OpenAI Chat Completions to Anthropic Messages

This page documents the conversion pair used when a client speaks OpenAI Chat Completions and the selected provider speaks Anthropic Messages.

Inbound

`openai_chat_completions` request payload

Normalize

`src/ingress/` parses Chat messages and tools

Translate request

Chat messages/tools -> Anthropic Messages request

Translate response

Anthropic Message/content blocks -> Chat choices

Outbound

`openai_chat_completions` response or SSE chunks

Request mapping

Chat concept	Anthropic Messages target	Notes
`messages[]`	`messages[]` plus normalized system placement	Chat system/developer instructions must not be duplicated as user-visible turns.
Assistant text	Assistant content blocks	Preserve ordered visible text.
`tools[]` function definitions	`tools[]`	Preserve name, description, and JSON schema where possible.
Assistant `tool_calls[]`	Anthropic `tool_use` blocks	Chat `tool_calls[].index` is stream-local; Anthropic content blocks use content-block indices.
Tool result messages	Anthropic `tool_result` blocks	Preserve call id linkage where target protocol can represent it.

Anthropic concept	Chat target	Notes
Text content block	`choices[0].message.content`	Chat has a single assistant message per choice.
Tool use block	`choices[0].message.tool_calls[]`	Assign stable call ids and function names.
Thinking/reasoning block	Chat-compatible reasoning extension when supported	Plain Chat has weak reasoning structure; do not mix reasoning into ordinary content unless required.
`stop_reason`	`finish_reason`	Map natural turns, token limits, tool use, and refusals carefully.

Anthropic event	Chat SSE expectation
`message_start`	Initialize choice and assistant role delta.
Text `content_block_delta`	Emit `delta.content` chunks.
Tool `input_json_delta`	Emit `delta.tool_calls[].function.arguments` chunks keyed by Chat tool index.
`message_delta.stop_reason`	Set final `finish_reason` when terminal state is known.
`message_stop`	Emit final chunk and `[DONE]`.

Chat has no native output item graph.
Reasoning support is weaker and often extension-based.
Chat streaming uses tool_calls[].index; Anthropic streaming uses content block index.
Chat [DONE] is a stream carrier terminator; Anthropic message_stop is the semantic terminal event.