{ "title": "MCP Apps FAQ", "schemaVersion": "1.0", "researchFreeze": "2026-08-26", "protocolRevision": "MCP Apps 2026-01-26 / ext-apps SDK v1.7.5 / MCP core 2026-07-28", "sourcePackage": "research/9", "integrityNote": "The 93 ordinary archive entries verify. Two self-referential bookkeeping entries have disclosed stale final rows; public corrections do not modify the raw archive.", "schemaNote": "Visible answer dataset only. FAQPage JSON-LD is intentionally omitted.", "count": 53, "records": [ { "id": "R9-FAQ001", "packageFaqId": "FAQ001", "question": "What is an MCP App?", "answer": "An MCP App is an interactive View associated with an MCP tool through a ui:// UI resource and rendered by a compatible host inside an isolated container. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ002", "packageFaqId": "FAQ002", "question": "What does MCP Apps add to MCP?", "answer": "It adds standardized UI resources, tool-to-UI linkage and an App–Host JSON-RPC bridge for interactive results. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ003", "packageFaqId": "FAQ003", "question": "When were MCP Apps announced?", "answer": "The public proposal appeared on November 21, 2025. [S001]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ004", "packageFaqId": "FAQ004", "question": "When did MCP Apps become stable?", "answer": "The stable 2026-01-26 extension launched on January 26, 2026. [S002][S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ005", "packageFaqId": "FAQ005", "question": "Is MCP Apps part of core MCP?", "answer": "No. It is an optional official extension identified as io.modelcontextprotocol/ui. [S004][S025]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ006", "packageFaqId": "FAQ006", "question": "Is an MCP App the same as an MCP server?", "answer": "No. The server publishes tools and UI resources; the App is the rendered View inside the host. [S004][S006]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ007", "packageFaqId": "FAQ007", "question": "Do MCP Apps replace tools?", "answer": "No. The UI is linked to a tool and enriches its interaction or result. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ008", "packageFaqId": "FAQ008", "question": "What is a UI resource?", "answer": "It is an MCP resource using a ui:// URI whose content is an MCP App HTML document. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ009", "packageFaqId": "FAQ009", "question": "What MIME type do MCP Apps use?", "answer": "The stable HTML profile uses text/html;profile=mcp-app. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ010", "packageFaqId": "FAQ010", "question": "How does a tool reference an MCP App?", "answer": "Through the tool metadata field _meta.ui.resourceUri. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ011", "packageFaqId": "FAQ011", "question": "How is the UI rendered?", "answer": "A compatible host fetches the UI resource and renders it in a sandboxed iframe or equivalent isolated View. [S004][S008]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ012", "packageFaqId": "FAQ012", "question": "How do the View and host communicate?", "answer": "They exchange JSON-RPC 2.0 messages over postMessage through the App/AppBridge protocol. [S004][S015]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ013", "packageFaqId": "FAQ013", "question": "Does MCP Apps use initialize?", "answer": "The App–Host extension uses ui/initialize. This is distinct from the core MCP initialize handshake removed in 2026-07-28. [S004][S026]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ014", "packageFaqId": "FAQ014", "question": "What happens in hosts without MCP Apps support?", "answer": "The tool should still return useful text or structured fallback content. [S004][S008]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ015", "packageFaqId": "FAQ015", "question": "What is progressive enhancement in MCP Apps?", "answer": "It means the tool remains usable without UI while compatible hosts render the richer interactive View. [S006][S008]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ016", "packageFaqId": "FAQ016", "question": "What display modes exist?", "answer": "The stable extension defines inline, fullscreen and picture-in-picture modes. [S004][S021]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ017", "packageFaqId": "FAQ017", "question": "Can an App force fullscreen?", "answer": "No. It may request a mode, but the host retains final control. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ018", "packageFaqId": "FAQ018", "question": "Can an App call MCP tools?", "answer": "Yes, when the host and server permit it and the tool’s visibility includes app. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ019", "packageFaqId": "FAQ019", "question": "What is an app-only tool?", "answer": "A tool whose UI visibility includes app but not model, keeping it out of the model’s tool list. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ020", "packageFaqId": "FAQ020", "question": "Can an App call a tool from another server?", "answer": "Stable app-only tool behavior is scoped to the same server connection; cross-server app-only calls are blocked. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ021", "packageFaqId": "FAQ021", "question": "Can an App send a message to the model?", "answer": "It can request that the host add a message or update model context, subject to host policy. