Repository navigation
Tool outputSchema is generated in validation mode, so serialization aliases and computed fields don't match structuredContent #3644
Description
On v2 (2118f14), a tool returning a model with a serialization_alias or a @computed_field advertises an outputSchema that its own structuredContent violates:
from pydantic import BaseModel, Field, computed_field
from mcp.server.mcpserver.utilities.func_metadata import func_metadata
class Out(BaseModel):
user_id: int = Field(serialization_alias="userId")
@computed_field
@property
def doubled(self) -> int:
return self.user_id * 2
def tool() -> Out:
return Out(user_id=3)
m = func_metadata(tool)
print(sorted(m.output_schema["properties"])) # ['user_id']
print(sorted(m.convert_result(Out(user_id=3)).structured_content)) # ['doubled', 'userId']Clients that validate structuredContent against outputSchema (as the spec says they should) reject these results. camelCase serialization aliases are common for JS-facing tools, so this hits anyone using them.
I had a fix for this in #3118 (closed in the v2 cleanup) and can port it to v2 with a regression test if you'd like to assign this to me.
Activity
jayzuccarelli commentedon Oct 6, 2026
@maxisbey since this is labeled for both v1 and v2: the fix I'd port is small. Generate the output schema with
mode="serialization"inFuncMetadata.model_post_init, and when a tool returns its ownCallToolResult, dump the validatedstructured_contentwithby_alias=Trueso it matches that schema. Regression test with a serialization alias and a computed field, on v2 first. Happy to take it if you assign it to me.Kludex commentedon Oct 10, 2026
This is the same validation-versus-serialization
outputSchemamismatch tracked in #3100, including serialization aliases and computed fields. Closing as a duplicate. AI-assisted triage; I reviewed both reports.