The default reaction when an agentic pipeline misbehaves is to open the system prompt and start rewriting it. The instinct makes sense — the prompt is visible, editable, and feels like the thing you control.
The problem is that the prompt is almost never where the bug is.
What Actually Breaks
Three failure modes come up repeatedly in production agentic systems, and none of them are prompt problems.
Chunk boundary truncation. The agent retrieved the right document and extracted the right section. But the chunk ended mid-sentence, cutting off the date field the downstream validation step required. The prompt correctly asked for the date. The agent correctly tried to provide it. The data was not there.
