This stays in your browser's local storage only — nothing is sent anywhere except directly to Anthropic's API if you add a key for grading.
Note: a Claude Pro/Max/Team subscription does not include an API key — it's a separate Console account at console.anthropic.com, billed separately. A live key can be used to run up charges on your account if it leaks, so please be prudent: create a short-lived key (e.g. 1-day expiry) just for this practice session rather than a permanent one. If it expires mid-course, generate a fresh one and paste it in here again — your progress isn't affected.
No key? You can still fly every mission — you'll self-assess against the reference answer instead of getting Claude's grading.