Don't Tweak Prompts First When Debugging Agents
hannune · reddit · 2026-07-17
The core argument of this post is that most bugs in agentic pipelines lie in the chain and data interfaces, not the prompt.
The author recommends implementing basic observability before modifying the system prompt:
- Add spans for every tool call
- Log structured records during context injection
- Increment counters at every router branch
This makes it easier to pinpoint real issues, such as:
- Retrieving the correct document, but the chunk boundary cutting off a date field needed downstream
- Selecting the right tool, but the returned field names don't match prompt expectations
- Running normally for the first few rounds, but hitting an anomalous input in the fifth round not covered by the eval dataset
The author stresses that these issues are fundamentally about data shapes, null checks, and tool output format drift—they can't be solved simply by rewriting prompts. The truly effective debugging loop is: trace the path → find the deviation point → fix the interface or data issue causing the deviation.
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