Merge pull request #539 from jlowin/codex/add-support-for-fastmcp-1.0-server

Add FastMCP 1.0 server support for in-memory Client / Testing

docs/clients/client.mdx

@@ -37,7 +37,7 @@ Clients must be initialized with a `transport`. You can either provide an alread
 The following inference rules are used to determine the appropriate `ClientTransport` based on the input type:
 
 1.  **`ClientTransport` Instance**: If you provide an already instantiated transport object, it's used directly.
-2.  **`FastMCP` Instance**: Creates a `FastMCPTransport` for efficient in-memory communication (ideal for testing).
+2.  **`FastMCP` Instance**: Creates a `FastMCPTransport` for efficient in-memory communication (ideal for testing). This also works with a **FastMCP 1.0 server** created via `mcp.server.fastmcp.FastMCP`.
 3.  **`Path` or `str` pointing to an existing file**:
     *   If it ends with `.py`: Creates a `PythonStdioTransport` to run the script using `python`.
     *   If it ends with `.js`: Creates a `NodeStdioTransport` to run the script using `node`.

docs/clients/transports.mdx

@@ -290,8 +290,8 @@ asyncio.run(main())
 ### FastMCP Transport
 
 - **Class:** `fastmcp.client.transports.FastMCPTransport`
-- **Inferred From:** An instance of `fastmcp.server.FastMCP`
-- **Use Case:** Connecting directly to a `FastMCP` server instance in the same Python process
+- **Inferred From:** An instance of `fastmcp.server.FastMCP` or a **FastMCP 1.0 server** (`mcp.server.fastmcp.FastMCP`)
+- **Use Case:** Connecting directly to a FastMCP server instance in the same Python process
 
 This is extremely useful for testing your FastMCP servers.
 

src/fastmcp/client/transports.py

@@ -19,6 +19,7 @@ from mcp.client.sse import sse_client
 from mcp.client.stdio import stdio_client
 from mcp.client.streamable_http import streamablehttp_client
 from mcp.client.websocket import websocket_client
+from mcp.server.fastmcp import FastMCP as FastMCP1Server
 from mcp.shared.memory import create_connected_server_and_client_session
 from pydantic import AnyUrl
 from typing_extensions import Unpack
@@ -448,15 +449,21 @@ class NpxStdioTransport(StdioTransport):
 
 
 class FastMCPTransport(ClientTransport):
-    """
-    Special transport for in-memory connections to an MCP server.
+    """In-memory transport for FastMCP servers.
 
-    This is particularly useful for testing or when client and server
-    are in the same process.
+    This transport connects directly to a FastMCP server instance in the same
+    Python process. It works with both FastMCP 2.x servers and FastMCP 1.0
+    servers from the low-level MCP SDK. This is particularly useful for unit
+    tests or scenarios where client and server run in the same runtime.
     """
 
-    def __init__(self, mcp: FastMCPServer):
-        self.server = mcp  # Can be FastMCP or MCPServer
+    def __init__(self, mcp: FastMCPServer | FastMCP1Server):
+        """Initialize a FastMCPTransport from a FastMCP server instance."""
+
+        # Accept both FastMCP 2.x and FastMCP 1.0 servers. Both expose a
+        # ``_mcp_server`` attribute pointing to the underlying MCP server
+        # implementation, so we can treat them identically.
+        self.server = mcp
 
     @contextlib.asynccontextmanager
     async def connect_session(
@@ -558,6 +565,7 @@ class MCPConfigTransport(ClientTransport):
 def infer_transport(
     transport: ClientTransport
     | FastMCPServer
+    | FastMCP1Server
     | AnyUrl
     | Path
     | MCPConfig
@@ -573,7 +581,7 @@ def infer_transport(
 
     The function supports these input types:
     - ClientTransport: Used directly without modification
-    - FastMCPServer: Creates an in-memory FastMCPTransport
+    - FastMCPServer or FastMCP1Server: Creates an in-memory FastMCPTransport
     - Path or str (file path): Creates PythonStdioTransport (.py) or NodeStdioTransport (.js)
     - AnyUrl or str (URL): Creates StreamableHttpTransport (default) or SSETransport (for /sse endpoints)
     - MCPConfig or dict: Creates MCPConfigTransport, potentially connecting to multiple servers
@@ -610,8 +618,8 @@ def infer_transport(
     if isinstance(transport, ClientTransport):
         return transport
 
-    # the transport is a FastMCP server
-    elif isinstance(transport, FastMCPServer):
+    # the transport is a FastMCP server (2.x or 1.0)
+    elif isinstance(transport, FastMCPServer | FastMCP1Server):
         inferred_transport = FastMCPTransport(mcp=transport)
 
     # the transport is a path to a script

tests/client/test_client.py

@@ -673,7 +673,7 @@ class TestInferTransport:
         assert transport.transport.command == "echo"
         assert transport.transport.args == ["hello"]
 
-    def test_infer_composite_client(config):
+    def test_infer_composite_client(self):
         config = {
             "mcpServers": {
                 "local": {
@@ -689,4 +689,17 @@ class TestInferTransport:
         transport = infer_transport(config)
         assert isinstance(transport, MCPConfigTransport)
         assert isinstance(transport.transport, FastMCPTransport)
-        assert len(transport.transport.server._mounted_servers) == 2
+        assert len(cast(FastMCP, transport.transport.server)._mounted_servers) == 2
+
+    def test_infer_fastmcp_server(self, fastmcp_server):
+        """FastMCP server instances should infer to FastMCPTransport."""
+        transport = infer_transport(fastmcp_server)
+        assert isinstance(transport, FastMCPTransport)
+
+    def test_infer_fastmcp_v1_server(self):
+        """FastMCP 1.0 server instances should infer to FastMCPTransport."""
+        from mcp.server.fastmcp import FastMCP as FastMCP1
+
+        server = FastMCP1()
+        transport = infer_transport(server)
+        assert isinstance(transport, FastMCPTransport)