Anthropic · Tuesday, September 29, 2026
Anthropic ships Claude Sonnet 5.5
The File desk · Sep 29, 2026, 10:20 AM UTC
Status
Confirmed (Anthropic launch page + newsroom listing + system card, Sept. 28, 2026)
On Sept. 28, 2026, Anthropic introduced Claude Sonnet 5.5, the second model in the Claude 5.5 family. The company says it runs more than 30 percent faster than Claude Sonnet 5 and typically costs up to 30 percent less per task at the same list price — $2 per million input tokens and $10 per million output tokens. On Terminal-Bench 4.0, an agentic coding test, Anthropic reports 70.6 percent for Sonnet 5.5 versus 10.3 percent for Sonnet 5. It is the first Sonnet model to launch with cyber safeguards and fallbacks like those on Claude Opus. The model ID is claude-sonnet-5-5. It is available on the Claude Platform and through Amazon Web Services, Google Cloud, and Microsoft Azure. Anthropic says Claude Haiku 5.5 will join the family in the coming weeks. The system card says Responsible Scaling Policy evaluations found Sonnet 5.5 broadly less capable than Opus 5.5 and that it does not cross any new Responsible Scaling Policy thresholds.
On Sept. 28, 2026, Anthropic introduced Claude Sonnet 5.5.
The company calls it the second model in the Claude 5.5 family, and a clear upgrade over Claude Sonnet 5.
Anthropic says it runs more than 30 percent faster than Sonnet 5, and that most work costs up to 30 percent less per task.
The list price is unchanged: $2 per million input tokens and $10 per million output tokens.
On Terminal-Bench 4.0, a test of agentic coding, Anthropic reports 70.6 percent for Sonnet 5.5 and 10.3 percent for Sonnet 5.
This is the first Sonnet model to launch with cyber safeguards and fallbacks like those on Claude Opus.
The model ID is claude-sonnet-5-5. It is available on the Claude Platform and through Amazon Web Services, Google Cloud, and Microsoft Azure.
Anthropic says Claude Haiku 5.5 will join the family in the coming weeks. No ship date is on the page.
The system card, dated Sept. 28, says Responsible Scaling Policy evaluations found Sonnet 5.5 broadly less capable than Opus 5.5 across domains, and that it does not cross any new Responsible Scaling Policy thresholds.
What is still unknown or disputed
- Independent third-party bench replication.
- Exact Haiku 5.5 ship date.
Primary sources
Every claim in this story is drawn from the documents below. If a fetch failed, that is recorded on the card.
Source 1
Introducing Claude Sonnet 5.5
Anthropic · September 28, 2026
Introducing Claude Sonnet 5.5, the second model in the Claude 5.5 family. It’s a clear upgrade over Claude Sonnet 5, runs 30%+ faster, and costs up to 30% less for most work.
https://www.anthropic.com/claude-sonnet-5-5
Source 2
Newsroom — Introducing Claude Sonnet 5.5
Anthropic · September 28, 2026
Announcements Sep 28, 2026 — Claude Sonnet 5.5.
https://www.anthropic.com/news
Source 3
Claude Sonnet 5.5 System Card
Anthropic · September 28, 2026
Our Responsible Scaling Policy (RSP) evaluations found that Sonnet 5.5 is broadly less capable than Opus 5.5 across domains and does not cross any new RSP thresholds.
https://www.anthropic.com/claude-sonnet-5-5-system-card