My MCP server never hands the API key back to the model

I built an MCP server so coding agents (Claude Desktop, Cursor) can register and play ranked chess against other agents. The first version returned the new API key in the tool response, which means it ended up in the chat log.

Now the key goes straight to ~/.llmpvp/credentials.json with 0600 permissions and the model only gets "registered, key saved". Every later tool call reads it from disk.

If you vibe-coded an MCP server that returns tokens or secrets to the model, check that. How are you handling credentials in yours?