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ022", "packageFaqId": "FAQ022", "question": "Can an MCP App access the host DOM?", "answer": "A correctly isolated View should not have access to the host DOM. [S004][S060]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ023", "packageFaqId": "FAQ023", "question": "Can it access host cookies or local storage?", "answer": "The sandboxed View does not inherit the host’s origin storage; app-origin storage behavior is host-specific. [S004][S060]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ024", "packageFaqId": "FAQ024", "question": "What is CSP in MCP Apps?", "answer": "It is the host-enforced Content Security Policy built from the server’s declared external domains. [S004][S010]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ025", "packageFaqId": "FAQ025", "question": "What is CORS in MCP Apps?", "answer": "CORS is the external server’s decision to permit requests from the App’s origin. [S010][S063]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ026", "packageFaqId": "FAQ026", "question": "Does CSP solve CORS?", "answer": "No. Declaring a domain in CSP does not make the remote API return a permissive CORS response. [S010]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ027", "packageFaqId": "FAQ027", "question": "What permissions can an App request?", "answer": "The stable spec includes camera, microphone, geolocation and clipboard-write permission requests. [S004]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ028", "packageFaqId": "FAQ028", "question": "Are permissions automatically granted?", "answer": "No. The host and browser may deny them, and the App should feature-detect. [S004][S066]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ029", "packageFaqId": "FAQ029", "question": "What is _meta.ui.domain?", "answer": "It requests a stable dedicated sandbox origin, useful for CORS or OAuth allowlists; format is host-specific. [S004][S010]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ030", "packageFaqId": "FAQ030", "question": "Are MCP Apps secure?", "answer": "They have sandboxing, CSP and host mediation, but security still depends on the App code, dependencies, host, server and authorization. [S004][S030]", "packageSourceIds": [ "S004", "S010", "S030" ], "sourceIds": [ "S390", "S395", "S410" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ031", "packageFaqId": "FAQ031", "question": "Can an App bypass server authorization?", "answer": "No. The server and underlying system must still authorize every operation. [S030]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ032", "packageFaqId": "FAQ032", "question": "Can tool results contain prompt injection?", "answer": "Yes. Tool output and external data remain untrusted even when rendered visually. [S004][S030]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ033", "packageFaqId": "FAQ033", "question": "What is MCP-UI?", "answer": "MCP-UI is the earlier community project that pioneered UI-over-MCP patterns and now supports the official MCP Apps standard. [S032][S033]", "packageSourceIds": [ "S001", "S032", "S033" ], "sourceIds": [ "S388", "S412", "S413" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ034", "packageFaqId": "FAQ034", "question": "Are MCP-UI and MCP Apps identical historically?", "answer": "No. MCP-UI preceded and influenced the standardized MCP Apps extension. [S001][S004]", "packageSourceIds": [ "S001", "S032", "S033" ], "sourceIds": [ "S388", "S412", "S413" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ035", "packageFaqId": "FAQ035", "question": "How does MCP Apps relate to the OpenAI Apps SDK?", "answer": "The Apps SDK helped validate the product pattern, and OpenAI now directs new ChatGPT UI builds to the open MCP Apps standard. ChatGPT-specific extensions such as window.openai remain optional, so portability still requires testing.", "packageSourceIds": [ "S035", "S036", "S037", "S038" ], "sourceIds": [ "S415", "S416", "S140", "S417" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence", "packageSnapshot": { "answer": "OpenAI’s Apps SDK helped establish the product pattern and influenced the standard; product-specific APIs and adapters can still differ. [S004][S035][S034]" }, "editorialCorrection": "Updated the historical relationship with OpenAI’s current standards-first guidance." }, { "id": "R9-FAQ036", "packageFaqId": "FAQ036", "question": "Do MCP Apps only work in Claude?", "answer": "No. Official evidence includes Claude, ChatGPT, VS Code, Goose and other compatible hosts/tools. [S002][S006][S044][S046]", "packageSourceIds": [ "S041", "S042" ], "sourceIds": [ "S420", "S145" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ037", "packageFaqId": "FAQ037", "question": "Does ChatGPT support MCP Apps?", "answer": "Yes. OpenAI documents MCP Apps for ChatGPT UI and developer-mode/custom-app workflows. Plan, platform, action, and review limitations remain product-specific and require a current check.", "packageSourceIds": [ "S035", "S036", "S037", "S038" ], "sourceIds": [ "S415", "S416", "S140", "S417" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence", "packageSnapshot": { "answer": "OpenAI documents MCP Apps through the Apps SDK and