You call Claude Opus 4.5 through Anthropic’s standard Messages API at https://api.anthropic.com/v1/messages, using the model ID claude-opus-4-5-20251101. In practice, this means sending a POST request with your API key in the x-api-key header, setting anthropic-version, and including "model": "claude-opus-4-5-20251101" in the JSON body along with messages and max_tokens. The official quickstart examples use the same structure—just replace other model names with Opus 4.5. A valid request includes a messages array where each item defines a role and content, typically simple text instructions.
A minimal curl command uses an environment variable like ANTHROPIC_API_KEY, then posts a JSON payload with the model name, user message, and output settings. The same pattern applies to the official Python and TypeScript SDKs: instantiate the client, then call client.messages.create() with model="claude-opus-4-5-20251101". Additional parameters like temperature and effort allow you to influence reasoning depth and latency. The API surface itself stays consistent across models, so adopting Opus 4.5 is mainly about swapping the model ID.
In production, this API call is commonly wrapped inside a backend service, gateway, or orchestration layer that also handles retrieval, logging, and tool execution. For retrieval-augmented generation, you may first query a vector database such as Milvus or Zilliz Cloud, then pass only the most relevant documents to Claude Opus 4.5 in each request. This reduces token usage and keeps application logic clean while leaving the model stateless. Once this pattern is in place, more advanced features—tool calls, structured outputs, file inputs—fit naturally into the same /v1/messages contract.