feat(types,spec): add tools field to McpUiHostCapabilities#656
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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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Pull request overview
Adds a missing tools capability to McpUiHostCapabilities so hosts can explicitly advertise support for app-declared tools, completing the host↔view capability handshake described in the MCP Apps draft spec.
Changes:
- Added
tools?: { listChanged?: boolean }toMcpUiHostCapabilitiesTypeScript types. - Updated the draft spec’s
HostCapabilitiesto document host requirements when advertisingtoolssupport.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| src/spec.types.ts | Extends McpUiHostCapabilities with a tools capability for app-declared tools support. |
| specification/draft/apps.mdx | Documents the new tools host capability and associated MUST-level host behaviors in the draft spec. |
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| /** @description Host supports calling tools declared by the app (oncalltool/onlisttools). When present, the host will call tools/list on the app after connect, surface the results to the LLM, and route matching tool calls back to the app via tools/call. */ | ||
| tools?: { | ||
| /** @description Host supports tools/list_changed notifications from the app and will re-query tools/list when received. */ | ||
| listChanged?: boolean; | ||
| }; |
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| /** | ||
| * Host supports calling tools declared by the app (oncalltool/onlisttools). | ||
| * When present, the host MUST: | ||
| * 1. Call tools/list on the app after connect and surface results to the LLM. | ||
| * 2. Route LLM tool calls matching app-declared tool names back to the app via tools/call. |
| /** | ||
| * Host supports calling tools declared by the app (oncalltool/onlisttools). | ||
| * When present, the host MUST: | ||
| * 1. Call tools/list on the app after connect and surface results to the LLM. |
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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.