Ensure headers are passed through proxy servers

src/fastmcp/client/transports.py

@@ -25,6 +25,7 @@ from pydantic import AnyUrl
 from typing_extensions import Unpack
 
 from fastmcp.server import FastMCP as FastMCPServer
+from fastmcp.server.dependencies import get_http_request
 from fastmcp.server.server import FastMCP
 from fastmcp.utilities.logging import get_logger
 from fastmcp.utilities.mcp_config import MCPConfig, infer_transport_type_from_url
@@ -34,6 +35,11 @@ if TYPE_CHECKING:
 
 logger = get_logger(__name__)
 
+EXCLUDE_HEADERS = {
+    "content-type",
+    "content-length",
+}
+
 
 class SessionKwargs(TypedDict, total=False):
     """Keyword arguments for the MCP ClientSession constructor."""
@@ -132,7 +138,21 @@ class SSETransport(ClientTransport):
     async def connect_session(
         self, **session_kwargs: Unpack[SessionKwargs]
     ) -> AsyncIterator[ClientSession]:
-        client_kwargs = {}
+        client_kwargs: dict[str, Any] = {
+            "headers": self.headers,
+        }
+
+        # load headers from an active HTTP request, if available. This will only be true
+        # if the client is used in a FastMCP Proxy, in which case the MCP client headers
+        # need to be forwarded to the remote server.
+        try:
+            active_request = get_http_request()
+            for name, value in active_request.headers.items():
+                if name not in self.headers and name not in EXCLUDE_HEADERS:
+                    client_kwargs["headers"][name] = str(value)
+        except RuntimeError:
+            client_kwargs["headers"] = self.headers
+
         # sse_read_timeout has a default value set, so we can't pass None without overriding it
         # instead we simply leave the kwarg out if it's not provided
         if self.sse_read_timeout is not None:
@@ -143,9 +163,7 @@ class SSETransport(ClientTransport):
             )
             client_kwargs["timeout"] = read_timeout_seconds.total_seconds()
 
-        async with sse_client(
-            self.url, headers=self.headers, **client_kwargs
-        ) as transport:
+        async with sse_client(self.url, **client_kwargs) as transport:
             read_stream, write_stream = transport
             async with ClientSession(
                 read_stream, write_stream, **session_kwargs
@@ -180,7 +198,23 @@ class StreamableHttpTransport(ClientTransport):
     async def connect_session(
         self, **session_kwargs: Unpack[SessionKwargs]
     ) -> AsyncIterator[ClientSession]:
-        client_kwargs = {}
+        client_kwargs: dict[str, Any] = {
+            "headers": self.headers,
+        }
+
+        # load headers from an active HTTP request, if available. This will only be true
+        # if the client is used in a FastMCP Proxy, in which case the MCP client headers
+        # need to be forwarded to the remote server.
+        try:
+            active_request = get_http_request()
+            for name, value in active_request.headers.items():
+                if name not in self.headers and name not in EXCLUDE_HEADERS:
+                    client_kwargs["headers"][name] = str(value)
+
+        except RuntimeError:
+            client_kwargs["headers"] = self.headers
+        print(client_kwargs)
+
         # sse_read_timeout has a default value set, so we can't pass None without overriding it
         # instead we simply leave the kwarg out if it's not provided
         if self.sse_read_timeout is not None:
@@ -188,9 +222,7 @@ class StreamableHttpTransport(ClientTransport):
         if session_kwargs.get("read_timeout_seconds", None) is not None:
             client_kwargs["timeout"] = session_kwargs.get("read_timeout_seconds")
 
-        async with streamablehttp_client(
-            self.url, headers=self.headers, **client_kwargs
-        ) as transport:
+        async with streamablehttp_client(self.url, **client_kwargs) as transport:
             read_stream, write_stream, _ = transport
             async with ClientSession(
                 read_stream, write_stream, **session_kwargs

tests/client/test_openapi.py

@@ -71,16 +71,40 @@ class TestClientHeaders:
             sys.exit(1)
         sys.exit(0)
 
-    @pytest.fixture(autouse=True, scope="class")
+    def run_proxy_server(self, host: str, port: int, remote_url: str) -> None:
+        try:
+            client = Client(transport=StreamableHttpTransport(remote_url))
+            app = FastMCP.as_proxy(client).http_app(transport="streamable-http")
+            server = uvicorn.Server(
+                config=uvicorn.Config(
+                    app=app,
+                    host=host,
+                    port=port,
+                    log_level="error",
+                    lifespan="on",
+                )
+            )
+            server.run()
+        except Exception as e:
+            print(f"Server error: {e}")
+            sys.exit(1)
+        sys.exit(0)
+
+    @pytest.fixture(scope="class")
     def shttp_server(self) -> Generator[str, None, None]:
         with run_server_in_process(self.run_shttp_server) as url:
             yield f"{url}/mcp"
 
-    @pytest.fixture(autouse=True, scope="class")
+    @pytest.fixture(scope="class")
     def sse_server(self) -> Generator[str, None, None]:
         with run_server_in_process(self.run_sse_server) as url:
             yield f"{url}/sse"
 
+    @pytest.fixture(scope="class")
+    def proxy_server(self, shttp_server: str) -> Generator[str, None, None]:
+        with run_server_in_process(self.run_proxy_server, shttp_server + "/mcp") as url:
+            yield f"{url}/mcp"
+
     async def test_client_headers_sse_resource(self, sse_server: str):
         async with Client(
             transport=SSETransport(sse_server, headers={"X-TEST": "test-123"})
@@ -155,3 +179,11 @@ class TestClientHeaders:
             assert isinstance(result[0], TextResourceContents)
             headers = json.loads(result[0].text)
             assert headers["x-server"] == "test-abc"
+
+    async def test_client_headers_proxy(self, proxy_server: str):
+        async with Client(transport=StreamableHttpTransport(proxy_server)) as client:
+            await client.ping()
+            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"