effort "off" does not disable thinking on Sonnet 5 / Opus 5 / Fable (DisableWire::None omits the parameter, which no longer means off)

[chrismurrph]

Claim

From the 0.24.1 changelog, introducing /effort:

default clears the override; off is a real level that disables reasoning.

On current Anthropic models it does not. effort: "off" (and /effort off) leaves thinking fully enabled on claude-sonnet-5, claude-opus-5 and claude-fable-5, so a user who explicitly opts out still pays the latency and the reasoning tokens — silently, on the one setting whose whole purpose is opting out.

Cause

adapter.rs gives each provider a DisableWire describing how thinking is turned off. Anthropic's is None — "omit the parameter":

("anthropic", EffortWire::AnthropicBudget, DisableWire::None),

Omitting thinking was a correct disable for Claude 4.6 and earlier. It stopped being one on the current generation:

Model	thinking omitted
Sonnet 4.6 / Opus 4.6	no thinking — omit still works
Sonnet 5	thinks (adaptive by default)
Opus 5	thinks (adaptive by default)
Fable 5	always thinks, cannot be disabled

So the wire shape didn't change; what omitting it means did.

The payload already exists in the codebase

Anthropic is one of only three providers left on DisableWire::None; five others carry real disable wires (ThinkingToggle, ChatTemplateKwargs, GeminiZeroBudget, OllamaThink).

Notably GLM — which mimics Anthropic's API — is on ThinkingToggle, which emits:

{"thinking": {"type": "disabled"}}

That is Anthropic's own documented shape. dirge already sends exactly the payload Anthropic needs; it just doesn't send it to Anthropic.

Why the fix is not a one-line table change

Model support is uneven, so a blanket switch to ThinkingToggle would trade this bug for 400s:

• Sonnet 5 accepts {"type": "disabled"}.
• Opus 5 accepts it only at effort high or below — pairing it with xhigh/max is rejected. Since off means no thinking at all, that combination shouldn't arise, but the constraint is worth encoding rather than assuming.
• Fable 5 rejects it outright — thinking is unconditional there. off can never be honoured; warning that the level is unavailable seems more honest than silently accepting it.
• Claude 4.6 and earlier must keep the current omit-the-parameter behaviour.

So it wants to be per-model, not per-provider — which may mean DisableWire needs a model-aware variant, or the choice moves to where the model id is known.

Impact

This is what forces a non-thinking tier to be Haiku 4.5. There is currently no way to get a fast, non-thinking turn out of Opus 5 or Sonnet 5 through dirge, even though the API supports it.

Happy to put up a PR if the shape above looks right — say if you'd rather have it structured differently (e.g. keep DisableWire per-provider and gate on the model at the call site).