Empty body sent to POST that expects JSON, if all parameters are omitted

[hniksic]

Which Fern component?

SDK Generator

How urgent is this?

P0 - Critical (Blocking work)

What's the issue?

Summary: When you define a POST endpoint that accepts a JSON body, and all its fields are optional, fern-generated SDK sends empty body to the endpoint, instead of sending the expected {}.

Details:

We define an OpenAPI schema with a POST endpoint that accepts a JSON object with all fields optional. Something like:

"requestBody": {
  "content": {
    "application/json": {
      "schema": {
        "type": "object",
        "properties": {
          "ids": {
            "description": "...",
            "type": ["array", "null"],
            "items": {
              "type": "integer",
              "format": "uint32",
              "minimum": 0.0
            }
          },
          "end_date": {
            "description": "...",
            "type": "string",
            "format": "date"
          },
          ...
        }
      }
    }
  },
  "required": true
},
...

Note, in particular, that none of the fields is marked as required, so they're all optional. The generated Python API respects that, and creates a function such as:

    def search_campaigns(
        self,
        *,
        ids: typing.Optional[typing.Sequence[int]] = OMIT,
        end_date: typing.Optional[str] = OMIT,
        ...
    ) -> PaginatedForCampaign:

Calling the function works as expected as long as I pass at least one parameter. But, if I call it without parameters, the backend complains of EOF while parsing JSON in request body. Adding further debugging to the backend shows that, indeed, an empty (zero-bytes) body is being sent, which is not valid JSON.

That's a bit surprising, because the generated code certainly looks like it's doing the right thing:

        _response = self._client_wrapper.httpx_client.request(
            "campaigns/processed/campaign",
            method="POST",
            json={
                "ids": ids,
                "end_date": end_date,
                ...
            },
            headers={
                "content-type": "application/json",
            },
            request_options=request_options,
            omit=OMIT,
        )

However, HttpClient.request generates JSON body by calling:

json_body, data_body = get_request_body(json=json, data=data, request_options=request_options, omit=omit)

and get_request_body does this:

    json_body = None
    data_body = None
    if data is not None:
        data_body = maybe_filter_request_body(data, request_options, omit)
    else:
        # If both data and json are None, we send json data in the event extra properties are specified
        json_body = maybe_filter_request_body(json, request_options, omit)

    # If you have an empty JSON body, you should just send None
    return (json_body if json_body != {} else None), data_body if data_body != {} else None

The last line is causing the issue. "Just sending None" means sending empty POST body instead of the expected {}. An empty document isn't even JSON, and this will cause problems on any POST endpoint that parses JSON with a JSON parser. For example, in Python:

>>> import json
>>> json.loads('{}')
{}
>>> json.loads('')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/ibz5rabb9z7hl5vy4w57q8dil5fqysjm-python3-3.11.11/lib/python3.11/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/ibz5rabb9z7hl5vy4w57q8dil5fqysjm-python3-3.11.11/lib/python3.11/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/ibz5rabb9z7hl5vy4w57q8dil5fqysjm-python3-3.11.11/lib/python3.11/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Could this behavior be changed? Or, if it is needed to make something else work, could it be made optional?

Fern CLI & Generator Versions

Fern CLI version: 0.57.17