Summary

jsonable_encoder forwards exclude_defaults to its recursive calls for list/tuple/set items, but omits it when recursing into dict keys and values. So a Pydantic model nested inside a dict keeps its default-valued fields under exclude_defaults=True, while the same model inside a list correctly drops them:

from pydantic import BaseModel
from fastapi.encoders import jsonable_encoder

class M(BaseModel):
    foo: str
    bar: str = "bar"

m = M(foo="foo", bar="bar")
jsonable_encoder([m], exclude_defaults=True)        # [{'foo': 'foo'}]                      ✅
jsonable_encoder({"m": m}, exclude_defaults=True)   # {'m': {'foo': 'foo', 'bar': 'bar'}}    ❌ default kept

The sibling flags exclude_unset and exclude_none are already forwarded in that same dict branch, so this is a copy/paste inconsistency rather than intended behavior. (include/exclude are intentionally top-level-only and are correctly not forwarded.)

Fix

Forward exclude_defaults in the two recursive jsonable_encoder calls inside the dict branch (for encoded_key and encoded_value), matching how the list/tuple/set branch already does it.

Test plan

Added test_encode_model_with_default_in_container to tests/test_jsonable_encoder.py, asserting a model-in-dict behaves the same as model-in-list under exclude_defaults / exclude_unset.

# fails on master (dict keeps defaults), passes with this change:
python -m pytest tests/test_jsonable_encoder.py -q
# 27 passed
ruff check fastapi/encoders.py tests/test_jsonable_encoder.py   # All checks passed!
ruff format fastapi tests --check                                # clean

No behavior change for the already-correct list/tuple/set path; only the dict path is brought into line.

Disclosure: authored with the assistance of an AI tool; fully reviewed and verified by me.