Anthropic API Key Authentication Failing

Authentication failures on the Anthropic API almost always come down to the header, the environment, or the key's workspace scope.

Use the right header

Anthropic uses x-api-key, not Authorization: Bearer. You must also send anthropic-version. A missing version header returns a confusing error that looks like an auth failure.

x-api-key: sk-ant-...
anthropic-version: 2023-06-01
content-type: application/json

Check for whitespace and truncation

Keys copied into .env files frequently pick up a trailing newline or quote characters. Print the key length in your app to confirm it matches.

Confirm the environment actually loaded

A key that works in a local shell but fails in production usually means the deployment environment never received the variable. Verify at runtime, not at build time.

Workspace and key scope

Keys are scoped to a workspace. A key from one workspace cannot access another workspace's models or spend limits, and a revoked or expired key returns the same 401.

Billing state

If your organisation has no active credit balance, requests fail even with a valid key.

Once authentication succeeds, the next errors you are likely to meet are rate limits and 529 overloads.