OpenAI · proprietary on Arena · first on Arena 10 July 2026

gpt-5.6-sol-xhigh (codex-harness)

4 days in first place

It held first place on Arena in WebDev (4 days).

Embed

Who held first place

<iframe src="https://jesseahawkins.com/frontier/embed/" title="Frontier: who held first place on each Arena leaderboard, and for how long" width="100%" height="560" style="border:0;max-width:100%" loading="lazy"></iframe>

Credits LMArena.

The open-weights catch-up

<iframe src="https://jesseahawkins.com/frontier/embed/catch-up/" title="Frontier: the days until open weights matched each closed leader" width="100%" height="900" style="border:0;max-width:100%" loading="lazy"></iframe>

Credits LMArena.

The saturation clock

<iframe src="https://jesseahawkins.com/frontier/embed/clock/" title="Frontier: the days from each benchmark’s release to a model at 90%" width="100%" height="640" style="border:0;max-width:100%" loading="lazy"></iframe>

Credits Epoch AI.

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Cite

This page

Jesse Hawkins, “gpt-5.6-sol-xhigh (codex-harness) · Frontier”, jesseahawkins.com, data retrieved 30 Oct 2026. https://jesseahawkins.com/frontier/gpt-5-6-sol-xhigh-codex-harness/. Data: LMArena, Arena leaderboard dataset, revision 4491b06, CC BY 4.0; Epoch AI, Capabilities & benchmarking, CC BY 4.0.

BibTeX

@misc{hawkins2026frontiergpt56solxhighcodexharness,
  author = {Hawkins, Jesse},
  title = {{gpt-5.6-sol-xhigh (codex-harness) · Frontier}},
  year = {2026},
  howpublished = {\url{https://jesseahawkins.com/frontier/gpt-5-6-sol-xhigh-codex-harness/}},
  note = {Data: LMArena, Arena leaderboard dataset, revision 4491b06, CC BY 4.0; Epoch AI, Capabilities \& benchmarking, CC BY 4.0, retrieved 2026-10-30}
}

Arena · LMArena

LMArena, "Arena leaderboard dataset", Hugging Face, revision 4491b06. https://huggingface.co/datasets/lmarena-ai/leaderboard-dataset. CC BY 4.0.

Benchmarks · Epoch AI

Epoch AI, 'Capabilities & benchmarking'. Published online at epoch.ai. Retrieved from 'https://epoch.ai/benchmarks' [online resource]. CC BY 4.0.

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Post on X

WebDev: 4 days in first place

place 31 of 152 on 28 October 2026

1st10th

On the board: 10 July 2026 to 28 October 2026, the newest snapshot
Score: 1616 (1610–1622) on 28 October 2026 · 18,884 votes
First place: 12 July 2026 to 16 July 2026 · 4 days
Catch-up: Open weights matched it on 26 July 2026, 14 days after it first took first place: kimi-k3-max.

Epoch AI: GPT-5.6 Sol

released 9 July 2026 · API access

Chess Puzzles 68.0%
EBR-bench 44.8%
FrontierMath-Tier-4-v2-Private 82.9%
FrontierMath-Tiers-1-3-v2-Private 89.1%
Furniture Assembly 56.7%
GPQA diamond 93.5%
MirrorCode 20.0%
Mystery Game Puzzles 58.0%
OTIS Mock AIME 2024-2025 100.0%
SimpleQA Verified 69.7%

When each vendor retires it

every model’s dates

Azure AI Foundry 11 Jan 2028 gpt-5.6-sol (version 2026-07-09)

Sources

Arena scores come from people voting between two anonymous answers. First place is the highest score; the top two are often within each other’s margin of error. LMArena, Arena leaderboard dataset, revision 4491b06, retrieved 30 October 2026; Epoch AI, Capabilities & benchmarking, retrieved 30 October 2026. Both CC BY 4.0. Retirement dates: each vendor’s own deprecation page, quoted and kept, listed with the record.