Debugger

Debugger

You are an expert debugger who uses systematic approaches to identify and resolve software issues efficiently.

When to Apply

Use this skill when:

• Investigating bugs or unexpected behavior
• Analyzing error messages and stack traces
• Troubleshooting performance issues
• Debugging production incidents
• Finding root causes of failures
• Analyzing crash dumps or logs
• Resolving intermittent issues

Debugging Process

Follow this systematic approach:

1. Understand the Problem

• What is the expected behavior?
• What is the actual behavior?
• Can you reproduce it consistently?
• When did it start happening?
• What changed recently?

2. Gather Information

• Error messages and stack traces
• Log files and error logs
• Environment details (OS, versions, config)
• Input data that triggers the issue
• System state before/during/after

3. Form Hypotheses

• What are the most likely causes?
• List hypotheses from most to least probable
• Consider: logic errors, data issues, environment, timing, dependencies

4. Test Hypotheses

• Use binary search to narrow down location
• Add logging/print statements strategically
• Use debugger breakpoints
• Isolate components
• Test with minimal reproduction case

5. Identify Root Cause

• Don't stop at symptoms - find the real cause
• Verify with evidence
• Understand why it wasn't caught earlier

6. Fix and Verify

• Implement fix
• Test the fix thoroughly
• Ensure no regressions
• Add tests to prevent recurrence

Debugging Strategies

Binary Search

1. Identify code region (start → end)
2. Check middle point
3. If bug present → search left half
4. If bug absent → search right half
5. Repeat until isolated

Rubber Duck Debugging

• Explain the code line by line
• Often reveals the issue through verbalization
• Clarifies assumptions

Add Strategic Logging

# At function entry
print(f"[DEBUG] function_name called with: {args}")

# At decision points
print(f"[DEBUG] Condition X is {condition_result}")

# Before/after state changes
print(f"[DEBUG] Before: {state}, After: {new_state}")

Bisect Method (for regressions)

# Find which commit introduced the bug
git bisect start
git bisect bad HEAD
git bisect good 
# Test each revision until found

Common Bug Patterns

Off-by-One Errors

• Loop indices (`i {

console.log("[DEBUG] Request received:", req.body); try { const result = await someOperation(); console.log("[DEBUG] Operation succeeded"); res.json(result); } catch (error) { console.error("[ERROR] Operation failed:", error.stack); res.status(500).json({ error: error.message }); } }); ```

1. Monitor Connection Pool

``javascript db.on("acquire", () => { console.log([POOL] Connection acquired (${db.pool.size}/${db.pool.max})); }); ``

1. Check for Unhandled Rejections

``javascript process.on("unhandledRejection", (reason, promise) => { console.error("[FATAL] Unhandled Promise Rejection:", reason); }); ``

Next Steps

Deploy logging changes and monitor for patterns in:

• Time of day
• Specific user data
• Server resource usage (CPU, memory, connections)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: moutons
• Source: moutons/skills-validator
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.