An agent that works once in a clean session meets four things in production it never saw: memory it cannot trust, untrusted input it does trust, multi-step tasks where a step silently fails, and repeated work it re-pays for every call.

No prompt fixes these. Each is fixed in the harness around the model: a deterministic gate before a write, a ground-truth check after a step, a tool the agent writes once and reuses. Five runnable demos, each reproducing the failure, then fixing it.

Outline:
• The demo lied to you
• Diagnose before you fix: break it on purpose
• Memory it cannot trust
• Memory it should not have trusted
• The step that silently failed
• Work it keeps re-paying for
• The decision rule + Q&A