Early benchmarks rank GPT-5.6 Sol near Fable 5 at lower cost

• GPT-5.6 shipped as Sol, Terra, and Luna across ChatGPT, Codex, and the API, with OpenAI's rollout note saying full availability would spread over 24 hours.
• Sol landed one point behind Fable 5 on the Artificial Analysis Intelligence Index but led the Coding Agent Index at lower per-task cost, according to Artificial Analysis.
• ARC Prize gave Sol 7.78% on ARC-AGI-3, up from Opus 4.8's 1.5%, while ARC Prize's failure analysis said deeper planning chains still broke.
• Cost was the through-line: bridgemindai's CursorBench table put Sol Max at 67.2% for $5.22 per task versus Fable 5 Max at 70.5% for $17.32.
• The caveats arrived quickly: jerryjliu0's ParseBench result found no document-understanding lift over GPT-5.5, and scaling01's math screenshot showed FrontierMath Tier 4 below GPT-5.5.

OpenAI's launch thread buried the product split inside a model launch: ChatGPT Work, a new desktop app, Codex in ChatGPT, and hosted Sites. The weirdest API additions were programmatic tool calling in hosted V8 runtimes and multi-agent requests that let Sol spawn concurrent subagents. dejavucoder caught the line that Sol was shown helping post-train Luna, while Cedric Chee flagged that the rumored 750 tok/s Cerebras path did not appear in OpenAI's GPT-5.6 docs.

OpenAI framed GPT-5.6 as a three-model family, not a single flagship.

• Sol: flagship model for complex coding, knowledge work, cybersecurity, science, and long-horizon agentic work, per OpenAI Developers.
• Terra: balanced model for everyday coding, reasoning, and agentic tasks at lower cost, per OpenAI Developers.
• Luna: fastest and lowest-cost model for high-volume tasks, per OpenAI Developers.
• Surfaces: ChatGPT, Codex, and API, with a gradual 24-hour rollout, per OpenAI.
• ChatGPT access: Plus, Pro, Business, and Enterprise users get Sol through medium and higher effort settings, while Pro and Enterprise also get GPT-5.6 Pro, per OpenAI.
• Product layer: Sam Altman listed ChatGPT Work, a new desktop app, and hosted Sites as the three major product launches alongside the model, in sama's livestream note.

OpenAI also reset limits after launch. thsottiaux said ChatGPT Work and Codex would receive two more resets over 24 hours so users could try ambitious tasks.

First-party

• Agents' Last Exam: Claude Fable 5 adaptive 40.5% → GPT-5.6 Sol 53.6%, +13.1 points, per OpenAI's launch thread.
• Terminal-Bench 2.1: Claude Mythos 5 88.0% → GPT-5.6 Sol Ultra 91.9%, +3.9 points, per the OpenAI benchmark chart.
• SEC-Bench Pro: GPT-5.5 45.8% → GPT-5.6 Sol Ultra 74.3%, +28.5 points, per the cybersecurity benchmark table.
• KernelGen 1P: GPT-5.5 baseline → GPT-5.6 Sol, +31.8 points, per the KernelGen chart.

Third-party evaluators

• Artificial Analysis Intelligence Index: GPT-5.5 55 → GPT-5.6 Sol 59, +4 points, per Artificial Analysis.
• Artificial Analysis Coding Agent Index: Claude Fable 5 77.2 → GPT-5.6 Sol 80.0, +2.8 points, per Lentils80's AA excerpt.
• GDPval-AA v2: GPT-5.5 1494 Elo → GPT-5.6 Sol 1748 Elo, +254 Elo, while Fable 5 stayed ahead at 1760, per Artificial Analysis.
• ARC-AGI-3: Claude Opus 4.8 1.5% → GPT-5.6 Sol 7.78%, +6.28 points, per ARC Prize.
• DeepSWE v1.1: GPT-5.5 67.0% → GPT-5.6 Sol 72.7%, +5.7 points, per DataCurve.
• CritPt: GPT-5.5 xhigh 27.1% → GPT-5.6 Sol max 32.3%, +5.2 points, per Artificial Analysis.

Customer-reported

• CursorBench: Fable 5 Max 70.5% → GPT-5.6 Sol Max 67.2%, -3.3 points, while cost fell from $17.32 to $5.22 per task, per bridgemindai's CursorBench table.
• Box AI Complex Work, healthcare: GPT-5.5 46% → GPT-5.6 Sol 58%, +12 points, per Aaron Levie's Box eval.
• Box AI Complex Work, public sector: GPT-5.5 63% → GPT-5.6 Sol 74%, +11 points, per Aaron Levie's Box eval.
• ProgramBench: GPT-5.5 9.5% → GPT-5.6 Sol 23.0%, +13.5 points, while Fable 5 led at 33.0%, per scaling01's ProgramBench table.
• Hermes Agent PR review: Terra 8.1 → Sol 8.8, +0.7 points, while Sol cost $1.66 versus Terra's $0.46 across five PRs, per Teknium's test.

Vals reported a different shape: Fable 5 remained ahead on the main Vals Index and Multimodal Index, but Sol ranked first on CyberBench, Excel Modeling Benchmark, Legal Research Bench, ProofBench, SWE-bench, and Terminal-Bench 2.1, according to ValsAI.

OpenAI's launch emphasized coding agents and knowledge work. The misses clustered around high-end SWE-Bench, math, document parsing, and design taste.

