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GPT 5.6 Luna low vs Opus 5 medium

Opus 5 medium leads on quality; GPT 5.6 Luna low on speed and cost.

Quality
+13.1 pts
Opus 5 medium
6–2 head to head, 16 tied
Not yet conclusive across all 136 comparisons on the board.
Speed
3.5× faster
GPT 5.6 Luna low
6.7 min vs 23.2 min per task
Cost
120.4× cheaper
GPT 5.6 Luna low
$0.05 vs $6.13 per task

24 tasksData from

Stet score

GPT 5.6 Luna lowcodex · low reasoningRank 15 of 17

45.095% range 39.1–50.4

Opus 5 mediummedium reasoningRank 2 of 17

58.195% range 50.4–66.113.1 points higher

Their 95% score ranges don’t overlap.

Quality

Head-to-head matchups

GPT 5.6 Luna low won2

Tied16

Opus 5 medium won6

Opus 5 medium’s win share 60%, 95% range 49–71%Ties are within judge noise.

Task by task

GPT 5.6 Luna low solved 0 tasks that Opus 5 medium didn’t, and Opus 5 medium solved 5 that GPT 5.6 Luna low didn’t. Both solved 10; neither solved 9.

stet-commit-bbbbae09Solved by 15 of 17 setups

  • GPT 5.6 Luna lowFailed
  • Opus 5 mediumPassed tests

A task counts as solved when its tests passed or a judge found the fix equivalent to the merged change, as in the leaderboard’s Solved column. Stet score also weighs code quality, so it can order the two differently.

All 24 compared tasks
Task by task
TaskGPT 5.6 Luna lowOpus 5 mediumSolved by
stet-232b50baPassed testsPassed testsBoth
stet-3169aefaPassed testsPassed testsBoth
stet-5276aa3fPassed testsPassed testsBoth
stet-89dfbc27Passed testsPassed testsBoth
stet-b28f4fe7Passed testsPassed testsBoth
stet-commit-bbbbae09FailedPassed testsOpus 5 medium only
stet-66762b4cEquivalent to the merged fixPassed testsBoth
stet-commit-5d766ddeEquivalent to the merged fixEquivalent to the merged fixBoth
stet-commit-f0caded9Equivalent to the merged fixPassed testsBoth
stet-43493c52Passed testsPassed testsBoth
stet-commit-f8ae1177FailedPassed testsOpus 5 medium only
stet-15439c21Equivalent to the merged fixEquivalent to the merged fixBoth
stet-2450ca2dFailedPassed testsOpus 5 medium only
stet-commit-342d0ac0FailedFailedNeither
stet-6f84e978FailedPassed testsOpus 5 medium only
stet-commit-60a0e6c2FailedFailedNeither
stet-commit-5e075641FailedFailedNeither
stet-commit-2826765eFailedEquivalent to the merged fixOpus 5 medium only
stet-commit-a8716b88FailedFailedNeither
stet-commit-e928166fFailedFailedNeither
stet-commit-30506f8eFailedFailedNeither
stet-commit-87814583FailedFailedNeither
stet-commit-d2ab0c25FailedFailedNeither
stet-commit-e41c2e2dFailedFailedNeither

Side by side

Side by side
MetricGPT 5.6 Luna lowOpus 5 medium
Harnesscodex—
Reasoning effortlowmedium
Rank15 of 172 of 17
Stet score45.095% range 39.1–50.458.195% range 50.4–66.1
Solved10/246 passed, 4 judged equivalent15/2412 passed, 3 judged equivalent
Cost / task$0.05$6.13
Agent time / task6.7 min23.2 min

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About this data

  • 24 tasks, each a merged change replayed from the repo as it was just before the merge.
  • Data from .
  • Both setups come from the same leaderboard run of 17 setups: bmk_d4ba310fdf50db4ccb2092e3.

See all 17 setups on the leaderboardHow Stet measures