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

[github-actions]

Cross-SDK consistency gap: enableMcpApps is added here and to ResumeSessionConfig in Node.js, but the equivalent option is missing from the other four SDK implementations.

The requestMcpApps wire flag won't be sent by Python, Go, .NET, or Rust sessions, so hosts using those SDKs can't opt in to MCP Apps support. Suggested additions to bring the other SDKs into parity:

SDK	Location	What to add
Python	python/copilot/client.py — create_session() + resume_session() signatures	enable_mcp_apps: bool = False parameter; payload["requestMcpApps"] = enable_mcp_apps
Go	go/types.go — SessionConfig + ResumeSessionConfig	EnableMcpApps bool field; set req.RequestMcpApps = Bool(true) in client.go when the flag is set
.NET	dotnet/src/Types.cs — SessionConfig + ResumeSessionConfig	public bool EnableMcpApps { get; set; } property; pass RequestMcpApps: config.EnableMcpApps ? true : null in Client.cs
Rust	rust/src/types.rs — SessionConfig + ResumeSessionConfig	pub request_mcp_apps: Option<bool> field + pub fn with_request_mcp_apps(mut self, enable: bool) -> Self builder

[mattdholloway]

@copilot Complete the implementation of the SDK surfaces across all implementations

[github-actions]

Cross-SDK Consistency Review ✅

This PR adds enableMcpApps / enable_mcp_apps / EnableMcpApps / request_mcp_apps to all five SDK implementations (Node.js, Python, Go, .NET, Rust) with excellent cross-language consistency. Here's a quick summary:

SDK	SessionConfig	ResumeSessionConfig	Wire field
Node.js	enableMcpApps?: boolean	✅ (via Pick<>)	requestMcpApps
Python	enable_mcp_apps: bool = False	✅	requestMcpApps
Go	EnableMcpApps bool	✅	requestMcpApps,omitempty
.NET	bool EnableMcpApps	✅	RequestMcpApps (null-when-false)
Rust	request_mcp_apps: Option<bool>	✅	requestMcpApps (skip_serializing_if=None)

Notes:

• mcpAppsSandbox.ts helpers are intentionally Node.js-only. buildMcpAppsCspHeader and buildMcpAppsAllowAttribute deal with browser-specific concepts (Content-Security-Policy headers, iframe Permission Policy). Python/Go/.NET/Rust are server-side runtimes that don't render iframes, so this asymmetry is by design and appropriate.

• Wire-format behavior is consistent within each SDK's existing pattern. Node.js and Python send requestMcpApps: false when disabled (following the same approach they use for all other request* flags). Go, .NET, and Rust omit the field when disabled via pointer/null/Option patterns. This pre-existing difference is maintained consistently across all new fields in this PR.

• Builder/accessor patterns follow each SDK's conventions: Rust gains with_request_mcp_apps(), Go uses direct struct field assignment, .NET exposes a property, Python uses a keyword argument, Node.js uses an optional interface field. All consistent with their respective SDKs.

No cross-SDK consistency issues identified. 🎉

Generated by SDK Consistency Review Agent for issue #1335 · ● 497.8K · ◷