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(
    openapi_spec=spec,
    client=client,
    mcp_component_fn=customize_components,
)

Request Parameter Handling

FastMCP intelligently handles different types of parameters in OpenAPI requests:

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

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

Headers

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

Get Started

Servers

Clients

Integrations

Patterns

Development

OpenAPI 🤝 FastMCP

Create a Server

Authentication

Route Mapping

Custom Route Maps

Excluding Routes

Advanced Route Mapping

Customization

Component Names

Tags

RouteMap Tags

Global Tags

OpenAPI Tags in Client Meta

Advanced Customization

Request Parameter Handling

Query Parameters

Path Parameters

Array Parameters

Headers

Get Started

Servers

Clients

Integrations

Patterns

Development

​Create a Server

​Authentication

​Route Mapping

​Custom Route Maps

​Excluding Routes

​Advanced Route Mapping

​Customization

​Component Names

​Tags

​RouteMap Tags

​Global Tags

​OpenAPI Tags in Client Meta

​Advanced Customization

​Request Parameter Handling

​Query Parameters

​Path Parameters

​Array Parameters

​Headers

Create a Server

Authentication

Route Mapping

Custom Route Maps

Excluding Routes

Advanced Route Mapping

Customization

Component Names

Tags

RouteMap Tags

Global Tags

OpenAPI Tags in Client Meta

Advanced Customization

Request Parameter Handling

Query Parameters

Path Parameters

Array Parameters

Headers