Head-to-head comparison across 1benchmark categories. Overall scores shown here use BenchLM's provisional ranking lane.
GPT-5.3 Codex
86
Mellum2-12B-A2.5B-Instruct
27
Pick GPT-5.3 Codex if you want the stronger benchmark profile. Mellum2-12B-A2.5B-Instruct only becomes the better choice if you would rather avoid the extra latency and token burn of a reasoning model.
Coding
+25.9 difference
GPT-5.3 Codex
Mellum2-12B-A2.5B-Instruct
$1.75 / $14
N/A
79 t/s
N/A
88.26s
N/A
400K
128K
Pick GPT-5.3 Codex if you want the stronger benchmark profile. Mellum2-12B-A2.5B-Instruct only becomes the better choice if you would rather avoid the extra latency and token burn of a reasoning model.
GPT-5.3 Codex is clearly ahead on the provisional aggregate, 86 to 27. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.
GPT-5.3 Codex's sharpest advantage is in coding, where it averages 63.1 against 37.2.
GPT-5.3 Codex is the reasoning model in the pair, while Mellum2-12B-A2.5B-Instruct 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. GPT-5.3 Codex gives you the larger context window at 400K, compared with 128K for Mellum2-12B-A2.5B-Instruct.
GPT-5.3 Codex is ahead on BenchLM's provisional leaderboard, 86 to 27.
GPT-5.3 Codex has the edge for coding in this comparison, averaging 63.1 versus 37.2. Mellum2-12B-A2.5B-Instruct stays close enough that the answer can still flip depending on your workload.
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