feat: MCP app sampling support#477
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alexhancock wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
feat: MCP app sampling support#477alexhancock wants to merge 1 commit intomodelcontextprotocol:mainfrom
alexhancock wants to merge 1 commit intomodelcontextprotocol:mainfrom
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@modelcontextprotocol/ext-apps
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@modelcontextprotocol/server-basic-vanillajs
@modelcontextprotocol/server-budget-allocator
@modelcontextprotocol/server-cohort-heatmap
@modelcontextprotocol/server-customer-segmentation
@modelcontextprotocol/server-map
@modelcontextprotocol/server-pdf
@modelcontextprotocol/server-scenario-modeler
@modelcontextprotocol/server-shadertoy
@modelcontextprotocol/server-sheet-music
@modelcontextprotocol/server-system-monitor
@modelcontextprotocol/server-threejs
@modelcontextprotocol/server-transcript
@modelcontextprotocol/server-video-resource
@modelcontextprotocol/server-wiki-explorer
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ochafik
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Mar 6, 2026
Alternative to #477. Instead of forking a stripped-down sampling schema, this reuses the stock `CreateMessageRequest` / `CreateMessageResult` / `CreateMessageResultWithTools` types from `@modelcontextprotocol/sdk` directly — same pattern already used for `tools/call`. - Spec: `sampling/createMessage` listed under `### Standard MCP Messages`, `sampling?: { tools?: {} }` added to `HostCapabilities` (mirrors MCP `ClientCapabilities.sampling` shape for easy pass-through). - SDK: `App.createSamplingMessage()` with overloads that narrow the return type based on whether `params.tools` is set; `AppBridge.oncreatesamplingmessage` setter; `CreateMessageRequest`/`CreateMessageResult*` added to the `AppRequest`/`AppResult` unions. - Picks up SEP-1577 tool-calling support (`tools`, `toolChoice`, `tool_use` content blocks, `stopReason: "toolUse"`, array content) for free — unblocks the "nested agents" use case motivated by the original PR. https://claude.ai/code/session_01ENGWTtsfcyP4S6fTWtUMAh
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Adds support for MCP apps to sample from the host's model connection, enabling Apps themselves to become AI-powered
See a more detailed proposal here: https://gist.github.com/alexhancock/2bb8b62500f33ccb29a0edb4aa7c5342
Motivation and Context
The idea is simply to let App code leverage the host's model connection, opening up a range of compelling app experiences (nested agents, specialized visual tutors, image generators, vision-based applications, etc)
How Has This Been Tested?
This concept has been implemented in goose (block/goose#7039) using the
onFallbackRequestparam, but this change elevates the idea to a first class citizen)Breaking Changes
None
Types of changes
Checklist
Additional context
This MCP SEP is related as it adds the supported modalities concept to client capabilities as well modelcontextprotocol/modelcontextprotocol#2278. This way we can use the same type in modeling host capabilities re: sampling