Advertise MCP App UI support to Codex Apps

codex-rs/analytics/src/analytics_client_tests.rs

@@ -793,12 +793,7 @@ fn sample_initialize_fact(connection_id: u64) -> AnalyticsFact {
                 title: None,
                 version: "1.0.0".to_string(),
             },
-            capabilities: Some(InitializeCapabilities {
-                experimental_api: false,
-                request_attestation: false,
-                opt_out_notification_methods: None,
-                mcp_server_openai_form_elicitation: false,
-            }),
+            capabilities: Some(InitializeCapabilities::default()),
         },
         product_client_id: DEFAULT_ORIGINATOR.to_string(),
         runtime: CodexRuntimeMetadata {
@@ -1674,12 +1669,7 @@ async fn initialize_caches_client_and_thread_lifecycle_publishes_once_initialize
                         title: None,
                         version: "1.0.0".to_string(),
                     },
-                    capabilities: Some(InitializeCapabilities {
-                        experimental_api: false,
-                        request_attestation: false,
-                        opt_out_notification_methods: None,
-                        mcp_server_openai_form_elicitation: false,
-                    }),
+                    capabilities: Some(InitializeCapabilities::default()),
                 },
                 product_client_id: DEFAULT_ORIGINATOR.to_string(),
                 runtime: CodexRuntimeMetadata {
@@ -1978,12 +1968,7 @@ async fn compaction_event_ingests_custom_fact() {
                         title: None,
                         version: "1.0.0".to_string(),
                     },
-                    capabilities: Some(InitializeCapabilities {
-                        experimental_api: false,
-                        request_attestation: false,
-                        opt_out_notification_methods: None,
-                        mcp_server_openai_form_elicitation: false,
-                    }),
+                    capabilities: Some(InitializeCapabilities::default()),
                 },
                 product_client_id: DEFAULT_ORIGINATOR.to_string(),
                 runtime: sample_runtime_metadata(),
@@ -2108,12 +2093,7 @@ async fn guardian_review_event_ingests_custom_fact_with_optional_target_item() {
                         title: None,
                         version: "1.0.0".to_string(),
                     },
-                    capabilities: Some(InitializeCapabilities {
-                        experimental_api: false,
-                        request_attestation: false,
-                        opt_out_notification_methods: None,
-                        mcp_server_openai_form_elicitation: false,
-                    }),
+                    capabilities: Some(InitializeCapabilities::default()),
                 },
                 product_client_id: DEFAULT_ORIGINATOR.to_string(),
                 runtime: sample_runtime_metadata(),

codex-rs/app-server-client/src/lib.rs

@@ -374,6 +374,7 @@ impl InProcessClientStartArgs {
                 Some(self.opt_out_notification_methods.clone())
             },
             mcp_server_openai_form_elicitation: self.mcp_server_openai_form_elicitation,
+            extensions: None,
         };
 
         InitializeParams {

codex-rs/app-server-client/src/remote.rs

@@ -101,6 +101,7 @@ impl RemoteAppServerConnectArgs {
                 Some(self.opt_out_notification_methods.clone())
             },
             mcp_server_openai_form_elicitation: self.mcp_server_openai_form_elicitation,
+            extensions: None,
         };
 
         InitializeParams {

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -2207,6 +2207,12 @@ mod tests {
                         "thread/started".to_string(),
                         "item/agentMessage/delta".to_string(),
                     ]),
+                    extensions: Some(std::collections::HashMap::from([(
+                        "io.modelcontextprotocol/ui".to_string(),
+                        json!({
+                            "mimeTypes": ["text/html;profile=mcp-app"],
+                        }),
+                    )])),
                 }),
             },
         };
@@ -2228,7 +2234,12 @@ mod tests {
                         "optOutNotificationMethods": [
                             "thread/started",
                             "item/agentMessage/delta"
-                        ]
+                        ],
+                        "extensions": {
+                            "io.modelcontextprotocol/ui": {
+                                "mimeTypes": ["text/html;profile=mcp-app"]
+                            }
+                        }
                     }
                 }
             }),
@@ -2255,7 +2266,12 @@ mod tests {
                     "optOutNotificationMethods": [
                         "thread/started",
                         "item/agentMessage/delta"
-                    ]
+                    ],
+                    "extensions": {
+                        "io.modelcontextprotocol/ui": {
+                            "mimeTypes": ["text/html;profile=mcp-app"]
+                        }
+                    }
                 }
             }
         }))?;
@@ -2278,6 +2294,12 @@ mod tests {
                             "thread/started".to_string(),
                             "item/agentMessage/delta".to_string(),
                         ]),
+                        extensions: Some(std::collections::HashMap::from([(
+                            "io.modelcontextprotocol/ui".to_string(),
+                            json!({
+                                "mimeTypes": ["text/html;profile=mcp-app"],
+                            }),
+                        )])),
                     }),
                 },
             }

