Claude Sonnet 4.5 Review – Best AI Coding Model Yet, Benchmarks, Pricing and Practical Use

Introduction

Anthropic has just unveiled Claude Sonnet 4.5, its newest “frontier” model that the company touts as the best coding AI on the market. Promising stronger computer‑use capabilities, longer multi‑step reasoning, and improved math and STEM performance—all at the same price as its predecessor—Sonnet 4.5 is generating buzz among developers, data scientists, and AI enthusiasts alike. This article breaks down the model’s specifications, benchmark results, pricing, safety features, and real‑world tooling, so you can decide whether it deserves a place in your development workflow.

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Model Overview

Claude Sonnet 4.5 builds on the solid foundation of Claude Sonnet 4, adding notable upgrades in three core areas:

• Computer use – more reliable interaction with terminals, file systems, and external tools.
• Multi‑step reasoning – deeper context handling for complex problem solving.
• Math & STEM – higher accuracy on quantitative tasks.

Anthropic also markets Sonnet 4.5 as its most aligned frontier model to date, released under ASL‑3 safeguards, which aim to curb unsafe or unintended behavior.

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Pricing and Availability

The model is priced at the same rates as Sonnet 4, making the upgrade financially painless:

• $3 per million input tokens
• $15 per million output tokens

These rates are especially attractive for long‑running sessions that consume large token volumes, such as code‑generation loops or extensive debugging sessions.

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Benchmark Performance

Anthropic released a comprehensive benchmark suite that pits Sonnet 4.5 against its rivals—Opus 4.1, GPT‑5, Gemini 2.5 Pro, and the older Sonnet 4. Below are the headline numbers (higher is better unless noted otherwise):

SWE‑Verified Agentic Coding

• Sonnet 4.5: 77.2 %
• Opus 4.1: 74.5 %
• Sonnet 4: 72.7 %
• GPT‑5: 72.8 %
• Gemini 2.5 Pro: 67.2 %

Terminal‑Style Coding (Terminal Bench)

• Sonnet 4.5: 50.0 %
• Opus 4.1: 46.5 %
• GPT‑5: 43.8 %
• Sonnet 4: 36.4 %
• Gemini 2.5 Pro: 25.3 %

Computer Use (OSWorld)

• Sonnet 4.5: 61.4 %
• Sonnet 4: 42.2 %
• Opus 4.1: 44.4 %

Reasoning‑Heavy Python Tasks (Aim 2025)

• Sonnet 4.5: 100 %
• GPT‑5: 99.6 %
• Gemini 2.5 Pro: 94.6 %
• Opus 4.1: 78.0 %
• Sonnet 4: 70.5 %

GPQA‑Diamond (General Knowledge)

• Sonnet 4.5: 83.4 %
• GPT‑5: 85.7 %
• Gemini 2.5 Pro: 86.4 %
• Opus 4.1: 81.0 %
• Sonnet 4: 76.1 %

Multilingual MMLU

• Sonnet 4.5: 89.1 %
• Opus 4.1: 89.5 %
• GPT‑5: 89.4 %

Visual Reasoning (MM‑Validation)

• Sonnet 4.5: 77.8 %
• GPT‑5: 84.2 %
• Gemini 2.5 Pro: 82.0 %
• Sonnet 4: 74.4 %

Finance Agent

• Sonnet 4.5: 55.3 %
• Opus 4.1: 50.9 %
• GPT‑5: 46.9 %
• Sonnet 4: 44.5 %
• Gemini 2.5 Pro: 29.4 %

Domain‑Specific Win Rates (Extended 16 k Context)

• Finance: 72 % (Sonnet 4.5) vs. low‑60 % for Opus 4.1 and ~50 % for Sonnet 4.
• STEM: 69 % (Sonnet 4.5) vs. 62 % for Opus 4.1 and 58 % for non‑extended Sonnet 4.5.

Overall, Sonnet 4.5 consistently outperforms its predecessor and many competitors, especially in coding‑centric and reasoning‑heavy tasks.

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Safety and Alignment

Anthropic highlights ASL‑3 (Alignment Safety Level 3) as the model’s safety tier. In internal misalignment tests, Sonnet 4.5 achieved the lowest error score among the evaluated models, indicating fewer unexpected or harmful outputs.

• Implication: When the model is used for browsing, file editing, or command execution, it is less likely to produce erratic behavior.
• Caveat: ASL‑3 still employs classifiers that may interrupt sessions in sensitive domains, occasionally generating false positives. In such cases, developers can fall back to Claude Sonnet 4 within the same thread.

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Practical Development Tools

Anthropic bundles Sonnet 4.5 with a set of developer‑focused utilities that streamline day‑to‑day coding.

Claude Code and Checkpoints

• Checkpoints let you save the model’s state mid‑task and instantly roll back if something goes awry—ideal for iterative debugging.
• The feature works both in the web UI and via the VS Code extension.

VS Code Extension

• Simple installation: add the extension, sign in with your Anthropic account, and connect to your workspace.
• Provides an experience comparable to Klein or GitHub Copilot, but with Sonnet 4.5’s superior coding abilities.
• Free tier includes a $25 credit, allowing unrestricted experimentation.

Claude Agent SDK

• Offers the same low‑level primitives Anthropic uses for its internal “Claude Code” system.
• Enables developers to build custom agentic workflows:
  • Controller agents orchestrate sub‑agents.
  • Testing agents run sandboxed commands.
  • Documentation agents generate summaries and changelogs.
  • Deployment agents act only after explicit approval.
• Supports parallel tool execution, maximizing actions per context window—a boon for CI pipelines.

Tip: While the SDK is powerful, effective use still requires thoughtful repository indexing and clear role definitions. A chaotic monorepo will not magically become manageable.

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Strengths and Limitations

Strengths

• Higher accuracy on coding, terminal, and math benchmarks.
• Improved alignment reduces risky behavior during autonomous tool use.
• Checkpoints simplify state management during long coding sessions.
• Flat pricing keeps token‑heavy workflows affordable.
• Integrated tooling (Claude Code, VS Code extension, Agent SDK) keeps the experience inside familiar environments.

Limitations

• ASL‑3 interruptions can still occur in edge‑case domains, requiring a manual fallback to Sonnet 4.
• Visual‑reasoning lags behind the top performer (GPT‑5) on certain metrics.
• Complex web‑scraping or highly dynamic pages may need extra supervision.
• Large, unstructured codebases still demand good repo organization; the model does not replace proper project hygiene.

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Conclusion

Claude Sonnet 4.5 represents a meaningful upgrade over its predecessor, delivering the strongest coding performance Anthropic has offered to date. Benchmarks confirm its lead in agentic coding, terminal interaction, and STEM reasoning, while the ASL‑3 safety tier provides a reassuring level of alignment for autonomous tasks.

For developers who value reliability, cost‑effective token usage, and deep integration with existing IDEs, Sonnet 4.5 is a compelling choice. Its new checkpoint system and robust SDK open doors to sophisticated, custom agentic workflows—provided you invest in proper repository structuring and policy design.

Stay tuned for upcoming hands‑on reviews that will put Sonnet 4.5 through real‑world developer pipelines. In the meantime, consider testing the model via the Ninja Chat platform (access to multiple top‑tier models in one UI) or directly through Anthropic’s API.

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