Head-to-head comparison across 2benchmark categories. Overall scores shown here use BenchLM's provisional ranking lane.
Claude Mythos Preview
99
GPT-5.3 Codex
89
Verified leaderboard positions: Claude Mythos Preview #1 · GPT-5.3 Codex unranked
Pick Claude Mythos Preview if you want the stronger benchmark profile. GPT-5.3 Codex only becomes the better choice if you want the cheaper token bill.
Agentic
+10.9 difference
Coding
+20.7 difference
Claude Mythos Preview
GPT-5.3 Codex
$25 / $125
$2.5 / $10
N/A
79 t/s
N/A
88.26s
1M
400K
Pick Claude Mythos Preview if you want the stronger benchmark profile. GPT-5.3 Codex only becomes the better choice if you want the cheaper token bill.
Claude Mythos Preview is clearly ahead on the provisional aggregate, 99 to 89. 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 coding, where it averages 83.8 against 63.1. The single biggest benchmark swing on the page is SWE-bench Pro, 77.8% to 56.8%.
Claude Mythos Preview is also the more expensive model on tokens at $25.00 input / $125.00 output per 1M tokens, versus $2.50 input / $10.00 output per 1M tokens for GPT-5.3 Codex. That is roughly 12.5x on output cost alone. Claude Mythos Preview gives you the larger context window at 1M, compared with 400K for GPT-5.3 Codex.
Claude Mythos Preview is ahead on BenchLM's provisional leaderboard, 99 to 89. The biggest single separator in this matchup is SWE-bench Pro, where the scores are 77.8% and 56.8%.
Claude Mythos Preview has the edge for coding in this comparison, averaging 83.8 versus 63.1. Inside this category, SWE-bench Pro is the benchmark that creates the most daylight between them.
Claude Mythos Preview has the edge for agentic tasks in this comparison, averaging 82.4 versus 71.5. Inside this category, OSWorld-Verified is the benchmark that creates the most daylight between them.
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