GPT-6 Sol vs Terra vs Luna vs Astra: Which Model Should You Use?

Start with gpt-6.1-sol for demanding everyday coding and agent work, gpt-6-luna for focused tasks you can check cheaply, and gpt-6-astra for the hardest multi-step work. GPT-6.1 Sol, released on September 29, 2026, is OpenAI's upgrade to GPT-6 Sol and now fills the Sol slot in the GPT-6 lineup. Keep gpt-5.6-terra in the trial if it already works for your team. Terra is a GPT-5.6 model, not a fourth GPT-6 tier. Those are starting choices from OpenAI's model guidance, not measured winners on your workload.

The practical question is whether a model clears your quality bar at an acceptable cost per accepted task. Per-token prices alone cannot answer that: output length, reasoning effort, tool use, retries, and human repair can change the result. The direct-API prices below are as of September 30, 2026, except Terra's, which are as of September 23, 2026.

Which names are actually GPT-6?

OpenAI's GPT-6 guide now lists three models: GPT-6 Astra (gpt-6-astra), GPT-6.1 Sol (gpt-6.1-sol), and GPT-6 Luna (gpt-6-luna). The version bump applies to Sol only; Astra and Luna are still GPT-6 models. The OpenAI API changelog records the GPT-6 Sol and Luna release on September 22, 2026, and GPT-6.1 Sol on September 29. The Terra model page names it gpt-5.6-terra. There is no gpt-6-terra in the GPT-6 lineup. Use full IDs in tests and configuration so an older model with a similar name cannot be mistaken for a new one.

The original gpt-6-sol is now the previous Sol. Its model page is still live and points to GPT-6.1 Sol, and the deprecations page lists no shutdown date for it as of September 30, 2026, but it has dropped out of the pricing page's flagship table.

These roles paraphrase OpenAI's GPT-6 guidance, model-selection guidance, and the Terra model page. They are evaluation priorities, not an independent ranking of speed or pass rate. If you are moving an existing gpt-6-sol integration, GPT-6.1 Sol: Pricing, Access, and What Breaks When You Switch covers the parameter changes. If your question is specifically about the older three-model lineup, see the GPT-5.6 Sol, Terra, and Luna comparison.

How much does each cost through the direct API?

The following are OpenAI direct-API Standard rates in USD per one million tokens for requests with at most 272,000 input tokens, from the official API pricing page and model pages. They are token rates, not ChatGPT subscription prices or included Codex usage.

The Sol–Terra rows deserve a careful read. GPT-6.1 Sol matches Terra on Standard short-context input ($2) and cache writes ($2.50), costs half as much for cached input ($0.10 versus $0.20), and charges $10 for output versus Terra's $12. The previous GPT-6 Sol differed from Terra only on output. For the same counted token quantities and the same billing mode, GPT-6.1 Sol's token charge is never higher than Terra's and is lower whenever cached input or output is billed. That does not establish a lower bill for the same completed job. The two models may use different output or reasoning tokens, produce different numbers of acceptable answers, or trigger different retries.

The rate relationship applies to a like-for-like token count. It is not a measured task-cost result.

Here is a deliberately hypothetical, uncached Standard request with 100,000 input tokens and 20,000 output tokens, no paid tools, and the short-context rate. Multiply each count in millions by its rate: Luna costs $0.02 (0.1 × $0.10 + 0.02 × $0.50); GPT-6.1 Sol costs $0.40 (0.1 × $2 + 0.02 × $10), the same as the previous GPT-6 Sol because nothing is cached; Terra costs $0.44 (0.1 × $2 + 0.02 × $12); Astra costs $2.00 (0.1 × $10 + 0.02 × $50).

Caching is where the two Sols separate: if 80,000 of those input tokens were billed as cached input, GPT-6.1 Sol would cost about $0.25 (0.02 × $2 + 0.08 × $0.10 + 0.02 × $10), GPT-6 Sol about $0.26, and Terra about $0.30. Both examples compare an identical token tally, not likely usage or quality on a real task. For a deeper Astra-specific rate breakdown, see GPT-6 Astra API pricing.

What happens after 272K input tokens?

Once input exceeds 272,000 tokens, the pricing rules apply the long-context rates to the entire request, not only the excess. For Standard processing, the long-context input/output pairs per million tokens are Luna $0.20/$0.75, GPT-6.1 Sol and GPT-6 Sol both $4/$15, Terra $4/$18, and Astra $20/$75. Long-context cached input is $0.20 for GPT-6.1 Sol and $0.40 for GPT-6 Sol. A 300K-input estimate calculated at short-context prices would therefore be wrong; check the pricing table for every category your request actually uses.

Batch and Flex are 50% below Standard where available, Fast mode is 2× Standard, and regional processing adds 10% where it applies. Tool calls can add charges. Keep the model, service tier, region, token categories, and input-length band together when you compare invoices. A shared 1,050,000-token context window on the GPT-6.1 Sol and GPT-6 Sol model pages does not make requests above 272K cheap or prove equal task performance.

