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fix: handle field names with leading underscores in Pydantic model creation #219

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fix: handle field names with leading underscores in Pydantic model creation #219

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2aad7c9
fix: handle field names with leading underscores in Pydantic model cr…
clawtom Mar 12, 2026
7bfb987
fix: strip exactly one leading underscore to avoid field name collisions
clawtom Mar 13, 2026
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20 changes: 15 additions & 5 deletions src/mxcp/server/interfaces/server/mcp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1137,6 +1137,7 @@ def _create_pydantic_model_from_schema(

# Create fields for the model
model_fields = {}
has_aliased_fields = False
for prop_name, prop_schema in properties.items():
prop_type = self._create_pydantic_model_from_schema(
prop_schema,
Expand All @@ -1147,19 +1148,28 @@ def _create_pydantic_model_from_schema(
# Extract field constraints for this property
field_kwargs = self._extract_field_constraints(prop_schema)

# Pydantic v2 rejects field names starting with underscores (treats them as
# private attributes). Map underscore-prefixed names to a safe Python identifier
# and record the original name via Field(alias=...) so validation still works.
python_name = prop_name
if prop_name.startswith("_"):
python_name = "field_" + prop_name[1:] # strip exactly one underscore to avoid name collisions
field_kwargs["alias"] = prop_name
has_aliased_fields = True

@cursor cursor Bot Mar 13, 2026

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Silent field collision with field_ prefix mapping

Low Severity

When an underscore-prefixed property like _name is renamed to field_name, it can silently collide with an existing non-underscore property also called field_name in the same schema. Both would map to the same key in model_fields, causing one to silently overwrite the other and be dropped from the generated model. No collision detection or error is raised.

Fix in Cursor Fix in Web


# Handle required vs optional fields
if prop_name in required_fields:
if field_kwargs:
model_fields[prop_name] = (prop_type, Field(**field_kwargs))
model_fields[python_name] = (prop_type, Field(**field_kwargs))
else:
model_fields[prop_name] = (prop_type, ...)
model_fields[python_name] = (prop_type, ...)
else:
# Optional field
if field_kwargs:
default = field_kwargs.pop("default", None)
model_fields[prop_name] = (prop_type | None, Field(default, **field_kwargs))
model_fields[python_name] = (prop_type | None, Field(default, **field_kwargs))
else:
model_fields[prop_name] = (prop_type | None, None)
model_fields[python_name] = (prop_type | None, None)

# Create the model with proper configuration

Expand All @@ -1172,7 +1182,7 @@ def _create_pydantic_model_from_schema(

final_type = create_model( # type: ignore[call-overload]
self._sanitize_model_name(model_name),
__config__=ConfigDict(extra=extra_mode),
__config__=ConfigDict(extra=extra_mode, populate_by_name=has_aliased_fields),
**model_fields,
)

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