From 37f6a3229cb0c47f09af29e3b1396bca52395184 Mon Sep 17 00:00:00 2001
From: David Soria Parra <davidsp@anthropic.com>
Date: Sat, 21 Dec 2024 00:26:59 +0000
Subject: [PATCH] fix: use pydantic Field with alias for _meta fields

Pydantic treats fields starting with underscore as private/hidden. To fix this,
we need to use Field with alias='_meta' to properly handle these fields while
keeping the external API unchanged. This fixes #103 where meta fields were not
being properly assigned in request contexts.
---
 src/mcp/shared/session.py | 2 +-
 src/mcp/types.py          | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/mcp/shared/session.py b/src/mcp/shared/session.py
index 0d0905e..5e114ec 100644
--- a/src/mcp/shared/session.py
+++ b/src/mcp/shared/session.py
@@ -221,7 +221,7 @@ class BaseSession(
                     )
                     responder = RequestResponder(
                         request_id=message.root.id,
-                        request_meta=validated_request.root.params._meta
+                        request_meta=validated_request.root.params.meta
                         if validated_request.root.params
                         else None,
                         request=validated_request,
diff --git a/src/mcp/types.py b/src/mcp/types.py
index a2b8974..2c27f83 100644
--- a/src/mcp/types.py
+++ b/src/mcp/types.py
@@ -1,6 +1,6 @@
 from typing import Any, Generic, Literal, TypeVar
 
-from pydantic import BaseModel, ConfigDict, FileUrl, RootModel
+from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel
 from pydantic.networks import AnyUrl
 
 """
@@ -39,14 +39,14 @@ class RequestParams(BaseModel):
 
         model_config = ConfigDict(extra="allow")
 
-    _meta: Meta | None = None
+    meta: Meta | None = Field(alias="_meta", default=None)
 
 
 class NotificationParams(BaseModel):
     class Meta(BaseModel):
         model_config = ConfigDict(extra="allow")
 
-    _meta: Meta | None = None
+    meta: Meta | None = Field(alias="_meta", default=None)
     """
     This parameter name is reserved by MCP to allow clients and servers to attach
     additional metadata to their notifications.
@@ -86,7 +86,7 @@ class Result(BaseModel):
 
     model_config = ConfigDict(extra="allow")
 
-    _meta: dict[str, Any] | None = None
+    meta: dict[str, Any] | None = Field(alias="_meta", default=None)
     """
     This result property is reserved by the protocol to allow clients and servers to
     attach additional metadata to their responses.