feat: surface capabilities.ui.mcpApps and warn on silent drop

Expose the runtime's response capability so consumers can detect when
their enableMcpApps opt-in was silently dropped by the runtime gate
(MCP_APPS feature flag / COPILOT_MCP_APPS env override unset).

For each SDK:
- Add mcpApps?: bool to the SessionUiCapabilities type
- After session.create / session.resume, if the consumer requested the
  opt-in but capabilities.ui.mcpApps is not true on the response, log
  a warning (console.warn / logger.warning / slog / tracing::warn /
  fmt.Fprintf(os.Stderr, ...)) so the silent drop is discoverable.

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

Author: mattdholloway

dotnet/src/Client.cs

@@ -653,6 +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)
         {
@@ -813,6 +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)
         {
@@ -1758,6 +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>

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; }
 }
 
 // ============================================================================

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
@@ -777,6 +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
 }
@@ -965,6 +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

@@ -804,6 +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.

nodejs/src/client.ts

@@ -86,6 +86,26 @@ function toWireProviderConfig(provider: ProviderConfig): Record<string, unknown>
  */
 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)
  */
@@ -849,10 +869,11 @@ export class CopilotClient {
             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;
@@ -990,10 +1011,11 @@ export class CopilotClient {
             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/types.ts

@@ -483,6 +483,14 @@ export interface SessionCapabilities {
     ui?: {
         /** Whether the host supports interactive elicitation dialogs. */
         elicitation?: boolean;
+        /**
+         * 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 absent or
+         * `false`, indicating the runtime silently dropped the opt-in.
+         */
+        mcpApps?: boolean;
     };
 }
 

python/copilot/client.py

@@ -75,6 +75,32 @@
 
 logger = logging.getLogger(__name__)
 
+
+def _warn_if_mcp_apps_dropped(
+    requested: bool,
+    capabilities: dict | None,
+    session_id: str,
+) -> None:
+    """Log a warning when ``enable_mcp_apps=True`` was requested 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``.
+    """
+    if not requested:
+        return
+    ui = (capabilities or {}).get("ui") or {}
+    if not ui.get("mcpApps"):
+        logger.warning(
+            "Session %s: enable_mcp_apps 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.",
+            session_id,
+        )
+
 # ============================================================================
 # Connection Types
 # ============================================================================
@@ -1684,6 +1710,7 @@ async def create_session(
             session._workspace_path = response.get("workspacePath")
             capabilities = response.get("capabilities")
             session._set_capabilities(capabilities)
+            _warn_if_mcp_apps_dropped(enable_mcp_apps, capabilities, actual_session_id)
         except BaseException as exc:
             with self._sessions_lock:
                 self._sessions.pop(actual_session_id, None)
@@ -2039,6 +2066,7 @@ async def resume_session(
             session._workspace_path = response.get("workspacePath")
             capabilities = response.get("capabilities")
             session._set_capabilities(capabilities)
+            _warn_if_mcp_apps_dropped(enable_mcp_apps, capabilities, session_id)
         except BaseException as exc:
             with self._sessions_lock:
                 self._sessions.pop(session_id, None)

python/copilot/session.py

@@ -373,6 +373,12 @@ class SessionUiCapabilities(TypedDict, total=False):
 
     elicitation: bool
     """Whether the host supports interactive elicitation dialogs."""
+    mcpApps: bool
+    """Whether the runtime has accepted the session's MCP Apps (SEP-1865) opt-in.
+    ``True`` when the consumer set ``enable_mcp_apps=True`` on create/resume and
+    the runtime's ``MCP_APPS`` feature flag (or ``COPILOT_MCP_APPS=true`` env
+    override) is on. Otherwise absent or ``False``, indicating the runtime
+    silently dropped the opt-in."""
 
 
 class SessionCapabilities(TypedDict, total=False):

rust/src/session.rs

@@ -855,6 +855,11 @@ impl Client {
             }));
         }
         *capabilities.write() = create_result.capabilities.unwrap_or_default();
+        warn_if_mcp_apps_dropped(
+            config.request_mcp_apps.unwrap_or(false),
+            &capabilities.read(),
+            &session_id,
+        );
 
         tracing::debug!(
             elapsed_ms = total_start.elapsed().as_millis(),
@@ -1020,6 +1025,11 @@ impl Client {
         }
 
         *capabilities.write() = resume_capabilities.unwrap_or_default();
+        warn_if_mcp_apps_dropped(
+            config.request_mcp_apps.unwrap_or(false),
+            &capabilities.read(),
+            &session_id,
+        );
 
         tracing::debug!(
             elapsed_ms = total_start.elapsed().as_millis(),
@@ -1044,6 +1054,34 @@ impl Client {
 
 type CommandHandlerMap = HashMap<String, Arc<dyn CommandHandler>>;
 
+/// Emit a `tracing::warn!` when the consumer set `request_mcp_apps: Some(true)`
+/// on create/resume but the runtime did not advertise `capabilities.ui.mcp_apps`
+/// 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`.
+fn warn_if_mcp_apps_dropped(
+    requested: bool,
+    capabilities: &SessionCapabilities,
+    session_id: &SessionId,
+) {
+    if !requested {
+        return;
+    }
+    let advertised = capabilities
+        .ui
+        .as_ref()
+        .and_then(|ui| ui.mcp_apps)
+        .unwrap_or(false);
+    if advertised {
+        return;
+    }
+    tracing::warn!(
+        session_id = %session_id,
+        "request_mcp_apps was set but the runtime did not advertise capabilities.ui.mcpApps; the MCP_APPS feature flag or COPILOT_MCP_APPS=true environment override is likely unset and the MCP Apps surface is unavailable for this session"
+    );
+}
+
 fn build_command_handler_map(commands: Option<&[CommandDefinition]>) -> Arc<CommandHandlerMap> {
     let map = match commands {
         Some(commands) => commands

rust/src/types.rs

@@ -3138,6 +3138,15 @@ pub struct UiCapabilities {
     /// Whether the host supports interactive elicitation dialogs.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub elicitation: Option<bool>,
+    /// Whether the runtime has accepted the session's MCP Apps (SEP-1865)
+    /// opt-in. `Some(true)` when the consumer set
+    /// [`SessionConfig::request_mcp_apps`] / [`ResumeSessionConfig::request_mcp_apps`]
+    /// to `Some(true)` on create/resume **and** the runtime's `MCP_APPS`
+    /// feature flag (or `COPILOT_MCP_APPS=true` env override) is on. Otherwise
+    /// absent or `Some(false)`, indicating the runtime silently dropped the
+    /// opt-in.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub mcp_apps: Option<bool>,
 }
 
 /// Options for the [`SessionUi::input`](crate::session::SessionUi::input) convenience method.