Migrate from OpenAI to Anthropic

Overview

Migrate from OpenAI/GPT to Anthropic/Claude. Covers the complete API mapping (endpoints, models, response shapes), SDK swap with before/after code, five key differences (max_tokens required, system as top-level param, alternating messages, response path, streaming events), and tool use migration.

API Mapping

OpenAI	Anthropic	Notes
openai.chat.completions.create()	anthropic.messages.create()	Different request/response shape
model: 'gpt-4o'	model: 'claude-sonnet-4-20250514'	Different model IDs
response.choices[0].message.content	response.content[0].text	Different response path
system in messages array	system as separate parameter	Claude uses top-level system
response_format: { type: 'json_object' }	System prompt: "Respond in JSON only"	No native JSON mode
tools / function_calling	tools (similar but different schema)	Input schema differences
openai.embeddings.create()	N/A — use Voyage or Cohere	No embeddings API

SDK Swap

Instructions

Step 1: Before (OpenAI)

import OpenAI from 'openai';
const openai = new OpenAI();

const response = await openai.chat.completions.create({
  model: 'gpt-4o',
  messages: [
    { role: 'system', content: 'You are helpful.' },
    { role: 'user', content: 'Hello' },
  ],
});
console.log(response.choices[0].message.content);