OpenAI provider incorrectly maps params(top_k=) to top_logprobs

[cpsievert]

Summary

chatlas/_provider_openai.py maps the standardized top_k chat parameter onto OpenAI's top_logprobs field:

# chatlas/_provider_openai.py:596-597
if "top_k" in params:
    res["top_logprobs"] = params["top_k"]

These are unrelated parameters — top_k controls sampling (limiting the candidate token pool), while top_logprobs controls how many log-probabilities are returned per token. Conflating them means setting top_k silently changes response metadata (logprobs count) instead of doing anything related to sampling, and vice versa.

ellmer just fixed the equivalent bug across its OpenAI-based providers (tidyverse/ellmer#1115, tidyverse/ellmer#1113): top_k is no longer sent as top_logprobs for chat_openai(), chat_deepseek(), or the generic OpenAI-compatible provider.

Expected fix

Stop mapping top_k → top_logprobs in chatlas/_provider_openai.py (and check chatlas/_provider_openai_completions.py / other OpenAI-compatible providers for the same pattern). Since OpenAI's API doesn't support top_k for sampling, the standardized top_k param should likely be dropped/warned-on for OpenAI-based providers rather than silently repurposed, matching ellmer's resolution.

References

• ellmer PR: Don't map top_k to top_logprobs tidyverse/ellmer#1115 (issue: params(top_k=) is sent as top_logprobs for OpenAI-compatible providers tidyverse/ellmer#1113)
• chatlas: chatlas/_provider_openai.py:596-597