[krubenok]

Summary

Add app-rendered form elicitation conventions to ModelContextProtocol.Extensions.Apps. The normative proposal is entirely contained in the MCP Apps extension: modelcontextprotocol/ext-apps#733. Core SEP-3118 is closed because no core protocol change is required.

Negotiation uses the nested elicitation: {} setting on the existing io.modelcontextprotocol/ui extension on both peers, together with core elicitation.form and Apps MIME support. McpAppElicitation.SetAppUiIfSupported attaches an absolute ui:// resource at _meta.ui.resourceUri without changing the core elicitation types. Unsupported clients retain native form rendering.

The request-context overload uses effective request-scoped client capabilities on 2026-07-28, with initialized-session fallback for older revisions. Metadata survives InputRequest.ForElicitation; the host returns the App's standard ElicitResult through the existing MRTR flow. Accepted content must match the requested schema; explicit decline/cancel results do not trigger fallback.

Includes capability types, metadata and result-validation helpers, documentation, and focused tests. No separate extension package, method, result type, or core protocol change is introduced.

TypeScript reference: modelcontextprotocol/typescript-sdk#2578.

Validation

Rebased on C# SDK main b9faee70.

• Apps and app-elicitation tests on .NET 8: 83 passed.
• Apps and app-elicitation tests on .NET 10: 83 passed.
• Apps package build for .NET Standard 2.0: passed, no warnings or errors.
• .NET 9 build succeeded; test execution could not run because this machine lacks the .NET 9 runtime.
• git diff --check: passed.

[krubenok]

The coordinated public references are now aligned to protocol revision 2026-07-28:

• MCP Apps draft: Support MCP Apps-rendered Elicitations ext-apps#733
• Core SEP: SEP-3118: App-Rendered Elicitations for MCP Apps modelcontextprotocol#3118
• C# SDK: Add app-rendered elicitation support to MCP Apps #1723
• TypeScript SDK: Add app-rendered elicitation MRTR example typescript-sdk#2578
• Reviewable Inspector fork PR: Add app-rendered elicitation reference implementation krubenok/inspector#1
• Inspector branch: https://github.com/krubenok/inspector/tree/agent/app-rendered-elicitations-reference
• Inspector reference commit: krubenok/inspector@3b524cd1

The primary flow uses request-scoped client capabilities, server capabilities from server/discover, and the core MRTR shape: the server embeds the ordinary elicitation/create request in InputRequiredResult; the host validates the App's standard ElicitResult, places it under the matching inputResponses key, and retries the originating request with requestState.

No second extension, custom method, or custom result type is introduced. The Inspector is pinned to ext-apps commit 89ab2bc. The MCP Apps View↔Host bridge independently negotiates Apps protocol version 2026-01-26; that is separate from core MCP 2026-07-28.

The references are generic and contain no product-specific example details.