Capabilities and fallbacks

Detect advertised Apps support without changing authorization or registration.

getAppSupport(ctx) from @toolgrain/mcp returns:

Value	Meaning
supported	The current client advertises the Apps extension and required HTML MIME type
not-advertised	Client capabilities are available but do not advertise that support
unknown	This invocation has no client-capability context

The extension key is io.modelcontextprotocol/ui; its mimeTypes must include text/html;profile=mcp-app.

Modern MCP requests carry their own capabilities. Legacy stdio retains claims from initialization. Independent legacy HTTP does not retain initialization state, so later calls return unknown. Toolgrain adds no session or transport-wide capability cache.

Registration stays static. Every UI-enabled tool must provide meaningful text/JSON fallback content even when UI support is advertised. Capabilities can guide optional presentation work, but they must not decide whether an operation is authorized.

The React iframe handshake negotiates host capabilities separately. useToolCall rejects when the host does not advertise server tool calls. Host themes and optional fields should have application fallbacks.

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React protocol support

The Toolgrain 0.3.0 API exposes the provider and four hooks. Start with AppProvider and useToolResult(), then add optional hooks as needed. An upstream advertisement describes what a host can accept; only implemented APIs can send the corresponding requests.

Wire feature	Toolgrain React behavior
ui/initialize and ui/notifications/initialized	Native provider handshake and revision check
tools/call	useToolCall; requires advertised serverTools
Tool input, result, and cancellation notifications	Retained initiating state through useToolResult
ui/notifications/tool-input-partial	Ignored; the specification permits this. Only complete input is exposed
Host context changes	useHostContext; optional theme/font application by the provider
ui/notifications/size-changed	Automatic size reporting when enabled and ResizeObserver is available
Host ping and ui/resource-teardown	Responded to natively; teardown closes the connection and cleans up
resources/read from the widget	No public React resource-reading API
ui/open-link, ui/message, ui/update-model-context	useHostActions; requires action and content-kind advertisements
ui/request-display-mode	useHostActions; requires intersection of host/provider modes. Provider defaults to inline
ui/download-file and sampling/createMessage	No public React API
App-exposed tools and app-initiated teardown	Deferred; the provider does not advertise app tools
Sandbox proxy setup	Host responsibility

useHostContext() reports the host’s current display mode and available modes. The new action hook requests changes separately and returns the actual selected mode; live context still follows host notifications. A tool that refreshes data uses useToolCall, independently of the initiating result.

The dedicated protocol types describe host capability advertisements, including optional actions outside this React API. They preserve extension fields. The reviewed upstream type file includes newer advertisements such as file downloads that are absent from the pinned stable prose specification; their presence is not a Toolgrain implementation or live-host compatibility claim.

Host actions currently have automated fixture coverage, with live-host acceptance pending. Missing advertisements reject locally, even for older hosts that implement unadvertised actions. Use the compatibility report for the exact tested host versions and transports.