Claude Fable 5.1 invalidates a thinking block the moment you edit an earlier turn, rebuild your system prompt, or compact a conversation. Claude Mythos 5.1 does not run that check at all. If your agent loop mutates history, that difference is the whole migration.
Anthropic shipped Claude Fable 5.1 and Claude Mythos 5.1 on September first, and most of the migration coverage has focused on Fable. The more useful decision for anyone running a real agent loop is which of the two you point at, and the deciding factor is a behavior almost nobody checks before switching models: whether the model validates that your conversation history is the same history it reasoned against.
On Claude Fable 5.1 it does. Every thinking block the model returns is bound to the exact system prompt, tools array, and conversation prefix that preceded it. Change any of those and the next request fails with a 400 telling you the signature in the thinking block is invalid because the block is bound to a different conversation. The patterns that trigger it are ordinary engineering, not abuse: editing or reordering earlier turns, rebuilding the system prompt between calls, regenerating the tools array from a registry, client side compaction that truncates, and injecting per turn context that you later strip out. Plenty of production agent frameworks do at least one of those on every single request.
Claude Mythos 5.1 does not enforce that check. Same family, same release, same day, and the constraint simply is not there. If your orchestration layer mutates history and you do not want to rewrite it, Mythos 5.1 is the model that lets you keep shipping while you decide whether the rewrite is worth doing.
The rest of the Mythos 5.1 surface is worth knowing before you point traffic at it. Its Anthropic API model ID is claude-mythos-5-1 and its Amazon Bedrock ID is anthropic.claude-mythos-5-1. It carries a one million token context window and a 128K maximum output. Adaptive thinking is always on and cannot be disabled, with effort configurable across low, medium, high, xhigh and max, defaulting to high. Sampling controls are largely gone: temperature must be 1.0 or unset, top_p must be at least 0.99 and below 1.0 or unset, and top_k is not supported at all. Code that sets temperature to 0.2 for determinism is code that needs changing.
Two constraints apply to both models and will stop a deploy cold if you miss them. Forced tool use is removed: a tool_choice of type any or type tool returns a 400 invalid_request_error, and only auto and none are supported. And both models require thirty day data retention, which means neither is available under a zero data retention arrangement unless Anthropic has expressly authorized it. If your organization negotiated zero data retention for compliance reasons, that is a contract conversation before it is an engineering one, and it is better had now than during a failed rollout.
The broader habit this rewards is simple. When a vendor ships two models together, read the differences rather than the benchmarks. The benchmarks will be close. The constraints will not be, and the constraints are what break in production.
Try it today
- Grep your agent code for anything that edits, reorders, truncates, or removes earlier messages, and for anywhere the system prompt or tools array is rebuilt per request rather than held constant.
- If you find any of those and you want Claude Fable 5.1, either make history append only or enable thinking.block_binding.prefix_mismatch_behavior set to drop_block, which requires the thinking-binding-controls-2026-08-01 value in the anthropic-beta header.
- If rewriting the orchestration is not worth it right now, point the workload at claude-mythos-5-1 instead, which does not run the binding check.
- Remove every temperature value other than 1.0, remove top_k entirely, and confirm top_p is either unset or at least 0.99 and below 1.0.
- Replace any forced tool_choice of type any or type tool with tool_choice auto plus an explicit instruction in the turn, since the forced types now return a 400.
- Confirm with your legal or compliance owner that thirty day data retention is in place, because neither model runs under a zero data retention arrangement without express authorization from Anthropic.