GPT-6 Sol

GPT-6 Sol, released September 22, 2026, was the mid-tier model of OpenAI's GPT-6 generation, launched with GPT-6 Luna nineteen days after GPT-6 Astra opened the line and on the same day as Anthropic's Claude Opus 5.5. OpenAI trained both with methods similar to Astra's and pitched them as the cost-efficiency end of the family: Astra stayed "our best model across the board", while Sol and Luna carried its gains in professional work, factuality, coding and computer use to faster, cheaper models. The price was the headline. Sol launched at $2.00 per million input tokens and $10.00 per million output — half the $4.00 and $20.00 OpenAI had charged for GPT-5.6 Sol under promotional pricing, and a fifth of Astra's $10.00 and $50.00 — with cached input at $0.20, cache writes at $2.50, and prompts past 272K input tokens at double on input and 1.5x on output.

The launch post argued the model on cost per task rather than on raw score. At xhigh effort it scored 33.2% on AutomationBench at $0.27 a task, which OpenAI set against 30.3% for GPT-6 Astra at low effort and 26.9% for Claude Opus 5 at max effort, at 9% of Opus 5's cost per task; on Agents' Last Exam it reached 56.4% at max effort, above Opus 5's best at 60% lower cost. In coding it posted 68.8% on DeepSWE v1.1, within 1.1 points of the 69.9% Claude Fable 5 had reached at xhigh effort and at roughly 80% lower cost per task, and OpenAI said it matched Claude Fable 5.1 on FrontierCode 1.1 without publishing the figure. On the offline set of OSWorld 2.0 it scored 60.5% at xhigh effort, level with Claude Opus 5 at medium effort. OpenAI also reported about half as many factual errors as GPT-5.6 Sol on its internal evaluation, approaching Astra's reliability at much lower cost.

The API surface was the 5.6 family's: a 1,050,000-token context window, 128,000 max output tokens, text and image in and text out, and a reasoning-effort control from none through low, medium, high and xhigh to max, with medium as the default, on the Responses, Chat Completions and Batch endpoints. OpenAI shipped improved prompt caching alongside the pair, with a 90% discount on cached reads and cache preserved across changes to reasoning effort and tool availability. Sol's knowledge cutoff was April 20, 2026. It reached ChatGPT Work and Codex on launch day for Plus, Pro, Business, Enterprise and Edu users, and the API as gpt-6-sol.