[2026-07-28] Extensions framework + Tasks + MCP Apps

Tracking issue for the MCP Spec 2026-07-28 release — Extensions framework + Tasks + MCP Apps milestone.

SEP-2133 establishes the extension mechanism (capability negotiation + extensible handler dispatch). Both SEP-2663 (Tasks) and SEP-1865 (MCP Apps) graduate to extensions on top of it. Land SEP-2133 first; Tasks and Apps depend on it.

SEPs covered

Sub-issues

Foundation (blocker for Tasks and Apps):

• #344 — SEP-2133: Extensions framework (capability negotiation + ExtensionRegistry + extensible dispatch)

SEP-2663 Tasks:

• #345 — Schema: Task shape, resultType discriminator, tasks/* request/result types
• #346 — Server: TasksExtension, capability gate, tasks/get/update/cancel handlers
• #347 — Server: TaskStoreInterface with InMemory and PSR-16 stores
• #348 — Server: Handlers return task handles; the application advances the task
• #349 — Client: Poll task handles and answer inputRequests via tasks/update

SEP-1865 MCP Apps:

• #350 — Schema: Content types, ui:// scheme, tool metadata fields
• #351 — Server: ext-apps registration, builder API, #[McpUiResource] attribute
• #352 — Server/Examples: Link tools to UI templates + example server + inspector tests

Notes

• All three SEPs are merged into the spec repo.
• SEP-2663 supersedes SEP-1686. Issue #139 (SEP-1686 Tasks) was closed and rescoped to this milestone — the SEP-1686 design was never shipped by the PHP SDK, so there is nothing to deprecate.
• SEP-1865 spec lives at modelcontextprotocol/ext-apps.

Start with sub-issue #344 and SEP-2133, since the Extensions framework blocks the Tasks and MCP Apps work. Then review sub-issues #345–#352 and their linked SEPs; the milestone is done when the foundation, Tasks, and MCP Apps items are implemented in dependency order.