feat: add MCP Apps (SEP-1865) support

dotnet/src/Client.cs

@@ -613,6 +613,7 @@ 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,
@@ -636,6 +637,7 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
             session.WorkspacePath = response.WorkspacePath;
             session.SetCapabilities(response.Capabilities);
             session.SetOpenCanvases(response.OpenCanvases);
+            WarnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionId);
         }
         catch (Exception ex)
         {
@@ -779,6 +781,7 @@ 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,
@@ -803,6 +806,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
             session.WorkspacePath = response.WorkspacePath;
             session.SetCapabilities(response.Capabilities);
             session.SetOpenCanvases(response.OpenCanvases);
+            WarnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionId);
         }
         catch (Exception ex)
         {
@@ -1697,6 +1701,23 @@ 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>
@@ -1917,6 +1938,7 @@ internal record CreateSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,
@@ -1988,6 +2010,7 @@ 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

@@ -1072,6 +1072,15 @@ public sealed 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="SessionConfigBase.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; }
 }
 
 // ============================================================================
@@ -2196,6 +2205,7 @@ protected SessionConfigBase(SessionConfigBase? 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;
@@ -2317,6 +2327,30 @@ protected SessionConfigBase(SessionConfigBase? other)
     /// <summary>Handler for auto-mode-switch requests from the server.</summary>
     public Func<AutoModeSwitchRequest, AutoModeSwitchInvocation, Task<AutoModeSwitchResponse>>? OnAutoModeSwitchRequest { 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>
     public SessionHooks? Hooks { get; set; }
 

go/client.go

@@ -34,6 +34,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log"
 	"net"
 	"os"
 	"os/exec"
@@ -52,6 +53,29 @@ import (
 	"github.com/github/copilot-sdk/go/rpc"
 )
 
+// warnIfMcpAppsDropped logs a warning via the standard log package 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.
+//
+// The warning is routed through log.Default(), so consumers can suppress or
+// redirect it with log.SetOutput / log.Default().SetOutput (e.g.
+// log.Default().SetOutput(io.Discard) to silence).
+func warnIfMcpAppsDropped(requested bool, capabilities *SessionCapabilities, sessionID string) {
+	if !requested {
+		return
+	}
+	if capabilities != nil && capabilities.UI != nil && capabilities.UI.McpApps {
+		return
+	}
+	log.Printf(
+		"[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.",
+		sessionID,
+	)
+}
+
 func validateSessionFsConfig(config *SessionFsConfig) error {
 	if config == nil {
 		return nil
@@ -647,6 +671,9 @@ func (c *Client) CreateSession(ctx context.Context, config *SessionConfig) (*Ses
 	if config.OnAutoModeSwitchRequest != nil {
 		req.RequestAutoModeSwitch = Bool(true)
 	}
+	if config.EnableMcpApps {
+		req.RequestMcpApps = Bool(true)
+	}
 
 	if config.Streaming != nil {
 		req.Streaming = config.Streaming
@@ -757,6 +784,7 @@ 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
 }
@@ -873,6 +901,9 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 	if config.OnAutoModeSwitchRequest != nil {
 		req.RequestAutoModeSwitch = Bool(true)
 	}
+	if config.EnableMcpApps {
+		req.RequestMcpApps = Bool(true)
+	}
 
 	traceparent, tracestate := getTraceContext(ctx)
 	req.Traceparent = traceparent
@@ -954,6 +985,7 @@ func (c *Client) ResumeSessionWithOptions(ctx context.Context, sessionID string,
 	session.workspacePath = response.WorkspacePath
 	session.setCapabilities(response.Capabilities)
 	session.setOpenCanvases(response.OpenCanvases)
+	warnIfMcpAppsDropped(config.EnableMcpApps, response.Capabilities, sessionID)
 
 	return session, nil
 }

go/client_test.go

@@ -946,6 +946,65 @@ func TestResumeSessionRequest_RequestElicitation(t *testing.T) {
 	})
 }
 
+func TestCreateSessionRequest_RequestMcpApps(t *testing.T) {
+	t.Run("sends requestMcpApps flag when EnableMcpApps is set", func(t *testing.T) {
+		req := createSessionRequest{
+			RequestMcpApps: Bool(true),
+		}
+		data, err := json.Marshal(req)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var m map[string]any
+		if err := json.Unmarshal(data, &m); err != nil {
+			t.Fatalf("Failed to unmarshal: %v", err)
+		}
+		if m["requestMcpApps"] != true {
+			t.Errorf("Expected requestMcpApps to be true, got %v", m["requestMcpApps"])
+		}
+	})
+
+	t.Run("does not send requestMcpApps when EnableMcpApps is unset", func(t *testing.T) {
+		req := createSessionRequest{}
+		data, _ := json.Marshal(req)
+		var m map[string]any
+		json.Unmarshal(data, &m)
+		if _, ok := m["requestMcpApps"]; ok {
+			t.Error("Expected requestMcpApps to be omitted when not set")
+		}
+	})
+}
+
+func TestResumeSessionRequest_RequestMcpApps(t *testing.T) {
+	t.Run("sends requestMcpApps flag when EnableMcpApps is set", func(t *testing.T) {
+		req := resumeSessionRequest{
+			SessionID:      "s1",
+			RequestMcpApps: Bool(true),
+		}
+		data, err := json.Marshal(req)
+		if err != nil {
+			t.Fatalf("Failed to marshal: %v", err)
+		}
+		var m map[string]any
+		if err := json.Unmarshal(data, &m); err != nil {
+			t.Fatalf("Failed to unmarshal: %v", err)
+		}
+		if m["requestMcpApps"] != true {
+			t.Errorf("Expected requestMcpApps to be true, got %v", m["requestMcpApps"])
+		}
+	})
+
+	t.Run("does not send requestMcpApps when EnableMcpApps is unset", func(t *testing.T) {
+		req := resumeSessionRequest{SessionID: "s1"}
+		data, _ := json.Marshal(req)
+		var m map[string]any
+		json.Unmarshal(data, &m)
+		if _, ok := m["requestMcpApps"]; ok {
+			t.Error("Expected requestMcpApps to be omitted when not set")
+		}
+	})
+}
+
 func TestResumeSessionRequest_ModeCallbackFlags(t *testing.T) {
 	req := resumeSessionRequest{
 		SessionID:             "s1",

