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03b62d2e3d* feat: handle error from the initialize middleware In some situation, the initialize middleware can check the status of the server and decide to raise an error. Example use case: in a FastMCPProxy, an initialization middleware overrides the on_initialize method and connect to the underlying proxied client. When client respond with error, I want to pass this error to the client. * docs update * test: use McpError assertions now that exception propagation is fixed - Update tests to catch McpError specifically instead of generic Exception - Remove commented-out code in low_level.py --------- Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
375 lines
13 KiB
Python
375 lines
13 KiB
Python
"""Tests for middleware support during initialization."""
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from typing import Any
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import mcp.types as mt
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import pytest
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from mcp import McpError
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from mcp.types import ErrorData
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from fastmcp import Client, FastMCP
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from fastmcp.server.middleware import CallNext, Middleware, MiddlewareContext
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class InitializationMiddleware(Middleware):
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"""Middleware that captures initialization details."""
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def __init__(self):
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super().__init__()
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self.initialized = False
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self.client_info = None
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self.session_data = {}
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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"""Capture initialization details and store session data."""
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self.initialized = True
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# Extract client info from the initialize params
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if hasattr(context.message, "params") and hasattr(
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context.message.params, "clientInfo"
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):
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self.client_info = context.message.params.clientInfo
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# Store data in the context state for cross-request access
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if context.fastmcp_context:
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context.fastmcp_context.set_state("client_initialized", True)
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if self.client_info:
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context.fastmcp_context.set_state(
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"client_name", getattr(self.client_info, "name", "unknown")
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)
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return await call_next(context)
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class ClientDetectionMiddleware(Middleware):
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"""Middleware that detects specific clients and modifies behavior.
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This demonstrates storing data in the middleware instance itself
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for cross-request access, since context state is request-scoped.
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"""
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def __init__(self):
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super().__init__()
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self.is_test_client = False
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self.tools_modified = False
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self.initialization_called = False
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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"""Detect test client during initialization."""
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self.initialization_called = True
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# For testing purposes, always set it to true
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# Store in instance variable for cross-request access
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self.is_test_client = True
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return await call_next(context)
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async def on_list_tools(
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self,
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context: MiddlewareContext[mt.ListToolsRequest],
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call_next: CallNext[mt.ListToolsRequest, list],
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) -> list:
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"""Modify tools based on client detection."""
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tools = await call_next(context)
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# Use the instance variable set during initialization
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if self.is_test_client:
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# Add a special annotation to tools for test clients
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for tool in tools:
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if not hasattr(tool, "annotations"):
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tool.annotations = mt.ToolAnnotations()
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if tool.annotations is None:
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tool.annotations = mt.ToolAnnotations()
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# Mark as read-only for test clients
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tool.annotations.readOnlyHint = True
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self.tools_modified = True
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return tools
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async def test_simple_initialization_hook():
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"""Test that the on_initialize hook is called."""
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server = FastMCP("TestServer")
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class SimpleInitMiddleware(Middleware):
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def __init__(self):
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super().__init__()
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self.called = False
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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self.called = True
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return await call_next(context)
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middleware = SimpleInitMiddleware()
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server.add_middleware(middleware)
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# Connect client
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async with Client(server):
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# Middleware should have been called
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assert middleware.called is True, "on_initialize was not called"
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async def test_middleware_receives_initialization():
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"""Test that middleware can intercept initialization requests."""
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server = FastMCP("TestServer")
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middleware = InitializationMiddleware()
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server.add_middleware(middleware)
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@server.tool
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def test_tool(x: int) -> str:
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return f"Result: {x}"
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# Connect client
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async with Client(server) as client:
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# Middleware should have been called during initialization
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assert middleware.initialized is True
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# Test that the tool still works
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result = await client.call_tool("test_tool", {"x": 42})
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assert result.content[0].text == "Result: 42" # type: ignore[attr-defined]
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async def test_client_detection_middleware():
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"""Test middleware that detects specific clients and modifies behavior."""
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server = FastMCP("TestServer")
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middleware = ClientDetectionMiddleware()
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server.add_middleware(middleware)
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@server.tool
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def example_tool() -> str:
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return "example"
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# Connect with a client
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async with Client(server) as client:
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# Middleware should have been called during initialization
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assert middleware.initialization_called is True
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assert middleware.is_test_client is True
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# List tools to trigger modification
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tools = await client.list_tools()
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assert len(tools) == 1
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assert middleware.tools_modified is True
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# Check that the tool has the modified annotation
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tool = tools[0]
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assert tool.annotations is not None
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assert tool.annotations.readOnlyHint is True
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async def test_multiple_middleware_initialization():
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"""Test that multiple middleware can handle initialization."""
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server = FastMCP("TestServer")
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init_mw = InitializationMiddleware()
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detect_mw = ClientDetectionMiddleware()
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server.add_middleware(init_mw)
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server.add_middleware(detect_mw)
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@server.tool
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def test_tool() -> str:
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return "test"
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async with Client(server) as client:
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# Both middleware should have processed initialization
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assert init_mw.initialized is True
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assert detect_mw.initialization_called is True
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assert detect_mw.is_test_client is True
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# List tools to check detection worked
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await client.list_tools()
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assert detect_mw.tools_modified is True
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async def test_initialization_middleware_with_state_sharing():
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"""Test that state set during initialization is available in later requests."""
