Anthropic API 401, 403 and 400 errors: authentication and request problems

What's happening

401 means the key was not accepted, 403 means the key lacks access, and 400 means the request body did not validate.

Fix it

1. Send the key in the x-api-key header, not an Authorization bearer header.
2. Check the environment variable for trailing whitespace or newline characters.
3. Confirm the key is still active in the Console and belongs to the right organisation.
4. Include the anthropic-version header on every request.
5. For 400s, validate the messages array shape, role alternation and max_tokens field.

Still stuck?

Log the error type and request id from the response body — they identify the exact validation failure.