Originally published on AI Tech Connect.
What changed Two models, one day. Claude Fable 5.1 (claude-fable-5-1) and Claude Mythos 5.1 (claude-mythos-5-1) shipped on 1 September 2026. Mythos 5.1 is the same set of capabilities, restricted to Project Glasswing participants. One price moved. Cache reads went from $1.00 to $0.25 per million tokens — a 75% cut. Base input, both cache-write tiers, output and batch pricing are all unchanged. Forced tool use is gone. A tool_choice of {"type":"any"} or {"type":"tool","name":"..."} now returns a 400. Thinking blocks travel forwards only. Fable 5.1 can read earlier models' thinking; no earlier model can read Fable 5.1's. History is effectively append-only. Editing anything before a Fable 5.1 thinking block invalidates that block and every block after it. Specs. A 1M-token context window as…
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