Head-to-head comparison across 1benchmark categories. Overall scores shown here use BenchLM's provisional ranking lane.
GPT-5.5 Pro
100
Mellum2-12B-A2.5B-Instruct
27
Pick GPT-5.5 Pro 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.
Knowledge
+16.3 difference
GPT-5.5 Pro
Mellum2-12B-A2.5B-Instruct
$30 / $180
N/A
N/A
N/A
N/A
N/A
1M
128K
Pick GPT-5.5 Pro 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-5.5 Pro is clearly ahead on the provisional aggregate, 100 to 27. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.
GPT-5.5 Pro's sharpest advantage is in knowledge, where it averages 57.2 against 40.9.
GPT-5.5 Pro 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-5.5 Pro gives you the larger context window at 1M, compared with 128K for Mellum2-12B-A2.5B-Instruct.
GPT-5.5 Pro is ahead on BenchLM's provisional leaderboard, 100 to 27.
GPT-5.5 Pro has the edge for knowledge tasks in this comparison, averaging 57.2 versus 40.9. Mellum2-12B-A2.5B-Instruct stays close enough that the answer can still flip depending on your workload.
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