Anthropic Claude API key quickstart

Published 2026-10-05 · Updated 2026-10-24

Anthropic keys look like sk-ant-api03- followed by a long token ending in AA. The API is close to OpenAI’s but not identical; here’s what actually matters.

Set the key

export ANTHROPIC_API_KEY="sk-ant-api03-..."

The curl shape is different

Anthropic doesn’t use Authorization: Bearer. It wants two headers: x-api-key and anthropic-version.

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-5",
    "max_tokens": 256,
    "messages": [{"role": "user", "content": "Say hello"}]
  }'

Two things in there that bite newcomers:

• max_tokens is required. Omit it and you get a 400, not a default.
• The endpoint is /v1/messages, not /v1/chat/completions.

Python SDK

pip install anthropic

from anthropic import Anthropic

client = Anthropic()  # reads ANTHROPIC_API_KEY

msg = client.messages.create(
    model="claude-sonnet-5",
    max_tokens=256,
    messages=[{"role": "user", "content": "Say hello"}],
)
print(msg.content[0].text)

Check a key cheaply

GET https://api.anthropic.com/v1/models with the x-api-key header lists models and costs nothing. It’s the cleanest way to confirm a key works before you burn tokens.

If you’d rather stay OpenAI-compatible

Claude models are also on OpenRouter, use an sk-or-v1- key, the openrouter.ai/api/v1 base URL, and model id anthropic/claude-sonnet-5. Your existing OpenAI-shaped code then works untouched. The trade-off is you pay OpenRouter’s pricing and lose a few Anthropic-only features.

Error cheat sheet

• 401: bad or revoked key.
• 400 with “max_tokens”: you forgot the field.
• 429: rate limit or exhausted credit.
• 403: key valid but lacks access to that model.

That’s the whole quickstart: header instead of bearer, messages instead of chat/completions, and never forget max_tokens.