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MCP Apps MCP Apps

Present MCP Apps is an extension to the Model Context Protocol (MCP) that lets an MCP server's tools declare an interactive UI — a chart, form, dashboard, or design canvas — that a compliant chat client renders inline, in the conversation, rather than as plain text or structured JSON. A tool definition declares a ui:// resource containing bundled HTML/JavaScript; when the LLM calls the tool, the host fetches that resource over the ordinary MCP resources/read call and displays it in a sandboxed iframe; a bidirectional channel then lets the host push tool data into the UI via notifications and lets the UI call other MCP tools through the host [1]. It solves the specific gap that 'MCP tools return text and structured data... but not... an interactive UI' [1], and it explicitly targets the same problem A2UI and AG-UI's generative-UI feature address, but scoped to MCP-native tool calls rather than a general-purpose agent-to-frontend event stream.

Overview

It does not define a new transport (it rides on ordinary MCP JSON-RPC), does not specify how a host discovers that a server supports it beyond ordinary MCP capability negotiation and per-tool ui:// metadata, and does not address payment, identity, or non-MCP agent frameworks.

Current state (as of 2026-09-05): the canonical repository is github.com/modelcontextprotocol/ext-apps, Apache-2.0 licensed, with a dated spec file at specification/2026-01-26/apps.mdx and a companion blog post on blog.modelcontextprotocol.io dated 2026-01-26 announcing it. The repo shows sustained activity (666 commits, latest merge 2026-08-12, 2.8k stars) and ships an official npm package (@modelcontextprotocol/ext-apps) plus a React binding and a server-helpers package. The repo's own README lists ChatGPT, Claude, and VS Code as supported clients, each with first-party integration docs, which is stronger implementation evidence than a single reference build [1]. A separate, earlier community project, MCP-UI (github.com/idosal/mcp-ui, now under the MCP-UI-Org), predates MCP Apps and originally proposed its own ui:///ui-app:// resource convention starting around April 2025; it has since repositioned itself explicitly as 'an SDK implementing the MCP Apps standard,' with a legacy rendering path kept only for pre-standard MCP-UI hosts [2]. The ext-apps repo itself credits MCP-UI's client as 'a fully-featured MCP Apps framework used by a few hosts,' alongside the possibility that a client rolls its own implementation.

What is disputed or unknown: this is a new record on agenticweb.wiki, not previously tracked, so there is no prior baseline to compare against. Its governance track is marked 'community' here because the ext-apps repo, while living under the modelcontextprotocol GitHub org, is an 'ext-' (extension) project and it is not yet confirmed whether it has been formally folded into the same Agentic AI Foundation/Linux Foundation governance this site's core mcp record cites — that should be checked against MCP's own governance documents on a future pass. No independent, counted adoption figure exists; the evidence here is qualitative (named supported clients, repo activity), not a probe-based census.

Discovery and probe

A tool's definition declares a ui:// resource; the host fetches it via the standard MCP resources/read call after a tool call, rather than via a separate well-known path · ui://<resource-id> (declared per-tool, not a fixed site-wide path)

POST {mcp-endpoint} — call tools/list then tools/call on a known MCP server, per JSON-RPC
parse:     Tool definition metadata includes a ui:// resource reference (or _meta.ui.resourceUri); a follow-up resources/read on that URI returns a resource with mimeType text/html (optionally ;profile=mcp-app)
on absent: Server exposes ordinary MCP tools with text/structured-data results only; no interactive UI capability

Not probed live: no-fixed-path. MCP Apps are declared inside an MCP server's tools/list; find the server first.

History

Instances

Every instance with its source is listed on the instances page.

See also

References

  1. Canonical repo github.com/modelcontextprotocol/ext-apps (Apache-2.0 license, 2.8k stars, 666 commits, latest commit 2026-08-12): 'MCP Apps extend the Model Context Protocol by letting tools declare UI resources' via a 4-step flow (tool definition, tool call, host renders in a sandboxed iframe, bidirectional communication via notifications/tool calls). Spec file at specification/2026-01-26/apps.mdx. Supported clients listed in the repo's badges/table include ChatGPT, Claude, and VS Code, each linking to first-party docs for building MCP Apps in that client. — https://github.com/modelcontextprotocol/ext-apps (2026-09-05) VERIFIED
  2. MCP-UI (github.com/MCP-UI-Org/mcp-ui) describes itself as 'an SDK implementing the MCP Apps standard for UI over MCP,' offering a client (@mcp-ui/client) that can render via the MCP Apps AppRenderer or via a legacy UIResourceRenderer for pre-standard MCP-UI hosts — evidence that MCP Apps formalized and superseded an earlier, informally-specified MCP-UI proposal from mid-2025. — https://github.com/MCP-UI-Org/mcp-ui (2026-09-05) VERIFIED

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