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Model API ReferenceAnthropic MessagesOverview

Anthropic Messages API

The Anthropic SDK base URL is https://api.fenno.ai. It adds /v1/messages itself; do not use the OpenAI /v1 base URL in this SDK.

MethodPathReference
POST/v1/messagesCreate a message
POST/v1/messages/count_tokensCount input tokens

Headers

x-api-key: <FENNO_API_KEY>
anthropic-version: 2023-06-01
Content-Type: application/json

Differences from Chat Completions

  • System guidance is the top-level system, not an arbitrary system message inside messages.
  • max_tokens is required for message generation.
  • content can contain text, images, documents, tool calls, tool results or model-specific blocks.
  • Tool schemas use input_schema; results use tool_result blocks in user messages.
  • Streaming uses named message/content-block events instead of Chat delta objects.

Model access, vision, thinking and caching remain model-specific. Do not assume support for upstream hosted tools, batches or files because the Anthropic SDK includes those APIs.

For general ID discovery, see model queries; a model-list entry alone does not prove native Messages support. See Claude Code setup and the official Messages documentation .

ModelsMessages