Node.js Debugger MCP Server

Instructions

Implementation Reference

• src/index.ts:499-557 (handler)

  The handler function for the 'set_breakpoint' tool. It checks for an active debug session, then uses the Chrome DevTools Protocol (CDP) Debugger.setBreakpointByUrl to set a breakpoint at the specified file URL and line number (converting to 0-based), optionally with a condition. Stores the breakpoint ID and returns success or error.

  private async setBreakpoint(args: { file: string; line: number; condition?: string }) {
    if (!this.debugSession.connected || !this.debugSession.client) {
      return {
        content: [
          {
            type: "text",
            text: "No active debug session. Please attach debugger first.",
          },
        ],
        isError: true,
      };
    }
  
    try {
      const { Debugger } = this.debugSession.client;
      
      // Use CDP to set the breakpoint
      const result = await Debugger.setBreakpointByUrl({
        lineNumber: args.line - 1, // CDP uses 0-based line numbers
        url: args.file,
        condition: args.condition
      });
  
      if (result.breakpointId) {
        // Store the breakpoint mapping
        const breakpointKey = `${args.file}:${args.line}`;
        this.debugSession.breakpoints!.set(breakpointKey, result.breakpointId);
  
        return {
          content: [
            {
              type: "text",
              text: `Set breakpoint at ${args.file}:${args.line}${args.condition ? ` (condition: ${args.condition})` : ""} - ID: ${result.breakpointId}`,
            },
          ],
        };
      } else {
        return {
          content: [
            {
              type: "text", 
              text: `Failed to set breakpoint at ${args.file}:${args.line}`,
            },
          ],
          isError: true,
        };
      }
    } catch (error) {
      return {
        content: [
          {
            type: "text",
            text: `Error setting breakpoint: ${error}`,
          },
        ],
        isError: true,
      };
    }
  }

• src/index.ts:187-202 (schema)

  Tool registration including name, description, and input schema definition for 'set_breakpoint'.

  {
    name: "set_breakpoint",
    description: "Set a breakpoint in the debugged process. Use full file:// URLs for reliable breakpoint hits.",
    inputSchema: {
      type: "object",
      properties: {
        file: { 
          type: "string", 
          description: "File URL or path (use file:///absolute/path/to/file.js for best results)" 
        },
        line: { type: "number", description: "Line number (1-based)" },
        condition: { type: "string", description: "Optional condition for the breakpoint" }
      },
      required: ["file", "line"],
    },
  },

• src/index.ts:256-257 (registration)

  Registration of the tool handler dispatch in the CallToolRequest switch statement.

  case "set_breakpoint":
    return await this.setBreakpoint(args as { file: string; line: number; condition?: string });