Claude Mythos Preview vs Claude Sonnet 4.5

Pick Claude Mythos Preview if you want the stronger benchmark profile. Claude Sonnet 4.5 only becomes the better choice if knowledge is the priority or you want the cheaper token bill.

Claude Mythos Preview is clearly ahead on the aggregate, 84 to 69. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.

Claude Mythos Preview's sharpest advantage is in agentic, where it averages 80.9 against 55.3. The single biggest benchmark swing on the page is Terminal-Bench 2.0, 82% to 50%. Claude Sonnet 4.5 does hit back in knowledge, so the answer changes if that is the part of the workload you care about most.

Claude Mythos Preview is also the more expensive model on tokens at $25.00 input / $125.00 output per 1M tokens, versus $3.00 input / $15.00 output per 1M tokens for Claude Sonnet 4.5. That is roughly 8.3x on output cost alone. Claude Mythos Preview is the reasoning model in the pair, while Claude Sonnet 4.5 is not. That usually helps on harder chain-of-thought-heavy tests, but it can also mean more latency and more token spend in real use. Claude Mythos Preview gives you the larger context window at 1M, compared with 200K for Claude Sonnet 4.5.