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from collections.abc import Sequence
from typing import Any, Literal
import mcp_types as types
from mcp import Client
from mcp.client import ClaimContext, ClientExtension, ResultClaim
from mcp.server.context import CallNext, HandlerResult, ServerRequestContext
from mcp.server.extension import Extension
from mcp.server.mcpserver import MCPServer, require_client_extension
EXTENSION_ID = "com.example/receipts"
class ReceiptResult(types.Result):
"""The claimed result shape; `result_type` pins the wire tag."""
result_type: Literal["receipt"] = "receipt"
receipt_token: str
class ReceiptIssuer(Extension):
"""Server half: answers `buy` with a receipt instead of a final result."""
identifier = EXTENSION_ID
async def intercept_tool_call(
self,
params: types.CallToolRequestParams,
ctx: ServerRequestContext[Any, Any],
call_next: CallNext,
) -> HandlerResult:
if params.name != "buy":
return await call_next(ctx)
require_client_extension(ctx, EXTENSION_ID)
return {"resultType": "receipt", "receiptToken": "r-117"}
class Receipts(ClientExtension):
"""Client half: claims the `receipt` shape and supplies the code that finishes it."""
identifier = EXTENSION_ID
def claims(self) -> Sequence[ResultClaim[Any]]:
return [ResultClaim(result_type="receipt", model=ReceiptResult, resolve=self._redeem)]
async def _redeem(self, claimed: ReceiptResult, ctx: ClaimContext) -> types.CallToolResult:
return await ctx.session.call_tool("redeem", {"token": claimed.receipt_token})
mcp = MCPServer("shop", extensions=[ReceiptIssuer()])
@mcp.tool()
def buy(item: str) -> types.CallToolResult:
"""Buy an item."""
raise NotImplementedError # ReceiptIssuer answers `buy` before the tool runs
@mcp.tool()
def redeem(token: str) -> str:
"""Exchange a receipt token for the goods."""
return f"goods for {token}"
async def main() -> None:
async with Client(mcp, extensions=[Receipts()]) as client:
result = await client.call_tool("buy", {"item": "lamp"})
print(result.content)
# [TextContent(text='goods for r-117')]