Improve OpenAPI param handling

docs/patterns/openapi.mdx

@@ -106,6 +106,38 @@ mcp = await FastMCP.from_openapi(
    - It sends the request through the provided httpx client
    - It translates the HTTP response to the appropriate MCP format
 
+### Request Parameter Handling
+
+FastMCP carefully handles different types of parameters in OpenAPI requests:
+
+#### Query Parameters
+
+By default, FastMCP will only include query parameters that have non-empty values. Parameters with `None` values or empty strings (`""`) are automatically filtered out of requests. This ensures that API servers don't receive unnecessary empty parameters that might cause issues.
+
+For example, if you call a tool with these parameters:
+```python
+await client.call_tool("search_products", {
+    "category": "electronics",  # Will be included
+    "min_price": 100,           # Will be included
+    "max_price": None,          # Will be excluded
+    "brand": "",                # Will be excluded
+})
+```
+
+The resulting HTTP request will only include `category=electronics&min_price=100`.
+
+#### Path Parameters
+
+For path parameters, which are typically required by REST APIs, FastMCP filters out `None` values and checks that all required path parameters are provided. If a required path parameter is missing or `None`, an error will be raised.
+
+```python
+# This will work
+await client.call_tool("get_product", {"product_id": 123})
+
+# This will raise ValueError: "Missing required path parameters: {'product_id'}"
+await client.call_tool("get_product", {"product_id": None})
+```
+
 ## Complete Example
 
 ```python [expandable]

src/fastmcp/server/openapi.py

@@ -149,19 +149,37 @@ class OpenAPITool(Tool):
         path = self._route.path
 
         # Replace path parameters with values from kwargs
+        # Path parameters should never be None as they're typically required
+        # but we'll handle that case anyway
         path_params = {
             p.name: kwargs.get(p.name)
             for p in self._route.parameters
             if p.location == "path"
+            and p.name in kwargs
+            and kwargs.get(p.name) is not None
         }
+
+        # Ensure all path parameters are provided
+        required_path_params = {
+            p.name
+            for p in self._route.parameters
+            if p.location == "path" and p.required
+        }
+        missing_params = required_path_params - path_params.keys()
+        if missing_params:
+            raise ValueError(f"Missing required path parameters: {missing_params}")
+
         for param_name, param_value in path_params.items():
             path = path.replace(f"{{{param_name}}}", str(param_value))
 
-        # Prepare query parameters
+        # Prepare query parameters - filter out None and empty strings
         query_params = {
             p.name: kwargs.get(p.name)
             for p in self._route.parameters
-            if p.location == "query" and p.name in kwargs
+            if p.location == "query"
+            and p.name in kwargs
+            and kwargs.get(p.name) is not None
+            and kwargs.get(p.name) != ""
         }
 
         # Prepare headers - fix typing by ensuring all values are strings
@@ -312,9 +330,18 @@ class OpenAPIResource(Resource):
                     for param_name, param_value in path_params.items():
                         path = path.replace(f"{{{param_name}}}", str(param_value))
 
+            # Filter any query parameters - get query parameters and filter out None/empty values
+            query_params = {}
+            for param in self._route.parameters:
+                if param.location == "query" and hasattr(self, f"_{param.name}"):
+                    value = getattr(self, f"_{param.name}")
+                    if value is not None and value != "":
+                        query_params[param.name] = value
+
             response = await self._client.request(
                 method=self._route.method,
                 url=path,
+                params=query_params,
                 timeout=self._timeout,
             )
 

tests/server/test_openapi.py

@@ -14,6 +14,7 @@ from pydantic.networks import AnyUrl
 
 from fastmcp import FastMCP
 from fastmcp.client import Client
+from fastmcp.exceptions import ClientError
 from fastmcp.server.openapi import (
     FastMCPOpenAPI,
     OpenAPIResource,
@@ -53,6 +54,22 @@ def fastapi_app(users_db: dict[int, User]) -> FastAPI:
         """Get all users."""
         return sorted(users_db.values(), key=lambda x: x.id)
 
+    @app.get("/search", tags=["search"])
+    async def search_users(
+        name: str | None = None, active: bool | None = None, min_id: int | None = None
+    ) -> list[User]:
+        """Search users with optional filters."""
+        results = list(users_db.values())