codex-rs/app-server-protocol/src/protocol/v1.rs

@@ -57,6 +57,13 @@ pub struct InitializeCapabilities {
     /// connection (for example `thread/started`).
     #[ts(optional = nullable)]
     pub opt_out_notification_methods: Option<Vec<String>>,
+    /// MCP extension settings declared by the app-server client.
+    ///
+    /// Codex currently consumes `io.modelcontextprotocol/ui` so widget-capable
+    /// hosts can advertise the same UI support to downstream MCP servers.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[ts(optional = nullable)]
+    pub extensions: Option<HashMap<String, serde_json::Value>>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]

codex-rs/app-server-test-client/src/lib.rs

@@ -1671,6 +1671,7 @@ impl CodexClient {
                             .collect(),
                     ),
                     mcp_server_openai_form_elicitation: false,
+                    extensions: None,
                 }),
             },
         };

codex-rs/app-server/README.md

@@ -93,6 +93,24 @@ App-server then advertises the downstream `openai/form` MCP extension for
 threads started, resumed, or forked by that connection. Clients that cannot
 handle the request envelope omit the field or set it to `false`.
 
+Clients that can render MCP App WebViews advertise the MCP UI extension:
+
+```json
+{
+  "capabilities": {
+    "extensions": {
+      "io.modelcontextprotocol/ui": {
+        "mimeTypes": ["text/html;profile=mcp-app"]
+      }
+    }
+  }
+}
+```
+
+App-server forwards this capability to the host-owned `codex_apps` MCP
+server during initialization and on each `tools/call`. Clients without an
+MCP App WebView omit the extension.
+
 Applications building on top of `codex app-server` should identify themselves via the `clientInfo` parameter.
 
 **Important**: `clientInfo.name` is used to identify the client for the OpenAI Compliance Logs Platform. If

codex-rs/app-server/src/message_processor.rs

@@ -225,6 +225,7 @@ pub(crate) struct InitializedConnectionSessionState {
     pub(crate) client_version: String,
     pub(crate) request_attestation: bool,
     pub(crate) supports_openai_form_elicitation: bool,
+    pub(crate) supports_mcp_app_ui_webview: bool,
 }
 
 impl Default for ConnectionSessionState {
@@ -281,6 +282,12 @@ impl ConnectionSessionState {
             .get()
             .is_some_and(|session| session.supports_openai_form_elicitation)
     }
+
+    pub(crate) fn supports_mcp_app_ui_webview(&self) -> bool {
+        self.initialized
+            .get()
+            .is_some_and(|session| session.supports_mcp_app_ui_webview)
+    }
     pub(crate) fn initialize(&self, session: InitializedConnectionSessionState) -> Result<(), ()> {
         self.initialized.set(session).map_err(|_| ())
     }
@@ -897,6 +904,7 @@ impl MessageProcessor {
         let app_server_client_name = session.app_server_client_name().map(str::to_string);
         let client_version = session.client_version().map(str::to_string);
         let supports_openai_form_elicitation = session.supports_openai_form_elicitation();
+        let supports_mcp_app_ui_webview = session.supports_mcp_app_ui_webview();
         let error_request_id = connection_request_id.clone();
         let rpc_gate = Arc::clone(&session.rpc_gate);
         let processor = Arc::clone(self);
@@ -913,6 +921,7 @@ impl MessageProcessor {
                         app_server_client_name,
                         client_version,
                         supports_openai_form_elicitation,
+                        supports_mcp_app_ui_webview,
                     )
                     .await;
                 if let Err(error) = result {
@@ -935,6 +944,7 @@ impl MessageProcessor {
         Ok(())
     }
 
