Fantasy's hardcoded responsesModelIDs list gates whether a model uses
the Responses API or Chat Completions code path. When a new model
(e.g. gpt-5.5) is added via `kit update-models` but fantasy hasn't
been updated yet, the type mismatch between *ResponsesProviderOptions
and *ProviderOptions causes a crash.
- Add isResponsesAPIModel()/isResponsesReasoningModel() helpers that
supplement fantasy's checks with prefix-based heuristics for modern
OpenAI model families (gpt-4.1+, gpt-5+, o-series, codex, chatgpt)
- Add RegisterResponsesModels() using go:linkname to append missing
model IDs from our database into fantasy's internal slices at init
time and after ReloadGlobalRegistry()
- Replace all direct openai.IsResponsesModel/IsResponsesReasoningModel
calls in providers.go with the new helpers
- Merge embedded + cached model databases instead of cache-only fallback
- Bump fantasy v0.19.0 -> v0.20.0 to match existing import usage
- Document the technique and model-family update process in AGENTS.md
Ed Zynda