Curiosity · AI Model
Claude Sonnet 4.5
Claude Sonnet 4.5 is Anthropic's September 2025 mid-tier model — the one that crossed 70% on SWE-bench Verified and briefly held the best-coding-model title. It introduced the Claude 4 agent stack (long tool loops, context editing, memory tools) and remained the default Sonnet tier until Sonnet 4.6 replaced it in early 2026.
Model specs
- Vendor
- Anthropic
- Family
- Claude 4
- Released
- 2025-09
- Context window
- 200,000 tokens
- Modalities
- text, vision, code
- Input price
- $3/M tok
- Output price
- $15/M tok
- Pricing as of
- 2026-04-20
Strengths
- First Sonnet to cross 70% on SWE-bench Verified at launch
- Stable across very long agent trajectories (many tool calls in a row)
- Strong computer-use numbers — good for browser automation
- Well-tuned prompt caching behaviour for long-context apps
Limitations
- Context window capped at 200K vs 1M in Sonnet 4.6
- Superseded on most benchmarks by Sonnet 4.6 and Opus 4.7
- Extended-thinking output tokens priced the same as normal outputs, inflating cost
Use cases
- Coding agents running multi-hour autonomous sessions
- Computer-use workflows via Anthropic's computer-use tool
- Tool-heavy support and ops agents
- RAG pipelines at Claude-4 quality with moderate cost
Benchmarks
| Benchmark | Score | As of |
|---|---|---|
| SWE-bench Verified | ≈77% | 2025-09 |
| OSWorld (computer use) | ≈62% | 2025-09 |
| MMLU-Pro | ≈82% | 2025-09 |
Frequently asked questions
What is Claude Sonnet 4.5?
Claude Sonnet 4.5 is Anthropic's September 2025 mid-tier model in the Claude 4 family. At launch it achieved best-in-class scores on coding benchmarks like SWE-bench Verified and introduced agent-friendly features such as context editing and the memory tool.
Should I use Sonnet 4.5 or Sonnet 4.6?
For new projects in 2026, use Sonnet 4.6 — it has a longer context window and slightly better benchmarks. Sonnet 4.5 remains a safe choice for existing pipelines tuned to its behaviour.
What is the context window of Claude Sonnet 4.5?
Claude Sonnet 4.5 supports a 200,000-token context window. For very long documents, Sonnet 4.6 or Opus 4.7 with 1M context are a better fit.
Does Claude Sonnet 4.5 support computer use?
Yes. Sonnet 4.5 scored around 62% on OSWorld at launch, making it the first Sonnet strong enough to recommend for production browser-automation agents using Anthropic's computer-use tool.
Sources
- Anthropic — Claude Sonnet 4.5 — accessed 2026-04-20
- Anthropic — Pricing — accessed 2026-04-20