Hand over the why, not just the code.
When a prototype becomes a real product, engineers may keep your code or rebuild it. Either way, what they need most is context.
The problem and who has it
The prototype link and repo
What's real and what's faked
Known shortcuts and risks
Your CLAUDE.md and key prompts
Test cases and edge cases found
What user feedback said
What data it touches
Your prototype is the spec. Your notes are the gold.