Blog · 2026-07-10
OpenAI's GPT-5.6 family ships in three tiers — Sol, Terra and Luna — and they are the same model line at three price points, not three different architectures. All three take text and image input, return text, carry a 1,050,000-token context window with up to 128,000 output tokens, support reasoning tokens, and share a February 2026 knowledge cutoff. Choosing between them is almost entirely a cost decision.
On apimodels.app the three sit at a clean 5 : 2.5 : 1 ratio. Sol costs $1.103 per million input tokens and $6.618 per million output. Terra is exactly half of that, $0.551 and $3.309. Luna is a fifth of Sol, $0.221 and $1.324. Cached input tokens are billed at a tenth of the input rate on every tier — $0.110, $0.055 and $0.022 respectively. For comparison, OpenAI's own list prices for the same three models are $5/$30, $2.5/$15 and $1/$6, so each tier lands at roughly a fifth of official.
A practical rule: start on Terra. It is the tier most workloads should default to — everyday chat, code generation, agentic tool calling — because doubling your spend to reach Sol buys reasoning depth you often will not use. Move up to Sol when a task genuinely fails on Terra: long multi-step planning, hard debugging, research synthesis. Move down to Luna when the job is bulk and mechanical: classification, extraction, rewriting, first-line support. At $0.221 per million input tokens, Luna makes batch pipelines that would be uneconomical on a frontier model suddenly cheap.
Upstream, OpenAI exposes reasoning depth as a suffix on the model name rather than a body parameter — gpt-5.6-sol-low through -ultra are separate model ids, all at the same per-token rate, differing only in how many reasoning tokens they emit. apimodels.app serves the three base tiers only: gpt-5.6-sol, gpt-5.6-terra and gpt-5.6-luna. The depth variants were retired on 2026-07-27 when the channel that carried them shut down, and the replacement upstream does not offer them; calling a suffixed id returns a clear error rather than silently substituting a different model. Pick the tier that matches the job — that is the decision that actually moves cost and quality.
Three behaviours are worth knowing before you build on any of the three. First, upstream injects a system preamble of roughly 1,450 tokens into every request, so usage.input_tokens never drops below that floor even for a one-word prompt — budget for it when you price short, high-frequency calls. Second, parallel_tool_calls: false is ignored: ask about the weather in Paris and Tokyo and you will get two tool_calls back regardless, so write your handler to iterate rather than assume one. Third, once a single request exceeds 272,000 input tokens the entire request is rebilled at 2x input and 1.5x output — the step is on the request, not on the tokens past the threshold.
All three answer on both OpenAI-compatible endpoints, /v1/chat/completions and /v1/responses, with function calling, streaming and multi-turn tool results verified on each. Point base_url at https://api.apimodels.app/v1, keep your existing OpenAI SDK, and switch tiers by editing one string. Two aliases exist for convenience: gpt-5.6 routes to Sol, and gpt-5.6-luna routes to gpt-5.6-luna-max, which is currently the only Luna depth the upstream serves — same price, so nothing changes on your bill.
GPT-5.6 Luna, at $0.221 per 1M input tokens and $1.324 per 1M output on apimodels.app, with cached input at $0.022. That is 20% of Sol's price and roughly a fifth of OpenAI's own $1 / $6 list. Call it as gpt-5.6-luna-max.
No. The depth variants were retired on 2026-07-27: the channel that served them shut down, and the replacement upstream carries only the three base tiers. A suffixed id now returns an explicit error instead of being silently mapped to the base model, so you always know which model answered you. Use gpt-5.6-sol, gpt-5.6-terra or gpt-5.6-luna — the tier choice, not the depth, is what actually moves cost and quality.
Yes. On apimodels.app each tier costs roughly a fifth of OpenAI's list price: Sol $1.103/$6.618 against $5/$30, Terra $0.551/$3.309 against $2.5/$15, and Luna $0.221/$1.324 against $1/$6. One API key also covers Claude, Gemini and every other model on the platform, it is reachable from China without organization verification, and failed requests are never billed.
The whole request is rebilled at 2x the input rate and 1.5x the output rate — not just the tokens past the threshold. For Sol that means $2.206 input and $9.926 output per 1M; Terra $1.102 and $4.964; Luna $0.442 and $1.986. This mirrors the upstream long-context rate step.
