Add a client extension API (#3034)

docs_src/apps/tutorial001.py

@@ -1,4 +1,5 @@
 from mcp import Client
+from mcp.client import advertise
 from mcp.server.apps import APP_MIME_TYPE, EXTENSION_ID, Apps, client_supports_apps
 from mcp.server.mcpserver import MCPServer
 from mcp.server.mcpserver.context import Context
@@ -32,7 +33,7 @@ mcp = MCPServer("clock", extensions=[apps])
 
 
 async def main() -> None:
-    async with Client(mcp, extensions={EXTENSION_ID: {"mimeTypes": [APP_MIME_TYPE]}}) as client:
+    async with Client(mcp, extensions=[advertise(EXTENSION_ID, {"mimeTypes": [APP_MIME_TYPE]})]) as client:
         result = await client.call_tool("get_time", {})
         print(result.content)
         # [TextContent(text='2026-06-26T12:00:00Z')]

docs_src/extensions/tutorial004.py

@@ -1,10 +1,11 @@
 from collections.abc import Sequence
-from typing import Any, Literal, cast
+from typing import Any, Literal
 
 import mcp_types as types
 from pydantic import Field
 
 from mcp import Client
+from mcp.client import advertise
 from mcp.server.context import ServerRequestContext
 from mcp.server.extension import Extension, MethodBinding
 from mcp.server.mcpserver import MCPServer, require_client_extension
@@ -51,8 +52,8 @@ mcp = MCPServer("catalog", extensions=[Search()])
 
 
 async def main() -> None:
-    async with Client(mcp, extensions={EXTENSION_ID: {}}) as client:
+    async with Client(mcp, extensions=[advertise(EXTENSION_ID)]) as client:
         request = SearchRequest(params=SearchParams(query="mcp", limit=3))
-        result = await client.session.send_request(cast("types.ClientRequest", request), SearchResult)
+        result = await client.session.send_request(request, SearchResult)
         print(result.items)
         # ['mcp-0', 'mcp-1', 'mcp-2']

docs_src/extensions/tutorial006.py

@@ -0,0 +1,70 @@
+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')]

docs_src/extensions/tutorial007.py

@@ -0,0 +1,50 @@
+from collections.abc import Sequence
+from typing import Any, Literal
+
+import mcp_types as types
+
+from mcp import Client
+from mcp.client import advertise
+from mcp.server.context import ServerRequestContext
+from mcp.server.extension import Extension, MethodBinding
+from mcp.server.mcpserver import MCPServer
+
+EXTENSION_ID = "com.example/jobs"
+
+
+class JobParams(types.RequestParams):
+    job_id: str
+
+
+class JobStatus(types.Result):
+    status: str
+
+
+class JobStatusRequest(types.Request[JobParams, Literal["com.example/jobs.status"]]):
+    method: Literal["com.example/jobs.status"] = "com.example/jobs.status"
+    params: JobParams
+    name_param = "jobId"  # params["jobId"] rides the Mcp-Name header
+
+
+async def job_status(ctx: ServerRequestContext[Any, Any], params: JobParams) -> JobStatus:
+    return JobStatus(status=f"{params.job_id} is running")
+
+
+class Jobs(Extension):
+    """An extension whose verb names its subject, so the header can route on it."""
+
+    identifier = EXTENSION_ID
+
+    def methods(self) -> Sequence[MethodBinding]:
+        return [MethodBinding("com.example/jobs.status", JobParams, job_status)]
+
+
+mcp = MCPServer("worker", extensions=[Jobs()])
+
+
+async def main() -> None:
+    async with Client(mcp, extensions=[advertise(EXTENSION_ID)]) as client:
+        request = JobStatusRequest(params=JobParams(job_id="job-7"))
+        result = await client.session.send_request(request, JobStatus)
+        print(result.status)
+        # job-7 is running