refactor: Make URI fields required and block all MCP scheme usage in resources

- Changed Tool and Prompt URI fields from optional to required (auto-generated)
- Updated Resource validator to block any URI using the MCP scheme (mcp://)
- Removed unnecessary None checks in URI validators since fields are now required
- Updated tests to match new error message

This provides stronger type safety and clearer API contracts while maintaining
backward compatibility through auto-generation.

Author: Tapan Chugh

src/mcp/types.py

@@ -440,12 +440,10 @@ class Resource(BaseMetadata):
 
     @model_validator(mode="after")
     def validate_uri_scheme(self) -> "Resource":
-        """Ensure resource URI doesn't use reserved schemes."""
+        """Ensure resource URI doesn't use reserved MCP scheme."""
         uri_str = str(self.uri)
-        if uri_str.startswith((TOOL_SCHEME, PROMPT_SCHEME)):
-            raise ValueError(
-                f"Resource URI cannot use reserved schemes '{TOOL_SCHEME}' or '{PROMPT_SCHEME}', got: {self.uri}"
-            )
+        if uri_str.startswith(f"{MCP_SCHEME}://"):
+            raise ValueError(f"Resource URI cannot use reserved MCP scheme '{MCP_SCHEME}://', got: {self.uri}")
         return self
 
 
@@ -642,7 +640,7 @@ class PromptArgument(BaseModel):
 class Prompt(BaseMetadata):
     """A prompt or prompt template that the server offers."""
 
-    uri: Annotated[AnyUrl, UrlConstraints(allowed_schemes=[MCP_SCHEME], host_required=False)] | None = None
+    uri: Annotated[AnyUrl, UrlConstraints(allowed_schemes=[MCP_SCHEME], host_required=False)]
     """URI for the prompt. Auto-generated if not provided."""
     description: str | None = None
     """An optional description of what this prompt provides."""
@@ -664,10 +662,9 @@ def __init__(self, **data: Any) -> None:
     @model_validator(mode="after")
     def validate_prompt_uri(self) -> "Prompt":
         """Validate that prompt URI starts with the correct prefix."""
-        if self.uri is not None:
-            uri_str = str(self.uri)
-            if not uri_str.startswith(f"{PROMPT_SCHEME}/"):
-                raise ValueError(f"Prompt URI must start with {PROMPT_SCHEME}/")
+        uri_str = str(self.uri)
+        if not uri_str.startswith(f"{PROMPT_SCHEME}/"):
+            raise ValueError(f"Prompt URI must start with {PROMPT_SCHEME}/")
         return self
 
 
@@ -878,7 +875,7 @@ class ToolAnnotations(BaseModel):
 class Tool(BaseMetadata):
     """Definition for a tool the client can call."""
 
-    uri: Annotated[AnyUrl, UrlConstraints(allowed_schemes=[MCP_SCHEME], host_required=False)] | None = None
+    uri: Annotated[AnyUrl, UrlConstraints(allowed_schemes=[MCP_SCHEME], host_required=False)]
     """URI for the tool. Auto-generated if not provided."""
     description: str | None = None
     """A human-readable description of the tool."""
@@ -907,10 +904,9 @@ def __init__(self, **data: Any) -> None:
     @model_validator(mode="after")
     def validate_tool_uri(self) -> "Tool":
         """Validate that tool URI starts with the correct prefix."""
-        if self.uri is not None:
-            uri_str = str(self.uri)
-            if not uri_str.startswith(f"{TOOL_SCHEME}/"):
-                raise ValueError(f"Tool URI must start with {TOOL_SCHEME}/")
+        uri_str = str(self.uri)
+        if not uri_str.startswith(f"{TOOL_SCHEME}/"):
+            raise ValueError(f"Tool URI must start with {TOOL_SCHEME}/")
         return self
 
 

tests/test_types.py

@@ -58,11 +58,11 @@ def test_resource_uri():
     assert resource.name == "test"
     assert str(resource.uri) == "file://test.txt/"  # AnyUrl adds trailing slash
 
-    # Should reject TOOL_SCHEME and PROMPT_SCHEME schemes
-    with pytest.raises(ValueError, match="reserved schemes"):
+    # Should reject MCP scheme
+    with pytest.raises(ValueError, match="reserved MCP scheme"):
         Resource(name="test", uri=AnyUrl(f"{TOOL_SCHEME}/test"))
 
-    with pytest.raises(ValueError, match="reserved schemes"):
+    with pytest.raises(ValueError, match="reserved MCP scheme"):
         Resource(name="test", uri=AnyUrl(f"{PROMPT_SCHEME}/test"))