Vio Labs

Notes AI models No. 02

September 2026 AI model releases and prices

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AI models

This note covers AI model releases from 1 to 30 September 2026, a month in which OpenAI, Anthropic, Google, xAI and Meta all shipped new proprietary models. The main releases were Anthropic's Claude Fable 5.1 (1 September), Google's Gemini 3.8 Flash (2 September), Meta's Muse Spark 1.3 (2 September), OpenAI's GPT-6 Astra (3 September), xAI's Grok 4.7 (21 September), Claude Opus 5.5 and OpenAI's GPT-6 Sol and GPT-6 Luna (22 September), Claude Sonnet 5.5 (28 September) and GPT-6.1 Sol (29 September). Open weight releases are left for a separate note.

September 2026 AI model releases in one table

Prices are the standard API rates in US dollars per million input and output tokens, as each lab published them. We checked them on 30 September 2026. Several labs charge more for very long prompts, and prices change, so check the linked pages before you budget.

DateModelLabInput / output per 1M tokens
1 SepClaude Fable 5.1Anthropic$10 / $50
2 SepGemini 3.8 FlashGoogle$0.75 / $3.75 (to 31 Dec 2026)
2 SepMuse Spark 1.3Meta$1.25 / $4.25
3 SepGPT-6 AstraOpenAI$10 / $50
21 SepGrok 4.7xAI$2 / $6 (prompts under 200K)
22 SepClaude Opus 5.5Anthropic$4 / $20
22 SepGPT-6 SolOpenAI$2 / $10
22 SepGPT-6 LunaOpenAI$0.10 / $0.50
28 SepClaude Sonnet 5.5Anthropic$2 / $10
29 SepGPT-6.1 SolOpenAI$2 / $10

OpenAI shipped GPT-6 Astra, then cheaper Sol and Luna

OpenAI released GPT-6 Astra on 3 September 2026. It has a 1,050,000 token context window, 128,000 tokens of maximum output and a knowledge cutoff of 30 April 2026. OpenAI's launch post said it was rolling out first to a limited set of organisations, then over the following days to ChatGPT Plus, Pro, Business and Enterprise users and through the API. OpenAI claims state-of-the-art results on several benchmarks, including FrontierMath Tier 4, ARC-AGI-3 and Terminal-Bench 4.0.

Astra's system card says it is OpenAI's first model to reach the Critical level for cybersecurity under its Preparedness Framework. In OpenAI's words, with the right tools and access it "can find previously unknown security flaws and develop new ways to exploit them" across many well-protected systems. The more permissive cyber configuration is offered through an application-based programme OpenAI calls Daybreak.

On 22 September OpenAI's API changelog recorded GPT-6 Sol and GPT-6 Luna, two cheaper reasoning models. A week later, at DevDay on 29 September, it released GPT-6.1 Sol. OpenAI's announcement says it "nearly matches" Astra on agentic coding, computer use and professional work, and its DevDay summary puts its standard token prices at one fifth of Astra's. OpenAI's own pricing page confirms the arithmetic: $2 and $10 against Astra's $10 and $50. GPT-6.1 Sol and GPT-6 Sol share a list price, but GPT-6.1 Sol charges half as much for cached input ($0.10 against $0.20).

DevDay also brought Codex Security Cloud, which runs scheduled vulnerability scans and prepares fixes, and an Ultrafast paid speed tier. Dots, OpenAI's new persistent agents, launched for Pro users in markets excluding the European Economic Area, Switzerland and the UK, according to OpenAI's help page on dots.

One release did not happen. Engadget, citing The Wall Street Journal, reported on 29 September that OpenAI had cancelled GPT-6.1 Astra, due in October, after internal tests showed higher levels of deception than earlier models. The report quotes OpenAI's Saachi Jain, but as of 30 September we had not found an announcement about it on OpenAI's own channels.

Anthropic's Fable 5.1, Opus 5.5 and Sonnet 5.5

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on 1 September. They are paired models: Mythos 5.1 has fewer safeguards and, Anthropic says, "is only available to a set of US organizations". The change in Fable 5.1 that matters most to security teams is that it "can now be used to discover software vulnerabilities" but not to develop exploits. Penetration testing, exploit generation and binary-based vulnerability scanning are still redirected to Opus models.

Claude Opus 5.5 followed on 22 September at $4 and $20, cheaper than Opus 5 at $5 and $25. Anthropic says it "performs at the level of Claude Fable 5.1 on most work and costs 40% less to run than Opus 5". As of 30 September, Anthropic's models overview told developers to start with Opus 5.5 for most workloads.

Claude Sonnet 5.5 arrived on 28 September at $2 and $10. Anthropic claims it runs more than 30% faster than Sonnet 5 and costs up to 30% less for most work. On Anthropic's own Terminal-Bench 4.0 chart it scores 70.6%, above Opus 5.5 at 66.4%. In the same announcement Anthropic said Claude Haiku 5.5 would join the family "in the coming weeks", without giving a date.

Google, xAI and Meta

Google released Gemini 3.8 Flash on 2 September. Its $0.75 and $3.75 prices are introductory and run to 31 December 2026, after which Google lists $1.50 and $7.50. The same day Google announced Gemini 3.8 Flash Cyber, a vulnerability-finding variant limited to governments, critical infrastructure operators and software maintainers through a new Fairwind Program. At I/O in May Google said it looked forward to rolling out Gemini 3.5 Pro "next month". On 30 September the newest Pro model on Google's API pricing page was still Gemini 3.1 Pro Preview.

xAI, whose site now carries the name SpaceXAI, released Grok 4.7 on 21 September at the same price as Grok 4.6. Its models page lists a 500K context window, and its pricing page lists higher rates, $4 and $12, for prompts of 200K tokens or more.

Meta released Muse Spark 1.3 on 2 September. Meta's API pricing includes an unusual Contributor tier at $0.10 and $0.20, in exchange for permission to train future Meta models on your prompts and completions.

Outside the API market, Apple began rolling out Siri AI in beta on 14 September, built on Apple Foundation Models made "in collaboration with Google and its Gemini models". It was not initially available in the EU on iPhone, iPad or Apple Watch.

What September means for teams choosing a model

Our reading of the month is that the bigger change was in price, not in the top score. Anthropic cut its Opus price by a fifth. OpenAI offered what it describes as near-Astra performance at one fifth of Astra's price, 26 days after launching Astra.

Two cautions. First, list price is not cost per task. Anthropic and OpenAI both argue that their new models use fewer tokens for the same work, and those claims are theirs to prove on your workload. Second, the most capable cyber configurations from Anthropic, OpenAI and Google are all gated behind vetting programmes. For UK and European teams, some of those programmes and products are US-only or exclude the UK at launch, so check eligibility before you plan around them.

Dates to watch

DateWhatSource
Not before 15 Oct 2026Earliest retirement date for Claude Haiku 4.5 listed by Anthropic on 30 SepAnthropic
No date givenClaude Haiku 5.5, which Anthropic said on 28 Sep would come "in the coming weeks"Anthropic
Not before 24 Nov 2026Earliest retirement date for Claude Opus 4.5 listed by Anthropic on 30 SepAnthropic
31 Dec 2026Gemini 3.8 Flash introductory pricing endsGoogle

Sources

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