New AI Models Released in September 2026
Every AI model released in September 2026, with dates, verified per-token prices and a primary source for each. Updated through the month as releases land.
One model has been released in September 2026 so far: Anthropic’s Claude Fable 5.1, on September 1, alongside its invitation-only twin Claude Mythos 5.1. That is the complete list as of September 2, and it is a genuinely quiet opening after a July that produced twenty-one releases.
The more useful September story is what happened to prices on the same day. September 1 was the date a Claude Sonnet 5 increase had been scheduled to take effect. It did not. Two Anthropic pricing decisions landed on one date and both moved in the customer’s favour.
September 1, 2026
Claude Fable 5.1 and Claude Mythos 5.1 released
Anthropic held Fable 5 rates at $10 input and $50 output per million tokens and cut only the cache read, from $1.00 to $0.25. Mythos 5.1 shares the specifications and the price but stays restricted to invited Project Glasswing participants.
September 1, 2026
The scheduled Claude Sonnet 5 increase did not happen
Sonnet 5 launched at $2 and $10 per million tokens, described as introductory pricing through August 31 with a rise to $3 and $15 due today. Anthropic cancelled it in August; the pricing documentation now records $2 and $10 as the standard rate.
Still pending
Google Gemini 3.5 Pro and OpenAI GPT-6
Gemini 3.5 Pro is announced and listed as coming soon, with no date. GPT-6 has never been officially announced; reporting and prediction markets expect it in the second half of 2026. Neither has a published price.
Every AI model released in September 2026
| Model | Provider | Released | Input / output per Mtok | What it is |
|---|---|---|---|---|
| Claude Fable 5.1 | Anthropic | Sep 1 | $10 / $50 | Frontier flagship. Fable 5 rates held, cache read cut to $0.25 |
| Claude Mythos 5.1 | Anthropic | Sep 1 | $10 / $50 | Same model, safeguards lifted in some areas. Project Glasswing invitation only |
Both rates are read from the Claude Platform pricing documentation rather than from the announcement, and both models carry a 1M-token context window with 128K maximum output.
Claude Fable 5.1: the sticker held, the cache read did not
Fable 5.1 is the month’s only launch and the only one that needed a pricing explanation. Every headline number matches Fable 5 exactly. Input $10, output $50, cache writes $12.50 for five minutes and $20 for an hour, batch at half price. The single change is the cache read, down from $1.00 to $0.25 per million tokens.
That rate is an exception in Anthropic’s own price list. Every other Claude model charges 10% of its base input rate for a cache hit. Fable 5.1 and Mythos 5.1 charge 2.5%.
| Tool | Cost per task | Multiple of baseline |
|---|---|---|
| Claude Fable 5 (until Sep 1) | $1.00 | - |
| Claude Opus 5 | $0.50 | - |
| Claude Fable 5.1 | $0.25 | - |
| Claude Sonnet 5 | $0.20 | - |
| Claude Haiku 4.5 | $0.10 | - |
Why that matters to a bill rather than a rate card: a coding agent re-reads the same file tree and conversation history on every step, so most of its input is cache hits. Modeled on the task profiles in the cost-per-task calculator, a long agentic run drops from $9.70 on Fable 5 to $7.68 on Fable 5.1, about 21%, with fresh input and output contributing nothing to the change. A small single-file edit saves only 11%, because less of its input comes from cache.
Anthropic quotes up to about 45% for highly agentic work. That figure needs a cache share above 93%, with essentially no output tokens. The full arithmetic, including why Fable 5.1 still never undercuts Opus 5 on a task that generates output, is in Claude Fable 5.1: pricing, benchmarks, cost.
The release pace collapsed
Two releases in the first two days is not, on its own, a slow month. Set it against the three months before it and the shape is clearer.
| Item | Value |
|---|---|
| June 2026 | 5 |
| July 2026 | 21 |
| August 2026 | 12 |
| September 2026 (to Sep 2) | 2 |
July was the outlier, not August. The GPT-5.6 family alone contributed three of those twenty-one, and Claude Opus 5, Kimi K3 and the Gemini 3.5 tiers filled most of the rest. August settled to twelve. Whether September lands nearer five or nearer twenty depends almost entirely on whether Gemini 3.5 Pro and GPT-6 arrive inside the month, and neither has a date.
