debugging-with-tools

name	debugging-with-tools
description	Use when encountering bugs or test failures - systematic debugging using debuggers, internet research, and agents to find root cause before fixing

<kimi_compat> This skill was ported to Kimi Code CLI. In Kimi:

• Load skills with /skill:name or let the model invoke them automatically.
• "SetTodoList" means use the SetTodoList tool.
• "Agent()" / "Agent" means use the built-in Agent tool.
• "parallel Agent calls with run_in_background=true" means dispatch multiple Agent calls with run_in_background=true.
• Claude-specific hooks and slash commands are not available; skip those steps. </kimi_compat> <skill_overview> Random fixes waste time and create new bugs. Always use tools to understand root cause BEFORE attempting fixes. Symptom fixes are failure. </skill_overview>

<rigidity_level> MEDIUM FREEDOM - Must complete investigation phases (tools → hypothesis → test) before fixing.

Can adapt tool choice to language/context. Never skip investigation or guess at fixes. </rigidity_level>

<quick_reference>

Phase	Tools to Use	Output
1. Investigate	Error messages, internet-researcher agent, debugger, codebase-investigator	Root cause understanding
2. Hypothesize	Form theory based on evidence (not guesses)	Testable hypothesis
3. Test	Validate hypothesis with minimal change	Confirms or rejects theory
4. Fix	Implement proper fix for root cause	Problem solved permanently

FORBIDDEN: Skip investigation → guess at fix → hope it works REQUIRED: Tools → evidence → hypothesis → test → fix

Key agents:

• internet-researcher - Search error messages, known bugs, solutions
• codebase-investigator - Understand code structure, find related code
• test-runner - Run tests without output pollution

</quick_reference>

<when_to_use> Use for ANY technical issue:

• Test failures
• Bugs in production or development
• Unexpected behavior
• Build failures
• Integration issues
• Performance problems

ESPECIALLY when:

• "Just one quick fix" seems obvious
• Under time pressure (emergencies make guessing tempting)
• Error message is unclear
• Previous fix didn't work </when_to_use>

<the_process>

Phase 1: Tool-Assisted Investigation

BEFORE attempting ANY fix, gather evidence with tools:

1. Read Complete Error Messages

• Entire error message (not just first line)
• Complete stack trace (all frames)
• Line numbers, file paths, error codes
• Stack traces show exact execution path

2. Search Internet FIRST (Use internet-researcher Agent)

Dispatch internet-researcher with:

"Search for error: [exact error message]
- Check Stack Overflow solutions
- Look for GitHub issues in [library] version [X]
- Find official documentation explaining this error
- Check if this is a known bug"

What agent should find:

• Exact matches to your error
• Similar symptoms and solutions
• Known bugs in your dependency versions
• Workarounds that worked for others

3. Use Debugger to Inspect State

Claude cannot run debuggers directly. Instead:

Option A - Recommend debugger to user:

"Let's use lldb/gdb/DevTools to inspect state at error location.
Please run: [specific commands]
When breakpoint hits: [what to inspect]
Share output with me."

Option B - Add instrumentation Claude can add:

// Add logging
println!("DEBUG: var = {:?}, state = {:?}", var, state);

// Add assertions
assert!(condition, "Expected X but got {:?}", actual);

4. Investigate Codebase (Use codebase-investigator Agent)

Dispatch codebase-investigator with:

"Error occurs in function X at line Y.
Find:
- How is X called? What are the callers?
- What does variable Z contain at this point?
- Are there similar functions that work correctly?
- What changed recently in this area?"

