Claude Sonnet 5.5 API

Technical Specifications of Claude Sonnet 5.5

What Is Claude Sonnet 5.5?

Claude Sonnet 5.5 is Anthropic's September 2026 Sonnet release and the second model in the Claude 5.5 family. Anthropic positions it as a faster, lower-cost complement to Claude Opus 5.5, with particular strength in well-scoped everyday work, software development, bug fixing, and producing polished documents, slides, and spreadsheets.

The model uses a 1M-token context window and supports text and image inputs with text output. Its adaptive thinking system adjusts reasoning depth through the effort setting, while the Claude API defaults to high effort. Anthropic also reports that Sonnet 5.5 generates output more than 30% faster than Sonnet 5.

Main Features of Claude Sonnet 5.5

• 1M-token long context: The large context window supports long documents, substantial codebases, and extended agentic workflows without requiring aggressive context reduction.
• Adaptive thinking: Sonnet 5.5 uses adaptive thinking, with effort controlling thinking depth, latency, and token consumption.
• Major coding improvement: Anthropic reports 70.6% on Terminal-Bench 4.0 versus 10.3% for Sonnet 5, alongside strong CursorBench and FrontierCode results.
• Fast iterative workflows: Anthropic says Sonnet 5.5 generates outputs 30%+ faster than Sonnet 5, making it suitable for repeated coding, review, and content-generation cycles.
• Knowledge-work performance: The model is designed for professional tasks across analysis, document production, computer use, and other long-horizon knowledge workflows.
• Image understanding and design: Sonnet 5.5 accepts image inputs and shows strong chart recognition and screenshot-based reasoning, while Anthropic highlights improvements in UI polish and presentation generation.

Benchmark Performance of Claude Sonnet 5.5

Anthropic reports the following results for Sonnet 5.5:

Benchmark figures are reported by Anthropic and should be interpreted within each benchmark's methodology and effort setting. For example, FrontierCode results vary with effort, and Anthropic notes that a pre-release structured-output bug may have understated some Artificial Analysis results.

Claude Sonnet 5.5 vs Claude Sonnet 5 vs Claude Opus 5.5

Anthropic describes Sonnet 5.5 as the faster, lower-cost complement to Opus 5.5. Its own evaluations show that Sonnet 5.5 can approach Opus 5.5 on some tasks at higher effort, while Anthropic still identifies Opus 5.5 as stronger for complex, open-ended work requiring sustained judgment.

Limitations and API Compatibility Notes

Claude Sonnet 5.5 introduces several migration considerations for applications previously using Sonnet 5:

• thinking: {"type": "disabled"} is no longer accepted; between_tools is the lowest thinking setting.
• Forced tool use with tool_choice types any and tool is not supported. auto and none are supported.
• Thinking blocks are tied to the model and conversation, which matters when switching models during an existing conversation.
• On the Claude API and Google Cloud, the earlier computer_20251124 computer-use tool is not accepted.
• Non-default temperature, top_p, or top_k values return a 400 error.
• Sonnet 5.5 has cybersecurity safeguards and fallbacks for higher-risk requests; some biology-related requests can also be flagged under its safety controls.

How to Access Claude Sonnet 5.5 API?

You can access Claude Sonnet 5.5 through CometAPI without setting up a separate Anthropic API integration. CometAPI provides a unified API layer for Claude and other leading AI models, allowing you to use one API key while keeping the model ID claude-sonnet-5-5 in your request.

Step 1: Create a CometAPI Account and Get an API Key

Go to CometAPI and sign up for an account. After logging in, open the API token section in your CometAPI console and create an API key.

Keep the API key on your server or in an environment variable. Do not expose it in browser-side JavaScript, mobile applications, public repositories, or client-side logs.

Step 2: Select Claude Sonnet 5.5

Open the Claude Sonnet 5.5 model page in CometAPI and select the model for your application.

Use the model ID:

claude-sonnet-5-5

For Claude-native integrations, use CometAPI's Anthropic-compatible Messages API. CometAPI's Claude integrations expose the /v1/messages endpoint for this workflow. If your application already uses OpenAI-compatible SDKs or a multi-model router, use the compatible chat-completions interface where supported.

Claude Sonnet 5.5 supports a 1-million-token context window and adaptive thinking. Anthropic's Claude Platform defaults to high effort for this model, while the effort setting can be adjusted when you need to trade reasoning depth against latency and token usage.

Step 3: Send Your First API Request

Configure your application with three essential values:

• Base URL: https://api.cometapi.com
• Model: claude-sonnet-5-5
• Authentication: Your CometAPI API key

Then send your prompt through the Messages API.

For a basic request, provide the model ID, a maximum output-token limit, and a messages array containing the user's prompt. The response will contain Claude Sonnet 5.5's generated text.

Before moving to production, test the exact parameters and response format used by your application in CometAPI's API documentation or Playground.

What Should You Know Before Migrating to Claude Sonnet 5.5?

If you are migrating an existing Claude integration, do not simply replace the previous model ID and assume every parameter remains compatible. Anthropic specifically notes that applications migrating to Sonnet 5.5 need to change workflows that previously disabled thinking with thinking: {"type": "disabled"}; the new between_tools setting should be used when up-front thinking needs to remain off.

It is also worth retesting tool calls, reasoning settings, output limits, and prompt behavior before deploying the new model to production.

Why Use CometAPI for Claude Sonnet 5.5?

CometAPI is useful when you want to evaluate Claude Sonnet 5.5 alongside models from other providers without maintaining separate API integrations. Instead of managing individual provider credentials and billing workflows, you can access supported models through a unified API layer and switch models by changing the model ID.

For developers building multi-model applications, this also makes it easier to compare Claude Sonnet 5.5 with other coding, reasoning, and multimodal models before committing to a production architecture.