One caution on reading that last column: it counts two days, and the labs that release most often have not yet moved. Alibaba, Z.ai and DeepSeek each shipped in the last week of August, and the Chinese labs in particular have run a cadence closer to fortnightly than monthly all year.
Did OpenAI release a new model in September 2026?
Not so far. OpenAI’s most recent model release remains the GPT-5.6 family, generally available July 9, 2026, with Terra and Luna repriced on July 30 and flagship Sol taking its own promotional cut to $4 and $20 per million tokens on August 21, held through at least November 21. GPT-6 has not been officially announced.
Did Google release a new model in September 2026?
Not so far. Gemini 3.7 Flash on August 13 is the most recent, and it carries a scheduled increase worth noting when budgeting: Google states on its own pricing page that the launch rate of $0.75 and $3.75 per million tokens doubles to $1.50 and $7.50 on January 1, 2027. Gemini 3.5 Pro is announced with no date.
Which September model should you actually use?
For most work, none of them, which is an unusual thing to say about a month’s releases and is a function of there being one.
Anthropic’s own guidance is to start with Claude Opus 5 and reach for Fable 5.1 only for demanding reasoning and long-horizon agentic work, or when Opus 5 at a higher effort setting still fails your evaluations. Opus 5 remains cheaper on every task profile that generates output.
The teams for whom September 1 was unambiguously good news are the ones already running Fable 5. For them this is roughly a fifth off the bill with no migration beyond three breaking API changes: forced tool use now returns an error, earlier models cannot read Fable 5.1 thinking blocks, and editing an earlier conversation turn invalidates them. The second one bites hardest on a mixed fleet, because a fallback path that drops out of Fable 5.1 into a cheaper model mid-conversation will break on the thinking history.
Current rates for every tracked model are in the AI model tracker, and the modeled cost comparison across all of them is in cost per coding task.
Frequently asked questions
- What AI models were released in September 2026?
- As of September 2, one: Anthropic released Claude Fable 5.1 on September 1, 2026, alongside Claude Mythos 5.1, which shares its specifications and pricing but is restricted to invited Project Glasswing participants. Both cost $10 per million input tokens and $50 per million output tokens.
- Did Anthropic release a new model in September 2026?
- Yes. Claude Fable 5.1 and Claude Mythos 5.1 were released on September 1, 2026. Fable 5.1 keeps the Fable 5 rates of $10 and $50 per million tokens and cuts the cache read from $1.00 to $0.25, which is worth about 21% on a long agentic task.
- Did OpenAI or Google release a new model in September 2026?
- Neither has so far. OpenAI last released the GPT-5.6 family on July 9, 2026, and Google last released Gemini 3.7 Flash on August 13, 2026. Google has announced Gemini 3.5 Pro as coming soon without a date, and GPT-6 has not been officially announced.
- How many AI models have been released in 2026 so far?
- The release archive here tracks 40 dated launches from June 2026 onward: 5 in June, 21 in July, 12 in August and 2 in September through September 2. Every row carries the price the model launched at and a link to the provider primary source.
- Why did Claude Sonnet 5 not get more expensive on September 1?
- Anthropic cancelled the increase. Sonnet 5 launched at $2 and $10 per million tokens as introductory pricing billed as running to August 31, 2026, with a rise to $3 and $15 scheduled for September 1. Anthropic announced in August that the increase would not occur and the pricing documentation now lists $2 and $10 as the standard rate.
Sources
Anthropic (2026). Claude Fable 5.1 and Claude Mythos 5.1. Anthropic announcement. https://www.anthropic.com/claude-fable-and-mythos-5-1 Verified 2026-09-02.
Anthropic (2026). Pricing. Claude Platform documentation, including the Claude Sonnet 5 introductory-pricing note and the Fable 5.1 cache-multiplier footnote. https://platform.claude.com/docs/en/about-claude/pricing Verified 2026-09-02.
Anthropic (2026). Claude Fable 5.1. Claude Platform model documentation. https://platform.claude.com/docs/en/models/fable-5-1/overview Verified 2026-09-02.
Anthropic (2026). Models overview. Claude Platform documentation. https://platform.claude.com/docs/en/models/overview Verified 2026-09-02.
Monthly release counts are derived from the release archive maintained on this site, whose every row carries the provider primary source it was verified against.