{ "slug": "mcp-apps-explained", "title": "MCP Apps Explained: The Official Interactive-UI Extension for MCP", "description": "What MCP Apps (SEP-1865) is: the ui:// resource scheme and sandboxed-iframe/JSON-RPC bridge it defines for MCP tools to return rendered UI instead of plain text, how it relates to the community MCP-UI project and OpenAI's Apps SDK, and which hosts support it.", "category": "Reference", "tags": [ "mcp-apps", "mcp", "protocols", "generative-ui", "resources", "agents", "standard" ], "updated": "2026-07-14", "canonical": "https://changegamer.ai/resources/mcp-apps-explained", "markdown": "https://changegamer.ai/resources/mcp-apps-explained.md", "outline": [ { "depth": 2, "text": "Key facts", "anchor": "key-facts" }, { "depth": 2, "text": "How a tool call becomes a rendered widget", "anchor": "how-a-tool-call-becomes-a-rendered-widget" }, { "depth": 2, "text": "Distinct from three adjacent ChangeGamer topics", "anchor": "distinct-from-three-adjacent-changegamer-topics" }, { "depth": 2, "text": "Security note", "anchor": "security-note" }, { "depth": 2, "text": "Verified sources", "anchor": "verified-sources" } ], "related": [ { "slug": "mcp-primitives", "title": "MCP Primitives: Resources, Prompts, Sampling, and Elicitation", "description": "Deep reference on the six MCP capability primitives beyond tools — who controls each, the exact JSON-RPC method names, and when to use Resources vs Tools — verified against the 2025-06-18 and 2025-11-25 spec revisions.", "url": "https://changegamer.ai/resources/mcp-primitives" }, { "slug": "nlweb-explained", "title": "NLWeb Explained: Microsoft's Natural-Language Query Protocol for Websites", "description": "What NLWeb is: the open, MIT-licensed protocol that lets a site answer natural-language questions over its own Schema.org data via /ask and /mcp endpoints, who built it, and how it differs from llms.txt, AGENTS.md, and a generic MCP server.", "url": "https://changegamer.ai/resources/nlweb-explained" }, { "slug": "mcp-2026-spec-revision", "title": "MCP Goes Stateless: The 2026-07-28 Spec Revision Explained", "description": "What changes in MCP's largest revision since launch: SEP-2575/SEP-2567 remove the session handshake and Mcp-Session-Id header for explicit state handles, new Mcp-Method/Mcp-Name routing headers, full JSON Schema 2020-12 tool schemas, and six authorization-hardening SEPs — an RC snapshot as of 2026-07-14; final ships 2026-07-28.", "url": "https://changegamer.ai/resources/mcp-2026-spec-revision" }, { "slug": "mcp-server-authentication", "title": "MCP Server Authentication: OAuth 2.1 for Remote Servers", "description": "How OAuth 2.1 works for remote MCP servers: transport differences, Protected Resource Metadata discovery, PKCE, Resource Indicators, and token-audience security — with a step-by-step client flow and honest notes on what ChangeGamer's own /mcp endpoint does.", "url": "https://changegamer.ai/resources/mcp-server-authentication" } ], "body": "Plain MCP tools return text and structured data — a chart or a form needs the host to build its own custom rendering logic. **MCP Apps** is the official MCP extension that closes that gap: a server pre-declares an HTML UI resource, links it to a tool, and a compatible host renders it in a sandboxed iframe that talks back over MCP's own JSON-RPC channel — instead of every client inventing its own bespoke widget protocol.\n\n## Key facts\n\n- MCP Apps is **SEP-1865** (\"MCP Apps - Interactive User Interfaces for MCP\"), proposed November 21, 2025 and merged as an accepted, live MCP extension January 26–28, 2026 — confirmed directly against the merged GitHub pull request and the MCP project's own blog.\n- It defines a `ui://` URI scheme for pre-declared UI resources served as `text/html;profile=mcp-app`, mandatory sandboxed-iframe rendering, and a bidirectional JSON-RPC-over-`postMessage` bridge (`ui/*` methods and notifications) between the widget and host.