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OpenAI model release dates: o3, GPT-5.5 and GPT-6 Astra, Sol and Luna in order

o3 is from April 2025, and the name Sol now means something different from what it meant in July.

By Think Facility · September 23, 2026 · OpenAI models release dates

OpenAI released o3 on April 16, 2025. It reached the API that day alongside o4-mini, and it's the date OpenAI's own changelog gives. If that's all you came for, you're done.

The full list matters more than it sounds, mostly because OpenAI's names stopped meaning one thing this summer. Here's every model since o3, in order, and then what the new names are for.

Every OpenAI model since o3, by release date

These are API dates. OpenAI keeps a developer changelog with an entry for each model the day it becomes callable, and it's the only first-party list I found that covers all of them in one place.

API release dates from OpenAI's developer changelog, read September 23, 2026.

A few things jump out once it's laid flat like that. GPT-5 to GPT-5.5 took eight and a half months, with GPT-5.1 through GPT-5.4 in between. Then GPT-5.6 and GPT-6 Astra arrived eight weeks apart.

The coding models ran on their own track. There's a Codex version of GPT-5, 5.1, 5.2 and 5.3, and then the line stops: nothing called GPT-5.4-Codex or GPT-5.5-Codex appears anywhere in the changelog. GPT-5.3 is the odd one, since its Codex model reached the API a week before any other GPT-5.3 model did.

What happened to o3 and the o-series

o3 was a reasoning model, and the changelog introduced it with o4-mini as two "o-series reasoning models" aimed at math, science, coding and visual reasoning. o3-pro followed in June 2025 as a version that "uses more compute to answer hard problems", and OpenAI cut o3's price the same day.

Then the o-series just stops. The last entries with an o in front are the deep research variants of o3 and o4-mini, on June 24, 2025. Six weeks later GPT-5 shipped, and every flagship since has been a GPT. (The "think harder" idea carried on under a different label: GPT-5 Pro, GPT-5.4 Pro and GPT-5.5 Pro are each described as the version that spends more compute on tougher problems.)

Why another site might give you a different day

The API date isn't always the day people first used a model. ChatGPT rollouts are staggered, and OpenAI says so. The GPT-6 Astra announcement has it "rolling out today to a limited set of organizations", with Plus, Pro, Business and Enterprise users due to get it over the following days.

So a date a day or two off the table can be a ChatGPT rollout date rather than a mistake. I've stuck to the changelog because it's the one record OpenAI keeps for every model with the same rule.

Astra, Sol, Terra, Luna: which GPT is which

This is the part that confused me. In July, OpenAI stopped using plain version numbers and gave the GPT-5.6 family three names: Sol "for frontier capability", Terra "for a balance of intelligence and cost" and Luna "for efficient, high-volume workloads". Sol was the top.

GPT-6 kept two of those names and moved one of them. Astra is the new top, and the models page now says to use it if you're "not sure where to start". GPT-6 Sol is the middle model, there to "balance intelligence and cost", which is the job Terra had. Luna is still the cheap one. There's no GPT-6 Terra on the page at all.

That makes "Sol" ambiguous. GPT-5.6 Sol is last generation's flagship and GPT-6 Sol is this generation's mid-tier. They're separate models with separate IDs (gpt-5.6-sol and gpt-6-sol), so check which one a benchmark or a forum post means before you compare anything.

API list prices from OpenAI's models page and changelog, September 23, 2026. GPT-5.6 Sol's price is promotional, held "at least through November 21, 2026".

The three GPT-6 models share a 1.05M-token context window and a 128K-token output cap. The knowledge cutoffs are the odd bit. Luna, the smallest, is the most current, at May 18, 2026, while Astra stops at April 30 and Sol at April 20.

OpenAI's Sol and Luna announcement frames the new pair as half the price of the old ones. GPT-6 Sol costs 50% less than GPT-5.6 Sol, and GPT-6 Luna 50% less than GPT-5.6 Luna, both measured against GPT-5.6's promotional rates.

Those promotional rates were already cuts. The changelog shows GPT-5.6 Luna getting 80% cheaper on July 30, three weeks after launch, and GPT-5.6 Sol dropping to $4 and $20 on August 21. So the cheap Luna tier, now ten cents per million input tokens on GPT-6, has had three prices in eleven weeks. If you budgeted from a July screenshot, it's worth redoing the sum.

Which ones you can use in ChatGPT

It depends on the model and on which part of ChatGPT you're in, which I hadn't expected. OpenAI's rollout plan for Astra covered Plus, Pro, Business and Enterprise. Pro, Business and Enterprise also get GPT-6 Astra Pro, and on Enterprise an administrator has to switch Astra on, because access starts off. How many Astra messages each plan gets is on our limits page.

Sol and Luna landed in ChatGPT Work and Codex for Plus, Pro, Business, Enterprise and Edu. Free and Go users get Luna, but only in the desktop app. And the announcement says plainly that the two models are "not yet available in Chat", the ordinary conversation view most people open first.

Even the older names aren't one model everywhere. A footnote on the Astra page says the GPT-5.6 Sol in ChatGPT's Chat view is "slightly different" from the version in the API, Codex and ChatGPT Work. I'd never have guessed that from the model picker.

What's being switched off

Older models don't disappear the day a new one lands. OpenAI lists retirements on its deprecations page, and a few of the models in the table above now have a date.

On June 11, 2026, OpenAI told developers that the original o3 and o3-pro snapshots and the August 2025 GPT-5, GPT-5 mini and GPT-5 nano snapshots, plus GPT-5 Pro, would be removed from the API on December 11, 2026. o3-mini and o4-mini go sooner, on October 23, 2026.

The recommended replacements on that page are all GPT-5.6 models: Sol for o3 and GPT-5, Terra for GPT-5 mini, Luna for GPT-5 nano. The notice predates GPT-6 by nearly three months (and the page still names them), so if you're migrating off o3 now, the replacement it names is already a generation old.

o3 stops answering API calls on December 11, 2026.