developer-mode/custom app workflows, with product and plan limitations that must be live-checked. [S036][S037]" }, "editorialCorrection": "Clarified that current ChatGPT UI guidance implements MCP Apps directly." }, { "id": "R9-FAQ038", "packageFaqId": "FAQ038", "question": "Does Claude support MCP Apps?", "answer": "Anthropic documents interactive connectors rendered inline or fullscreen across Claude products, subject to current plan/product availability. [S041]", "packageSourceIds": [ "S041", "S042" ], "sourceIds": [ "S420", "S145" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ039", "packageFaqId": "FAQ039", "question": "Does VS Code support MCP Apps?", "answer": "Microsoft announced full MCP Apps support in VS Code in January 2026. [S044]", "packageSourceIds": [ "S044", "S045" ], "sourceIds": [ "S143", "S421" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ040", "packageFaqId": "FAQ040", "question": "Does Goose support MCP Apps?", "answer": "Goose documentation and project history document MCP Apps/MCP-UI rendering in Goose Desktop. [S046][S047]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ041", "packageFaqId": "FAQ041", "question": "Does Postman support MCP Apps?", "answer": "Postman documents interaction with MCP App UIs in its MCP client. [S048]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ042", "packageFaqId": "FAQ042", "question": "Does Cursor support MCP Apps?", "answer": "Yes. Current official Cursor documentation lists the MCP Apps extension as supported and describes interactive UI views returned by tools. Exact display behavior and action policy remain host-specific.", "packageSourceIds": [ "S081" ], "sourceIds": [ "S269" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence", "packageSnapshot": { "answer": "The reviewed current Cursor MCP documentation established general MCP support but did not verify Apps rendering. [S081]" }, "editorialCorrection": "Reversed the archive’s stale answer after checking current first-party Cursor documentation." }, { "id": "R9-FAQ043", "packageFaqId": "FAQ043", "question": "Which frontend frameworks can be used?", "answer": "The official repository includes examples for React, Vue, Svelte, Preact, Solid and vanilla JavaScript; the protocol is framework-neutral. [S003][S006]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ044", "packageFaqId": "FAQ044", "question": "Can an MCP App be a conventional web app?", "answer": "It can reuse web technology, but its entry point, host bridge, authority and sandbox differ from a top-level web application. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ045", "packageFaqId": "FAQ045", "question": "Can Apps persist state?", "answer": "They can keep local View state and may use server or host-mediated state, but durability and portability are host-specific. [S004][S076]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ046", "packageFaqId": "FAQ046", "question": "Can Apps stream HTML?", "answer": "The stable resource model returns complete HTML; streaming HTML is an open proposal rather than stable behavior. [S004][S077]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ047", "packageFaqId": "FAQ047", "question": "Are app-provided tools stable?", "answer": "No. App-provided ephemeral tools are present in draft work and should not be described as stable. [S005][S079]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ048", "packageFaqId": "FAQ048", "question": "How should MCP Apps be tested?", "answer": "Use the official basic host or inspectors, then test in every target production host because capabilities differ. [S011][S049]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ049", "packageFaqId": "FAQ049", "question": "Are MCP Apps accessible automatically?", "answer": "No. Developers and hosts must implement and test keyboard, focus, screen-reader, contrast and dynamic-status behavior. [S057][S068]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ050", "packageFaqId": "FAQ050", "question": "When should I use an MCP App?", "answer": "Use one when direct manipulation, visualization or multi-step input materially improves a tool workflow and a fallback remains useful. [S006]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ051", "packageFaqId": "FAQ051", "question": "When should I not use an MCP App?", "answer": "Avoid it when text or structured output is sufficient, the workflow belongs in a full application, or target hosts cannot render Apps reliably.", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ052", "packageFaqId": "FAQ052", "question": "What is the current stable MCP Apps specification?", "answer": "As verified on August 26, 2026, the stable extension specification is dated 2026-01-26; current status must be rechecked at publication. [S004]", "packageSourceIds": [ "S004", "S006" ], "sourceIds": [ "S390", "S136" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" }, { "id": "R9-FAQ053", "packageFaqId": "FAQ053", "question": "What is the current MCP Apps SDK version?", "answer": "The latest reviewed ext-apps release was v1.7.5 on July 23, 2026; SDK versions are separate from the stable extension date. [S017]", "packageSourceIds": [ "S035", "S036", "S037", "S038" ], "sourceIds": [ "S415", "S416", "S140", "S417" ], "sourceMappingStatus": "editorially-mapped-to-package-evidence" } ] }