Phase 2: Form Hypothesis

Based on evidence (not guesses):

1. State what you know (from investigation)
2. Propose theory explaining the evidence
3. Make prediction that tests the theory

Example:

Known: Error "null pointer" at auth.rs:45 when email is empty
Theory: Empty email bypasses validation, passes null to login()
Prediction: Adding validation before login() will prevent error
Test: Add validation, verify error doesn't occur with empty email

NEVER:

• Guess without evidence
• Propose fix without hypothesis
• Skip to "try this and see"

Phase 3: Test Hypothesis

Minimal change to validate theory:

1. Make smallest change that tests hypothesis
2. Run test/reproduction case
3. Observe result

If confirmed: Proceed to Phase 4 If rejected: Return to Phase 1 with new information

Phase 4: Implement Fix

After understanding root cause:

1. Write test reproducing bug (RED phase - use test-driven-development skill)
2. Implement proper fix addressing root cause
3. Verify test passes (GREEN phase)
4. Run full test suite (regression check)
5. Commit fix

The fix should:

• Address root cause (not symptom)
• Be minimal and focused
• Include test preventing regression

</the_process>

Developer encounters test failure, immediately tries "obvious" fix without investigation Test error: ``` FAIL: test_login_expired_token AssertionError: Expected Err(TokenExpired), got Ok(User) ```

Developer thinks: "Obviously the token expiration check is wrong"

Makes change without investigation:

// "Fix" - just check if token is expired
if token.expires_at < now() {
    return Err(AuthError::TokenExpired);
}

Commits without testing other cases.

<why_it_fails> No investigation:

• Didn't read error completely
• Didn't check what expires_at contains
• Didn't debug to see token state
• Didn't search for similar issues

What actually happened: Token expires_at was being parsed incorrectly, always showing future date. The "fix" adds dead code that never runs.

Result: Bug not fixed, new dead code added, time wasted. </why_it_fails>

**Phase 1 - Investigate with tools:**

# 1. Read complete error
FAIL: test_login_expired_token at line 45
Expected: Err(TokenExpired)
Got: Ok(User { id: 123 })
Token: { expires_at: "2099-01-01", ... }

Dispatch internet-researcher:

"Search for: token expiration always showing future date
- Check date parsing bugs
- Look for timezone issues
- Find JWT expiration handling"

Add instrumentation:

println!("DEBUG: expires_at = {:?}, now = {:?}, expired = {:?}",
         token.expires_at, now(), token.expires_at < now());

Run test again:

DEBUG: expires_at = 2099-01-01T00:00:00Z, now = 2024-01-15T10:30:00Z, expired = false

Phase 2 - Hypothesis: "Token expires_at is being set to 2099, not actual expiration. Problem is in token creation, not validation."

Phase 3 - Test: Check token creation code:

// Found the bug!
fn create_token() -> Token {
    Token {
        expires_at: "2099-01-01".parse()?, // HARDCODED!
        ...
    }
}

Phase 4 - Fix root cause:

fn create_token(duration: Duration) -> Token {
    Token {
        expires_at: now() + duration,  // Correct
        ...
    }
}

Result: Root cause fixed, test passes, no dead code.

Developer skips internet search, reinvents solution to known problem Error: ``` error: linking with `cc` failed: exit status: 1 ld: symbol(s) not found for architecture arm64 ```

Developer thinks: "Must be a linking issue, I'll add flags"

Spends 2 hours trying different linker flags:

[target.aarch64-apple-darwin]
rustflags = ["-C", "link-arg=-undefined dynamic_lookup"]
# Doesn't work, tries more flags...

<why_it_fails> Skipped internet search:

• This is a common error with known solutions
• Stack Overflow has exact fix
• Official docs explain the issue
• Wasted 2 hours reinventing solution

Why it happens: Impatience, thinking "I can figure this out faster" </why_it_fails>

**Dispatch internet-researcher FIRST:**

"Search for: 'symbol not found for architecture arm64' Rust linking
- Check Stack Overflow solutions
- Look for Xcode/macOS specific fixes
- Find Cargo configuration for Apple Silicon"

Agent returns (30 seconds):

Found on Stack Overflow (2.4k upvotes):
This occurs when Xcode Command Line Tools aren't installed or outdated.

Solution:
xcode-select --install

Or update existing:
softwareupdate --all --install --force

Apply solution:

xcode-select --install
# Wait for installation
cargo build
# Success!

Result: Fixed in 5 minutes, not 2 hours.

