feat: add MCP Apps (SEP-1865) support

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,
@@ -652,6 +653,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
 
             session.WorkspacePath = response.WorkspacePath;
             session.SetCapabilities(response.Capabilities);
+            WarnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionId);
         }
         catch (Exception ex)
         {
@@ -793,6 +795,7 @@ 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,
@@ -811,6 +814,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
 
             session.WorkspacePath = response.WorkspacePath;
             session.SetCapabilities(response.Capabilities);
+            WarnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionId);
         }
         catch (Exception ex)
         {
@@ -1756,6 +1760,23 @@ private void RemoveSession(string sessionId)
         _sessions.TryRemove(sessionId, out _);
     }
 
+    /// <summary>
+    /// Emit a warning log when the consumer set <c>EnableMcpApps=true</c> on create/resume
+    /// but the runtime did not advertise <c>capabilities.ui.mcpApps</c> in the response.
+    /// The runtime silently drops the opt-in when its <c>MCP_APPS</c> feature flag (or
+    /// <c>COPILOT_MCP_APPS=true</c> env override) is unset, so without this warning a
+    /// consumer trying to use MCP Apps would see no error -- just tools that never expose
+    /// <c>_meta.ui.resourceUri</c>.
+    /// </summary>
+    private void WarnIfMcpAppsDropped(bool requested, SessionCapabilities? capabilities, string sessionId)
+    {
+        if (!requested) return;
+        if (capabilities?.Ui?.McpApps == true) return;
+        _logger?.LogWarning(
+            "Session {SessionId}: EnableMcpApps was requested but the runtime did not advertise capabilities.ui.mcpApps. The runtime's MCP_APPS feature flag or COPILOT_MCP_APPS=true environment override is likely unset; the MCP Apps surface is unavailable for this session.",
+            sessionId);
+    }
+
     /// <summary>
     /// Disposes the <see cref="CopilotClient"/> synchronously.
     /// </summary>
@@ -2034,6 +2055,7 @@ internal record CreateSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,
@@ -2096,6 +2118,7 @@ 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

@@ -1066,6 +1066,16 @@ public class SessionUiCapabilities
     /// Whether the host supports interactive elicitation dialogs.
     /// </summary>
     public bool? Elicitation { get; set; }
+
+    /// <summary>
+    /// Whether the runtime has accepted the session's MCP Apps (SEP-1865) opt-in.
+    /// <c>true</c> when the consumer set <see cref="SessionConfig.EnableMcpApps"/>
+    /// (or <see cref="ResumeSessionConfig.EnableMcpApps"/>) to <c>true</c> on
+    /// create/resume <b>and</b> the runtime's <c>MCP_APPS</c> feature flag (or
+    /// <c>COPILOT_MCP_APPS=true</c> env override) is on. Otherwise absent or
+    /// <c>false</c>, indicating the runtime silently dropped the opt-in.
+    /// </summary>
+    public bool? McpApps { get; set; }
 }
 
 // ============================================================================
@@ -2065,6 +2075,7 @@ 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;
@@ -2217,6 +2228,30 @@ 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> <b>and</b> the runtime has MCP Apps enabled (via the
+    /// <c>MCP_APPS</c> feature flag or <c>COPILOT_MCP_APPS=true</c> environment override), 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,diagnose}</c>
+    /// JSON-RPC methods.
+    /// </para>
+    /// <para>
+    /// If the runtime gate is off, the opt-in is silently dropped server-side (the runtime logs a
+    /// warning); the session is created normally but the MCP Apps surface is unavailable. Inspect
+    /// the runtime's <c>capabilities.ui.mcpApps</c> on the create/resume response to detect this.
+    /// </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>
@@ -2377,6 +2412,7 @@ 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
@@ -2507,6 +2543,12 @@ 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

@@ -54,6 +54,25 @@ import (
 
 const noResultPermissionV2Error = "permission handlers cannot return 'no-result' when connected to a protocol v2 server"
 
+// warnIfMcpAppsDropped emits a stderr warning when the consumer set
+// EnableMcpApps=true on create/resume but the runtime did not advertise
+// capabilities.ui.mcpApps in the response. The runtime silently drops the
+// opt-in when its MCP_APPS feature flag (or COPILOT_MCP_APPS=true env
+// override) is unset, so without this warning a consumer trying to use MCP
+// Apps would see no error -- just tools that never expose _meta.ui.resourceUri.
+func warnIfMcpAppsDropped(requested bool, capabilities *SessionCapabilities, sessionID string) {
+	if !requested {
+		return
+	}
+	if capabilities != nil && capabilities.UI != nil && capabilities.UI.McpApps {
+		return
+	}
+	fmt.Fprintf(os.Stderr,
+		"[copilot-sdk] Session %s: EnableMcpApps was requested but the runtime did not advertise capabilities.ui.mcpApps. The runtime's MCP_APPS feature flag or COPILOT_MCP_APPS=true environment override is likely unset; the MCP Apps surface is unavailable for this session.\n",
+		sessionID,
+	)
+}
+
 func validateSessionFsConfig(config *SessionFsConfig) error {
 	if config == nil {
 		return nil
@@ -670,6 +689,9 @@ 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)
@@ -774,6 +796,7 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 
 	session.workspacePath = response.WorkspacePath
 	session.setCapabilities(response.Capabilities)
+	warnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionID)
 
 	return session, nil
 }
@@ -882,6 +905,9 @@ 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
@@ -959,6 +985,7 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 
 	session.workspacePath = response.WorkspacePath
 	session.setCapabilities(response.Capabilities)
+	warnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionID)
 
