September 2026 began with the densest 48 hours of frontier releases since the August wave. Anthropic shipped Claude Fable 5.1 and Mythos 5.1 on September 1 at an unchanged list price with three breaking API changes. On September 2 Google released Gemini 3.8 Flash at the same introductory price as 3.7 Flash, with the end of that price now printed, alongside a Fairwind-gated Cyber variant; and Meta released Muse Spark 1.3 with a contributor tier listed the same evening.
This ledger is the September edition of the dated tracker we have kept since May and August. Each row is dated on the vendor’s own announcement, priced from the vendor or the OpenRouter listing where the vendor publishes no price, and linked to the post where we covered it. Releases after September 3 are added to this page rather than given new posts, unless one forces a decision on its own.
- 01Three frontier models in two days, none of them a price cut.Fable 5.1 keeps Fable 5’s $10 and $50; only cache reads fell. Gemini 3.8 Flash keeps 3.7’s $0.75 and $3.75 and now carries a January doubling. Muse Spark 1.3 keeps 1.2’s $1.25 and $4.25. The month’s cheaper rows are all late-August carry-ins.
- 02Two of the six September rows are gated or conditional.Mythos 5.1 is available only through Anthropic’s verification programmes, and Gemini 3.8 Flash Cyber only through Google’s new Fairwind Program. Neither has a public price.
- 03Listing dates and launch dates disagree by up to six days this month.IBM’s Granite 4.2 8B carries an August 25 release date on its model card and an August 31 OpenRouter listing. The frontier releases listed within hours. The rule stands: the vendor’s date is the date.
- 04Four dated events are already fixed for the rest of the month and beyond.Mercury 2.5’s 80% discount ends September 8 at 07:00 UTC; Claude Code’s weekly-limit change lands September 14; Anthropic’s Enterprise Frontier Safeguards are “this fall”; Gemini 3.8 Flash’s intro price expires December 31.
01 — The ledgerThe September ledger.
| Date | Vendor · model | What it is | Price | Context | Primary source | Replaces |
|---|---|---|---|---|---|---|
| September 1–2 · verified against vendor primaries | ||||||
| Sep 1 | Anthropic · Claude Fable 5.1 and Claude Mythos 5.1 | Same model, two safeguard levels; Mythos 5.1 gated to Anthropic’s trusted-access programmes. Three breaking API changes: forced tool choice removed, thinking bound one way by model, history edits invalidate thinking (enforced for accounts created on or after Aug 31) | $10 / $50 per M, unchanged from Fable 5; cache reads cut to $0.25 (was $1.00); batch $5 / $25 | 1M in, 128K out | anthropic.com announcement; platform.claude.com migration guide. OpenRouter listed Sep 1, 18:03 UTC | Fable 5 (still available) |
| Sep 1 | Perplexity · Hybrid Compute on Mac, with PPLX Qwen 3.8 27B | Computer tasks start in the cloud; sensitive steps and private-file access run locally on Apple silicon (macOS 15+, 24 GB minimum, 32 GB recommended) behind an on-device PII classifier; local work uses no cloud credits | Included in Perplexity Pro, Max and Enterprise | Local model; not applicable | perplexity.ai/hub (returns 403 to fetchers); tier-one same-day coverage used, as stated in our post | Cloud-only Computer tasks |
| Sep 2 | Google · Gemini 3.8 Flash | Third Flash release in six weeks by Google’s count. HLE-Verified 54.9% (vendor-run); Artificial Analysis index 59, cost per task $0.58 vs $0.40 for 3.7 Flash at the same per-token price. Live in the API, AI Studio, Antigravity, Gemini app, AI Mode and Sheets | $0.75 / $3.75 per M to Dec 31, 2026; $1.50 / $7.50 from Jan 1, 2027 (Google’s footnote) | 1,048,576 (OpenRouter) | blog.google launch post. OpenRouter listed Sep 2, 15:14 UTC | 3.7 Flash “remains fully supported” |
