Problem

Claude Fable 5 (released 2026-06-09) uses a new thinking API contract that OpenClaw's current Anthropic messages adapter does not support.

Fable 5 requires thinking.type = "adaptive" with output_config.effort to control thinking behavior. It rejects both thinking.type = "enabled" (extended thinking) and thinking.type = "disabled" which are the only two modes the current adapter sends.

Error messages from API

• With thinking=low: "thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
• With thinking=off: "thinking.type.disabled" is not supported for this model. Thinking defaults to adaptive mode when not specified.

What needs to change

The Anthropic messages adapter needs to support a third thinking mode: thinking.type = "adaptive" + output_config.effort for models that only support adaptive thinking.

The model entry config may also need a field to declare the thinking type a model supports (adaptive-only vs enabled/disabled).

Workaround

None currently. The model resolves correctly but all requests fail at the API level due to the thinking parameter mismatch.