github
diff --git a/‎dotnet/src/Client.cs‎
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diff --git a/‎dotnet/src/Types.cs‎
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diff --git a/‎go/client.go‎
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diff --git a/‎go/client_test.go‎
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diff --git a/‎go/types.go‎
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diff --git a/‎java/src/main/java/com/github/copilot/SessionRequestBuilder.java‎
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diff --git a/‎java/src/main/java/com/github/copilot/rpc/CreateSessionRequest.java‎
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diff --git a/‎java/src/main/java/com/github/copilot/rpc/ResumeSessionConfig.java‎
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diff --git a/‎java/src/main/java/com/github/copilot/rpc/ResumeSessionRequest.java‎
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@@ -878,6 +878,7 @@ 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,
@@ -1059,6 +1060,7 @@ 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,
@@ -2171,6 +2173,7 @@ internal record CreateSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,
@@ -2243,6 +2246,7 @@ internal record ResumeSessionRequest(
         InfiniteSessionConfig? InfiniteSessions,
         IList<CommandWireDefinition>? Commands = null,
         bool? RequestElicitation = null,
+        bool? RequestMcpApps = null,
         string? Traceparent = null,
         string? Tracestate = null,
         ModelCapabilitiesOverride? ModelCapabilities = null,
 
@@ -1130,6 +1130,16 @@ 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>
+    [Experimental(Diagnostics.Experimental)]
+    public bool? McpApps { get; set; }
 }
 
 // ============================================================================
@@ -2346,6 +2356,7 @@ 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;
@@ -2507,6 +2518,31 @@ 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>
+    [Experimental(Diagnostics.Experimental)]
+    public bool EnableMcpApps { get; set; }
+
     /// <summary>Hook handlers for session lifecycle events.</summary>
     public SessionHooks? Hooks { get; set; }
 
 
@@ -659,6 +659,9 @@ 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
@@ -985,6 +988,9 @@ 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
 
@@ -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",
 
@@ -987,6 +987,29 @@ 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.
+	//
+	// Experimental: EnableMcpApps is part of an experimental wire-protocol
+	// surface (SEP-1865) and may change or be removed in a future release.
+	//
+	// 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.
@@ -1089,6 +1112,15 @@ 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.
+	//
+	// Experimental: McpApps is part of an experimental wire-protocol surface
+	// (SEP-1865) and may change or be removed in a future release.
+	McpApps bool `json:"mcpApps,omitempty"`
 }
 
 // ElicitationResult is the user's response to an elicitation dialog.
@@ -1275,6 +1307,12 @@ 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.
+	//
+	// Experimental: EnableMcpApps is part of an experimental wire-protocol
+	// surface (SEP-1865) and may change or be removed in a future release.
+	EnableMcpApps bool
 	// Canvases declares canvases this session provides. Sent over the wire on
 	// `session.resume`. See SessionConfig.Canvases.
 	Canvases []CanvasDeclaration
@@ -1534,6 +1572,7 @@ 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"`
@@ -1599,6 +1638,7 @@ 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"`
 
@@ -144,6 +144,9 @@ static CreateSessionRequest buildCreateRequest(SessionConfig config, String sess
         if (config.getOnElicitationRequest() != null) {
             request.setRequestElicitation(true);
         }
+        if (config.isEnableMcpApps()) {
+            request.setRequestMcpApps(true);
+        }
         if (config.getOnExitPlanMode() != null) {
             request.setRequestExitPlanMode(true);
         }
@@ -238,6 +241,9 @@ static ResumeSessionRequest buildResumeRequest(String sessionId, ResumeSessionCo
         if (config.getOnElicitationRequest() != null) {
             request.setRequestElicitation(true);
         }
+        if (config.isEnableMcpApps()) {
+            request.setRequestMcpApps(true);
+        }
         if (config.getOnExitPlanMode() != null) {
             request.setRequestExitPlanMode(true);
         }
 
@@ -114,6 +114,9 @@ public final class CreateSessionRequest {
     @JsonProperty("requestElicitation")
     private Boolean requestElicitation;
 
+    @JsonProperty("requestMcpApps")
+    private Boolean requestMcpApps;
+
     @JsonProperty("requestExitPlanMode")
     private Boolean requestExitPlanMode;
 
@@ -503,6 +506,21 @@ public void clearRequestElicitation() {
         this.requestElicitation = null;
     }
 
+    /** Gets the requestMcpApps flag. @return the flag */
+    public Boolean getRequestMcpApps() {
+        return requestMcpApps;
+    }
+
+    /** Sets the requestMcpApps flag. @param requestMcpApps the flag */
+    public void setRequestMcpApps(boolean requestMcpApps) {
+        this.requestMcpApps = requestMcpApps;
+    }
+
+    /** Clears the requestMcpApps setting, reverting to the default behavior. */
+    public void clearRequestMcpApps() {
+        this.requestMcpApps = null;
+    }
+
     /** Gets the requestExitPlanMode flag. @return the flag */
     public Boolean getRequestExitPlanMode() {
         return requestExitPlanMode;
 
