Add a client extension API (#3034)

src/mcp/server/extension.py

@@ -19,7 +19,6 @@ extensions remain importable without constructing an `MCPServer`.
 
 from __future__ import annotations
 
-import re
 from collections.abc import Awaitable, Callable, Sequence
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any
@@ -30,31 +29,14 @@ from pydantic import BaseModel
 
 from mcp.server.context import CallNext, HandlerResult, ServerMiddleware, ServerRequestContext
 
+# Re-exported from `mcp.shared.extension` (shared with the client surface) for existing importers.
+from mcp.shared.extension import validate_extension_identifier as validate_extension_identifier
+
 if TYPE_CHECKING:
     from mcp.server.mcpserver.resources import Resource
 
 RequestHandler = Callable[[ServerRequestContext[Any, Any], Any], Awaitable[HandlerResult]]
 
-# Extension identifiers follow the `_meta` key grammar with a mandatory prefix
-# (SEP-2133 / basic/index.mdx): dot-separated labels, each starting with a
-# letter and ending with a letter or digit (hyphens interior), then `/`, then a
-# name that starts and ends alphanumeric (`.`/`_`/`-` interior).
-_LABEL = r"[A-Za-z](?:[A-Za-z0-9-]*[A-Za-z0-9])?"
-_NAME = r"[A-Za-z0-9](?:[A-Za-z0-9._-]*[A-Za-z0-9])?"
-_IDENTIFIER_RE = re.compile(rf"{_LABEL}(?:\.{_LABEL})*/{_NAME}")
-
-
-def validate_extension_identifier(identifier: Any, *, owner: str) -> None:
-    """Raise `TypeError` unless `identifier` is a `vendor-prefix/name` string.
-
-    SEP-2133 requires extension identifiers to carry a reverse-DNS prefix.
-    """
-    if not isinstance(identifier, str) or not _IDENTIFIER_RE.fullmatch(identifier):
-        raise TypeError(
-            f"{owner}.identifier must be a `vendor-prefix/name` string "
-            f"(reverse-DNS prefix required), got {identifier!r}"
-        )
-
 
 @dataclass(frozen=True)
 class ToolBinding: