Reports

could you tell me why it doesn't work when passed as JSON?

When you register a route with

@app.post('/ints')
def post_int(x: int, y: int):
    return x, y

in FastApi, the endpoint is added in

decorator (routing.py)
add_api_route (routing.py)
__init__ (routing.py)
get_dependant (dependencies/utils.py)
analyze_param (dependencies/utils.py)

Near the end of analyze_param, you can find

if is_path_param:
    # We might check here that `default_value is RequiredParam`, but the fact is that the same
    # parameter might sometimes be a path parameter and sometimes not. See
    # `tests/test_infer_param_optionality.py` for an example.
    field_info = params.Path(annotation=use_annotation)
elif is_uploadfile_or_nonable_uploadfile_annotation(
    type_annotation
) or is_uploadfile_sequence_annotation(type_annotation):
    field_info = params.File(annotation=use_annotation, default=default_value)
elif not field_annotation_is_scalar(annotation=type_annotation):
    field_info = params.Body(annotation=use_annotation, default=default_value)
else:
    field_info = params.Query(annotation=use_annotation, default=default_value)

https://github.com/fastapi/fastapi/blob/master/fastapi/dependencies/utils.py#L460

There you can see that scalar parameters have field info type query. Lists aren't scalar values and have field info type body.

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