MCP4Modal Sandbox

Instructions

        Reads the content of a file in the sandbox.
        
        Parameters:
        - sandbox_id: The unique identifier of the sandbox
        - path: Path to the file to read
        
        Returns a SandboxReadFileContentResponse containing:
        - content: String content of the file
        
        This tool is useful for:
        - Viewing file contents without downloading
        - Debugging sandbox operations
        - Checking operation results
        - Quick file inspection
        
        The tool will:
        1. Verify sandbox and file exist
        2. Read file contents
        3. Return file content as string
        

Implementation Reference

• src/mcp4modal_sandbox/backend/mcp_server.py:438-454 (handler)

  Main handler function: gets sandbox by ID, checks if running, reads file content using helper in thread executor, returns response with content.

  async def read_file_content_from_sandbox(self, sandbox_id: str, path: str) -> SandboxReadFileContentResponse:
      # Get sandbox from Modal using from_id
      modal_sandbox = await modal.Sandbox.from_id.aio(sandbox_id)
      
      # Check if sandbox is running before reading file
      sandbox_status = await modal_sandbox.poll.aio()
      if sandbox_status is not None:
          raise ToolError(f"Sandbox {sandbox_id} is not running")
      
      # Read from sandbox using thread executor
      content = await self._read_sandbox_file_in_thread(modal_sandbox, path, 'rb')
      
      logger.info(f"Read file content from {path} in sandbox {sandbox_id}")
      
      return SandboxReadFileContentResponse(
          content=content
      )

• src/mcp4modal_sandbox/backend/response_schema.py:33-34 (schema)

  Pydantic response schema defining the output: file content as string.

  class SandboxReadFileContentResponse(BaseModel):
      content: str

• src/mcp4modal_sandbox/backend/mcp_server.py:146-149 (registration)

  Tool registration in MCP app with name and description, binding the handler method.

  mcp_app.tool(
      name="read_file_content_from_sandbox",
      description=ToolDescriptions.READ_FILE_CONTENT_FROM_SANDBOX,
  )(self.read_file_content_from_sandbox)

• src/mcp4modal_sandbox/backend/mcp_server.py:84-90 (helper)

  Supporting helper: reads file synchronously in thread pool executor to handle Modal sandbox file I/O without blocking the async loop.

  async def _read_sandbox_file_in_thread(self, modal_sandbox, file_path: str, mode: str = 'rb'):
      def _sync_read():
          with modal_sandbox.open(file_path, mode) as f:
              return f.read()
      
      loop = asyncio.get_event_loop()
      return await loop.run_in_executor(self.thread_pool_executor, _sync_read)