Claude Sonnet 4.5

Anthropic·Sep 2025reasoningproprietary

65.5

modhub Index

Claude Sonnet 4.5 (September 2025) was billed as the best coding model in the world at launch and proved capable of maintaining focus across 30+ hours of autonomous multi-step work. It drove the explosive growth of Claude Code through late 2025. Sonnet 4.6 has since replaced it as the mid-tier default, but 4.5 remains available, deeply benchmarked, and a common pin for stable agent deployments.

Benchmark results

Where it shines

  • Exceptional long-horizon agentic endurance
  • Strong computer-use and tool-calling reliability
  • Mature ecosystem of prompts and integrations

Alternatives to Claude Sonnet 4.5

GLM-4.6

The budget coding-agent favorite of late 2025 — near-Sonnet coding utility with MIT weights and a $3/month plan.

Kimi K2 Thinking

A trillion-parameter open reasoning agent that can chain 200–300 tool calls — the open-weights agentic standout of late 2025.

DeepSeek V3.2

The sparse-attention release that halved DeepSeek's already-lowest-in-class prices while keeping GPT-class quality.

OpenAI o3

OpenAI's dedicated 2025 reasoning model that pioneered thinking-with-images and agentic tool use within chain-of-thought.

Frequently asked questions

How much does the Claude Sonnet 4.5 API cost?
Claude Sonnet 4.5 costs $3 per million input tokens and $15 per million output tokens, with cached input at $0.3 per million. A workload of 10M input and 1.5M output tokens per month costs about $52.50.
What is the context window of Claude Sonnet 4.5?
Claude Sonnet 4.5 supports a context window of 200,000 tokens (200K), with up to 64K output tokens per response.
Is Claude Sonnet 4.5 open source?
No — Claude Sonnet 4.5 is a proprietary model available through Anthropic's API and partner platforms.
What are the best alternatives to Claude Sonnet 4.5?
The closest alternatives by overall capability are GLM-4.6, Kimi K2 Thinking, DeepSeek V3.2, OpenAI o3. See the comparison pages for detailed head-to-head breakdowns.