feat(types,spec): add tools field to McpUiHostCapabilities#656
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saaage wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
feat(types,spec): add tools field to McpUiHostCapabilities#656saaage wants to merge 1 commit intomodelcontextprotocol:mainfrom
saaage wants to merge 1 commit intomodelcontextprotocol:mainfrom
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McpUiAppCapabilities already allows widgets to declare tools: {}
to signal they expose oncalltool/onlisttools, but McpUiHostCapabilities
had no corresponding field. This left the handshake one-sided: widgets
could declare intent but had no way to check whether the host would
honor it, and hosts had no spec-defined way to advertise support.
Add tools?: { listChanged?: boolean } to McpUiHostCapabilities in both
the TypeScript types and the draft spec, with MUST-level requirements on
what a host advertising this capability is expected to implement:
- Call tools/list on the app after connect and surface results to the LLM
- Route matching LLM tool calls back to the app via tools/call
- Re-query tools/list on notifications/tools/list_changed (if listChanged: true)
Fixes: modelcontextprotocol#655
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Closes #655.
Problem
McpUiAppCapabilitiesallows widgets to declaretools: {}signalling they exposeoncalltool/onlisttools, butMcpUiHostCapabilitieshas no corresponding field. The handshake is one-sided:tools: {}inui/initialize)getHostCapabilities()?.toolsbecause the field doesn't existbridge.listTools()— the spec gap is likely a contributing factorFix
Add
tools?: { listChanged?: boolean }toMcpUiHostCapabilitiesin bothsrc/spec.types.tsandspecification/draft/apps.mdx, with MUST-level requirements describing what a host advertising this capability is expected to implement:tools/liston the app after connect and surface results to the LLMtools/calltools/listonnotifications/tools/list_changedfrom the app (iflistChanged: true)Precedent
This follows the same pattern as PR #653, which added
updateModelContextandmessagetoMcpUiHostCapabilities— fields that were already in the TypeScript types but missing from the spec document.