# 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 References to `PathValidator` in this document are outdated.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.
