> For clean Markdown content of this page, append .md to this URL. For the complete documentation index, see https://buildwithfern.com/learn/llms.txt.

## 5.29.3

**`(fix):`** Cache Pydantic field alias metadata per model class instead of rebuilding the field name to alias maps on
every validation. The `mode="before"` validator now skips copying the input mapping when no key needs to be
rewritten, and `parse_obj_as` reuses the cached metadata instead of rescanning fields on each call. This
roughly halves the parsing cost of streaming responses, where the validator runs once per nested model per
event. As a result the validator can now hand the caller's own `dict` to Pydantic rather than a copy of it;
Pydantic does not mutate or retain that mapping, so parsed models are unaffected.