go/types.go

@@ -915,6 +915,26 @@ type SessionConfig struct {
 	// OnAutoModeSwitchRequest is a handler for auto-mode-switch requests from the server.
 	// When provided, enables autoModeSwitch.request callbacks for the session.
 	OnAutoModeSwitchRequest AutoModeSwitchRequestHandler
+	// 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.
@@ -1017,6 +1037,12 @@ 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.
@@ -1190,6 +1216,9 @@ type ResumeSessionConfig struct {
 	// OnAutoModeSwitchRequest is a handler for auto-mode-switch requests from the server.
 	// See SessionConfig.OnAutoModeSwitchRequest.
 	OnAutoModeSwitchRequest AutoModeSwitchRequestHandler
+	// EnableMcpApps enables MCP Apps (SEP-1865) UI passthrough on resume.
+	// See SessionConfig.EnableMcpApps.
+	EnableMcpApps bool
 	// Canvases declares canvases this session provides. Sent over the wire on
 	// `session.resume`. See SessionConfig.Canvases.
 	Canvases []CanvasDeclaration
@@ -1426,6 +1455,7 @@ 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"`
@@ -1486,6 +1516,7 @@ 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"`
 	Canvases                       []CanvasDeclaration            `json:"canvases,omitempty"`

java/src/main/java/com/github/copilot/sdk/CopilotClient.java

@@ -34,6 +34,7 @@
 import com.github.copilot.sdk.json.PingResponse;
 import com.github.copilot.sdk.json.ResumeSessionConfig;
 import com.github.copilot.sdk.json.ResumeSessionResponse;
+import com.github.copilot.sdk.json.SessionCapabilities;
 import com.github.copilot.sdk.json.SessionConfig;
 import com.github.copilot.sdk.json.SessionLifecycleHandler;
 import com.github.copilot.sdk.json.SessionListFilter;
@@ -290,6 +291,30 @@ private static boolean isUnsupportedConnectMethod(JsonRpcException ex) {
         return ex.getCode() == METHOD_NOT_FOUND_ERROR_CODE || "Unhandled method connect".equals(ex.getMessage());
     }
 
+    /**
+     * Logs a warning when the consumer set {@code enableMcpApps=true} on
+     * create/resume but the runtime did not advertise
+     * {@code capabilities.ui.mcpApps} in the response. The runtime silently drops
+     * the opt-in when its {@code MCP_APPS} feature flag (or
+     * {@code 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 {@code _meta.ui.resourceUri}.
+     */
+    private static void warnIfMcpAppsDropped(boolean requested, SessionCapabilities capabilities, String sessionId) {
+        if (!requested) {
+            return;
+        }
+        boolean advertised = capabilities != null && capabilities.getUi() != null
+                && capabilities.getUi().getMcpApps().orElse(false);
+        if (advertised) {
+            return;
+        }
+        LOG.warning("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.");
+    }
+
     /**
      * Disconnects from the Copilot server and closes all active sessions.
      * <p>
@@ -467,6 +492,7 @@ public CompletableFuture<CopilotSession> createSession(SessionConfig config) {
                         rpcNanos);
                 session.setWorkspacePath(response.workspacePath());
                 session.setCapabilities(response.capabilities());
+                warnIfMcpAppsDropped(config.isEnableMcpApps(), response.capabilities(), sessionId);
                 // If the server returned a different sessionId (e.g. a v2 CLI that ignores
                 // the client-supplied ID), re-key the sessions map.
                 String returnedId = response.sessionId();
@@ -552,6 +578,7 @@ public CompletableFuture<CopilotSession> resumeSession(String sessionId, ResumeS
                         rpcNanos);
                 session.setWorkspacePath(response.workspacePath());
                 session.setCapabilities(response.capabilities());
+                warnIfMcpAppsDropped(config.isEnableMcpApps(), response.capabilities(), sessionId);
                 // If the server returned a different sessionId than what was requested, re-key.
                 String returnedId = response.sessionId();
                 if (returnedId != null && !returnedId.equals(sessionId)) {