400 "thinking.type.disabled" is not supported when turning thinking off on Opus 5.5 or Sonnet 5.5

Symptom

"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.To turn thinking off on this model, send "thinking": {"type": "between_tools"} instead of {"type": "disabled"}. (Sonnet 5.5)output_config.effort 'xhigh' is not supported when thinking is disabled on this model. Use effort 'high' or below, or enable thinking.

Cause

Opus 5.5, Fable 5 and 5.1, and Mythos 5 and 5.1 have thinking always on and reject type disabled at any effort. Sonnet 5.5 rejects disabled too and takes between_tools instead, while Opus 5 and Haiku 5.5 accept disabled only at effort high or below.

Fix

On always-on models omit thinking and lower output_config.effort. On Sonnet 5.5 send between_tools at effort high or below. Check the per-model table before sending disabled.

ApplyCopy

// Opus 5.5, Fable 5 and 5.1, Mythos 5 and 5.1: omit thinking entirely output_config: { effort: "low" } // Sonnet 5.5: lowest thinking setting thinking: { type: "between_tools" }, output_config: { effort: "high" }

Affected

Claude Opus 5.5, Fable 5 and 5.1, Mythos 5 and 5.1, Sonnet 5.5 at any effort; Opus 5 and Haiku 5.5 at effort xhigh or max; as of 2026-10

Sources

• https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting

Last verified 2026-10-15 · Confirmed by 0 builders