1 min lesson
Debug mode for the cross-layer bug
Connect "Cursor's Debug mode (a distinct mode in the agent selector) is built for exactly that" to the decision you would make.
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Debug mode for the cross-layer bugwhen the evidence won't line up
Some bugs sit between layers - a failure that spans services or hides in the seam between logic layers, where standard agent mode's bias for action just guesses and over-edits. Cursor's Debug modeA mode that diagnoses a failure: it reproduces the issue, adds instrumentation and watches the logs, rather than reviewing a pull request. Press Enter for the full definition. (a distinct mode in the agent selector) is built for exactly that. It instruments the codebase to find the root cause instead of patching the first thing it sees, and it works on anything that can fire a log line, including a compiled executable.
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Full explanation
Debug Mode, Step by Step
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It validates hypotheses against a real execution path, then fixes only the broken lines and removes its own instrumentation.
Standard agent mode is biased toward action: on a subtle bug it will guess a cause and edit broadly to make the symptom go away. Debug modeA mode that diagnoses a failure: it reproduces the issue, adds instrumentation and watches the logs, rather than reviewing a pull request. Press Enter for the full definition. trades that speed for a confirmed execution path, so the fix lands on the actual broken lines instead of the nearest plausible ones.
You're relying on the agent to just look at a very specific part of the codebase and only fix what's broken.
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Optional practice
Practice: Debug mode for the cross-layer bug
QA user's Cursor completions silently stop and a HAR capture shows the completions endpoint returning 407. What's the cause and what is it NOT?