Claude Mythos 5.2 changelog

Claude Fable 5.2 and Mythos 5.2 return 400 errors when the prefix changes

Both models enforce the prefix check for every account. Requests that edit the system prompt, tools, or an earlier message, then send back a thinking block from a later turn, return a 400 error.

Who is affected:

Apps that edit earlier context while reusing thinking blocks will fail on these models.

Suggested next step:

Check whether your code edits the prefix before you switch models.

Claude Fable 5.2 and Mythos 5.2 launch with outputs up to 30% faster

Claude Fable 5.2 and Claude Mythos 5.2 are upgrades that show improvements in coding, reasoning, and vision. Testers found they generate outputs up to 30% faster than Claude Fable 5.1 and Claude Mythos 5.1.

Claude Mythos 5.2 launches only for organizations verified by Anthropic programs

Claude Mythos 5.2 offers the same capabilities only to organizations verified through Anthropic's verification programs, such as the Cyber Verification Program.

Who is affected:

Only verified organizations can use Claude Mythos 5.2.

Suggested next step:

Check whether your organization qualifies for a verification program before planning to use it.

Claude Mythos 5.2 upgrades Claude Mythos 5.1 with better vulnerability finding

Claude Mythos 5.2 is announced as an upgrade to Claude Mythos 5.1. Anthropic says it found 14.4% more vulnerabilities than Claude Mythos 5.1 on its internal CVE Rediscovery evaluation.