| Sep 2 | Google · Gemini 3.8 Flash Cyber | Cyber variant with looser mitigations, gated through the new Fairwind Program for governments, critical-infrastructure operators and core technology platforms. CWE-Bench pass@1 47.2% and other figures are vendor-run | Not published | Not published | blog.google launch post; deepmind.google/fairwind-program | 3.5 Flash Cyber (July) |
| Sep 2 | Meta · Muse Spark 1.3 | Asks clarifying questions, asks for help when stuck, confirms before consequential actions. Meta: ~20% fewer tool calls, ~25% fewer tokens than 1.2 (vendor-run). Artificial Analysis: index 61 at xhigh, $0.55 per task, ~57% more input tokens per task. Max reasoning “coming shortly” | $1.25 / $4.25 per M, cache hit $0.15, unchanged | 1,048,576 (OpenRouter) | research.meta.ai launch post; artificialanalysis.ai. OpenRouter listed Sep 2, 19:45 UTC | Muse Spark 1.2 (August) |
| Sep 2 | Meta · Muse Spark 1.3 Contributor | Discounted tier on which Meta trains on your traffic; the same arrangement as the 1.2 contributor tier | $0.10 / $0.20 per M, cached input $0.002 | 1,048,576 (OpenRouter) | OpenRouter listing Sep 2, 20:38 UTC (listing date; Meta’s launch post does not name the tier) | Muse Spark 1.2 Contributor |
Each September row has a post behind it. The Anthropic row’s breaking changes are in our Fable 5.1 cost and breaking-changes post; the Gemini row’s January price is the subject of its own post; the Muse row’s two conflicting token numbers are explained in the Muse Spark 1.3 post; and the Perplexity row is covered in our Hybrid Compute post, which states that Perplexity’s own hub page could not be fetched and that tier-one same-day coverage was used instead.
02 — ContextLate-August carry-ins.
Six releases in the last week of August are the price floor September is being read against, and two of them still have live dates inside September. They are recorded here once, dated on the vendor where a vendor date exists, so that a reader comparing Gemini 3.8 Flash’s $0.75 against GLM-5.3-Flash’s $0.075 has both rows on one page.
| Date | Vendor · model | What it is | Price | Context | Primary source | Replaces |
|---|---|---|---|---|---|---|
| August 25–31 · carry-ins | ||||||
| Aug 25 | IBM · Granite 4.2 8B | Dense 8B open model with reasoning effort levels; release date per IBM’s Hugging Face model card | $0.10 / $0.15 per M on OpenRouter | 131,072 | huggingface.co/ibm-granite/granite-4.2-8b. OpenRouter listed Aug 31, 20:06 UTC, six days after the card’s release date | Not stated on the model card |
| Aug 26 | Z.ai · GLM-5.3-Flash | MIT-licensed open-weight Flash model, revealed as the stealth ox-alpha model | $0.075 / $0.25 per M on OpenRouter | 1,310,720 | z.ai/blog/glm-5.3-flash. OpenRouter listed Aug 26, 13:59 UTC | GLM-5.2-class Flash |
| Aug 26 | Alibaba · Qwen3.8-Flash | Flash-class release covered in our August 26–31 days | $0.15 / $0.47 per M on OpenRouter | 1,000,000 | OpenRouter listed Aug 26, 19:37 UTC; vendor coverage in the August ledger | Earlier Qwen Flash tier |
| Aug 28 | Tencent · Hy4 preview | Preview of Tencent’s next flagship; covered in the August ledger | $0.834 / $2.501 per M on OpenRouter | 1,048,576 | OpenRouter listed Aug 28, 06:09 UTC | Hy3 |
| Aug 28 | Z.ai · GLM-5.3 (753B) open weights | Weights released under a bespoke licence after a two-week hold Z.ai attributed to safety evaluation of faster-than-expected cyber capability | Open weights; hosted pricing varies | Per host | z.ai/blog/glm-5.3 | GLM-5.2 |
| Aug 31 | Inception · Mercury 2.5 Preview | Diffusion language model; 1,107 tokens per second claimed (vendor-run). Comparison set named by Inception: GPT-5.6 Luna (Low), Gemini 3.5 Flash-Lite, Claude Haiku 4.5 | $0.04 / $0.15 per M at an 80% discount until Sep 8, 07:00 UTC; list $0.20 / $0.75 | 260,000 in, 65,536 out | Inception site; OpenRouter listed Aug 31, 20:57 UTC | Mercury 2 (February) |
03 — CalendarDated events still to come.