Developer fixes symptom without understanding root cause, bug returns different way Bug: Users can delete other users' posts

Developer notices: DELETE /posts/:id doesn't check ownership

"Fix":

@app.delete("/posts/{post_id}")
def delete_post(post_id: int, user: User):
    post = get_post(post_id)
    if post.author_id != user.id:
        raise Forbidden()
    delete(post)

Bug "fixed"... but bug report comes back: "Users can edit other users' posts"

<why_it_fails> Symptom fix:

• Fixed DELETE endpoint
• Didn't investigate root cause
• Didn't check other endpoints

Root cause: No authorization middleware checking ownership across ALL endpoints.

Result: Whack-a-mole fixing symptoms, not the underlying issue. </why_it_fails>

**Phase 1 - Investigate with codebase-investigator:**

"User can delete other users' posts. Investigate:
- What authorization exists across all post endpoints?
- Is there a pattern for ownership checks?
- Which other endpoints access posts?
- Is there middleware handling this?"

Agent reports:

Found:
- DELETE /posts/:id - NO ownership check
- PUT /posts/:id - NO ownership check
- GET /posts/:id - NO ownership check (allows viewing private posts!)
- No authorization middleware found
- Similar pattern in comments, likes endpoints

Phase 2 - Hypothesis: "Missing authorization layer. Need middleware checking resource ownership across ALL endpoints."

Phase 4 - Fix root cause:

# Add authorization middleware
class OwnershipMiddleware:
    def check_ownership(self, resource, user):
        if resource.author_id != user.id:
            raise Forbidden()

# Apply to all endpoints
@app.delete("/posts/{post_id}")
@require_ownership(Post)
def delete_post(...):
    ...

@app.put("/posts/{post_id}")
@require_ownership(Post)
def update_post(...):
    ...

Result: Root cause fixed, ALL endpoints secured, not just one symptom.

<critical_rules>

Rules That Have No Exceptions

1. Tools before fixes → Never guess without investigation

   • Use internet-researcher for errors
   • Use debugger or instrumentation for state
   • Use codebase-investigator for context

2. Evidence-based hypotheses → Not guesses or hunches

   • State what tools revealed
   • Propose theory explaining evidence
   • Make testable prediction

3. Test hypothesis before fixing → Minimal change to validate

   • Smallest change that tests theory
   • Observe result
   • If wrong, return to investigation

4. Fix root cause, not symptom → One fix, many symptoms prevented

   • Understand why problem occurred
   • Fix the underlying issue
   • Don't play whack-a-mole

Common Excuses

All of these mean: Stop, use tools to investigate:

• "The fix is obvious"
• "I know what this is"
• "Just a quick try"
• "No time for debugging"
• "Error message is clear enough"
• "Internet search will take too long"

</critical_rules>

<verification_checklist>

Before proposing any fix:

• Read complete error message (not just first line)
• Dispatched internet-researcher for unclear errors
• Used debugger or added instrumentation to inspect state
• Dispatched codebase-investigator to understand context
• Formed hypothesis based on evidence (not guesses)
• Tested hypothesis with minimal change
• Verified hypothesis confirmed before fixing

Before committing fix:

• Written test reproducing bug (RED phase)
• Verified test fails before fix
• Implemented fix addressing root cause
• Verified test passes after fix (GREEN phase)
• Ran full test suite (regression check)

</verification_checklist>

This skill calls:

• internet-researcher (search errors, known bugs, solutions)
• codebase-investigator (understand code structure, find related code)
• test-driven-development (write test for bug, implement fix)
• test-runner (run tests without output pollution)

This skill is called by:

• fixing-bugs (complete bug fix workflow)
• root-cause-tracing (deep debugging for complex issues)
• Any skill when encountering unexpected behavior

Agents used:

• /skill:internet-researcher (search for error solutions)
• /skill:codebase-investigator (understand codebase context)
• /skill:test-runner (run tests, return summary only)

Detailed guides:

• Debugger reference - LLDB, GDB, DevTools commands
• Debugging session example - Complete walkthrough

When stuck:

• Error unclear → Dispatch internet-researcher with exact error text
• Don't understand code flow → Dispatch codebase-investigator
• Need to inspect runtime state → Recommend debugger to user or add instrumentation
• Tempted to guess → Stop, use tools to gather evidence first