OpenAI Retunes GPT-5.6 Sol in ChatGPT and Makes Luna Free Default
On August 6, 2026, OpenAI rolled out a ChatGPT-focused GPT-5.6 Sol update with a thinking-effort slider for Plus and Pro, said internal evals cut factual-error rates ~68% vs GPT-5.5 Instant for Sol and ~62% for Luna, and made GPT-5.6 Luna the Free/Go default with unlimited text chats and a Think button on the way.
TLDR
OpenAI on August 6, 2026 updated ChatGPT defaults across paid and free tiers. Plus and Pro get a ChatGPT-tuned GPT‑5.6 Sol (separate from the Sol used in Codex / ChatGPT Work) plus a thinking-effort slider that replaces separate Instant vs Thinking modes. Free and Go users get GPT‑5.6 Luna as default, with unlimited text chats and a Think button rolling out on a staggered schedule. OpenAI says one billion people now use ChatGPT weekly.
What changed by tier
| Tier | Model default | New controls | Notes (OpenAI + release notes) |
|---|---|---|---|
| Plus / Pro | Updated GPT‑5.6 Sol (ChatGPT path) | Thinking slider (how much “thought” per response) | More focused answers; less excess formatting; adapts detail to question difficulty |
| Free / Go | GPT‑5.6 Luna | Think button for harder questions (rolling) | Unlimited text chats next (abuse guardrails remain); limits stay on files, images, voice, image gen, other tools |
| Codex / Work Sol | Unchanged | — | ChatGPT Sol retune is product-path specific |
Reliability claims (lab-internal)
OpenAI reports an internal evaluation on finance, medicine, and law prompts requiring factual detail:
| Model | Claim vs GPT‑5.5 Instant |
|---|---|
| GPT‑5.6 Sol (updated ChatGPT) | ~68% fewer responses with ≥1 factual error |
| GPT‑5.6 Luna | ~62% fewer responses with ≥1 factual error |
These are company-run figures—not independent arena or third-party factuality boards. Treat as lab-reported until Artificial Analysis / Arena-style audits catch up.
Ranking / product context
- Not a new base-model family GA—GPT‑5.6 Sol/Terra/Luna already public from the July family launch and subsequent price cuts (Luna −80%, Terra −20% on Jul 30).
- This is a consumer ChatGPT distribution + retune day: free tier moves fully onto Luna, paid chat unifies around Sol + continuous effort control.
- Independent LMArena / Artificial Analysis ranks for the original Sol/Luna tiers still apply as the backbone; this post does not publish a new public Elo. Note when ChatGPT-path Sol diverges from API/Codex Sol in user experience.
Why this story matters
OpenAI is splitting the free tier into unlimited cheap intelligence (Luna text) while selling deeper, cleaner Sol behind Plus/Pro—with a slider that productizes “how hard should the model think?” as UX instead of model-picker jargon. Competitors must answer on free-tier generosity and paid factual reliability. Watch independent factuality checks, whether unlimited free text raises abuse/cost pressure, and whether Work/Codex Sol stays deliberately different from chat Sol.
Sources
- OpenAI: Improving GPT‑5.6 Sol in ChatGPT—and expanding access to GPT-5.6 Luna for free users (August 6, 2026)
- OpenAI ChatGPT release notes (August 6, 2026 section)
- The Decoder: OpenAI improves GPT-5.6 Sol and free-tier Luna shift (August 6, 2026)
- Times of AI prior: GPT-5.6 Luna/Terra price cuts (July 30, 2026)