+    #[allow(clippy::too_many_arguments)]
     async fn handle_initialized_client_request(
         self: Arc<Self>,
         connection_request_id: ConnectionRequestId,
@@ -943,6 +953,7 @@ impl MessageProcessor {
         app_server_client_name: Option<String>,
         client_version: Option<String>,
         supports_openai_form_elicitation: bool,
+        supports_mcp_app_ui_webview: bool,
     ) -> Result<(), JSONRPCErrorError> {
         let connection_id = connection_request_id.connection_id;
         let request_id = ConnectionRequestId {
@@ -1099,6 +1110,7 @@ impl MessageProcessor {
                         app_server_client_name.clone(),
                         client_version.clone(),
                         supports_openai_form_elicitation,
+                        supports_mcp_app_ui_webview,
                         request_context,
                     )
                     .await
@@ -1117,6 +1129,7 @@ impl MessageProcessor {
                         client_version.clone(),
                         /*supports_openai_form_elicitation*/
                         supports_openai_form_elicitation,
+                        supports_mcp_app_ui_webview,
                     )
                     .await
             }
@@ -1129,6 +1142,7 @@ impl MessageProcessor {
                         client_version.clone(),
                         /*supports_openai_form_elicitation*/
                         supports_openai_form_elicitation,
+                        supports_mcp_app_ui_webview,
                     )
                     .await
             }
@@ -1330,6 +1344,7 @@ impl MessageProcessor {
                         client_version.clone(),
                         /*supports_openai_form_elicitation*/
                         supports_openai_form_elicitation,
+                        supports_mcp_app_ui_webview,
                     )
                     .await
             }
@@ -1346,7 +1361,7 @@ impl MessageProcessor {
             }
             ClientRequest::ThreadRealtimeStart { params, .. } => {
                 self.turn_processor
-                    .thread_realtime_start(&request_id, params)
+                    .thread_realtime_start(&request_id, params, supports_mcp_app_ui_webview)
                     .await
             }
             ClientRequest::ThreadRealtimeAppendAudio { params, .. } => {
@@ -1373,7 +1388,9 @@ impl MessageProcessor {
                 self.turn_processor.thread_realtime_list_voices().await
             }
             ClientRequest::ReviewStart { params, .. } => {
-                self.turn_processor.review_start(&request_id, params).await
+                self.turn_processor
+                    .review_start(&request_id, params, supports_mcp_app_ui_webview)
+                    .await
             }
             ClientRequest::McpServerOauthLogin { params, .. } => {
                 self.mcp_processor.mcp_server_oauth_login(params).await
@@ -1393,7 +1410,7 @@ impl MessageProcessor {
             }
             ClientRequest::McpServerToolCall { params, .. } => {
                 self.mcp_processor
-                    .mcp_server_tool_call(&request_id, params)
+                    .mcp_server_tool_call(&request_id, params, supports_mcp_app_ui_webview)
                     .await
             }
             ClientRequest::WindowsSandboxSetupStart { params, .. } => {

codex-rs/app-server/src/request_processors/initialize_processor.rs

@@ -71,6 +71,8 @@ impl InitializeRequestProcessor {
         let experimental_api_enabled = capabilities.experimental_api;
         let request_attestation = capabilities.request_attestation;
         let supports_openai_form_elicitation = capabilities.mcp_server_openai_form_elicitation;
+        let supports_mcp_app_ui_webview =
+            codex_mcp::supports_mcp_app_ui_webview(capabilities.extensions.as_ref());
         let opt_out_notification_methods = capabilities
             .opt_out_notification_methods
             .unwrap_or_default();
@@ -98,6 +100,7 @@ impl InitializeRequestProcessor {
                 client_version: version,
                 request_attestation,
                 supports_openai_form_elicitation,
+                supports_mcp_app_ui_webview,
             })
             .is_err()
         {