Tree-Hugger-JS MCP Server

Instructions

Implementation Reference

• src/index.ts:1055-1104 (handler)

  The core handler function that executes the insert_code tool. It uses the tree-hugger-js transform API to insert code before or after matching nodes based on the pattern, handles preview mode, updates the AST state, and logs the transformation.

  private async insertCode(args: { pattern: string; code: string; position: "before" | "after"; preview?: boolean }) {
    if (!this.currentAST) {
      return {
        content: [{
          type: "text",
          text: "No AST loaded. Please use parse_code first.",
        }],
        isError: true,
      };
    }
  
    try {
      const transformer = this.currentAST.tree.transform();
      const transformed = args.position === "before" 
        ? transformer.insertBefore(args.pattern, args.code)
        : transformer.insertAfter(args.pattern, args.code);
  
      const result = transformed.toString();
      
      if (!args.preview) {
        this.currentAST.sourceCode = result;
        this.currentAST.tree = parse(result);
        this.currentAST.timestamp = new Date();
      }
  
      const transformResult: TransformResult = {
        operation: "insert_code",
        parameters: { pattern: args.pattern, text: args.code },
        preview: result.slice(0, 500) + (result.length > 500 ? '...' : ''),
        timestamp: new Date(),
      };
      
      this.transformHistory.push(transformResult);
  
      return {
        content: [{
          type: "text",
          text: `${args.preview ? 'Preview: ' : ''}Inserted code ${args.position} pattern "${args.pattern}"\n\n${args.preview ? 'Preview:\n' : 'Result:\n'}${result}`,
        }],
      };
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: `Error inserting code: ${error instanceof Error ? error.message : String(error)}`,
        }],
        isError: true,
      };
    }
  }

• src/index.ts:382-404 (schema)

  The input schema defining the parameters for the insert_code tool: pattern (string), code (string), position (enum: before/after), optional preview (boolean).

  inputSchema: {
    type: "object",
    properties: {
      pattern: {
        type: "string",
        description: "Pattern to match: 'function_declaration', 'class[name=\"MyClass\"]', 'method_definition[async]'"
      },
      code: {
        type: "string",
        description: "Code to insert. Will be formatted with proper indentation automatically."
      },
      position: {
        type: "string",
        enum: ["before", "after"],
        description: "Insert position: 'before' (above) or 'after' (below) the matched nodes"
      },
      preview: {
        type: "boolean",
        description: "Return preview only without applying changes (default: false). Always preview first!"
      }
    },
    required: ["pattern", "code", "position"],
  },

• src/index.ts:379-405 (registration)

  Registration of the insert_code tool in the ListToolsRequestSchema handler, including name, detailed description with usage examples, and input schema.

  {
    name: "insert_code",
    description: "Insert code before or after nodes with smart formatting. Professional-quality code insertion with proper indentation.\n\nExamples:\n• Add logging: insert_code('function_declaration', 'console.log(\"Function started\");', 'after')\n• Add validation: insert_code('method_definition[name=\"save\"]', 'if (!this.isValid()) return;', 'after')\n• Add comments: insert_code('class_declaration', '// Main user management class', 'before')\n• Add error handling: insert_code('function[async]', 'try {', 'after') + insert_code('function[async]', '} catch(e) { console.error(e); }', 'after')\n• Add metrics: insert_code('function[name*=\"api\"]', 'performance.mark(\"api-start\");', 'after')\n• Debug mode: insert_code('call[text*=\"fetch\"]', 'console.log(\"API call:\", url);', 'before')",
    inputSchema: {
      type: "object",
      properties: {
        pattern: {
          type: "string",
          description: "Pattern to match: 'function_declaration', 'class[name=\"MyClass\"]', 'method_definition[async]'"
        },
        code: {
          type: "string",
          description: "Code to insert. Will be formatted with proper indentation automatically."
        },
        position: {
          type: "string",
          enum: ["before", "after"],
          description: "Insert position: 'before' (above) or 'after' (below) the matched nodes"
        },
        preview: {
          type: "boolean",
          description: "Return preview only without applying changes (default: false). Always preview first!"
        }
      },
      required: ["pattern", "code", "position"],
    },
  },

• src/index.ts:446-447 (registration)

  Dispatch registration in the CallToolRequestSchema switch statement that routes calls to the insertCode handler method.

  case "insert_code":
    return await this.insertCode(args as { pattern: string; code: string; position: "before" | "after"; preview?: boolean });

• src/index.ts:1081-1085 (helper)

  Helper code within the handler that creates a TransformResult record for the insert_code operation, used for history tracking.

    operation: "insert_code",
    parameters: { pattern: args.pattern, text: args.code },
    preview: result.slice(0, 500) + (result.length > 500 ? '...' : ''),
    timestamp: new Date(),
  };