mirror of
https://github.com/PrefectHQ/fastmcp.git
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de14ed1b7cRemoves pins added while making the auto-default suite pass that weren't actually testing older-protocol-only behavior, and keeps (with a stated reason) the ones that are. Along the way, fixes two real defects the audit surfaced in the modern protocol path: PingMiddleware could leak a _active_sessions entry when a connection's exit_stack closed before its keepalive task got its first scheduler turn, and FastMCP(experimental_ capabilities=...) was silently dropped from server/discover responses (it only ever reached the legacy initialize handshake).
435 lines
16 KiB
Python
435 lines
16 KiB
Python
"""Tests for middleware support during initialization.
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`on_initialize` only fires for the `initialize` handshake, which is unique to
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the older protocol version; the modern version connects without it, so a
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default client never triggers this hook. Most tests below pin `mode="legacy"`
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for that reason. `test_session_state_persists_across_tool_calls` pins for a
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different reason: it exercises `ctx.set_state`/`get_state` persisting across
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multiple tool calls in the same client session, which requires the
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handshake-era's persistent session (see `test_session_visibility.py` for the
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same distinction applied to a different feature).
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"""
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from collections.abc import Sequence
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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 TextContent
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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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from fastmcp.tools.base import Tool
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class InitializationMiddleware(Middleware):
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"""Middleware that captures initialization details.
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Note: Session state is NOT available during on_initialize because
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the MCP session has not been established yet. Use instance variables
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to store data that needs to persist across the session.
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"""
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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, mt.InitializeResult | None],
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) -> mt.InitializeResult | None:
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"""Capture initialization details."""
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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.client_info
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# Store in instance for cross-request access
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# (session state is not available during on_initialize)
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self.session_data["client_initialized"] = True
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if self.client_info:
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self.session_data["client_name"] = getattr(
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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, mt.InitializeResult | None],
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) -> mt.InitializeResult | 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, Sequence[Tool]],
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) -> Sequence[Tool]:
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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.read_only_hint = 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, mt.InitializeResult | None],
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) -> mt.InitializeResult | 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, mode="legacy"):
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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, mode="legacy") 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 isinstance(result.content[0], TextContent)
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assert result.content[0].text == "Result: 42"
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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, mode="legacy") 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.read_only_hint 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, mode="legacy") 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_session_state_persists_across_tool_calls():
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"""Test that session-scoped state persists across multiple tool calls.
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Session state is only available after the session is established,
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so it can't be set during on_initialize. This test shows state set
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during one tool call is accessible in subsequent tool calls.
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"""
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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.call_count = 0
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self.state_values = []
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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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self.call_count += 1
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if context.fastmcp_context:
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# Read existing state
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counter = await context.fastmcp_context.get_state("call_counter")
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self.state_values.append(counter)
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# Increment and save
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new_counter = (counter or 0) + 1
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await context.fastmcp_context.set_state("call_counter", new_counter)
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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, mode="legacy") as client:
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# First call - state should be None initially
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result = await client.call_tool("test_tool", {})
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assert isinstance(result.content[0], TextContent)
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assert result.content[0].text == "success"
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# Second call - state should show previous value (1)
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result = await client.call_tool("test_tool", {})
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assert isinstance(result.content[0], TextContent)
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# Third call - state should show previous value (2)
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result = await client.call_tool("test_tool", {})
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assert isinstance(result.content[0], TextContent)
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# Verify state persisted across calls within the session
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assert middleware.call_count == 3
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# First call saw None, second saw 1, third saw 2
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assert middleware.state_values == [None, 1, 2]
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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, mode="legacy"):
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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.server_info.name == "TestServer"
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assert middleware.initialize_result.protocol_version 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, mt.InitializeResult | None],
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) -> mt.InitializeResult | None:
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raise MCPError(
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code=mt.INVALID_PARAMS, message="Invalid initialization parameters"
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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, mode="legacy"):
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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, mt.InitializeResult | None],
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) -> mt.InitializeResult | None:
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raise MCPError(code=mt.INVALID_REQUEST, message="Request validation failed")
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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, mode="legacy"):
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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(code=mt.INTERNAL_ERROR, message="Post-processing failed")
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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, mode="legacy"):
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pass
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assert middleware.error_raised is True
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async def test_state_isolation_between_streamable_http_clients():
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"""Test that different HTTP clients have isolated session state.
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Each client should have its own session ID and isolated state.
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"""
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from fastmcp.server.context import Context
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from fastmcp.utilities.tests import asgi_server
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server = FastMCP("TestServer")
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@server.tool
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async def store_and_read(value: str, ctx: Context) -> dict:
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"""Store a value and return session info."""
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existing = await ctx.get_state("client_value")
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await ctx.set_state("client_value", value)
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return {
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"existing": existing,
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"stored": value,
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"session_id": ctx.session_id,
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}
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async with asgi_server(server, transport="streamable-http") as running_server:
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import json
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# Client 1 stores its value
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async with running_server.client() as client1:
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result1 = await client1.call_tool(
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"store_and_read", {"value": "client1-value"}
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)
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data1 = json.loads(result1.content[0].text)
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assert data1["existing"] is None
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assert data1["stored"] == "client1-value"
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session_id_1 = data1["session_id"]
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# Client 2 should have completely isolated state
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async with running_server.client() as client2:
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result2 = await client2.call_tool(
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"store_and_read", {"value": "client2-value"}
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)
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data2 = json.loads(result2.content[0].text)
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# Should NOT see client1's value
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assert data2["existing"] is None
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assert data2["stored"] == "client2-value"
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session_id_2 = data2["session_id"]
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# Session IDs should be different
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assert session_id_1 != session_id_2
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