Claude Mythos Preview

Claude Mythos Preview (API name claude-mythos-preview) is the invitation-only research preview that preceded Claude Mythos 5. It was never a public product: access ran through Project Glasswing, Anthropic's gated programme for approved customers, and the model has no launch post, no benchmark table and no published price of its own. What documents it is Anthropic's own migration guide, which now lists it as deprecated and points users at claude-mythos-5. Technically it already had the shape of the family: a one-million-token context window by default and up to 128,000 output tokens per request, sharing a tokenizer with Mythos 5 and Fable 5 — Anthropic states that token counts come out roughly unchanged when moving between the three. The interesting differences are in control. On the preview a developer still steered reasoning by hand, enabling extended thinking with an explicit budget_tokens value; on Mythos 5 that call returns a 400 error, because thinking is adaptive and always on. Assistant prefill was already refused on the preview, and it stays refused. Like every Claude from Opus 4.7 onward, the preview rejects temperature, top_p and top_k at non-default values with a 400 error — sampling knobs are simply gone from this generation, replaced by prompting. The profile exists here because a catalogue that only lists finished products misreads how frontier models actually reach the world. This one reached a closed list of customers first, ran long enough to acquire a documented migration path, and is now being wound down — the ordinary life cycle of a restricted-access model, which the public rarely gets to see. Anthropic has not published a retirement date for it. Claude Mythos 5 has its own profile in this catalogue; older versions are never removed here.

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