Answers a query's format: option for InfluxElixir.Client.Local as Client.HTTP answers it from InfluxDB 3 (verified against Core):

• :json / :jsonl (the default) — the rows as they are
• :csv — the rows as the engine's CSV parses: every value a string, rendered as the engine renders it (1.5 → "1.5", 1.0e16 → "1e16", true → "true"), an empty value absent like a null one; timestamps stay DateTimes. The engine cannot write a nested value (array_agg, a selector_* struct) as CSV: it has sent 200 and closes the connection, so the answer is the same {:connection_error, %Mint.TransportError{reason: :closed}}
• :parquet — refused by name: the double holds no Parquet writer
• :pretty / :json_lines and their string forms — the engine accepts them but the client cannot parse them: {:unsupported_format, format}
• anything else — the engine's 400 for an unknown variant

The engine reads format with the request, before it plans the query: an unknown format is its 400 whatever the query, as is the refusal of :parquet here; the others are answered from the query's result.