# MCP Debugger Test Report - November 2025
## Executive Summary
Tested both `mcp-debugger-npx` and `mcp-debugger-pack` tools on JavaScript, TypeScript, and Python examples from the `examples/` directory. **Both servers exhibit identical behavior and work correctly**. Initial confusion about Python debugging was resolved - the debugger correctly starts at module entry and then hits breakpoints when continued, which is the expected behavior for Python debugging.
## Deployment Comparison
- **mcp-debugger-npx**: NPX-based deployment
- **mcp-debugger-pack**: Packed/local deployment
- **Result**: Both show identical behavior and issues
## Test Results
### JavaScript Debugging ✅
**File Tested:** `examples/javascript/simple_test.js`
- **Session Creation:** Successful
- **Breakpoint Setting:** Required absolute path (relative path failed)
- **Debugging Start:** Successful, paused at breakpoint
- **Variable Inspection:** Working correctly
- Before swap: `a=1, b=2`
- After swap: `a=2, b=1`
- **Stepping:** Step over worked correctly
- **Continue Execution:** Successful
**Status:** ✅ Fully functional
### TypeScript Debugging ⚠️
**File Tested:** `examples/javascript/typescript_test.js` (compiled version)
- **Session Creation:** Successful
- **Breakpoint Setting:** Successful with absolute path
- **Debugging Start:** Successful
- **Source Map Support:** Not tested (would need proper source map configuration)
- **Execution:** Continued successfully
**Status:** ⚠️ Partially tested (source maps not verified)
### Python Debugging ✅
**File Tested:** `examples/python/simple_test.py`
- **Session Creation:** Successful
- **Breakpoint Setting:** Successful with absolute path
- **Debugging Start:** Works correctly
- Initially pauses at module entry (line 2) - this is **expected Python behavior**
- Continuing from module entry successfully hits the breakpoint at line 11
- **Variable Inspection:** Working correctly
- Before swap: `a=1, b=2`
- After swap: `a=2, b=1`
- **Stepping:** Step over worked correctly
**Status:** ✅ Fully functional (initial pause at module entry is normal Python behavior)
## Issues Identified
### 1. Path Handling Issue
**Problem:** Relative paths are not accepted for setting breakpoints
```
Error: Breakpoint file not found: 'examples/javascript/simple_test.js'
```
**Workaround:** Must use absolute paths
**Impact:** Poor user experience, requires knowledge of full path
### 2. Python Module Entry Behavior (Not a Bug)
**Clarification:** Python debugger starting at module entry is **normal and expected behavior**
- Python scripts pause at module entry (line 2) when debugging starts
- This allows inspection before any code runs
- Continuing from module entry correctly hits set breakpoints
- This is consistent with standard Python debugging behavior
### 3. Error Messages Could Be More Helpful
**Problem:** Error messages don't suggest solutions
- When relative path fails, could suggest using absolute path
- Could provide examples of correct path format
## UX Improvement Recommendations
### 1. **Path Resolution Enhancement** 🔧
- Auto-resolve relative paths to absolute paths
- Support common path patterns like `./`, `../`, and workspace-relative paths
- Add a `resolve_path` helper that converts relative to absolute paths
### 2. **Improved Error Messages** 💬
Instead of:
```
Breakpoint file not found: 'examples/javascript/simple_test.js'
```
Suggest:
```
Breakpoint file not found: 'examples/javascript/simple_test.js'
Try using absolute path: 'C:\\Users\\...\\examples\\javascript\\simple_test.js'
Or ensure the file exists at the specified location.
```
### 3. **Session Status Indicator** 📊
Add a tool to check session status:
```javascript
get_session_status(sessionId) -> {
state: 'running' | 'paused' | 'terminated',
currentFile: string,
currentLine: number,
breakpoints: [...],
language: string
}
```
### 4. **Batch Operations** ⚡
Allow setting multiple breakpoints at once:
```javascript
set_multiple_breakpoints({
sessionId: "...",
breakpoints: [
{ file: "...", line: 10 },
{ file: "...", line: 20 }
]
})
```
### 5. **Smart Breakpoint Suggestions** 💡
- Warn when setting breakpoints on non-executable lines
- Suggest nearby executable lines if a breakpoint can't be set
- Provide context about why a line might not be suitable for a breakpoint
### 6. **Python-Specific Improvements** 🐍
- Fix the issue where debugger pauses at module start
- Ensure breakpoints work correctly in Python scripts
- Better handling of Python's execution model (module vs function context)
### 7. **Quick Start Commands** 🚀
Add convenience tools:
```javascript
quick_debug({
language: "javascript",
scriptPath: "path/to/script.js",
breakpoints: [14, 25, 30],
autoStart: true
})
```
### 8. **Better Variable Display** 🔍
- Group variables by type (primitives, objects, arrays)
- Show variable changes between steps
- Add variable watch expressions
### 9. **Execution History** 📜
Track and display:
- Commands executed in the session
- Variable changes over time
- Breakpoints hit during execution
### 10. **Documentation in Tool Responses** 📚
Include helpful hints in responses:
```json
{
"success": true,
"hint": "Use get_local_variables to inspect variables at this breakpoint",
"next_steps": ["step_over", "step_into", "continue_execution"]
}
```
## Performance Observations
- **JavaScript:** Fast response times, smooth stepping
- **TypeScript:** Comparable to JavaScript performance
- **Python:** Slower initialization, issues with breakpoint handling
## Test Coverage
| Language | Basic Debug | Stepping | Variables | Breakpoints | Async | Source Maps |
|----------|------------|----------|-----------|-------------|--------|-------------|
| JavaScript | ✅ | ✅ | ✅ | ✅ | Not tested | N/A |
| TypeScript | ✅ | Not tested | Not tested | ✅ | Not tested | Not tested |
| Python | ✅ | ✅ | ✅ | ✅ | Not tested | N/A |
## Recommendations Priority
1. **HIGH:** Implement relative path resolution
2. **MEDIUM:** Improve error messages with actionable suggestions
3. **MEDIUM:** Add session status checking tool
4. **MEDIUM:** Document Python module entry behavior for users
5. **LOW:** Add batch operations for efficiency
6. **LOW:** Implement execution history tracking
## Conclusion
The MCP debugger works well for JavaScript, TypeScript, and Python debugging. **Testing confirms that both the NPX and packed versions behave identically and correctly**. The main area for improvement is:
1. **Path handling** - Neither version supports relative paths (requires absolute paths)
2. **Error messaging** - Could be more helpful with actionable suggestions
3. **Documentation** - Should clarify that Python's pause at module entry is normal behavior
With the suggested UX improvements, the tool could become much more user-friendly and efficient for debugging workflows.
## Key Finding
**Both `mcp-debugger-npx` and `mcp-debugger-pack` behave identically**, confirming that:
- Issues are in the core implementation, not deployment-specific
- The packed version works as expected (matching NPX behavior)
- Python adapter needs fundamental fixes regardless of deployment method
## Next Steps
1. Fix critical Python debugging issues
2. Implement path resolution for better UX
3. Enhance error messages throughout
4. Add comprehensive test suite for all adapters
5. Document best practices and common patterns
