Use lowercase name for headers

src/fastmcp/client/transports.py

@@ -148,6 +148,7 @@ class SSETransport(ClientTransport):
         try:
             active_request = get_http_request()
             for name, value in active_request.headers.items():
+                name = name.lower()
                 if name not in self.headers and name not in EXCLUDE_HEADERS:
                     client_kwargs["headers"][name] = str(value)
         except RuntimeError:
@@ -208,6 +209,7 @@ class StreamableHttpTransport(ClientTransport):
         try:
             active_request = get_http_request()
             for name, value in active_request.headers.items():
+                name = name.lower()
                 if name not in self.headers and name not in EXCLUDE_HEADERS:
                     client_kwargs["headers"][name] = str(value)
 

src/fastmcp/server/openapi.py

@@ -35,6 +35,26 @@ logger = get_logger(__name__)
 
 HttpMethod = Literal["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS", "HEAD"]
 
+
+def _get_mcp_client_headers() -> dict[str, str]:
+    """
+    Extract headers from the current MCP client HTTP request if available.
+
+    These headers will take precedence over OpenAPI-defined headers when both are present.
+
+    Returns:
+        Dictionary of header name-value pairs (lowercased names), or empty dict if no HTTP request is active.
+    """
+    try:
+        http_request = get_http_request()
+        return {
+            name.lower(): str(value) for name, value in http_request.headers.items()
+        }
+    except RuntimeError:
+        # No active HTTP request (e.g., STDIO transport), return empty dict
+        return {}
+
+
 # Type definitions for the mapping functions
 RouteMapFn = Callable[[HTTPRoute, "MCPType"], "MCPType | None"]
 ComponentFn = Callable[
@@ -367,26 +387,20 @@ class OpenAPITool(Tool):
         # Prepare headers - fix typing by ensuring all values are strings
         headers = {}
 
-        # Try to get headers from the current MCP client HTTP request
-        try:
-            http_request = get_http_request()
-            # Add headers from the MCP client request
-            for name, value in http_request.headers.items():
-                # Don't override headers that are already set on the client
-                if name not in self._client.headers:
-                    headers[name] = str(value)
-        except RuntimeError:
-            # No active HTTP request (e.g., STDIO transport), continue without client headers
-            pass
-
-        # Add any OpenAPI-defined header parameters (these take precedence over client headers)
+        # Start with OpenAPI-defined header parameters
+        openapi_headers = {}
         for p in self._route.parameters:
             if (
                 p.location == "header"
                 and p.name in kwargs
                 and kwargs[p.name] is not None
             ):
-                headers[p.name] = str(kwargs[p.name])
+                openapi_headers[p.name.lower()] = str(kwargs[p.name])
+        headers.update(openapi_headers)
+
+        # Add headers from the current MCP client HTTP request (these take precedence)
+        mcp_headers = _get_mcp_client_headers()
+        headers.update(mcp_headers)
 
         # Prepare request body
         json_data = None
@@ -536,16 +550,8 @@ class OpenAPIResource(Resource):
 
             # Prepare headers from MCP client request if available
             headers = {}
-            try:
-                http_request = get_http_request()
-                # Add headers from the MCP client request
-                for name, value in http_request.headers.items():
-                    # Don't override headers that are already set on the client
-                    if name not in self._client.headers:
-                        headers[name] = str(value)
-            except RuntimeError:
-                # No active HTTP request (e.g., STDIO transport), continue without client headers
-                pass
+            mcp_headers = _get_mcp_client_headers()
+            headers.update(mcp_headers)
 
             response = await self._client.request(
                 method=self._route.method,
@@ -991,8 +997,3 @@ class FastMCPOpenAPI(FastMCP):
         logger.debug(
             f"Registered TEMPLATE: {uri_template_str} ({route.method} {route.path}) with tags: {route.tags}"
         )
-
-    async def _mcp_call_tool(self, name: str, arguments: dict[str, Any]) -> Any:
-        """Override the call_tool method to return the raw result without converting to content."""
-        result = await self._tool_manager.call_tool(name, arguments)
-        return result

tests/client/test_openapi.py

@@ -169,7 +169,7 @@ class TestClientHeaders:
             headers = json.loads(result[0].text)
             assert headers["x-test"] == "test-123"
 
-    async def test_client_doesnt_override_server_headers(self, shttp_server: str):
+    async def test_client_overrides_server_headers(self, shttp_server: str):
         async with Client(
             transport=StreamableHttpTransport(
                 shttp_server, headers={"X-SERVER": "test-client"}
@@ -178,7 +178,7 @@ class TestClientHeaders:
             result = await client.read_resource("resource://get_headers_headers_get")
             assert isinstance(result[0], TextResourceContents)
             headers = json.loads(result[0].text)
-            assert headers["x-server"] == "test-abc"
+            assert headers["x-server"] == "test-client"
 
     async def test_client_headers_proxy(self, proxy_server: str):
         """