Four events already have dates. Two are price changes, one is a usage-limit change covered in our Claude Code September 14 post, and one is a safeguards product Anthropic has announced but not shipped.
Mercury 2.5 discount ends
Inception’s 80% launch discount on Mercury 2.5 Preview expires. Anyone who routed a fast-cheap tier to it in the first week should have the list price in the forecast from this date.
Claude Code weekly limit change
The weekly-limit reduction Anthropic scheduled for September 14 lands. Details and the decision it forces are in our dedicated post; this row exists so the date is on the calendar.
Anthropic Enterprise Frontier Safeguards
Anthropic’s launch post says eligible customers can use Fable 5.1 under zero data retention until EFS arrives, phased “starting this fall.” Not available today; do not plan on it for a Q3 deployment.
Gemini 3.8 Flash intro price expires
Google’s launch-post footnote. The only frontier intro price this month with a published end date; the price index carries both numbers.
Two items are real but undated and therefore not rows: Muse Spark 1.3’s max reasoning level, which Meta says is “coming shortly after we finish additional safety testing,” and the Muse Spark open-weights release, listed in Meta’s roadmap with no date and tracked on our withheld-weights ledger. They become rows when they get dates.
04 — AnalysisHow to read the month so far.
The pattern across the six September rows is that per-token prices held and the mechanisms around them moved. Anthropic changed the cache-read price and three API behaviours. Google left the price and published its expiry. Meta left the price and changed the model’s behaviour so that it reads more on a fixed task. A buyer comparing the month by list price sees nothing happening; a buyer comparing cost per task, which Artificial Analysis now publishes for every row above, sees Gemini 3.8 Flash up about 40% on its predecessor and Muse Spark 1.3 up from $0.40 to $0.55. The frontier model API price index carries the per-token rows; this ledger is where the dates live.
The second pattern is gating. Two of six September rows require an application, and both vendors’ gated models are the versions with cyber safeguards loosened. That is now the standing shape of a frontier launch, and the eligibility rules differ enough by lab that they need their own table rather than a column here. The third pattern is that the cheap end of the market is entirely open-weight or diffusion and entirely late-August: GLM-5.3-Flash, Qwen3.8-Flash, Granite 4.2 8B and Mercury 2.5 between them cover $0.04 to $0.15 per million input tokens. Whether September adds a frontier-lab entry at that price is the question the rest of this ledger will answer.
05 — MethodMethodology.
A dated ledger, not a roundup: each row is a verified release with a vendor primary, and the page is refreshed in place rather than replaced by later instalments.
- What was collected
- Model releases with a vendor announcement dated September 1–30, 2026, plus late-August releases whose pricing or dated events carry into September. For each: date, vendor and model, what it is, price, context window, primary source, and what it replaces.
- Sources
- Vendor announcements (anthropic.com, platform.claude.com, blog.google, deepmind.google, research.meta.ai, z.ai, IBM’s Hugging Face model card, Inception); Artificial Analysis for independent index and cost-per-task figures; a live pull of the OpenRouter models API on September 3, 2026, filtered on created time, for listing times, prices and context lengths where the vendor publishes none.
- As-of date
- Collected September 3, 2026. The page is dated September 2 for the releases it opens with; this row is the only statement of the collection date, and it moves with each refresh.
- Dating rule
- The vendor’s announcement date is the row date. An OpenRouter listing time is corroboration only; it can lead or lag the real date by days or months, as the Granite row shows. Where no vendor date could be located, the row says so and gives the listing time.
- Exclusions
- Post-trains of open-weight models by third parties (they are listed on the host’s page, not here); models with no public listing or announcement; benchmark-only previews without a name.
06 — ConclusionSame prices, moved mechanisms.
Two days, three frontier models, zero list-price cuts. The changes are in cache rates, token usage, expiry dates and who is allowed in.
The September ledger opens with the vendors holding their prices and changing everything around them: what a cache read costs, how many tokens a task uses, when an introductory price ends, and which customers get the version with the safeguards off.
This page will carry every dated September release as it lands, on the vendor’s date, with its primary. If a release forces a decision on its own it gets a post and a link from its row; if it does not, the row is the coverage.