Anthropic has confirmed Claude Sonnet 5.5. It has not said when. The only official line sits in the September 22, 2026 announcement of Claude Opus 5.5: “Claude Sonnet 5.5 and Claude Haiku 5.5 will follow in the coming weeks”. As of September 23, 2026, 14:40 UTC, Anthropic’s model documentation lists claude-sonnet-5 as the current Sonnet, its release notes and pricing page carry no Sonnet 5.5, and OpenRouter’s Anthropic catalog has no entry for it.
That puts this tracker in an unusual spot: the model is real by the vendor’s own word, and every spec attached to it is still rumor. This page keeps the two apart, dates every claim, and becomes the release record on this URL the day Anthropic ships.
If you searched for Sonnet 5.1, there is none. Anthropic went from Claude Sonnet 5, released June 30, 2026, to announcing Sonnet 5.5, the number it gave Opus.
On this page · 11 sectionsOpen
Claude Sonnet 5.5 is real by Anthropic’s own word and undated. The September 22, 2026 Opus 5.5 announcement says Sonnet 5.5 and Haiku 5.5 will follow in the coming weeks. That sentence is the whole official record: no identifier, price, model card or benchmark exists as of September 23.
There is no Sonnet 5.1. Anthropic went from Claude Sonnet 5 (June 30, 2026) straight to announcing Sonnet 5.5, the same number it gave Opus. Claude Sonnet 5 is the current model, identifier claude-sonnet-5, at $2 per million input tokens and $10 output.
The specs are rumors from X. Since August 4 one account has posted that Sonnet 5.5 is in the final stages, with a Fennec codename and a 2M-token context window; a September 4 post added near Fable 5.1 performance at Sonnet pricing. One August post notes Fennec was already tied to the earlier Sonnet 5 leak.
Anthropic’s Sonnet gaps have run 141 and 133 days. A Sonnet 5.5 three weeks after the announcement would land about 105 days after Sonnet 5. In the last .5 generation the whole family shipped within 56 days, Sonnet first; this time Opus went first.
CellCog runs every Claude tier on Claude Opus 5.5 since its release day and does not route to Sonnet today. A Sonnet 5.5 near Opus 5.5 quality at a lower price would be a same-day routing decision, and this page becomes its release record on this URL.
§ 01What we actually know
| Claim | Status |
|---|---|
| Sonnet 5.5 exists and is coming | Anthropic, September 22: in the coming weeks |
| Release date | None given; coming weeks is the whole statement |
| Model identifier | None in Anthropic’s docs; claude-sonnet-5 is current |
| Price | Not published; Sonnet 5 lists $2 input and $10 output per million tokens |
| Codename Fennec | X accounts from August 9; one notes it was already tied to the Sonnet 5 leak |
| 2M-token context window | X rumor, August 9 and September 4; no Anthropic document |
| Near Fable 5.1 performance at Sonnet pricing | X rumor, September 4; no benchmark |
| Already finished, waiting for release | X post, September 23; no source named |
| Benchmark results | None exist for Sonnet 5.5 |
§ 02What Anthropic said, word for word
One sentence, in the announcement of Claude Opus 5.5 on September 22, 2026: “Claude Sonnet 5.5 and Claude Haiku 5.5 will follow in the coming weeks”. The same announcement says Opus 5.5 “performs at the level of Claude Fable 5.1 on most work”, which sets the bar a Sonnet 5.5 will be measured against on release day.
Two things follow from that sentence. The version jumps a whole half step, from Sonnet 5 to Sonnet 5.5 with no 5.1 in between. And the order flipped: in the last .5 generation Sonnet shipped first and Opus last; this time Opus went first.
§ 03The rumor chain, dated
| Date | Event | Source |
|---|---|---|
| Jun 30 | Claude Sonnet 5 released at $2 and $10 per million tokens | Anthropic |
| Aug 4, 09:51 | Sonnet 5.5 in the final stages before release, as early as next month | @Mr_Salio on X |
| Aug 9, 08:52 | Codename Fennec, rumored 2M-token context window | @Mr_Salio on X |
| Aug 13, 04:32 | Fennec already associated with the earlier Sonnet 5 leak | @vepsi__ on X |
| Aug 20, 13:43 | Nearing release, will approach Fable 5 level | @Mr_Salio on X |
| Sep 4, 00:22 | Could land this month, near Fable 5.1 at Sonnet pricing | @0x0SojalSec on X |
| Sep 22, 16:31 | Sonnet 5.5 and Haiku 5.5 follow in the coming weeks | Anthropic |
| Sep 23, 12:05 | Already finished, waiting to be released | @Mr_Salio on X |
| Sep 23, 14:40 | Docs, release notes, pricing, OpenRouter catalog: no Sonnet 5.5 | Our read |
The August posts pointed at September, and a September 4 post said “could land this month”. September has one week left, and Anthropic’s own phrase does not commit to it. The codename is the weakest link: on August 13 @vepsi__ wrote that “that trail was already associated with the earlier Sonnet 5 leak”, so Fennec may name no specific model at all. The September 4 post’s spec line, “Expected to approach Fable 5.1-level performance while keeping Sonnet pricing”, is plausible given what Opus 5.5 did, and it is still a sentence with no document behind it.