@@ -74,6 +74,7 @@ public class ResumeSessionConfig {
     private ElicitationHandler onElicitationRequest;
     private ExitPlanModeHandler onExitPlanMode;
     private AutoModeSwitchHandler onAutoModeSwitch;
+    private boolean enableMcpApps;
     private String gitHubToken;
     private String remoteSession;
 
@@ -972,6 +973,31 @@ public ResumeSessionConfig setOnElicitationRequest(ElicitationHandler onElicitat
         return this;
     }
 
+    /**
+     * Returns whether MCP Apps (SEP-1865) UI passthrough is enabled on resume.
+     *
+     * @return {@code true} if the consumer has opted into MCP Apps, otherwise
+     *         {@code false}
+     * @see #setEnableMcpApps(boolean)
+     */
+    public boolean isEnableMcpApps() {
+        return enableMcpApps;
+    }
+
+    /**
+     * Enables MCP Apps (SEP-1865) UI passthrough on the resumed session. See
+     * {@link SessionConfig#setEnableMcpApps(boolean)} for full semantics (runtime
+     * gate, capability inspection, renderer requirement).
+     *
+     * @param enableMcpApps
+     *            {@code true} to opt into MCP Apps support on resume
+     * @return this config for method chaining
+     */
+    public ResumeSessionConfig setEnableMcpApps(boolean enableMcpApps) {
+        this.enableMcpApps = enableMcpApps;
+        return this;
+    }
+
     /**
      * Gets the exit-plan-mode request handler.
      *
@@ -1129,6 +1155,7 @@ public ResumeSessionConfig clone() {
         copy.onElicitationRequest = this.onElicitationRequest;
         copy.onExitPlanMode = this.onExitPlanMode;
         copy.onAutoModeSwitch = this.onAutoModeSwitch;
+        copy.enableMcpApps = this.enableMcpApps;
         copy.gitHubToken = this.gitHubToken;
         copy.remoteSession = this.remoteSession;
         return copy;
 
@@ -119,6 +119,9 @@ public final class ResumeSessionRequest {
     @JsonProperty("requestElicitation")
     private Boolean requestElicitation;
 
+    @JsonProperty("requestMcpApps")
+    private Boolean requestMcpApps;
+
     @JsonProperty("requestExitPlanMode")
     private Boolean requestExitPlanMode;
 
@@ -528,6 +531,21 @@ public void clearRequestElicitation() {
         this.requestElicitation = null;
     }
 
+    /** Gets the requestMcpApps flag. @return the flag */
+    public Boolean getRequestMcpApps() {
+        return requestMcpApps;
+    }
+
+    /** Sets the requestMcpApps flag. @param requestMcpApps the flag */
+    public void setRequestMcpApps(boolean requestMcpApps) {
+        this.requestMcpApps = requestMcpApps;
+    }
+
+    /** Clears the requestMcpApps setting, reverting to the default behavior. */
+    public void clearRequestMcpApps() {
+        this.requestMcpApps = null;
+    }
+
     /** Gets the requestExitPlanMode flag. @return the flag */
     public Boolean getRequestExitPlanMode() {
         return requestExitPlanMode;
Original file line number	Diff line number	Diff line change
`@@ -144,6 +144,9 @@ static CreateSessionRequest buildCreateRequest(SessionConfig config, String sess`
`144`	`144`	`if (config.getOnElicitationRequest() != null) {`
`145`	`145`	`request.setRequestElicitation(true);`
`146`	`146`	`}`
	`147`	`+ if (config.isEnableMcpApps()) {`
	`148`	`+ request.setRequestMcpApps(true);`
	`149`	`+ }`
`147`	`150`	`if (config.getOnExitPlanMode() != null) {`
`148`	`151`	`request.setRequestExitPlanMode(true);`
`149`	`152`	`}`
`@@ -238,6 +241,9 @@ static ResumeSessionRequest buildResumeRequest(String sessionId, ResumeSessionCo`
`238`	`241`	`if (config.getOnElicitationRequest() != null) {`
`239`	`242`	`request.setRequestElicitation(true);`
`240`	`243`	`}`
	`244`	`+ if (config.isEnableMcpApps()) {`
	`245`	`+ request.setRequestMcpApps(true);`
	`246`	`+ }`
`241`	`247`	`if (config.getOnExitPlanMode() != null) {`
`242`	`248`	`request.setRequestExitPlanMode(true);`
`243`	`249`	`}`