• SWE-Bench Pro: Claude Mythos 5 80.3% → GPT-5.6 Sol 64.6%, -15.7 points, per the benchmark screenshot.
• FrontierMath Tier 4: GPT-5.5 72.5% → GPT-5.6 Sol 65.9%, -6.6 points, while Claude Fable scored 87.8%, per scaling01's math table.
• ParseBench average: GPT-5.5 64.4 → GPT-5.6 Sol 62.1, -2.3 points, in jerryjliu0's document-understanding result.
• Every Senior Engineer benchmark: Fable 91/100 → Sol 56/100, -35 points, though danshipper's review called Sol an excellent implementer.

Artificial Analysis found a small accuracy lift on AA-Omniscience, but its breakdown also reported a higher hallucination rate versus GPT-5.5.

ARC-AGI-3 had a scoring caveat. scaling01 argued Sol would score 0% under a $10k cap, while the plotted run used a higher budget.

The pricing grid stayed simple, with one new gotcha: OpenAI added cache-write pricing.

• API pricing: Sol is $5 input and $30 output per 1M tokens, Terra is $2.50 and $15, Luna is $1 and $6, per TheRealAdamG's pricing excerpt.
• Cache writes: GPT-5.6 bills cache writes at 1.25x uncached input, while cache reads keep the 90% cached-input discount, per the pricing excerpt.
• Context: Vals measured Sol with a 1M context window and 128K max output tokens, per ValsAI.
• Codex context: one Codex screenshot put the usable context at 353K, with 373K total and reserved space for compaction prompts, per Lentils80.
• Reasoning levels: GPT-5.6 adds max and Ultra, and reach_vb warned Ultra can strain usage limits.
• Ultra mode: OpenAI described Ultra as parallel agent coordination that trades higher token use for stronger and faster results on demanding tasks, in its launch thread.
• Programmatic tool calling: GPT-5.6 can write and run JavaScript to coordinate tool workflows inside isolated hosted V8 runtimes, per OpenAI Developers.
• Multi-agent API: the Responses API beta can spawn concurrent subagents inside a single request, per OpenAI Developers.
• Computer Use: OpenAI said GPT-5.6 made Computer Use faster and more token-efficient, with batching, parallel operations, and picture-in-picture supervision, per OpenAI Developers.

OpenAI's public launch materials cited here do not state whether GPT-5.6 is a new pretrain or a further-trained GPT-5.5. haider1 claimed it is further-trained from 5.5, while Simon Willison's roundup described the release as a new GPT-5.6 family without resolving the training-run question.

Hands-on reports converged on one split: Fable often sounded smarter, while Sol kept grinding.

• petergostev called Fable a "wise owl" and Sol a "rottweiler," then put Sol ahead on robustness, code-pattern adherence, subagents, computer use, token efficiency, and multi-day runs.
• The same comparison said a benchmark-building task took Sol six hours to two days and produced a tested suite, while Fable returned in about 40 minutes with self-judged work that did not hold up.
• MatthewBerman said he spent 25B tokens over two months, used Sol for days-long /goal runs, a six-day Excel recreation loop, Supabase resizing, and a Google Workspace migration with MX, SPF, and DKIM changes.
• Berman also reported rough edges: Ultra often took longer than the extra thinking was worth, Sol still made confident mistakes, and internal process occasionally leaked into answers.
• danshipper's review called Sol an A-tier coder but below Fable on complex code, stronger at writing, improved but not top-tier at design, and most changed for whole loops of knowledge work.
• emollick split usage by task: Sol for back-and-forth work where the target was still forming, Fable for long tasks with a clear spec, and Sol Pro for hard problems.
• Legal testing was unusually bullish: deredleritt3r put GPT-5.6 Sol Pro at 91/99 on prinzbench, up from GPT-5.5 Pro at 82/99 and GPT-5.4 Pro at 79/99.

The sharpest safety-adjacent anecdote came from the same legal thread: Sol reportedly solved about 70 CAPTCHAs to download old New York legislative-history documents, according to deredleritt3r.

The rollout landed in the tools engineers already use.

• Cursor added Sol, Terra, and Luna on day one, and cursor_ai reported Sol at 67.2% on CursorBench.
• VS Code said GPT-5.6 Sol, Terra, and Luna were rolling out, according to code.
• Devin made GPT-5.6 available in Cloud, Desktop, and CLI, with cognition saying Sol reached top performance at nearly half the cost of the next best FrontierCode 1.1 Extended model.
• Lovable said GPT-5.6 used roughly 25% fewer steps, 35% to 48% fewer tool calls, and reduced stuck runs by 15%, per Lovable.
• OpenRouter shipped all three models and described Sol as the flagship, Terra as the balanced tier, and Luna as the fast low-cost tier, in OpenRouter's rollout thread.
• Vercel AI Gateway exposed openai/gpt-5.6-sol, openai/gpt-5.6-terra, and openai/gpt-5.6-luna, per vercel_dev.
• Perplexity added Terra and Sol to Perplexity and Perplexity Computer, per perplexity_ai.
• Letta said GPT-5.6 was available in Letta Agent and Code, with Codex or ChatGPT subscriptions usable to get started, per Letta_AI.
• Agent Arena added Sol, Terra, and Luna for Battle Mode, Agent Mode, text, vision, document, code, and search for Sol, per arena.

Codex is already a scaled surface, not just a demo. TheRealAdamG's r/Codex screenshot says more than 5 million people use Codex weekly, twice as many as three months earlier, and that OpenAI shipped 150 Codex features and improvements in the same period.