python-sdk/docs_src/middleware/tutorial002.py at main · modelcontextprotocol/python-sdk · GitHub

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from typing import Any

from mcp import MCPError

from mcp.server import Server, ServerRequestContext

from mcp.server.context import CallNext, HandlerResult, ServerMiddleware

from mcp.types import (

CallToolRequestParams,

CallToolResult,

ListToolsResult,

PaginatedRequestParams,

TextContent,

Tool,

)

# MCP defines no "busy" error, so this server picks its own code.

SERVER_BUSY = 1

async def on_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:

return ListToolsResult(

tools=[

Tool(

name="search_books",

description="Search the catalog by title or author.",

input_schema={

"type": "object",

"properties": {"query": {"type": "string"}},

"required": ["query"],

)

]

)

async def on_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:

query = (params.arguments or {})["query"]

return CallToolResult(content=[TextContent(type="text", text=f"Found 3 books matching {query!r}.")])

def max_concurrent_tool_calls(limit: int) -> ServerMiddleware[Any]:

running = 0

async def middleware(ctx: ServerRequestContext, call_next: CallNext) -> HandlerResult:

nonlocal running

if ctx.method != "tools/call":

return await call_next(ctx)

if running >= limit:

raise MCPError(code=SERVER_BUSY, message=f"Server busy: tool call limit reached ({limit} in progress).")

running += 1

try:

return await call_next(ctx)

finally:

running -= 1

return middleware

server = Server("Bookshop", on_list_tools=on_list_tools, on_call_tool=on_call_tool)

server.middleware.append(max_concurrent_tool_calls(4))

python-sdk/docs_src/middleware/tutorial002.py at main · modelcontextprotocol/python-sdk · GitHub

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