nullcipher

Anthropic API keys (sk-ant-…)

sk-ant- plus 20 or more letters, digits, dashes or underscores (sk-ant-api03-…, sk-ant-admin01-…). Below: the exact rule, a live check, why it matters, and what to do if one already leaked.

Live check: Anthropic API key

What an AI chat would receive after masking

[ANTHROPIC_KEY_1]

1 × Anthropic API key found and masked.

How nullcipher recognises it

sk-ant- followed by at least 20 letters, digits, dashes or underscores (sk-ant-api03-…, sk-ant-admin01-…).

This is the rule text of the “Anthropic API key” detector in the masking engine, copied from the engine when this page was built. Detectors run in a fixed order and, where two overlap, the earlier one wins.

Example (fake)

sk-ant-api03-EXAMPLE-EXAMPLE-EXAMPLE-EXAMPLE-notarealkey

nullcipher replaces it with [ANTHROPIC_KEY_1]. The part after sk-ant-api03- spells EXAMPLE and notarealkey. Anthropic never issued it.

Why it matters in an AI chat

An Anthropic API key lets whoever holds it send requests to the Claude API as your organisation, and the usage is charged to your account.

Anthropic automatically deactivates a key that GitHub detects in a public repository, and e-mails the owner. That protection covers public GitHub repositories. A key pasted into a chat, a support ticket or a private log is not scanned, and stays live until you delete it.

It is also the key people most often paste into Claude itself while debugging code that calls the API.

If it already leaked

  1. Log in to the Claude Console, go to the API keys page from your profile, click the ⋯ (three dots) menu next to the key and select Delete API Key. [1]
  2. Create a new key, put it wherever the old one was used, and keep it in an environment variable or secrets manager rather than in code you might paste.

Sources

  1. API Key Best Practices: Keeping Your Keys Safe and Secure — Claude Help Center — checked 2026-09-23.

Open the full masking tool to mask a whole log, .env file or e-mail — every type at once — and restore the values in the AI's answer. Or see all 16 secret types and how to delete a chat you already sent.