isReasoningModel matched the literal "gpt-5" prefix plus the GPT-6 Sol/Luna
spellings, so gpt-6 and gpt-6-astra (both already in the model catalog and
price list) fell into the sampling branch. Anthropic Messages and Chat
Completions requests converted to the Responses API then forwarded
temperature/top_p, which the Responses API rejects for these models with
"Unsupported parameter".
Key the check on the parsed GPT generation number instead, keeping the
existing Sol/Luna spelling match, so later families are covered without
another edit. Non-generation GPT families (gpt-image-1, gpt-audio) and
gpt-4.x are unaffected.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>