GPT-5.6 Sol: OpenAI's Former Flagship, Explained

GPT-5.6 Sol went generally available on July 9, 2026, rolled out simultaneously to ChatGPT, Codex, and the API. It arrived with a new naming scheme worth thirty seconds of your attention, because it finally makes OpenAI's lineup legible: the number is the generation, the name is the capability tier. GPT-5.6 is the generation; Sol, Terra, and Luna are the tiers within it.

Sol was the flagship, its API identifier is gpt-5.6-sol, and it was the only tier with the two headline features: max reasoning effort and ultra mode. The scheme carried into GPT-6, where Astra sits above Sol and Luna. Where GPT-5.6 Sol sat against Anthropic's frontier tier at the time is what the rest of this guide covers; today's lineup is on the models hub.

Data verified September 23, 2026 against OpenAI's published pricing and launch materials. The full family, side by side:

The Terra line deserved a call-out: OpenAI positioned it as roughly GPT-5.5 performance at 60% lower token prices, which quietly made it the value pick of the generation, more on that in the decision guide below. The current version of that decision, GPT-6.1 Sol vs Astra vs Luna, is in the best Codex model guide.

Sol's two exclusives are what separate it from Terra beyond raw capability. Max reasoning effort is the highest thinking budget in the family, for problems where you want the model to grind. Ultra mode is more interesting: Sol natively fans a task out to parallel subagents, multiple copies of itself working a problem concurrently. OpenAI's own numbers show what that buys: 91.9% in ultra mode on Terminal-Bench 2.1, up from 88.8% standard, per OpenAI's launch materials.

Note what ultra mode parallelizes: work within one task. It does not parallelize your backlog. Running several independent features at once, each on its own branch, is an orchestration-layer job, which is exactly the layer AIDEN provides on top of the Codex CLI, see AIDEN vs Codex CLI.

OpenAI led its launch with Terminal-Bench 2.1 and, notably, did not publish SWE-bench scores. Here is the verified picture, including how Sol compares with Claude Fable 5:

Data verified September 23, 2026. OpenAI also claims leads on ALE (53.6) and the Artificial Analysis Coding Agent Index (80.0); treat both as OpenAI's claims until independently reproduced.

At launch, Sol, Terra, and Luna were the current generation in Codex, and Codex auto-selected a model by default, so most sessions never touched the picker. Also selectable at the time: GPT-5.5, GPT-5.4, GPT-5.4 Mini, and GPT-5.3 Codex Spark, a text-only research preview limited to ChatGPT Pro. OpenAI had deprecated gpt-5.2 and gpt-5.3-codex for ChatGPT sign-in, and GPT-5.5, the previous flagship, overlapped almost entirely with Terra at 2.5x Terra's token prices.

That roster has since moved on. The GPT-6 family is the current generation, OpenAI recommends GPT-6.1 Sol for complex coding, and the GPT-5.6 models stay selectable during the GPT-6 rollout. GPT-5.5 retires from Codex with ChatGPT sign-in on October 14, 2026; the API is unaffected. The tier-by-tier picking logic, reasoning effort, and the retirement dates live in the best Codex model guide.

This was the routing rule inside the GPT-5.6 family, and it still holds if you run these models. For new work, start on GPT-6.1 Sol instead.

One more axis: you do not have to pick a vendor at all. AIDEN runs your Codex CLI next to your Claude Code CLI on one kanban board, each feature on its own branch, so you can split today's work between GPT-6.1 Sol and Claude Opus 5.5 feature by feature, and let the per-feature results, not the launch keynotes, decide who gets the next card.