[app-server] model/list API (#5382)

Adds a `model/list` paginated API that returns the list of models
supported by Codex.

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

@@ -1,6 +1,7 @@
 use crate::error_code::INTERNAL_ERROR_CODE;
 use crate::error_code::INVALID_REQUEST_ERROR_CODE;
 use crate::fuzzy_file_search::run_fuzzy_file_search;
+use crate::models::supported_models;
 use crate::outgoing_message::OutgoingMessageSender;
 use crate::outgoing_message::OutgoingNotification;
 use codex_app_server_protocol::AddConversationListenerParams;
@@ -29,6 +30,8 @@ use codex_app_server_protocol::InterruptConversationResponse;
 use codex_app_server_protocol::JSONRPCErrorError;
 use codex_app_server_protocol::ListConversationsParams;
 use codex_app_server_protocol::ListConversationsResponse;
+use codex_app_server_protocol::ListModelsParams;
+use codex_app_server_protocol::ListModelsResponse;
 use codex_app_server_protocol::LoginApiKeyParams;
 use codex_app_server_protocol::LoginApiKeyResponse;
 use codex_app_server_protocol::LoginChatGptCompleteNotification;
@@ -111,7 +114,6 @@ use uuid::Uuid;
 
 // Duration before a ChatGPT login attempt is abandoned.
 const LOGIN_CHATGPT_TIMEOUT: Duration = Duration::from_secs(10 * 60);
-
 struct ActiveLogin {
     shutdown_handle: ShutdownHandle,
     login_id: Uuid,
@@ -172,6 +174,9 @@ impl CodexMessageProcessor {
             ClientRequest::ListConversations { request_id, params } => {
                 self.handle_list_conversations(request_id, params).await;
             }
+            ClientRequest::ListModels { request_id, params } => {
+                self.list_models(request_id, params).await;
+            }
             ClientRequest::ResumeConversation { request_id, params } => {
                 self.handle_resume_conversation(request_id, params).await;
             }
@@ -831,6 +836,58 @@ impl CodexMessageProcessor {
         self.outgoing.send_response(request_id, response).await;
     }
 
+    async fn list_models(&self, request_id: RequestId, params: ListModelsParams) {
+        let ListModelsParams { page_size, cursor } = params;
+        let models = supported_models();
+        let total = models.len();
+
+        if total == 0 {
+            let response = ListModelsResponse {
+                items: Vec::new(),
+                next_cursor: None,
+            };
+            self.outgoing.send_response(request_id, response).await;
+            return;
+        }
+
+        let effective_page_size = page_size.unwrap_or(total).max(1).min(total);
+        let start = match cursor {
+            Some(cursor) => match cursor.parse::<usize>() {
+                Ok(idx) => idx,
+                Err(_) => {
+                    let error = JSONRPCErrorError {
+                        code: INVALID_REQUEST_ERROR_CODE,
+                        message: format!("invalid cursor: {cursor}"),
+                        data: None,
+                    };
+                    self.outgoing.send_error(request_id, error).await;
+                    return;
+                }
+            },
+            None => 0,
+        };
+
+        if start > total {
+            let error = JSONRPCErrorError {
+                code: INVALID_REQUEST_ERROR_CODE,
+                message: format!("cursor {start} exceeds total models {total}"),
+                data: None,
+            };
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
+
+        let end = start.saturating_add(effective_page_size).min(total);
+        let items = models[start..end].to_vec();
+        let next_cursor = if end < total {
+            Some(end.to_string())
+        } else {
+            None
+        };
+        let response = ListModelsResponse { items, next_cursor };
+        self.outgoing.send_response(request_id, response).await;
+    }
+
     async fn handle_resume_conversation(
         &self,
         request_id: RequestId,