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What Are MCP Resources?

Resources are data that an MCP server exposes for reading. Each resource is identified by a URI and has a MIME type. Unlike tools (which execute actions), resources represent data that can be fetched — files, structured data, or (in MCP Apps) interactive HTML UIs.

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Discovery (resources/list)

The host calls resources/list to discover available resources. Each resource declares:
FieldDescription
uriUnique identifier (e.g., "ui://weather/view.html")
nameHuman-readable name
descriptionWhat the resource contains
mimeTypeContent type (e.g., "text/html", "application/json")
{
  "uri": "ui://weather/view.html",
  "name": "Weather View",
  "description": "Interactive weather display",
  "mimeType": "text/html;profile=mcp-app"
}

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URIs

Resources are identified by URIs. Servers can use any URI scheme. MCP Apps uses the ui:// scheme to identify UI resources that should be rendered in iframes.

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MIME Types

Each resource declares its content type via mimeType. MCP Apps uses the MIME type text/html;profile=mcp-app (exported as RESOURCE_MIME_TYPE) to identify HTML resources that hosts should render as interactive Views.

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Reading (resources/read)

The host calls resources/read with a resource URI to fetch its contents. The server returns an array of content items. 
// Request
{ "method": "resources/read", "params": { "uri": "ui://weather/view.html" } }

// Response
{
  "contents": [{
    "uri": "ui://weather/view.html",
    "mimeType": "text/html;profile=mcp-app",
    "text": "<!DOCTYPE html>..."
  }]
}

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In MCP Apps

MCP Apps extends resources with _meta.ui metadata on content items. This lets servers declare security policies (CSP), iframe permissions, rendering preferences, and stable sandbox origins for their UIs. 
{
  "uri": "ui://weather/view.html",
  "mimeType": "text/html;profile=mcp-app",
  "text": "<!DOCTYPE html>...",
  "_meta": {
    "ui": {
      "csp": { "connectDomains": ["https://api.example.com"] },
      "permissions": { "geolocation": {} }
    }
  }
}

registerAppResource

Register HTML resources on the MCP server.

Resource _meta

CSP, permissions, domain, and rendering metadata.