+
+        if name is not None:
+            results = [u for u in results if name.lower() in u.name.lower()]
+        if active is not None:
+            results = [u for u in results if u.active == active]
+        if min_id is not None:
+            results = [u for u in results if u.id >= min_id]
+
+        return sorted(results, key=lambda x: x.id)
+
     @app.get("/users/{user_id}", tags=["users", "detail"])
     async def get_user(user_id: int) -> User | None:
         """Get a user by ID."""
@@ -304,7 +321,7 @@ class TestResources:
         """
         async with Client(fastmcp_openapi_server) as client:
             resources = await client.list_resources()
-        assert len(resources) == 3
+        assert len(resources) == 4
         assert resources[0].uri == AnyUrl("resource://openapi/get_users_users_get")
         assert resources[0].name == "get_users_users_get"
 
@@ -904,7 +921,7 @@ class TestMountFastMCP:
         # Check that resources are available with prefixed URIs
         async with Client(mcp) as client:
             resources = await client.list_resources()
-        assert len(resources) == 3
+        assert len(resources) == 4  # Updated to account for new search endpoint
         # We're checking the key used by mcp to store the resource
         # The prefixed URI is used as the key, but the resource's original uri is preserved
         prefixed_uri = "fastapi+resource://openapi/get_users_users_get"
@@ -932,3 +949,88 @@ class TestMountFastMCP:
         async with Client(mcp) as client:
             prompts = await client.list_prompts()
         assert len(prompts) == 0
+
+
+async def test_empty_query_parameters_not_sent(
+    fastapi_app: FastAPI, api_client: httpx.AsyncClient
+):
+    """Test that empty and None query parameters are not sent in the request."""
+
+    # Create a TransportAdapter to track requests
+    class RequestCapture(httpx.AsyncBaseTransport):
+        def __init__(self, wrapped):
+            self.wrapped = wrapped
+            self.requests = []
+
+        async def handle_async_request(self, request):
+            self.requests.append(request)
+            return await self.wrapped.handle_async_request(request)
+
+    # Use our transport adapter to wrap the original one
+    capture = RequestCapture(api_client._transport)
+    api_client._transport = capture
+
+    # Create the OpenAPI server with new route map to make search endpoint a tool
+    openapi_spec = fastapi_app.openapi()
+    mcp_server = FastMCPOpenAPI(
+        openapi_spec=openapi_spec,
+        client=api_client,
+        route_maps=[
+            RouteMap(methods=["GET"], pattern=r".*", route_type=RouteType.TOOL)
+        ],
+    )
+
+    # Call the search tool with mixed parameter values
+    async with Client(mcp_server) as client:
+        await client.call_tool(
+            "search_users_search_get",
+            {
+                "name": "",  # Empty string should be excluded
+                "active": None,  # None should be excluded
+                "min_id": 2,  # Has value, should be included
+            },
+        )
+
+    # Verify that the request URL only has min_id parameter
+    assert len(capture.requests) > 0
+    request = capture.requests[-1]  # Get the last request
+
+    # URL should only contain min_id=2, not name= or active=
+    url = str(request.url)
+    assert "min_id=2" in url, f"URL should contain min_id=2, got: {url}"
+    assert "name=" not in url, f"URL should not contain name=, got: {url}"
+    assert "active=" not in url, f"URL should not contain active=, got: {url}"
+
+    # More direct check - parse the URL to examine query params
+    from urllib.parse import parse_qs, urlparse
+
+    parsed_url = urlparse(url)
+    query_params = parse_qs(parsed_url.query)
+
+    assert "min_id" in query_params
+    assert "name" not in query_params
+    assert "active" not in query_params
+
+
+async def test_none_path_parameters_rejected(
+    fastapi_app: FastAPI, api_client: httpx.AsyncClient
+):
+    """Test that None values for path parameters are properly rejected."""
+    # Create the OpenAPI server
+    openapi_spec = fastapi_app.openapi()
+    mcp_server = FastMCPOpenAPI(
+        openapi_spec=openapi_spec,
+        client=api_client,
+    )
+
+    # Create a client and try to call a tool with a None path parameter
+    async with Client(mcp_server) as client:
+        # get_user has a required path parameter user_id
+        with pytest.raises(ClientError, match="Missing required path parameters"):
+            await client.call_tool(
+                "update_user_name_users__user_id__name_patch",
+                {
+                    "user_id": None,  # This should cause an error
+                    "name": "New Name",
+                },
+            )