MCP File Editor Server

Instructions

Implementation Reference

• src/index.ts:92-120 (handler)

  The handler function that reads the file content, performs global regex replacement if allowed, and writes the updated content back to the file.

  execute: async ({ file_path, regex_source, target, multiple }) => {
    const absolutePath = validateAbsolutePath(file_path, 'file_path');
    validateFileExists(absolutePath);
  
    try {
      const content = fs.readFileSync(absolutePath, 'utf-8');
      const regex = new RegExp(regex_source, 'g');
      const matches = content.match(regex);
  
      if (!matches) {
        throw new UserError(`No matches found for regex pattern "${regex_source}" in file "${absolutePath}".`);
      }
  
      if (!multiple && matches.length > 1) {
        throw new UserError(
          `Multiple matches found (${matches.length}) for regex pattern "${regex_source}" in file "${absolutePath}", ` +
          `but multiple=false. Either set multiple=true or refine your regex to match only one occurrence.`
        );
      }
  
      const newContent = content.replace(regex, target);
      fs.writeFileSync(absolutePath, newContent, 'utf-8');
  
      return `Successfully replaced ${matches.length} occurrence(s) of "${regex_source}" with "${target}" in "${absolutePath}".`;
    } catch (error: any) {
      if (error instanceof UserError) throw error;
      throw new UserError(`Error performing replacement in file "${absolutePath}": ${error.message}`);
    }
  }

• src/index.ts:86-91 (schema)

  Zod schema defining the input parameters for the replace_in_file tool.

  parameters: z.object({
    file_path: z.string().describe('Absolute path to the file'),
    regex_source: z.string().describe('Regular expression pattern to search for'),
    target: z.string().describe('String to replace matches with'),
    multiple: z.boolean().optional().describe('Allow multiple replacements. If false, fails if multiple matches found.')
  }),

• src/index.ts:83-121 (registration)

  Registers the replace_in_file tool with the FastMCP server instance.

  server.addTool({
    name: 'replace_in_file',
    description: 'Replace all occurrences of a regex pattern with a target string in a file.',
    parameters: z.object({
      file_path: z.string().describe('Absolute path to the file'),
      regex_source: z.string().describe('Regular expression pattern to search for'),
      target: z.string().describe('String to replace matches with'),
      multiple: z.boolean().optional().describe('Allow multiple replacements. If false, fails if multiple matches found.')
    }),
    execute: async ({ file_path, regex_source, target, multiple }) => {
      const absolutePath = validateAbsolutePath(file_path, 'file_path');
      validateFileExists(absolutePath);
  
      try {
        const content = fs.readFileSync(absolutePath, 'utf-8');
        const regex = new RegExp(regex_source, 'g');
        const matches = content.match(regex);
  
        if (!matches) {
          throw new UserError(`No matches found for regex pattern "${regex_source}" in file "${absolutePath}".`);
        }
  
        if (!multiple && matches.length > 1) {
          throw new UserError(
            `Multiple matches found (${matches.length}) for regex pattern "${regex_source}" in file "${absolutePath}", ` +
            `but multiple=false. Either set multiple=true or refine your regex to match only one occurrence.`
          );
        }
  
        const newContent = content.replace(regex, target);
        fs.writeFileSync(absolutePath, newContent, 'utf-8');
  
        return `Successfully replaced ${matches.length} occurrence(s) of "${regex_source}" with "${target}" in "${absolutePath}".`;
      } catch (error: any) {
        if (error instanceof UserError) throw error;
        throw new UserError(`Error performing replacement in file "${absolutePath}": ${error.message}`);
      }
    }
  });