After Debug Mode cracked a stubborn race condition last week, a developer now starts every task in it. Today's task is a validation rule they can already name, in a file they can already point at, and the session opens by adding log statements and asking them to reproduce a failure. Which choice matches Cursor's stated division between these two modes?

The principle — The line between Agent mode and Debug Mode is drawn by what you do not yet know, not by which mode is more capable. Cursor states the pair together: use Agent mode when you know what to build, and use Debug Mode when something is not working and you need to find out why. One mode implements a specified change; the other runs an investigation and produces a diagnosis.

Why the key is correct — A validation rule the developer can already name, in a file they can already point at, is a specification. Agent mode takes a specification and applies it. Debug Mode instead generates hypotheses, adds log statements, hands control back so the failure can be reproduced, reads the collected logs, makes a targeted fix and then removes its instrumentation. That whole cycle exists to convert an unexplained failure into evidence. Run against a task that was never a mystery, it produces the same edit far more slowly, and it asks the developer to reproduce a bug that does not exist.

Why the others are wrong — Ranking Debug Mode as simply the stronger mode misreads the difference as capability. Cursor describes it as a different approach, one that uses runtime evidence rather than guessing at fixes, which is decisive when the cause is hidden in the source and irrelevant when nothing is hidden. Choosing by the file's history picks the wrong axis: whether that code misbehaved last month says nothing about whether today's work is specified or unexplained. And splitting the modes by new files against existing ones invents a rule the documentation never makes, since Agent mode edits existing code as its normal work and Debug Mode is defined by the failure, not by the file's age.

Remember this — Ask which thing is missing. If you are missing the change, that is Agent mode; if you are missing the cause, that is Debug Mode.

Sources — Cursor's Debug mode help documentation.