I use my own API key in private projects, and it seems to work like this:

# .env file
ANTHROPIC_API_KEY=sk-...

In the project:

# .claude/settings.json
{
  "apiKeyHelper": "source .env && echo $ANTHROPIC_API_KEY"
}

Then invoke claude in inside the project root and verify with /status command:

Claude Code v1.0.85
  L Session ID: 69...
...
 Account
  L Auth Token: apiKeyHelper
  L API Key: apiKeyHelper

Source: https://github.com/anthropics/claude-code/issues/441#issuecomment-3207226042

If someone get this error: Error getting API key from apiKeyHelper (in settings or ~/.claude.json): /bin/sh: 1: source: not found

That error comes from the fact that your shell (likely /bin/sh) doesn’t recognize the source command — it’s a Bash built-in, not POSIX-standard.

Here's the fix (Force bash):

{ "apiKeyHelper": "bash -c 'source .env && echo $ANTHROPIC_API_KEY'" }

Never teach people to put a secret key in a non-popular env var convention file. Always put it in the popular environment variable file.