feat: add MCP Apps option to Python, Go, .NET, Rust SDKs

Mirror nodejs enableMcpApps across the other four SDKs so hosts using
them can opt into MCP Apps (SEP-1865) UI passthrough by sending
requestMcpApps on session.create / session.resume.

- python: enable_mcp_apps kwarg on create_session / resume_session
- go: EnableMcpApps field on SessionConfig / ResumeSessionConfig
- dotnet: EnableMcpApps property on SessionConfig / ResumeSessionConfig
- rust: request_mcp_apps field + with_request_mcp_apps builder

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mattdholloway

dotnet/src/Client.cs

@@ -634,6 +634,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
                 config.InfiniteSessions,
                 Commands: config.Commands?.Select(c => new CommandWireDefinition(c.Name, c.Description)).ToList(),
                 RequestElicitation: config.OnElicitationRequest != null,
+                RequestMcpApps: config.EnableMcpApps ? true : null,
                 Traceparent: traceparent,
                 Tracestate: tracestate,
                 ModelCapabilities: config.ModelCapabilities,
@@ -793,6 +794,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
                 config.InfiniteSessions,
                 Commands: config.Commands?.Select(c => new CommandWireDefinition(c.Name, c.Description)).ToList(),
                 RequestElicitation: config.OnElicitationRequest != null,
+                RequestMcpApps: config.EnableMcpApps ? true : null,
                 Traceparent: traceparent,
                 Tracestate: tracestate,
                 ModelCapabilities: config.ModelCapabilities,
@@ -2025,6 +2027,7 @@ internal record CreateSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,
@@ -2087,6 +2090,7 @@ internal record ResumeSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,

dotnet/src/Types.cs

@@ -2058,6 +2058,7 @@ protected SessionConfig(SessionConfig? other)
         Agent = other.Agent;
         DisabledSkills = other.DisabledSkills is not null ? [.. other.DisabledSkills] : null;
         EnableConfigDiscovery = other.EnableConfigDiscovery;
+        EnableMcpApps = other.EnableMcpApps;
         ExcludedTools = other.ExcludedTools is not null ? [.. other.ExcludedTools] : null;
         Hooks = other.Hooks;
         InfiniteSessions = other.InfiniteSessions;
@@ -2210,6 +2211,23 @@ protected SessionConfig(SessionConfig? other)
     /// </summary>
     public AutoModeSwitchHandler? OnAutoModeSwitch { get; set; }
 
+    /// <summary>
+    /// Enable MCP Apps (SEP-1865) UI passthrough on this session.
+    /// <para>
+    /// When <c>true</c>, the runtime adds the <c>mcp-apps</c> capability to the session, which
+    /// causes it to advertise the <c>extensions.io.modelcontextprotocol/ui</c> extension to MCP
+    /// servers (so they expose <c>_meta.ui.resourceUri</c> on tools) and to expose the
+    /// <c>session.rpc.mcp.apps.{listTools,callTool,readResource,setHostContext,getHostContext}</c>
+    /// JSON-RPC methods.
+    /// </para>
+    /// <para>
+    /// SDK consumers MUST set this to <c>true</c> only when they have an iframe renderer that can
+    /// display <c>ui://</c> MCP App bundles. Setting it without a renderer will cause MCP servers
+    /// to register UI-enabled tool variants the consumer cannot display.
+    /// </para>
+    /// </summary>
+    public bool EnableMcpApps { get; set; }
+
     /// <summary>
     /// Hook handlers for session lifecycle events.
     /// </summary>
@@ -2370,6 +2388,7 @@ protected ResumeSessionConfig(ResumeSessionConfig? other)
         EnableConfigDiscovery = other.EnableConfigDiscovery;
         ContinuePendingWork = other.ContinuePendingWork;
         ExcludedTools = other.ExcludedTools is not null ? [.. other.ExcludedTools] : null;
+        EnableMcpApps = other.EnableMcpApps;
         Hooks = other.Hooks;
         InfiniteSessions = other.InfiniteSessions;
         McpServers = other.McpServers is not null
@@ -2500,6 +2519,12 @@ protected ResumeSessionConfig(ResumeSessionConfig? other)
     /// </summary>
     public AutoModeSwitchHandler? OnAutoModeSwitch { get; set; }
 