§ 04What Anthropic’s own rhythm says
Anthropic does not pre-announce dates, so the calibration data is its shipping record. Dates below are each model’s first listing in OpenRouter’s Anthropic catalog, which matched Anthropic’s release day for every model here.
| Model | First listed | Days since previous Sonnet |
|---|---|---|
| Claude Sonnet 4.5 | September 29, 2025 | first of this run |
| Claude Sonnet 4.6 | February 17, 2026 | 141 |
| Claude Sonnet 5 | June 30, 2026 | 133 |
| Sonnet 5.5, if three weeks out (October 13) | rumored | 105 |
A Sonnet 5.5 three weeks after the announcement would be the fastest Sonnet turnover of this run, 85 days of Sonnet 5 plus the wait. The family precedent points the same way: in the 4.5 generation, Haiku 4.5 followed Sonnet 4.5 by 16 days and Opus 4.5 closed the family 56 days after the first release.
| Generation | First | Second | Third |
|---|---|---|---|
| 4.5 (2025) | Sonnet 4.5, Sep 29 | Haiku 4.5, Oct 15 (+16 days) | Opus 4.5, Nov 24 (+56 days) |
| 5.5 (2026) | Opus 5.5, Sep 22 | Sonnet 5.5, coming weeks | Haiku 5.5, coming weeks |
§ 05What it would cost
Anthropic has published no Sonnet 5.5 price. The current reference is Claude Sonnet 5 at $2 per million input tokens and $10 output, and Anthropic’s pricing page notes that the introductory price is now standard: “The previously scheduled increase to $3/$15 per million input/output tokens on September 1, 2026 will not occur.” Opus 5.5 came in 20 percent under Opus 5 on input and output. The same cut on Sonnet would read $1.60 and $8; that is arithmetic on a pattern of one, not a forecast.
§ 06What would confirm it
Any one of these flips this page from tracker to record:
- A new identifier in Anthropic’s model documentation beside claude-sonnet-5.
- A Sonnet 5.5 row on Anthropic’s pricing page.
- An entry in the platform release notes or an Anthropic announcement page.
- A catalog line on Amazon Bedrock, Google Cloud, Microsoft Foundry or OpenRouter.
- A system card.
A screenshot of a benchmark sheet, a codename in a config file or another post saying it is finished would not confirm it. Those go in the rumor table.
§ 07Where CellCog stands
Every Claude tier in CellCog has run on Claude Opus 5.5 since its release day (what changed); we do not route to Sonnet today. A Sonnet 5.5 close to Opus 5.5 on long agentic runs at a lower price would be a routing decision on its release day, and the decision and its reasons would land here.
§ 08What we are watching for
- Anthropic’s model docs, pricing page and release notes, read on every scan.
- Provider catalogs for a new claude-sonnet identifier.
- A spec from Anthropic: context window, output limit, knowledge cutoff.
- Independent scores after release: Terminal-Bench, SWE-Bench, Artificial Analysis.
- Whether Sonnet or Haiku ships first.
§ 09The tracker
This page updates when facts change, not when posts get louder. As of September 23, 2026, 14:40 UTC: Sonnet 5.5 is confirmed by name, undated, unpriced and absent from every Anthropic document except one sentence. Every spec in circulation traces to X posts.
§ 10Update log
As of September 23, 2026, 14:40 UTC: page opened. Anthropic’s statement is in the coming weeks; the rumored September window has one week left.
§ 11Sources
- Anthropic, Introducing Claude Opus 5.5, September 22, 2026.
- Anthropic, models overview, pricing and release notes, read September 23, 2026, 14:40 UTC.
- OpenRouter, Anthropic catalog, read September 23, 2026, 14:40 UTC; first-listing dates from the models API.
- X: @Mr_Salio, August 4, August 9, August 20 and September 23; @vepsi__, August 13; @0x0SojalSec, September 4. All read directly; times computed from the post ids.
Q1Has Anthropic confirmed Claude Sonnet 5.5?
Yes, by name and without a date. The September 22, 2026 announcement of Claude Opus 5.5 says Claude Sonnet 5.5 and Claude Haiku 5.5 will follow in the coming weeks. Anthropic’s model documentation, release notes and pricing page, read September 23, carry no Sonnet 5.5 entry yet.
Q2What is the Fennec codename?
A name X accounts attached to Sonnet 5.5 from August 9, 2026. On August 13 another account pointed out that the Fennec trail was already associated with the earlier Sonnet 5 leak. No Anthropic document mentions it, so treat it as unconfirmed.
Q3Will Sonnet 5.5 have a 2M-token context window?
That is an X rumor from August 9 and September 4, 2026, with no document behind it. Anthropic has published no context window, output limit or knowledge cutoff for Sonnet 5.5.
Q4Why did Anthropic skip Sonnet 5.1?
Anthropic has not explained its numbering. It released Claude Opus 5.5 on September 22 and named the next Sonnet and Haiku 5.5 as well, so the whole family now carries one number. The same jump happened in 2025, when Sonnet 4.5, Haiku 4.5 and Opus 4.5 followed Sonnet 4 and Opus 4.1.
Q5Should I wait for Sonnet 5.5?
Not on this evidence. Claude Sonnet 5 is live at $2 and $10 per million tokens, and Anthropic made that introductory price standard. If you are choosing where your agents run rather than which API to call, the layer around the model matters more than the version: memory that survives the conversation, a task board, an inbox and approvals a human can see.