The tenth prompt tweak usually feels like progress.
The eleventh reveals the problem: the fix that stopped the agent from inventing refund policies also made it refuse legitimate refund questions, call the wrong tool, or ask for clarification when it already had enough context.
This is where prompt-only development breaks down.
AI agents are not just chat prompts. They are multi-step systems that combine model reasoning, retrieved context, tool calls, memory, permissions, retries, timeouts, and user-facing text. A prompt change can improve one behavior while silently degrading five others.
That is why the core engineering artifact for production agents is not a better prompt.
