Welcome Sonnet 4.5 to Theia AI (and Theia IDE)!

Developers and tool builders can use Anthropic’s Sonnet 4.5 directly in Theia AI and the AI-powered Theia IDE, without any additional glue code. Just add "sonnet-4.5" to your model list in your settings—and you’re ready to go.

If you’re new to Theia AI or the AI-powered Theia IDE, check out our Theia AI introduction and AI Theia IDE overview, or download the AI-powered Theia IDE here.

For background on how Theia’s architecture supports any LLM, see Why Theia supports any LLM.

What We Know (and What We Expect) About Sonnet 4.5

Anthropic announced Claude Sonnet 4.5 on September 29, 2025, positioning it as their “most capable” model for agentic, coding, and computer-use tasks. (TechRadar)

From public reports, here are the highlights—along with what they mean for tool builders and IDE users:

• Long-running autonomous work (up to 30 hours) – Anthropic reports Sonnet 4.5 can sustain autonomous operation far beyond the ~7 hours of Opus 4.1 (Axios). For tool builders, this opens possibilities for agents that handle extended workflows like large-scale code transformations or multi-step simulations. For IDE users, it means agents that can keep going on complex coding tasks without interruption.

• Improved coding, reasoning, and math – Marketed as the “best coding model in the world” with substantial gains in accuracy (TOI). Tool builders can rely on fewer hallucinations and better structured reasoning for integrating agents into specialized tools. IDE users should see more reliable completions, debugging support, and multi-file reasoning.

• Better computer use and tool integration – Sonnet 4.5 advances Anthropic’s “computer use” paradigm with stronger handling of files, interactions, and multi-step tool sequences (TechRadar). Tool builders gain a model that orchestrates workflows more effectively, making it easier to build sophisticated agents. IDE users can expect smoother multi-step automation, like making edits, and running tests, fixing issues in one flow.

• Balanced power and cost – As the mid-tier model in the Claude lineup, Sonnet 4.5 is designed to balance capability with efficiency (Anthropic). For tool builders, it offers a practical default choice when embedding AI without incurring top-tier costs. For IDE users, it becomes the “just right” model: powerful enough for demanding work, but efficient enough to use as the everyday default.

• Seamless adoption – Sonnet 4.5 is being rolled out at the same price point, with easy upgrade paths for existing users (TechRadar). Tool builders can add it immediately to their model lists in Theia AI, and IDE users benefit from having it available out of the box—no more switching between Sonnet 4 for everyday tasks and Opus 4 for heavy lifting.

Of course, these claims mostly still come from Anthropic and press reporting; independent benchmarking is still limited. context window limits).

Configuring and Using Sonnet 4.5 in Theia IDE

Thanks to Theia AI’s open, model-agnostic architecture, adding Sonnet 4.5 is purely a settings update—no internal code changes. Once configured, Sonnet 4.5 is instantly available to all AI agents in the Theia IDE (e.g. Theia Coder).

In the demo below, you’ll see:

1. How to configure Sonnet 4.5 in the Theia IDE
2. How Sonnet 4.5 can add itself to the default model list, so that future versions of Theia will ship with it enabled

From zero to full integration, it takes just a few steps to get Sonnet 4.5 fully working and default inside your Theia workflows.

Why We’re Personally Happy

We’re really glad we don’t have to switch between Sonnet 4 and Opus 4 anymore—that overhead is now eliminated. :-)

We look forward to extensive testing and real-world feedback, and we’ll share findings on Sonnet 4.5’s strengths, limitations, and best practices in Theia-based tool development.

For more background, see our previous posts: Introducing the AI-powered Theia IDE and Introducing Theia AI.

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