Claude Sonnet 4.5

Sonnet 4.5 shipped on September 29, 2025, as the smartest Sonnet yet in its price band. Top coding and agent numbers landed with this release, and the quick response that kept Sonnet the daily Claude Code choice never slowed down. At $3 input and $15 output per million tokens, most developers set it once and left it alone.

Key Specs

This release hit the sweet spot: real intelligence at a rate that supports all-day use.

What Landed With Sonnet 4.5

Top scores across its tier. At launch, Sonnet 4.5 posted the highest results on most evaluation tasks in its price band. Complex reasoning held up longer. Code generation showed measurable quality gains. Multi-step problems dropped fewer threads than earlier Sonnet releases.

Agent runs got more reliable. Chain tool calls together to hit a goal (the agentic pattern), and the improvement here was visible. Fewer runs abandoned midway. Better recovery after a tool failed. Smarter judgment about when to pause for a question versus push ahead.

1M context in beta. For long sessions and big repos, the beta window meant working an entire feature without bumping the context wall. API access came through a beta header (or the Console), and it made extended engineering sessions possible that earlier Sonnets could not sustain.

Best-in-class coding numbers. On SWE-bench, HumanEval, and practical coding completion tasks, Sonnet 4.5 sat at or close to the top of its tier. The usual gap between Sonnet and Opus narrowed enough that Claude Code users stopped reaching for Opus by reflex.

Sonnet 4.5 vs Sonnet 4

The upgrade from Sonnet 4 was substantial. Ambiguous prompts got handled with less thrashing. Generated code read as more idiomatic across a wider set of languages. Long conversations stayed coherent further in instead of unraveling at the edges.

Sonnet 4.5 vs Opus 4.1

Against Opus 4.1, the comparison got interesting. Many coding tasks came out equal or better on Sonnet, and it ran faster at lower cost per token on top of that. For most Claude Code workflows, the mid-tier model turned into the smarter pick over last generation's flagship.

Opus at the time cost roughly five times more per token, and Sonnet 4.5 was hitting 90% of its capability or better. The $3/$15 rate kept Claude Code cheap enough to run all day without thinking about the bill.

Daily Driver Status

For most Claude Code users, this was the one you set as default and rarely touched. Speed, intelligence, and cost made it the obvious daily pick.

Deeper reasoning problems still warranted a switch up to Opus. Simple, high-volume jobs went down to Haiku 4.5 for a fraction of the spend. The middle band of daily work ran on Sonnet 4.5 with no real tradeoff.

Current Status

Sonnet 4.6 has taken over. It brings Opus-tier intelligence while keeping the $3/$15 rate. The older model stays reachable through its full API identifier, and Anthropic now points every workload at 4.6 as the default Sonnet pick.

Related Pages

• All Claude Models for the complete model timeline
• Opus 4.1 for the previous generation's top model
• Haiku 4.5 for the budget option in the same family
• Model selection guide for strategic model switching
• Usage optimization for managing costs across models