The Claude Sonnet 4.5 API is our coding-agent default — it ships multi-file PRs that pass review. The 5% RouterBase discount is pure margin.
The Claude Sonnet 4.5 API is Anthropic's best model for coding and agentic workflows, balanced for speed and cost — a 200K-token context window, extended thinking, vision, tool use, and prompt caching. The Claude Sonnet 4.5 API is OpenAI-compatible: point any existing SDK at RouterBase and swap the model name to claude-sonnet-4-5. Input at $2.85 / 1M tokens, output at $14.25 / 1M tokens, cache reads at $0.285 / 1M tokens — 5% below the standard rate, all through one REST endpoint.
Input
Claude Sonnet 4.5
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Claude Sonnet 4.5 API: Chat
Use the Claude Sonnet 4.5 API to run Anthropic's best coding and agent model — a 200K-token context, extended thinking, and prompt caching.
The Claude Sonnet 4.5 API is Anthropic's best model for coding and agentic workflows, balanced for speed and cost — a 200K-token context window, extended thinking, vision, tool use, and prompt caching. Routed through RouterBase, the Claude Sonnet 4.5 API is fully OpenAI-compatible: point any existing SDK at RouterBase's base URL, swap the model name to claude-sonnet-4-5, and your first call goes through immediately.
Pricing is $2.85 / 1M input tokens, $14.25 / 1M output tokens, and $0.285 / 1M cached-input reads — 5% below the standard rate. One RouterBase key — no Anthropic account required.
Six reasons teams ship on the Claude Sonnet 4.5 API
From best-in-class coding to 5%-off pricing — what makes the Claude Sonnet 4.5 API stand out.
Best-in-class coding
The Claude Sonnet 4.5 API leads on real-world coding and refactoring — it edits multi-file repos, writes tests, and fixes bugs with the reliability teams ship to production.
Built for agents
Long-horizon tool use, planning, and state tracking make the Claude Sonnet 4.5 API a strong agent backbone — it stays on task across many steps without losing the thread.
200K-token context
Accepts up to 200,000 tokens per request — long documents, multi-file codebases, or full conversation history fit in a single Claude Sonnet 4.5 API call without chunking.
Extended thinking
Enable extended thinking to let the Claude Sonnet 4.5 API reason step-by-step on hard problems, or keep it off for the fastest responses — you control the depth per request.
Prompt caching
Cache long system prompts and shared context; the Claude Sonnet 4.5 API bills cached reads at $0.285 / 1M, cutting cost up to 90% on repeated prefixes.
OpenAI-compatible endpoint
The Claude Sonnet 4.5 API speaks the OpenAI chat-completions wire format. Point any OpenAI SDK at RouterBase — no Anthropic SDK or separate credentials needed.
Quickstart
Get started with the Claude Sonnet 4.5 API in 3 steps
From sign-up to your first response in under 5 minutes.
Create a RouterBase API key
Sign up and generate an API key — one key reaches the Claude Sonnet 4.5 API and every other model in the catalog.
Send your first message
Point any OpenAI-compatible SDK at routerbase.com/v1 and set the model to claude-sonnet-4-5. The response follows the standard chat-completions schema — streaming and tool use supported.
Inspect usage
Every response includes a detailed token breakdown — input, output, and cache-read tokens — so cost and cache-hit rate are visible on every call.
Pay only for what you use
RouterBase passes through partner-tier pricing. Compared against the model's official published API rate.
What teams build with the Claude Sonnet 4.5 API
Real production loads running on the Claude Sonnet 4.5 API and the RouterBase model catalog.
We swapped our agent backbone to the Claude Sonnet 4.5 API and watched task-completion rates climb. It stays on task across dozens of tool calls.
Prompt caching on the Claude Sonnet 4.5 API dropped our agent bill by more than half — the long system prompt is basically free now.
Extended thinking on the Claude Sonnet 4.5 API lets us dial reasoning depth per task. RouterBase makes it 5% cheaper and routes around outages automatically.
200K of context means I can drop a whole repo in. The Claude Sonnet 4.5 API refactors across files without losing the thread.
RouterBase puts the Claude Sonnet 4.5 API and 200+ other models behind one key. Our team stopped filing requests for new provider accounts.
We A/B the Claude Sonnet 4.5 API against Opus per request — same SDK, one model field. Most traffic stays on Sonnet and the bill dropped sharply.
We migrated our coding agent to the Claude Sonnet 4.5 API over a weekend. The only PR comment was 'wait, that's all?'.
At $2.85 / 1M input minus 5%, the Claude Sonnet 4.5 API gave us flagship-grade coding without the flagship bill. The ROI math was instant.
Frequently Asked Questions
Common questions about the Claude Sonnet 4.5 API.
It is RouterBase's pass-through to Anthropic's Claude Sonnet 4.5 — Anthropic's best coding and agent model, with a 200K-token context, extended thinking, vision, tool use, and prompt caching, served via an OpenAI-compatible REST interface.