What loads when you open ChatGPT depends entirely on what you're paying. Free users get one model with no picker at all. Paying users get a whole lineup: GPT-5.5, and as of this month, the new GPT-5.6 family split into three tiers named Sol, Terra, and Luna.

Here's what's in there and which one to reach for.

The quick answer

On the free plan, you're on GPT-5.5 Instant with no model picker. It's a solid daily driver. On Plus and above, GPT-5.5 Instant still handles casual chat by default, but raising the reasoning or effort setting opens the GPT-5.6 family: Sol for the hardest problems, Terra for balanced everyday work at lower cost, and Luna for fast, cheap, high-volume tasks.

GPT-5.5 Instant: what free users get

GPT-5.5 launched April 23, 2026, first to Plus, Pro, Business, and Enterprise users, described by OpenAI as its smartest and most intuitive model at the time, built to handle messy, multi-part tasks: research, coding, document work, and moving across tools without step-by-step hand-holding. A lighter version, GPT-5.5 Instant, later became the default for free-tier users. It's not the frontier model anymore, but it's a genuinely capable everyday assistant, not a stripped-down demo.

GPT-5.6: Sol, Terra, and Luna

GPT-5.6 reached general availability on July 9, 2026. Instead of one flagship model, OpenAI split it into three tiers built for different budgets and workloads:

  • Sol is the flagship. OpenAI built it for the hardest work: complex coding, security research, and long-running agent tasks. It set new highs on OpenAI's own benchmarks for long-horizon professional work and coding-agent performance at launch.
  • Terra is the balanced option: OpenAI says it performs competitively with GPT-5.5 at a meaningfully lower cost, aimed at high-volume business tasks like customer support and document analysis.
  • Luna is the fastest and cheapest of the three, built for routine work like summarizing, drafting, and everyday automation rather than depth.

OpenAI's own framing is worth knowing: "the number identifies the generation, while Sol, Terra, and Luna are durable capability tiers that can advance on their own cadence." In other words, expect these three names to stick around and get updated in place rather than being replaced by a new number every few months.

Who gets access to what

In the regular ChatGPT chat interface, Plus, Pro, Business, and Enterprise users can access Sol by raising the reasoning or effort setting; GPT-5.5 Instant stays the default for quick, everyday questions. Pro and Enterprise users can additionally select Sol Pro, the highest-capability option, for the most demanding tasks. Free users aren't left out of GPT-5.6 entirely: OpenAI has been extending access to Terra on lower tiers over time, but the reliable, no-picker experience on the free plan is still GPT-5.5 Instant.

Plans and pricing

ChatGPT's individual plans run Free ($0, GPT-5.5 Instant), Go (a lower-cost tier with higher limits than Free), Plus ($20/month, opens the GPT-5.6 family through reasoning settings), and Pro (the highest tier, with the most access to Sol Pro and the largest usage limits). Business and Enterprise plans are seat-based or custom. Plan names and prices shift often enough that OpenAI's own pricing page is worth checking before you commit to a tier.

Bottom line

If you're on the free plan, GPT-5.5 Instant is genuinely fine for most everyday questions, drafts, and quick research. If you're paying for Plus, don't assume the default is doing its best work on a hard problem: raise the reasoning setting and you're talking to Sol. If you're doing serious coding or research work regularly, Pro's access to Sol Pro is built for exactly that. And if you're running high volumes of simple, repetitive tasks, Terra or Luna will get it done without paying for capability you don't need.

The names will keep changing. The habit of matching the model to the task, instead of assuming whatever loads by default is the best ChatGPT has to offer, is the part worth keeping.

Sources:
openai.com/index/gpt-5-6 · openai.com/index/introducing-gpt-5-5 · openai.com/chatgpt/pricing