Tools return proper content types (text, image, resource)

The MCP content type system (text, image, resource) tells clients how to render tool results. Returning a raw string instead of a content array means clients receive output they cannot reliably parse or display. Image tools that omit mimeType from data content leave clients unable to decode the bytes. These are protocol conformance failures — a client following the MCP spec will reject or misrender non-conformant responses, turning working tool calls into user-visible errors.

Return a content array with the correct type field from every tool handler. Never return a raw string.

// src/tools/screenshot.ts
server.tool('screenshot', ..., async () => ({
  content: [{
    type: 'image',
    data: base64ImageData,   // required for image
    mimeType: 'image/png',  // required for image
  }]
}))

// Text result
server.tool('read_file', ..., async ({ path }) => ({
  content: [{ type: 'text', text: await fs.readFile(path, 'utf-8') }]
}))

For tools returning multiple items, include them as separate entries in the content array — not as a concatenated string.