+    /// <summary>
+    /// Enable MCP Apps (SEP-1865) UI passthrough on the resumed session.
+    /// See <see cref="SessionConfig.EnableMcpApps"/>.
+    /// </summary>
+    public bool EnableMcpApps { get; set; }
+
     /// <summary>
     /// Hook handlers for session lifecycle events.
     /// </summary>

go/client.go

@@ -663,6 +663,9 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 	if config.OnAutoModeSwitch != nil {
 		req.RequestAutoModeSwitch = Bool(true)
 	}
+	if config.EnableMcpApps {
+		req.RequestMcpApps = Bool(true)
+	}
 
 	if config.Streaming {
 		req.Streaming = Bool(true)
@@ -866,6 +869,9 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 	if config.OnAutoModeSwitch != nil {
 		req.RequestAutoModeSwitch = Bool(true)
 	}
+	if config.EnableMcpApps {
+		req.RequestMcpApps = Bool(true)
+	}
 
 	traceparent, tracestate := getTraceContext(ctx)
 	req.Traceparent = traceparent

go/types.go

@@ -688,6 +688,19 @@ type SessionConfig struct {
 	// OnAutoModeSwitch is a handler for auto-mode-switch requests from the server.
 	// When provided, enables autoModeSwitch.request callbacks for the session.
 	OnAutoModeSwitch AutoModeSwitchHandler
+	// EnableMcpApps enables MCP Apps (SEP-1865) UI passthrough on this session.
+	//
+	// When true, the runtime adds the mcp-apps capability to the session, which
+	// causes it to advertise the extensions.io.modelcontextprotocol/ui extension
+	// to MCP servers (so they expose _meta.ui.resourceUri on tools) and to expose
+	// the session.rpc.mcp.apps.{listTools,callTool,readResource,setHostContext,
+	// getHostContext} JSON-RPC methods.
+	//
+	// SDK consumers MUST set this to true only when they have an iframe renderer
+	// that can display ui:// MCP App bundles. Setting it without a renderer will
+	// cause MCP servers to register UI-enabled tool variants the consumer cannot
+	// display.
+	EnableMcpApps bool
 	// GitHubToken is an optional per-session GitHub token used for authentication.
 	// When provided, the session authenticates as the token's owner instead of
 	// using the global client-level auth.
@@ -947,6 +960,9 @@ type ResumeSessionConfig struct {
 	// OnAutoModeSwitch is a handler for auto-mode-switch requests from the server.
 	// See SessionConfig.OnAutoModeSwitch.
 	OnAutoModeSwitch AutoModeSwitchHandler
+	// EnableMcpApps enables MCP Apps (SEP-1865) UI passthrough on resume.
+	// See SessionConfig.EnableMcpApps.
+	EnableMcpApps bool
 }
 type ProviderConfig struct {
 	// Type is the provider type: "openai", "azure", or "anthropic". Defaults to "openai".
@@ -1168,6 +1184,7 @@ type createSessionRequest struct {
 	InfiniteSessions               *InfiniteSessionConfig         `json:"infiniteSessions,omitempty"`
 	Commands                       []wireCommand                  `json:"commands,omitempty"`
 	RequestElicitation             *bool                          `json:"requestElicitation,omitempty"`
+	RequestMcpApps                 *bool                          `json:"requestMcpApps,omitempty"`
 	GitHubToken                    string                         `json:"gitHubToken,omitempty"`
 	RemoteSession                  rpc.RemoteSessionMode          `json:"remoteSession,omitempty"`
 	Cloud                          *CloudSessionOptions           `json:"cloud,omitempty"`
@@ -1224,6 +1241,7 @@ type resumeSessionRequest struct {
 	InfiniteSessions               *InfiniteSessionConfig         `json:"infiniteSessions,omitempty"`
 	Commands                       []wireCommand                  `json:"commands,omitempty"`
 	RequestElicitation             *bool                          `json:"requestElicitation,omitempty"`
+	RequestMcpApps                 *bool                          `json:"requestMcpApps,omitempty"`
 	GitHubToken                    string                         `json:"gitHubToken,omitempty"`
 	RemoteSession                  rpc.RemoteSessionMode          `json:"remoteSession,omitempty"`
 	Traceparent                    string                         `json:"traceparent,omitempty"`