Which model should you try first for your work?

Choose the first candidate from the cost of a mistake, then give every candidate the same acceptance test. If a bad result is easy to catch with code or a quick review, start with Luna. If the work requires substantial coding or tool decisions, start with GPT-6.1 Sol. If a task crosses several tools or is costly to repair, test Astra. If Terra is already deployed successfully, retain it as a measured baseline instead of assuming the newer generation must replace it. This is a trial order, not a claim that one candidate will be faster or better in your environment.

An extraction queue illustrates the tradeoff. A low token rate helps only while validation catches mistakes and reviewers are not spending the difference on fixes. A repository migration has a different failure cost: one superficially plausible but broken change can outweigh many cheap calls. For either workflow, measure end-to-end completion, not just the first model response.

OpenAI describes GPT-6.1 Sol as "near-Astra performance for complex work at a lower cost" and positions Luna for focused, repeatable work. The Astra page describes its higher-capability role. On Artificial Analysis's Intelligence Index v4.3.2, as of September 30, 2026, GPT-6.1 Sol at high reasoning effort scored 50 at $0.32 per index task, against 51 at $1.73 for Astra at the same effort; that is a third-party harness, not your prompts. Whether Astra's gap is worth paying for is the subject of GPT-6.1 Sol vs GPT-6 Astra vs GPT-6 Sol: When Astra Is Worth It. The same context limit across models is a capacity limit, not evidence of equal comprehension of a long document.

Run an acceptance test before changing the default

1. Collect representative tasks. Include ordinary cases, expensive failures, long inputs if you use them, and the tool actions your production workflow really needs. Define a clear pass rule before reading the model answers: for example, tests pass and the patch meets review criteria, or extracted fields match checked source records.
2. Fix the comparison setup. Record the exact model ID, prompt, input, tools, service tier, region, and reasoning effort. Compare candidates with equivalent settings where the API supports them; when support differs, record that difference instead of calling the trials identical. GPT-6 Astra and GPT-6.1 Sol do not support none reasoning effort; GPT-6 Sol and GPT-6 Luna do. OpenAI suggests low in its place. Tool calling on GPT-6.1 Sol and Astra requires the Responses API.
3. Record outcomes and usage. For each attempt, log whether it met the pass rule, input categories, output tokens (including billed reasoning output), tool charges, elapsed time, retries, and review or repair time. A token estimate from text length is weaker than returned usage and an actual bill.
4. Compare accepted-task cost. Divide total measured spend—including failed attempts and paid tools—by the number of accepted tasks. Keep human repair time and latency as separate columns, or translate repair time to money using your team's explicit rate. Choose the least costly candidate that meets your quality and latency requirements; keep the baseline if no challenger does.

This diagram shows the evaluation method; the comparison cells are deliberately unfilled.

For example, 100 attempted tasks with 80 accepted outcomes and $8 in model/tool spend yield $0.10 per accepted task before labor. If another model spends $6 but delivers only 40 accepted outcomes, its corresponding figure is $0.15. Those numbers illustrate the calculation; they are not observed results for any of these models. A controlled trial still needs enough representative tasks to reveal expensive failure modes.

Does the API comparison apply to Work or Codex?

No direct conversion exists between this per-token API table and a Work or Codex plan allowance. OpenAI's Work/Codex model guide lists GPT-6.1 Sol, GPT-6 Sol, and GPT-6 Luna in Work and Codex. Chat changed on October 7–8, 2026: GPT-6 in Chat is now powered by GPT-6 Sol on Plus, Pro, Business, and Enterprise, and by GPT-6 Luna on Free and Go, rolling out in stages. GPT-6.1 Sol remains a Work and Codex model, and Astra appears in Chat only as GPT-6 Pro on Pro, Business, and Enterprise plans. GPT-6.1 Sol is rolling out to Plus, Pro, Business, Enterprise, and Edu; on Enterprise and Edu it stays off until an administrator enables it, and Free and Go are not included at launch. GPT-5.6 Sol, Terra, and Luna remain available during the rollout. In the Codex CLI, codex -m gpt-6.1-sol selects the new Sol.

What appears in a particular picker depends on the account, workspace, client, and rollout. Selecting an ID does not grant access or change workspace permissions. For a sense of scale, OpenAI's pricing page estimates 15–160 local GPT-6.1 Sol messages per five hours on Plus, against 350–3,000 for Luna, and says these are not fixed message limits.

For API development, check model access in the relevant project and use the exact ID in your request. For Work or Codex, check the actual model picker and your applicable plan or credit rules. The Standard table above cannot tell you your subscription limit, purchased-credit rate, or availability in a specific account. If your current workflow is already reliable, run the acceptance test before changing a saved default.