MCP Apps: Spec compliance tracking (2026-01-26)

[aharvard]

MCP Apps Spec Compliance — 2026-01-26

Tracking full compliance with the MCP Apps specification (2026-01-26).

Current status: ~95% compliant

All issues below are labeled mcp-apps-compliance for filtering.

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High Priority

• Respect MCP visibility flags #7467 — Respect MCP visibility flags (_meta.ui.visibility filtering + enforcement) — feat: filter tools by MCP Apps visibility metadata #8022
• MCP App iframe does not support fullscreen display mode (e.g. Excalidraw) #7142 — Fullscreen close button must always persist regardless of app capabilities — fix(desktop): fullscreen header bar + always-visible close controls #8033

Medium Priority

• feat: Implement ui/update-model-context for MCP Apps #6472 — Implement ui/update-model-context for MCP Apps

Low Priority

• MCP Apps: Report sandbox permissions and CSP in hostCapabilities #8013 — Report sandbox permissions and CSP in hostCapabilities
• MCP Apps: Security hardening — audit logging and resource integrity #8014 — Security hardening — audit logging and resource integrity
• MCP Apps: Plumb toolInfo.id (JSON-RPC request ID) to frontend #8015 — Plumb toolInfo.id (JSON-RPC request ID) to frontend
• MCP Apps: Support appCapabilities.tools (View-exposed tools) #8016 — Support appCapabilities.tools (View-exposed tools)

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Completed

• Tool visibility filtering — _meta.ui.visibility parsing, model-side filtering, app-side enforcement with 403 (Respect MCP visibility flags #7467, feat: filter tools by MCP Apps visibility metadata #8022)
• Fullscreen header bar with title, always-visible close/PiP controls (MCP App iframe does not support fullscreen display mode (e.g. Excalidraw) #7142, fix(desktop): fullscreen header bar + always-visible close controls #8033)
• Respect MCP visibility flags — _meta.ui.visibility filtering (Respect MCP visibility flags #7467, feat: filter tools by MCP Apps visibility metadata #8022)
• Extension capability negotiation — io.modelcontextprotocol/ui + mimeTypes (feat(mcp): upgrade rmcp to 0.15.0 and advertise MCP Apps UI extension capability #6927)
• MCP-compliant theme tokens — CSS variables with light-dark(), custom fonts (refactor: MCP-compliant theme tokens and CSS class rename #7275)
• Container dimensions — containerDimensions replacing viewport (fix(ui): revert app-driven iframe width and send containerDimensions per ext-apps spec #7300)
• Display modes — fullscreen and PiP with negotiation (feat(ui): implement fullscreen and pip display modes for MCP Apps #7312)
• toolInfo in hostContext — tool name, description, inputSchema
• sandbox-proxy-ready notification naming
• appCapabilities.availableDisplayModes parsing
• ui/request-display-mode handler
• host-context-changed auto-diffing (handled by @mcp-ui/client SDK v6.1.0)
• UI resource format — ui:// scheme, text/html;profile=mcp-app MIME type
• CSP enforcement — connectDomains, resourceDomains, frameDomains, baseUriDomains
• Permission Policy — camera, microphone, geolocation, clipboardWrite
• All standard MCP messages — tools/call, resources/read, ping, notifications/message
• All host→view notifications — tool-input, tool-input-partial, tool-result, tool-cancelled, size-changed, resource-teardown
• sampling/createMessage support via onFallbackRequest