Claude Fable 5 and Claude Mythos 5 support only adaptive thinking mode.

Builders must remove disabled thinking, manual thinking budgets, and assistant prefill or requests return 400 errors.

For migrations from Claude Mythos Preview, integrations need adaptive thinking only and no assistant prefill.

• Jun 11, 2026OpenAI modelcard diff shows the API docs navigation content removed.
• Jun 11, 2026OpenAI pricing page removed visible API pricing content for GPT-5.5, GPT-5.4, and realtime.
• Jun 11, 2026Anthropic launched Claude Fable 5 and Claude Mythos 5 with 1M context and 128k max output.

This is the world we build in. It moves this fast. We move with it.