Root-Cause Analysis with Auto-Instrumentation

Cursor's Debug Mode (accessible via /debug in Composer or the CLI) transforms the agent into a dedicated debugging specialist. Instead of immediately trying fixes, the agent enters a methodical root-cause analysis workflow that dramatically increases first-attempt fix rates.

How Debug Mode Works

1. Hypothesis Generation: The agent analyzes the error, stack trace, or failing test and generates 3-5 ranked hypotheses for the root cause — ordered by likelihood.
2. Auto-Instrumentation: For each hypothesis, the agent automatically inserts targeted logging statements, breakpoints, or diagnostic assertions into the relevant code paths. This is non-destructive — all instrumentation is tracked and removed after diagnosis.
3. Execution & Observation: The agent re-runs the failing scenario with instrumentation active, collects output, and narrows down the root cause based on observed behavior.
4. Fix & Verify: Once the root cause is confirmed, the agent proposes a fix, applies it, removes all instrumentation, and re-runs the original test to verify the fix.

Activating Debug Mode

// In Composer, type:
/debug The authentication middleware returns 401 for valid tokens

// From the CLI:
cursor debug "Tests in auth.test.ts fail with timeout"

// The agent will:
// 1. Read the failing test and related source files
// 2. Generate hypotheses (token expiry? wrong secret? async timing?)
// 3. Add console.log / assertions at key points
// 4. Re-run and analyze output
// 5. Identify root cause and apply fix
// 6. Clean up all debug instrumentation
// 7. Verify the fix passes

Debug Mode vs Regular Agent