\n- It was co-developed by Anthropic, OpenAI, and the community **MCP-UI** project (`@mcp-ui/server` / `@mcp-ui/client`), which had already been shipping the same iframe-plus-postMessage pattern experimentally — MCP Apps standardizes what MCP-UI and OpenAI's separately-branded **Apps SDK** each prototyped first.\n- The extension is optional and backward-compatible, negotiated per-connection through MCP's existing capability mechanism — a server or client that doesn't implement it just gets a plain tool result, nothing breaks.\n- Adoption named on the launch announcement itself: Anthropic (Claude), OpenAI (ChatGPT, rolling out), Block (Goose), Microsoft (VS Code Insiders), JetBrains, AWS, and Google DeepMind.\n\n## How a tool call becomes a rendered widget\n\nThe server registers a `ui://` resource template and links it to a tool — the MCP Apps standard key is `_meta.ui.resourceUri` on the tool descriptor (OpenAI's Apps SDK instead used its own `_meta[\"openai/outputTemplate\"]` key; check both if a tool must target either host — WebSearch-corroborated, not primary-fetched, see sources below). The model calls the tool as normal via `tools/call`; the result points at the linked resource. The host fetches and caches the `ui://` resource once, then renders it sandboxed. Interaction inside the widget (a button, a form) sends JSON-RPC messages back over the bridge, which can trigger further tool calls — the same trust boundary as any other MCP message, not an open postMessage channel.\n\n## Distinct from three adjacent ChangeGamer topics\n\n- **Not one of the six core primitives** (/resources/mcp-primitives): MCP Apps is an optional *extension* layered on the Resources primitive, not itself one of the base six.\n- **Not AG-UI** (/resources/generative-ui-for-agents): AG-UI is an SSE event vocabulary an agent backend streams to a frontend you build; MCP Apps ships the UI *inside the MCP resource itself* and renders inside the host's own chat surface — no separate frontend to build.\n- **Not covered by the general server-building guide** (/resources/building-mcp-servers): its \"three server primitives\" section predates this extension and stops at plain tool/resource/prompt results, with no UI-rendering path.\n\n## Security note\n\nA rendered widget is untrusted, model-adjacent content running in the user's own session — treat it as another agent-exposed UI surface (see /resources/agentic-security-checklist and /resources/agentic-browsers): validate everything the iframe sends back before acting on it, and rely on the mandatory sandboxing rather than the widget's own good behavior.\n\n## Verified sources\n\nPrimary (fetched directly this session):\n\n- SEP-1865 pull request (merged; title and technical summary confirmed): https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1865\n- MCP Apps launch post, \"now live\" (Jan 26, 2026): https://blog.modelcontextprotocol.io/posts/2026-01-26-mcp-apps/\n- MCP Apps proposal post (Nov 21, 2025): https://blog.modelcontextprotocol.io/posts/2025-11-21-mcp-apps/\n\nSecondary — WebSearch-corroborated only (developers.openai.com, mcpui.dev, and a wikipedia.org control fetch all returned HTTP 403 to direct WebFetch this session, so these specific details are not primary-fetched and should be reverified when those domains are reachable):\n\n- OpenAI Apps SDK reference (page title + 3 independent developer write-ups agree on the `_meta` key names): https://developers.openai.com/apps-sdk/reference\n- MCP-UI project docs on the Apps SDK relationship: https://mcpui.dev/guide/apps-sdk\n- Shopify Engineering, \"MCP UI: Breaking the text wall with interactive components\": https://shopify.engineering/mcp-ui-breaking-the-text-wall", "sources": [ "https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1865", "https://blog.modelcontextprotocol.io/posts/2026-01-26-mcp-apps/", "https://blog.modelcontextprotocol.io/posts/2025-11-21-mcp-apps/", "https://developers.openai.com/apps-sdk/reference", "https://mcpui.dev/guide/apps-sdk", "https://shopify.engineering/mcp-ui-breaking-the-text-wall" ] }