python/copilot/client.py

@@ -1354,6 +1354,7 @@ async def create_session(
         on_elicitation_request: ElicitationHandler | None = None,
         on_exit_plan_mode: ExitPlanModeHandler | None = None,
         on_auto_mode_switch: AutoModeSwitchHandler | None = None,
+        enable_mcp_apps: bool = False,
         create_session_fs_handler: CreateSessionFsHandler | None = None,
         github_token: str | None = None,
         remote_session: RemoteSessionMode | None = None,
@@ -1497,6 +1498,7 @@ async def create_session(
         payload["requestElicitation"] = bool(on_elicitation_request)
         payload["requestExitPlanMode"] = bool(on_exit_plan_mode)
         payload["requestAutoModeSwitch"] = bool(on_auto_mode_switch)
+        payload["requestMcpApps"] = bool(enable_mcp_apps)
 
         # Serialize commands (name + description only) into payload
         if commands:
@@ -1725,6 +1727,7 @@ async def resume_session(
         on_elicitation_request: ElicitationHandler | None = None,
         on_exit_plan_mode: ExitPlanModeHandler | None = None,
         on_auto_mode_switch: AutoModeSwitchHandler | None = None,
+        enable_mcp_apps: bool = False,
         create_session_fs_handler: CreateSessionFsHandler | None = None,
         github_token: str | None = None,
         remote_session: RemoteSessionMode | None = None,
@@ -1883,6 +1886,7 @@ async def resume_session(
         payload["requestElicitation"] = bool(on_elicitation_request)
         payload["requestExitPlanMode"] = bool(on_exit_plan_mode)
         payload["requestAutoModeSwitch"] = bool(on_auto_mode_switch)
+        payload["requestMcpApps"] = bool(enable_mcp_apps)
 
         # Serialize commands (name + description only) into payload
         if commands:

rust/src/types.rs

@@ -1078,6 +1078,23 @@ pub struct SessionConfig {
     /// Defaults to `Some(true)` via [`SessionConfig::default`].
     #[serde(skip_serializing_if = "Option::is_none")]
     pub request_elicitation: Option<bool>,
+    /// Enable MCP Apps (SEP-1865) UI passthrough on this session.
+    ///
+    /// When `Some(true)`, the runtime adds the `mcp-apps` capability to the
+    /// session, which causes it to advertise the
+    /// `extensions.io.modelcontextprotocol/ui` extension to MCP servers (so
+    /// they expose `_meta.ui.resourceUri` on tools) and to expose the
+    /// `session.rpc.mcp.apps.{listTools,callTool,readResource,setHostContext,
+    /// getHostContext}` JSON-RPC methods.
+    ///
+    /// SDK consumers MUST set this to `Some(true)` only when they have an
+    /// iframe renderer that can display `ui://` MCP App bundles. Setting it
+    /// without a renderer will cause MCP servers to register UI-enabled tool
+    /// variants the consumer cannot display.
+    ///
+    /// Defaults to `None` (disabled).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub request_mcp_apps: Option<bool>,
     /// Skill directory paths passed through to the GitHub Copilot CLI.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub skill_directories: Option<Vec<PathBuf>>,
@@ -1208,6 +1225,7 @@ impl std::fmt::Debug for SessionConfig {
             .field("request_exit_plan_mode", &self.request_exit_plan_mode)
             .field("request_auto_mode_switch", &self.request_auto_mode_switch)
             .field("request_elicitation", &self.request_elicitation)
+            .field("request_mcp_apps", &self.request_mcp_apps)
             .field("skill_directories", &self.skill_directories)
             .field("instruction_directories", &self.instruction_directories)
             .field("disabled_skills", &self.disabled_skills)
@@ -1271,6 +1289,7 @@ impl Default for SessionConfig {
             request_exit_plan_mode: Some(true),
             request_auto_mode_switch: Some(true),
             request_elicitation: Some(true),
+            request_mcp_apps: None,
             skill_directories: None,
             instruction_directories: None,
             disabled_skills: None,
@@ -1491,6 +1510,13 @@ impl SessionConfig {
         self
     }
 
