oai2ant

Zero-dependency, pure-function converters between the Anthropic Messages API and the OpenAI Chat Completions API. It lets an Anthropic-Messages client talk to an OpenAI-compatible backend: translate the request to OpenAI, then translate the response (and stream) back to Anthropic.

• antReqToOaiReq(req) — Anthropic request → OpenAI request
• oaiResToAntRes(res) — OpenAI response → Anthropic message
• oaiStreamToAntStream(chunks) — OpenAI chunk stream → Anthropic SSE events

Types come from the official @anthropic-ai/sdk and openai packages (dev dependencies only); the published code has no runtime dependencies. Non-standard fields used by OpenAI-compatible backends — reasoning_content/reasoning, chat_template_kwargs, defer_loading, tool_reference — are modeled by the OpenAI* types in src/types/openai.ts, which extend the official SDK types.

Usage

import { antReqToOaiReq, oaiResToAntRes, oaiStreamToAntStream } from "oai2ant";

// 1. Anthropic request -> OpenAI request, send to an OpenAI-compatible backend.
const oaiReq = antReqToOaiReq(anthropicRequest);

// 2. Non-streaming: OpenAI response -> Anthropic message.
const antRes = oaiResToAntRes(await openai.chat.completions.create(oaiReq));

// 3. Streaming: OpenAI chunks -> Anthropic SSE events.
const stream = await openai.chat.completions.create({ ...oaiReq, stream: true });
for await (const event of oaiStreamToAntStream(stream)) {
  // event is an Anthropic RawMessageStreamEvent: write it to your SSE response.
}

What it converts

• Messages: system prompt, multi-turn text, images (base64 → data URI, url), tool calls (tool_use ↔ tool_calls), and tool results (tool_result → tool role messages).
• Tools: definitions, tool_choice (incl. parallel-use toggle), and defer_loading / tool_reference for chat-template deferred tool loading.
• Reasoning: thinking config → reasoning_effort, and thinking blocks ↔ reasoning_content/reasoning (with chat_template_kwargs thinking control).
• Stop reasons, token usage, refusals, and the full streaming event lifecycle.

Development

bun install
bun test          # 100% coverage is enforced
bun run typecheck
bun run lint