MCP File Editor Server

Instructions

Implementation Reference

• src/index.ts:180-209 (handler)

  The execute function implementing the insert_into_file tool: validates path, reads file, verifies target line if applicable, inserts new content before/after specified line or appends, writes back to file, and returns success message.

  execute: async ({ file_path, line_number, line_contents, where, contents }) => {
    const absolutePath = validateAbsolutePath(file_path, 'file_path');
    validateFileExists(absolutePath);
  
    try {
      const content = fs.readFileSync(absolutePath, 'utf-8');
      const lines = content.split('\n');
  
      if (line_number === 0) {
        // Append to end
        lines.push(contents);
      } else {
        // Verify line content
        verifyLineContent(absolutePath, line_number, line_contents);
  
        // Insert at specified position
        const insertIndex = where === 'after' ? line_number : line_number - 1;
        lines.splice(insertIndex, 0, contents);
      }
  
      const newContent = lines.join('\n');
      fs.writeFileSync(absolutePath, newContent, 'utf-8');
  
      const position = line_number === 0 ? 'end of file' : `${where} line ${line_number}`;
      return `Successfully inserted content ${position} in "${absolutePath}".`;
    } catch (error: any) {
      if (error instanceof UserError) throw error;
      throw new UserError(`Error inserting content into file "${absolutePath}": ${error.message}`);
    }
  }

• src/index.ts:173-178 (schema)

  Zod parameter schema for the insert_into_file tool defining input validation for file_path, line_number (0 for append), line_contents (for verification), where (before/after), and contents to insert.

  parameters: z.object({
    file_path: z.string().describe('Absolute path to the file'),
    line_number: z.number().int().min(0).describe('Line number to insert at (1-based). Use 0 to append to end.'),
    line_contents: z.string().describe('Expected content of the target line (used for verification)'),
    where: z.enum(['before', 'after']).describe('Whether to insert before or after the target line'),
    contents: z.string().describe('Content to insert')

• src/index.ts:170-210 (registration)

  Registration of the insert_into_file tool with the FastMCP server via server.addTool, including name, description, schema, and execute handler.

  server.addTool({
    name: 'insert_into_file',
    description: 'Insert content into a file at a specific line position.',
    parameters: z.object({
      file_path: z.string().describe('Absolute path to the file'),
      line_number: z.number().int().min(0).describe('Line number to insert at (1-based). Use 0 to append to end.'),
      line_contents: z.string().describe('Expected content of the target line (used for verification)'),
      where: z.enum(['before', 'after']).describe('Whether to insert before or after the target line'),
      contents: z.string().describe('Content to insert')
    }),
    execute: async ({ file_path, line_number, line_contents, where, contents }) => {
      const absolutePath = validateAbsolutePath(file_path, 'file_path');
      validateFileExists(absolutePath);
  
      try {
        const content = fs.readFileSync(absolutePath, 'utf-8');
        const lines = content.split('\n');
  
        if (line_number === 0) {
          // Append to end
          lines.push(contents);
        } else {
          // Verify line content
          verifyLineContent(absolutePath, line_number, line_contents);
  
          // Insert at specified position
          const insertIndex = where === 'after' ? line_number : line_number - 1;
          lines.splice(insertIndex, 0, contents);
        }
  
        const newContent = lines.join('\n');
        fs.writeFileSync(absolutePath, newContent, 'utf-8');
  
        const position = line_number === 0 ? 'end of file' : `${where} line ${line_number}`;
        return `Successfully inserted content ${position} in "${absolutePath}".`;
      } catch (error: any) {
        if (error instanceof UserError) throw error;
        throw new UserError(`Error inserting content into file "${absolutePath}": ${error.message}`);
      }
    }
  });