# Line Context Feature Implementation Summary
> **⚠️ ARCHIVED DOCUMENTATION**
> References to `PathValidator` in this document are outdated. The project now uses a **TRUE HANDS-OFF** approach with `SimpleFileChecker` for existence validation only.
## Overview
Successfully implemented line context functionality for the MCP debugger, enabling AI agents to make intelligent decisions about breakpoint placement and providing source context for debugging operations.
## What Was Implemented
### 1. Line Reader Utility (`src/utils/line-reader.ts`)
- **Purpose**: Efficiently read specific lines from files with surrounding context
- **Features**:
- LRU cache for recently accessed files (max 20 files, 5-minute TTL)
- Binary file detection to avoid processing non-text files
- Handles various line endings (Unix/Windows)
- File size limits (10MB default) to prevent memory issues
- Empty file handling
### 2. Enhanced Breakpoint Response
- **Location**: `src/server.ts` - `set_breakpoint` handler
- **Enhancement**: Now includes line context when setting breakpoints
- **Response Format**:
```json
{
"success": true,
"breakpointId": "abc-123",
"file": "example.py",
"line": 10,
"verified": false,
"message": "Breakpoint set at example.py:10",
"context": {
"lineContent": " # TODO: implement this function",
"surrounding": [
{ "line": 8, "content": "def calculate_total(items):" },
{ "line": 9, "content": " \"\"\"Calculate the total price of items\"\"\"" },
{ "line": 10, "content": " # TODO: implement this function" },
{ "line": 11, "content": " pass" },
{ "line": 12, "content": "" }
]
}
}
```
### 3. Implemented `get_source_context` Tool
- **Location**: `src/server.ts` - `handleGetSourceContext` method
- **Purpose**: Get source code context around a specific line
- **Parameters**:
- `sessionId`: Debug session ID
- `file`: Path to source file
- `line`: Line number to get context for
- `linesContext`: Number of lines before/after (default: 5)
## Benefits Achieved
1. **Language Agnostic**: Works with any text file format without language-specific parsing
2. **AI-Friendly**: LLMs can easily interpret if a line is appropriate for breakpoints
3. **Efficient**: Only reads necessary lines, not entire files
4. **Cached**: Reduces file I/O for multiple operations on the same file
5. **Error Resilient**: Gracefully handles binary files, missing files, and edge cases
## Test Coverage
### Unit Tests (`tests/unit/utils/line-reader.spec.ts`)
- ✅ Valid file and line number handling
- ✅ Edge cases (start/end of file)
- ✅ Out of range line numbers
- ✅ Binary file detection
- ✅ Large file rejection
- ✅ Cache functionality
- ✅ Empty file handling
- ✅ Windows line endings
- ✅ Multi-line context extraction
- ✅ Error handling
### Manual Test (`tests/manual/line-context-test/`)
- Python test script with various line types (code, comments, blank lines)
- JavaScript test runner demonstrating both features
## Usage Examples
### Setting a Breakpoint with Context
```javascript
const result = await client.callTool('set_breakpoint', {
sessionId: sessionId,
file: '/path/to/script.py',
line: 15
});
// Response includes context to help AI determine if line is appropriate
if (result.context) {
console.log(`Line ${result.line}: "${result.context.lineContent}"`);
// AI can analyze if this is a comment, blank line, or executable code
}
```
### Getting Source Context
```javascript
const context = await client.callTool('get_source_context', {
sessionId: sessionId,
file: '/path/to/script.py',
line: 25,
linesContext: 3 // Get 3 lines before and after
});
// Use context for debugging visualization or AI analysis
```
## Implementation Notes
1. **Path Validation**: Leverages existing `PathValidator` for consistent file path handling
2. **Error Handling**: Context retrieval failures don't break breakpoint setting
3. **Logging**: Debug logging added for troubleshooting
4. **Type Safety**: Full TypeScript types for all interfaces
## Future Enhancements
1. **Configurable Cache**: Allow cache size/TTL configuration
2. **Syntax Highlighting**: Add optional syntax highlighting metadata
3. **Multi-file Context**: Support for getting context across multiple files
4. **Performance Metrics**: Add timing metrics for cache hits/misses
## Success Criteria Met
✅ Breakpoint responses include line context when file is readable
✅ AI agents can identify inappropriate breakpoint locations from context
✅ No performance degradation for normal operations
✅ All existing tests continue to pass
✅ New tests verify context functionality
✅ `get_source_context` tool implemented
## Conclusion
The line context feature successfully enhances the MCP debugger by providing crucial information for intelligent breakpoint placement and source code inspection. The implementation is efficient, well-tested, and maintains backward compatibility while significantly improving the AI agent experience.
