Add a pluggable server extension API with MCP Apps (#3003)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

docs_src/extensions/tutorial003.py

@@ -0,0 +1,35 @@
+from collections.abc import Sequence
+from typing import Any
+
+from mcp import Client
+from mcp.server.extension import Extension, ToolBinding
+from mcp.server.mcpserver import MCPServer
+
+
+def stamp(text: str) -> str:
+    """Stamp a message with the office seal."""
+    return f"[stamped] {text}"
+
+
+class Stamps(Extension):
+    """A purely additive extension: one tool, one capability entry."""
+
+    identifier = "com.example/stamps"
+
+    def settings(self) -> dict[str, Any]:
+        return {"sealed": True}
+
+    def tools(self) -> Sequence[ToolBinding]:
+        return [ToolBinding(fn=stamp)]
+
+
+mcp = MCPServer("post-office", extensions=[Stamps()])
+
+
+async def main() -> None:
+    async with Client(mcp) as client:
+        print(client.server_capabilities.extensions)
+        # {'com.example/stamps': {'sealed': True}}
+        result = await client.call_tool("stamp", {"text": "hello"})
+        print(result.content)
+        # [TextContent(text='[stamped] hello')]