feat: add MCP Apps (SEP-1865) support

[mattdholloway]

Adds opt-in MCP Apps (SEP-1865) support to the SDK.

What

• New enableMcpApps session capability (SessionConfig + ResumeSessionConfig). When true, the runtime advertises the extensions.io.modelcontextprotocol/ui extension to MCP servers and exposes the session.rpc.mcp.apps.{listTools,callTool,readResource,setHostContext,getHostContext} JSON-RPC methods. Defaults to false so hosts without an iframe renderer don't accidentally register UI-enabled tool variants they can't display.
• Two new pure helpers in the Node SDK for hosts that render ui:// MCP App bundles in iframes:
  • buildMcpAppsCspHeader(csp) — builds the Content-Security-Policy header per SEP-1865 §UI Resource Format + §Security Implications. Emits the restrictive default (connect-src 'none') when _meta.ui.csp is absent, and the constructed default (connect-src 'self' + declared domains, etc.) when it is declared (even with empty arrays).
  • buildMcpAppsAllowAttribute(permissions) — maps _meta.ui.permissions to the iframe allow attribute (Permission Policy), including the hyphenated clipboard-write name.
• Regenerated RPC/session-event types across Node, Python, Go, .NET, and Rust to pick up the new schema.

 sequenceDiagram
     autonumber
     participant Host as Host App
     participant SDK as Copilot SDK
     participant Sandbox as mcpAppsSandbox helpers
     participant Conn as JSON-RPC Connection
     participant Runtime as Copilot Runtime
     participant Iframe as ui iframe
 
     Note over Host,Runtime: 1. Opt-in via SDK config
     Host->>SDK: createSession({ enableMcpApps: true })
     SDK->>Conn: session.create { ..., requestMcpApps: true }
     Conn->>Runtime: JSON-RPC request
     Runtime-->>Conn: CreateSessionResponse
     Conn-->>SDK: response
     SDK-->>Host: CopilotSession
 
     Note over Runtime,Host: 2. Tool execution events forwarded unchanged
     Runtime-->>Conn: tool.execution_complete { uiResource, toolDescription._meta.ui }
     Conn-->>SDK: SessionEvent (untyped pass-through)
     SDK-->>Host: onEvent(event)
 
     Note over Host,Iframe: 3. Host builds sandboxed iframe using SDK helpers
     Host->>Sandbox: buildMcpAppsCspHeader(uiResource._meta.ui.csp)
     Sandbox-->>Host: CSP header value
     Host->>Sandbox: buildMcpAppsAllowAttribute(uiResource._meta.ui.permissions)
     Sandbox-->>Host: allow attribute value
     Host->>Iframe: render with CSP + allow + uiResource
 
     Note over Iframe,Runtime: 4. App-to-server RPCs (no typed SDK surface)
     Iframe->>Host: postMessage RPC
     Host->>Conn: raw JSON-RPC: session.rpc.mcp.apps.{listTools|callTool|readResource}
     Conn->>Runtime: forwarded
     Runtime-->>Conn: result (+ optional ephemeral mcp_app.tool_call_complete event)
     Conn-->>SDK: SessionEvent (untyped pass-through, callTool only)
     SDK-->>Host: onEvent(event)
     Conn-->>Host: RPC response
     Host-->>Iframe: postMessage response
 
     Note over Host,Runtime: 5. Resume preserves opt-in
     Host->>SDK: resumeSession({ enableMcpApps: true })
     SDK->>Conn: session.resume { ..., requestMcpApps: true }

Loading

Closes https://github.com/github/copilot-mcp-core/issues/1715
Depends on https://github.com/github/copilot-agent-runtime/pull/7605

[mattdholloway]

@copilot resolve the merge conflicts in this pull request

[github-actions]

Generated by SDK Consistency Review Agent for issue #1335 · ● 793.2K

[mattdholloway]

@copilot resolve the merge conflicts in this pull request

[github-actions]

Cross-SDK Consistency Review

Summary: This PR adds enableMcpApps (SEP-1865) support consistently across Node.js, Python, Go, .NET, and Rust. However, the Java SDK is missing this feature.

✅ SDKs Updated

SDK	Config Field	Wire Field	Warn Helper
Node.js	enableMcpApps?: boolean	requestMcpApps	warnIfMcpAppsDropped()
Python	enable_mcp_apps: bool = False	requestMcpApps	_warn_if_mcp_apps_dropped()
Go	EnableMcpApps bool	requestMcpApps	warnIfMcpAppsDropped()
.NET	EnableMcpApps bool	RequestMcpApps	WarnIfMcpAppsDropped()
Rust	request_mcp_apps: Option<bool>	request_mcp_apps	warn_if_mcp_apps_dropped()

❌ Java SDK — Missing enableMcpApps Support

The Java SDK needs equivalent changes in the following files:

1. java/src/main/java/com/github/copilot/sdk/json/SessionConfig.java — add enableMcpApps field (analogous to onElicitationRequest at line 70)
2. java/src/main/java/com/github/copilot/sdk/json/ResumeSessionConfig.java — add enableMcpApps field (analogous to onElicitationRequest at line 70)
3. java/src/main/java/com/github/copilot/sdk/json/CreateSessionRequest.java — add requestMcpApps wire field (analogous to requestElicitation at line 113)
4. java/src/main/java/com/github/copilot/sdk/json/ResumeSessionRequest.java — add requestMcpApps wire field (analogous to requestElicitation at line 117)
5. java/src/main/java/com/github/copilot/sdk/json/SessionUiCapabilities.java — add mcpApps capability flag (analogous to elicitation)
6. java/src/main/java/com/github/copilot/sdk/SessionRequestBuilder.java — wire enableMcpApps → requestMcpApps (analogous to the requestElicitation block at lines 144–145 and 238–239)
7. A warnIfMcpAppsDropped helper (analogous to the pattern in all other SDKs)

Would you like to include Java in this PR, or should a follow-up issue be filed to track it?

Generated by SDK Consistency Review Agent for issue #1335 · ● 12.7M · ◷