MCP SmallEdit

Instructions

Implementation Reference

• src/index.ts:760-799 (handler)

  The main handler function for the 'diff_preview' tool. It creates a temporary copy of the file, applies the specified command using the chosen tool (perl, sed, or awk), generates a unified diff between original and modified temp file, returns the diff preview, and cleans up the temp file.

  case 'diff_preview': {
    const { file, command, tool = 'perl' } = args;
    
    if (!existsSync(file)) {
      throw new Error(`File not found: ${file}`);
    }
    
    // Create temp file
    const tempFile = `${file}.preview.tmp`;
    await execAsync(`rm -f .bak; cp '${file}' '${tempFile}'`);
    
    // Apply command to temp file
    let editCmd;
    switch (tool) {
      case 'perl':
        editCmd = `perl -i -pe '${command}' '${tempFile}'`;
        break;
      case 'sed':
        editCmd = `sed -i.tmp '${command}' '${tempFile}' && rm -f '${tempFile}.tmp'`;
        break;
      case 'awk':
        editCmd = `awk '${command}' '${tempFile}' > '${tempFile}.new' && mv '${tempFile}.new' '${tempFile}'`;
        break;
    }
    
    await execAsync(editCmd);
    
    // Generate diff
    const { stdout } = await execAsync(`diff -u '${file}' '${tempFile}' || true`);
    
    // Cleanup
    await execAsync(`rm -f '${tempFile}'`);
    
    return {
      content: [{
        type: 'text',
        text: stdout ? `Preview of changes:\n${stdout}` : 'No changes would be made'
      }]
    };
  }

• src/index.ts:203-226 (registration)

  Tool registration object including name, description, and input schema, added to the tools array for server.setTools().

  {
    name: 'diff_preview', 
    description: 'Preview what changes would be made by showing a diff',
    inputSchema: {
      type: 'object',
      properties: {
        file: {
          type: 'string',
          description: 'File to preview changes for'
        },
        command: {
          type: 'string',
          description: 'Command that would make changes (e.g., "s/old/new/g")'
        },
        tool: {
          type: 'string',
          enum: ['sed', 'perl', 'awk'],
          default: 'perl',
          description: 'Which tool to use for the preview'
        }
      },
      required: ['file', 'command']
    }
  },

• src/index.ts:207-224 (schema)

  Input schema defining the parameters for the diff_preview tool: file, command (required), and optional tool (perl/sed/awk).

  type: 'object',
  properties: {
    file: {
      type: 'string',
      description: 'File to preview changes for'
    },
    command: {
      type: 'string',
      description: 'Command that would make changes (e.g., "s/old/new/g")'
    },
    tool: {
      type: 'string',
      enum: ['sed', 'perl', 'awk'],
      default: 'perl',
      description: 'Which tool to use for the preview'
    }
  },
  required: ['file', 'command']

• src/index.ts:493-511 (helper)

  Documentation and usage examples for the diff_preview tool in the helpContent object.

    diff_preview: `diff_preview - Preview changes
  ============================
  See what changes would be made before applying.
  
  Examples:
    // Preview perl substitution
    diff_preview({ file: "config.json", command: "s/8080/3000/g" })
    
    // Preview with sed syntax
    diff_preview({ file: "data.txt", command: "10,20d", tool: "sed" })
    
    // Preview AWK processing  
    diff_preview({ file: "log.csv", command: "BEGIN{FS=\",\"} {print $2}", tool: "awk" })
  
  Output:
  - Shows unified diff format
  - No changes made to original file
  - Temp files are cleaned up
  `,