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server = FastMCP("TestServer")
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class StateTrackingMiddleware(Middleware):
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def __init__(self):
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super().__init__()
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self.init_state = {}
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self.tool_state = {}
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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# Store some state during initialization
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if context.fastmcp_context:
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context.fastmcp_context.set_state("init_timestamp", "2024-01-01")
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context.fastmcp_context.set_state("client_id", "test-123")
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self.init_state["timestamp"] = "2024-01-01"
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self.init_state["client_id"] = "test-123"
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return await call_next(context)
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async def on_call_tool(
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self,
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context: MiddlewareContext[mt.CallToolRequestParams],
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call_next: CallNext[mt.CallToolRequestParams, Any],
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) -> Any:
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# Try to access state from initialization
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if context.fastmcp_context:
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timestamp = context.fastmcp_context.get_state("init_timestamp")
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client_id = context.fastmcp_context.get_state("client_id")
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self.tool_state["timestamp"] = timestamp
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self.tool_state["client_id"] = client_id
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return await call_next(context)
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middleware = StateTrackingMiddleware()
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server.add_middleware(middleware)
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@server.tool
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def test_tool() -> str:
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return "success"
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async with Client(server) as client:
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# Initialization should have set state
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assert middleware.init_state["timestamp"] == "2024-01-01"
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assert middleware.init_state["client_id"] == "test-123"
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# Call a tool - state should be accessible
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result = await client.call_tool("test_tool", {})
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assert result.content[0].text == "success" # type: ignore[attr-defined]
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# State should have been accessible during tool call
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# Note: State is request-scoped, so it won't persist across requests
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# This test shows the pattern, but actual cross-request state would need
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# external storage (Redis, DB, etc.)
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# The middleware.tool_state might be None if state doesn't persist
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async def test_middleware_can_access_initialize_result():
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"""Test that middleware can access the InitializeResult from call_next().
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This verifies that the initialize response is returned through the middleware
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chain, not just sent directly via the responder (fixes #2504).
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"""
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server = FastMCP("TestServer")
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class ResponseCapturingMiddleware(Middleware):
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def __init__(self):
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super().__init__()
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self.initialize_result: mt.InitializeResult | None = None
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, mt.InitializeResult | None],
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) -> mt.InitializeResult | None:
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# Call next and capture the result
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result = await call_next(context)
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self.initialize_result = result
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return result
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middleware = ResponseCapturingMiddleware()
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server.add_middleware(middleware)
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async with Client(server):
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# Middleware should have captured the InitializeResult
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assert middleware.initialize_result is not None
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assert isinstance(middleware.initialize_result, mt.InitializeResult)
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# Verify the result contains expected server info
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assert middleware.initialize_result.serverInfo.name == "TestServer"
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assert middleware.initialize_result.protocolVersion is not None
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assert middleware.initialize_result.capabilities is not None
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async def test_middleware_mcp_error_during_initialization():
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"""Test that McpError raised in middleware during initialization is sent to client."""
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server = FastMCP("TestServer")
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class ErrorThrowingMiddleware(Middleware):
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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raise McpError(
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ErrorData(
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code=mt.INVALID_PARAMS, message="Invalid initialization parameters"
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)
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)
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server.add_middleware(ErrorThrowingMiddleware())
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with pytest.raises(McpError) as exc_info:
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async with Client(server):
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pass
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assert exc_info.value.error.message == "Invalid initialization parameters"
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assert exc_info.value.error.code == mt.INVALID_PARAMS
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async def test_middleware_mcp_error_before_call_next():
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"""Test McpError raised before calling next middleware."""
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server = FastMCP("TestServer")
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class EarlyErrorMiddleware(Middleware):
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, None],
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) -> None:
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raise McpError(
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ErrorData(code=mt.INVALID_REQUEST, message="Request validation failed")
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)
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server.add_middleware(EarlyErrorMiddleware())
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with pytest.raises(McpError) as exc_info:
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async with Client(server):
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pass
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assert exc_info.value.error.message == "Request validation failed"
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assert exc_info.value.error.code == mt.INVALID_REQUEST
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async def test_middleware_mcp_error_after_call_next():
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"""Test that McpError raised after call_next doesn't break the connection.
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When an error is raised after call_next, the responder has already completed,
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so the error is caught but not sent to the responder (checked via _completed flag).
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"""
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server = FastMCP("TestServer")
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class PostProcessingErrorMiddleware(Middleware):
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def __init__(self):
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super().__init__()
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self.error_raised = False
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async def on_initialize(
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self,
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context: MiddlewareContext[mt.InitializeRequest],
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call_next: CallNext[mt.InitializeRequest, mt.InitializeResult | None],
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) -> mt.InitializeResult | None:
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await call_next(context)
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self.error_raised = True
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raise McpError(
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ErrorData(code=mt.INTERNAL_ERROR, message="Post-processing failed")
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)
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middleware = PostProcessingErrorMiddleware()
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server.add_middleware(middleware)
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# Error is logged but not re-raised to prevent duplicate response
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async with Client(server):
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pass
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assert middleware.error_raised is True
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