extended_thinking="off" does not actually disable thinking on Claude Opus 5 / Sonnet 5

This issue was posted by Claude Code using claude-opus-4-8 on behalf of David.

Reported from an adversarially-verified review; David directed the review and the finding was confirmed against the current main. Issue text is AI-drafted.

Where: code_puppy/model_utils.py → resolve_anthropic_thinking_payload

What. if extended_thinking not in ("enabled", "adaptive"): return None, and make_model_settings only sets anthropic_thinking when the payload is non-None. pydantic-ai's AnthropicModel._translate_thinking then returns OMIT (verified: if thinking is None or thinking is False: return OMIT), so the thinking parameter is absent from the wire request. The function never emits {"type": "disabled"} for any model.

Impact. A user on claude-opus-5 or claude-sonnet-5 who selects Extended Thinking = "off" in /model_settings (choices are ["enabled", "adaptive", "off"]) still gets adaptive thinking on every request and pays for the thinking tokens, with no error and no UI signal. It also interacts with the max_tokens cap: thinking and response text share that budget, so a turn sized for a thinking-free answer can truncate mid-response.

Suggested fix. Return {"type": "disabled"} when the target model's profile reports anthropic_supports_adaptive_thinking, and keep returning None only for classic models where omission genuinely means off. Clamp effort to high when thinking is disabled on Opus 5 — pydantic-ai encodes this as anthropic_disallows_top_effort_when_thinking_disabled and raises a UserError for xhigh/max. Fable 5 rejects {"type":"disabled"} at any effort, so that family must keep the omit behaviour.

Start in code_puppy/model_utils.py at resolve_anthropic_thinking_payload, then trace make_model_settings and pydantic-ai's AnthropicModel._translate_thinking behavior. Verify how the Opus 5, Sonnet 5, classic, and Fable 5 profiles handle disabled thinking and effort limits. Done means "off" is represented correctly for supported models without breaking classic or Fable behavior.