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

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

docs_src/extensions/tutorial005.py

@@ -0,0 +1,34 @@
+import logging
+from typing import Any
+
+from mcp_types import CallToolRequestParams
+
+from mcp.server.context import CallNext, HandlerResult, ServerRequestContext
+from mcp.server.extension import Extension
+from mcp.server.mcpserver import MCPServer
+
+logger = logging.getLogger(__name__)
+
+
+class AuditLog(Extension):
+    """Observe every tools/call without touching its result."""
+
+    identifier = "com.example/audit"
+
+    async def intercept_tool_call(
+        self,
+        params: CallToolRequestParams,
+        ctx: ServerRequestContext[Any, Any],
+        call_next: CallNext,
+    ) -> HandlerResult:
+        logger.info("tool %r called", params.name)
+        return await call_next(ctx)
+
+
+mcp = MCPServer("audited", extensions=[AuditLog()])
+
+
+@mcp.tool()
+def add(a: int, b: int) -> int:
+    """Add two numbers."""
+    return a + b