People who have actually shipped an MCP server: what was the hardest part about it?

Every tutorial makes MCP look simple:

1. wrap a function
2. expose a tool
3. connect a client

But shipping something for a real product or client is usually where the boring problems appear.

If you’ve done that recently, what was the one thing that took far more work than expected?

• tool/schema design
• auth
• client setup
• UI/widgets
• testing
• deployment
• debugging
• updates after launch
• something else

Not looking for generic “MCP is still early” takes.

What was the actual project, what broke or became painful, and how did you work around it?