---
id: "2025-09-29-claude-sonnet-4-5"
url: "https://postcutoff.com/e/2025-09-29-claude-sonnet-4-5/"
as_of: "2026-11-13T13:49:00+01:00"
date: "2025-09-29"
date_precision: day
category: model-release
importance: 4
confidence: high
status: [Confirmed]
sources: 3
editor: human editor
human_review: null
version: "2026-10-11"
---

As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/e/2025-09-29-claude-sonnet-4-5/

# Anthropic releases Claude Sonnet 4.5

Claude Sonnet 4.5 became the state-of-the-art model on SWE-bench Verified and OSWorld, able to maintain focus on complex tasks for over 30 hours; Anthropic also launched the Claude Agent SDK and Claude Code 2.0. Claude Haiku 4.5 followed on 15 October 2025.

## Key facts
- Released 29 September 2025
- SWE-bench Verified: 77.2%, per Anthropic
- OSWorld: 61.4%, per Anthropic
- Observed working autonomously for more than 30 hours on complex tasks
- Same price as Sonnet 4: $3 / $15 per million tokens

## What happened

Anthropic released its best coding and computer-use model at the time, alongside the building blocks behind Claude Code as a general agent SDK.

## Why it matters

Pushed the length of tasks AI agents can reliably do and made agent-building infrastructure broadly available.

## Your AI and this story

- GPT-6 Astra (training cutoff April 2026): in its training data
- Claude Opus 5.5 (training cutoff June 2026): in its training data
- Gemini 4 Hydrogen (training cutoff March 2026): in its training data
- Grok 4.7 (training cutoff May 2026): in its training data

## Sources

1. [Anthropic Engineering: Effective context engineering for AI agents (Sept 29, 2025)](https://www.anthropic.com/engineering/effective-context-engineering-for-ai-agents) (anthropic.com, official)
2. [Introducing Claude Sonnet 4.5 (Anthropic)](https://www.anthropic.com/news/claude-sonnet-4-5) (anthropic.com, official)
3. [Introducing Claude Haiku 4.5 (Anthropic)](https://www.anthropic.com/news/claude-haiku-4-5) (anthropic.com, official)

## Changes

- 2026-10-11: Added related Anthropic engineering post link(s)
- 2026-09-29 (filed): Created

## Related

- 2025-11-24: [Anthropic releases Claude Opus 4.5](https://postcutoff.com/e/2025-11-24-claude-opus-4-5/index.md)
- 2025-05-22: [Anthropic releases Claude Opus 4 and Sonnet 4](https://postcutoff.com/e/2025-05-22-claude-4/index.md)