Validate metadata keys in types.RequestParams.Meta

Author: samchenatti

src/mcp/types.py

@@ -1,7 +1,8 @@
 from collections.abc import Callable
 from typing import Annotated, Any, Generic, Literal, TypeAlias, TypeVar
+import re
 
-from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel
+from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel, model_validator
 from pydantic.networks import AnyUrl, UrlConstraints
 from typing_extensions import deprecated
 
@@ -52,6 +53,49 @@ class Meta(BaseModel):
 
         model_config = ConfigDict(extra="allow")
 
+        @model_validator(mode="before")
+        @classmethod
+        def validate_metadata_keys(cls, data: Any) -> Any:
+            """
+            Validate if metadata keys follows the protocol specification
+            See section "General fields" at https://modelcontextprotocol.io/specification/
+            """
+            for metadata_key in data.keys():
+                key_parts = metadata_key.split("/")
+
+                match len(key_parts):
+                    case 1:
+                        cls._validate_metadata_name(key_parts[0])
+
+                    case 2:
+                        cls._validate_metadata_prefix(key_parts[0])
+                        cls._validate_metadata_name(key_parts[1])
+
+                    case _:
+                        raise ValueError(f"The metadata key {metadata_key} does not comply with MCP specification")
+
+            return data
+
+        @classmethod
+        def _validate_metadata_prefix(cls, prefix: str):
+            if len(prefix) == 0:
+                raise ValueError(
+                    "One of the metadata keys is empty, and therefore does not comply with MCP specification"
+                )
+
+            for label in prefix.split("."):
+                cls._validate_prefix_label(prefix=prefix, label=label)
+
+        @classmethod
+        def _validate_metadata_name(cls, name: str):
+            if re.match(r"^[a-zA-Z0-9]([a-zA-Z0-9-._]*[a-zA-Z0-9])?$", name) is None:
+                raise ValueError(f"The metadata name {name} does not comply with MCP specification")
+
+        @classmethod
+        def _validate_prefix_label(cls, label: str, prefix: str):
+            if re.match(r"^[a-zA-Z][a-zA-Z0-9-]*[a-zA-Z0-9]$", label) is None:
+                raise ValueError(f"The label {label} inside of prefix {prefix} does not comply with MCP specification")
+
     meta: Meta | None = Field(alias="_meta", default=None)
 
 

tests/types/__init__.py

@@ -0,0 +1,64 @@
+import pytest
+from mcp import types
+
+
+@pytest.mark.parametrize(
+    argnames="key",
+    argvalues=[
+        # Simple keys without reserved prefix
+        "clientId",
+        "request-id",
+        "api_version",
+        "product-id",
+        "x-correlation-id",
+        "my-key",
+        "info",
+        "data-1",
+        "label-key",
+        # Keys with reserved prefix
+        "modelcontextprotocol.io/request-id",
+        "mcp.dev/debug-mode",
+        "api.modelcontextprotocol.org/api-version",
+        "tools.mcp.com/validation-status",
+        "my-company.mcp.io/internal-flag",
+        "modelcontextprotocol.io/a",
+        "mcp.dev/b-c",
+        # Keys with non-reserved prefix
+        "my-app.com/user-preferences",
+        "internal.api/tracking-id",
+        "org.example/resource-type",
+        "custom.domain/status",
+    ],
+)
+def test_metadata_valid_keys(key: str):
+    """
+    Asserts that valid metadata keys does not raise ValueErrors
+    """
+    types.RequestParams.Meta(**{key: "value"})
+
+
+@pytest.mark.parametrize(
+    argnames="key",
+    argvalues=[
+        # Invalid key names (without prefix)
+        "-leading-hyphen",
+        "trailing-hyphen-",
+        "with space",
+        "key/with/slash",
+        "no@special-chars",
+        "...",
+        # Invalid prefixes
+        "mcp.123/key",
+        "my.custom./key",
+        "my-app.com//key",
+        # Invalid combination of prefix and name
+        "mcp.dev/-invalid",
+        "org.example/invalid-name-",
+    ],
+)
+def test_metadata_invalid_keys(key: str):
+    """
+    Asserts that invalid metadata keys raise ValueErrors
+    """
+    with pytest.raises(ValueError):
+        types.RequestParams.Meta(**{key: "value"})