OpenAI API vs Anthropic API: Which Should You Use?

Both APIs are excellent. The real question is which one fits your use case. Here's a no-hype breakdown from a developer's perspective.

TL;DR

API Setup

Both are straightforward. OpenAI:

npm install openai

import OpenAI from "openai";

const client = new OpenAI({ apiKey: process.env.OPENAI_API_KEY });

const response = await client.chat.completions.create({
  model: "gpt-4o",
  messages: [{ role: "user", content: "Hello!" }],
});

Anthropic:

npm install @anthropic-ai/sdk

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY });

const response = await client.messages.create({
  model: "claude-sonnet-4-20250514",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Hello!" }],
});

The main structural difference: Anthropic separates system from messages, while OpenAI puts everything in the messages array.

When to Pick OpenAI

Function calling / tool use — OpenAI's function calling is battle-tested and has better ecosystem support (LangChain, LlamaIndex, etc. target it first).

JSON mode — Pass response_format: { type: 'json_object' } and GPT-4o will always return valid JSON. Anthropic requires you to prompt for it.

Wider model range — GPT-4o mini is a great cheap model for simple tasks. Whisper (speech-to-text) and DALL-E (image generation) are in the same API.

Ecosystem maturity — Most tutorials, SDKs, and starter kits target OpenAI first.

When to Pick Anthropic

Long documents — Claude's 200k context window (vs 128k) matters when you're processing books, codebases, or long transcripts.

Coding tasks — Claude 3.5 Sonnet consistently scores at the top of coding benchmarks. It's particularly strong at understanding and refactoring existing code.

Writing quality — Claude tends to produce more natural, less robotic prose. Worth testing side-by-side if output quality matters for your product.

Instruction following — Claude is notably good at following complex, multi-step instructions without drifting.

Streaming — Both Work the Same Way

OpenAI:

const stream = await client.chat.completions.create({
  model: "gpt-4o",
  stream: true,
  messages,
});

for await (const chunk of stream) {
  process.stdout.write(chunk.choices[0]?.delta?.content ?? "");
}

Anthropic:

const stream = await client.messages.stream({
  model: "claude-sonnet-4-20250514",
  max_tokens: 1024,
  messages,
});

for await (const chunk of stream) {
  if (chunk.type === "content_block_delta") {
    process.stdout.write(chunk.delta.text);
  }
}

The Honest Answer

For most new projects, start with whichever you have credits for and switch if you hit a wall. The APIs are close enough in capability that the decision rarely matters early on.

The exceptions:

• Processing very long documents → Anthropic
• Heavy tool/function calling → OpenAI
• Matching an existing codebase → use what's already there

Run evals on your actual use case before committing either way. A benchmark that matters is one you ran yourself.