+    /// Enable MCP Apps (SEP-1865) UI passthrough on this session. Defaults
+    /// to `None` (disabled). See [`SessionConfig::request_mcp_apps`].
+    pub fn with_request_mcp_apps(mut self, enable: bool) -> Self {
+        self.request_mcp_apps = Some(enable);
+        self
+    }
+
     /// Set skill directory paths passed through to the CLI.
     pub fn with_skill_directories<I, P>(mut self, paths: I) -> Self
     where
@@ -1680,6 +1706,10 @@ pub struct ResumeSessionConfig {
     /// Advertise elicitation provider capability on resume.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub request_elicitation: Option<bool>,
+    /// Enable MCP Apps (SEP-1865) UI passthrough on resume. See
+    /// [`SessionConfig::request_mcp_apps`].
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub request_mcp_apps: Option<bool>,
     /// Skill directory paths passed through to the GitHub Copilot CLI on resume.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub skill_directories: Option<Vec<PathBuf>>,
@@ -1790,6 +1820,7 @@ impl std::fmt::Debug for ResumeSessionConfig {
             .field("request_exit_plan_mode", &self.request_exit_plan_mode)
             .field("request_auto_mode_switch", &self.request_auto_mode_switch)
             .field("request_elicitation", &self.request_elicitation)
+            .field("request_mcp_apps", &self.request_mcp_apps)
             .field("skill_directories", &self.skill_directories)
             .field("instruction_directories", &self.instruction_directories)
             .field("disabled_skills", &self.disabled_skills)
@@ -1852,6 +1883,7 @@ impl ResumeSessionConfig {
             request_exit_plan_mode: Some(true),
             request_auto_mode_switch: Some(true),
             request_elicitation: Some(true),
+            request_mcp_apps: None,
             skill_directories: None,
             instruction_directories: None,
             disabled_skills: None,
@@ -2043,6 +2075,13 @@ impl ResumeSessionConfig {
         self
     }
 
+    /// Enable MCP Apps (SEP-1865) UI passthrough on resume. Defaults to
+    /// `None` (disabled). See [`SessionConfig::request_mcp_apps`].
+    pub fn with_request_mcp_apps(mut self, enable: bool) -> Self {
+        self.request_mcp_apps = Some(enable);
+        self
+    }
+
     /// Set skill directory paths passed through to the CLI on resume.
     pub fn with_skill_directories<I, P>(mut self, paths: I) -> Self
     where
@@ -3366,6 +3405,7 @@ mod tests {
         assert_eq!(cfg.request_elicitation, Some(true));
         assert_eq!(cfg.request_exit_plan_mode, Some(true));
         assert_eq!(cfg.request_auto_mode_switch, Some(true));
+        assert_eq!(cfg.request_mcp_apps, None);
     }
 
     #[test]
@@ -3376,6 +3416,7 @@ mod tests {
         assert_eq!(cfg.request_elicitation, Some(true));
         assert_eq!(cfg.request_exit_plan_mode, Some(true));
         assert_eq!(cfg.request_auto_mode_switch, Some(true));
+        assert_eq!(cfg.request_mcp_apps, None);
     }
 
     #[test]