 	return session, nil
 }

go/types.go

@@ -697,6 +697,26 @@ 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 AND the runtime has MCP Apps enabled (via the MCP_APPS feature
+	// flag or COPILOT_MCP_APPS=true environment override), 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,diagnose} JSON-RPC methods.
+	//
+	// If the runtime gate is off, the opt-in is silently dropped server-side
+	// (the runtime logs a warning); the session is created normally but the
+	// MCP Apps surface is unavailable. Inspect the runtime's
+	// capabilities.ui.mcpApps on the create/resume response to detect this.
+	//
+	// 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.
@@ -784,6 +804,12 @@ type SessionCapabilities struct {
 type UICapabilities struct {
 	// Elicitation indicates whether the host supports interactive elicitation dialogs.
 	Elicitation bool `json:"elicitation,omitempty"`
+	// McpApps indicates whether the runtime has accepted the session's MCP Apps
+	// (SEP-1865) opt-in. True when the consumer set EnableMcpApps=true on
+	// create/resume AND the runtime's MCP_APPS feature flag (or
+	// COPILOT_MCP_APPS=true env override) is on. Otherwise false, indicating
+	// the runtime silently dropped the opt-in.
+	McpApps bool `json:"mcpApps,omitempty"`
 }
 
 // ElicitationResult is the user's response to an elicitation dialog.
@@ -956,6 +982,9 @@ 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".
@@ -1177,6 +1206,7 @@ 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"`
@@ -1233,6 +1263,7 @@ 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"`

nodejs/src/client.ts

@@ -86,6 +86,26 @@
  */
 const MIN_PROTOCOL_VERSION = 2;
 
+/**
+ * Emit a `console.warn` when the consumer set `enableMcpApps: true` on
+ * create/resume but the runtime did not advertise `capabilities.ui.mcpApps`
+ * in the response. The runtime silently drops the opt-in when its `MCP_APPS`
+ * feature flag (or `COPILOT_MCP_APPS=true` env override) is unset, so without
+ * this warning a consumer trying to use MCP Apps would see no error — just
+ * tools that never expose `_meta.ui.resourceUri`.
+ */
+function warnIfMcpAppsDropped(
+    requested: boolean | undefined,
+    capabilities: { ui?: { mcpApps?: boolean } } | undefined,
+    sessionId: string
+): void {
+    if (requested && !capabilities?.ui?.mcpApps) {
+        console.warn(
+            `[copilot-sdk] Session ${sessionId}: enableMcpApps was requested but the runtime did not advertise capabilities.ui.mcpApps. The runtime's MCP_APPS feature flag or COPILOT_MCP_APPS=true environment override is likely unset; the MCP Apps surface is unavailable for this session.`
+        );
+    }
+}
+
 /**
  * Check if value is a Zod schema (has toJSONSchema method)
  */
@@ -823,6 +843,7 @@
                 requestPermission: true,
                 requestUserInput: !!config.onUserInputRequest,
                 requestElicitation: !!config.onElicitationRequest,
+                requestMcpApps: !!config.enableMcpApps,
                 requestExitPlanMode: !!config.onExitPlanMode,
                 requestAutoModeSwitch: !!config.onAutoModeSwitch,
                 hooks: !!(config.hooks && Object.values(config.hooks).some(Boolean)),
@@ -848,10 +869,11 @@
             const { workspacePath, capabilities } = response as {
                 sessionId: string;
                 workspacePath?: string;
-                capabilities?: { ui?: { elicitation?: boolean } };
+                capabilities?: { ui?: { elicitation?: boolean; mcpApps?: boolean } };
             };
             session["_workspacePath"] = workspacePath;
             session.setCapabilities(capabilities);
+            warnIfMcpAppsDropped(config.enableMcpApps, capabilities, sessionId);
         } catch (e) {
             this.sessions.delete(sessionId);
             throw e;
@@ -962,6 +984,7 @@
                     config.onPermissionRequest !== defaultJoinSessionPermissionHandler,
                 requestUserInput: !!config.onUserInputRequest,
                 requestElicitation: !!config.onElicitationRequest,
+                requestMcpApps: !!config.enableMcpApps,
                 requestExitPlanMode: !!config.onExitPlanMode,
                 requestAutoModeSwitch: !!config.onAutoModeSwitch,
                 hooks: !!(config.hooks && Object.values(config.hooks).some(Boolean)),
@@ -988,10 +1011,11 @@
             const { workspacePath, capabilities } = response as {
                 sessionId: string;
                 workspacePath?: string;
-                capabilities?: { ui?: { elicitation?: boolean } };
+                capabilities?: { ui?: { elicitation?: boolean; mcpApps?: boolean } };
             };
             session["_workspacePath"] = workspacePath;
             session.setCapabilities(capabilities);
+            warnIfMcpAppsDropped(config.enableMcpApps, capabilities, sessionId);
         } catch (e) {
             this.sessions.delete(sessionId);
             throw e;

nodejs/src/index.ts

@@ -10,6 +10,12 @@
 
 export { CopilotClient } from "./client.js";
 export { CopilotSession, type AssistantMessageEvent } from "./session.js";
+export {
+    buildMcpAppsAllowAttribute,
+    buildMcpAppsCspHeader,
+    type McpAppsCspInput,
+    type McpAppsPermissionsInput,
+} from "./mcpAppsSandbox.js";
 export {
     defineTool,
     approveAll,