diff --git a/server/packages/sandbox-agent/src/router.rs b/server/packages/sandbox-agent/src/router.rs
index d61c494..a2d1efe 100644
--- a/server/packages/sandbox-agent/src/router.rs
+++ b/server/packages/sandbox-agent/src/router.rs
@@ -978,6 +978,7 @@ pub(crate) struct SessionManager {
 struct ModelCatalogState {
     models: HashMap<AgentId, AgentModelsResponse>,
     in_flight: HashMap<AgentId, Arc<Notify>>,
+    codex_unavailable_models: HashSet<String>,
 }
 
 /// Shared Codex app-server process that handles multiple sessions via JSON-RPC.
@@ -987,11 +988,15 @@ struct CodexServer {
     /// Sender for writing to the process stdin
     stdin_sender: mpsc::UnboundedSender<String>,
     /// Pending JSON-RPC requests awaiting responses, keyed by request ID
-    pending_requests: std::sync::Mutex<HashMap<i64, oneshot::Sender<Value>>>,
+    pending_requests: std::sync::Mutex<HashMap<i64, oneshot::Sender<CodexRequestResult>>>,
+    /// Optional mapping from request ID to session ID for routing request-scoped errors
+    request_sessions: std::sync::Mutex<HashMap<i64, String>>,
     /// Next request ID for JSON-RPC
     next_id: AtomicI64,
     /// Whether initialize/initialized handshake has completed
     initialized: std::sync::Mutex<bool>,
+    /// Serializes initialize handshakes so only one request is in flight at a time.
+    initialize_lock: Mutex<()>,
     /// Mapping from thread_id to session_id for routing notifications
     thread_sessions: std::sync::Mutex<HashMap<String, String>>,
 }
@@ -1009,8 +1014,10 @@ impl CodexServer {
         Self {
             stdin_sender,
             pending_requests: std::sync::Mutex::new(HashMap::new()),
+            request_sessions: std::sync::Mutex::new(HashMap::new()),
             next_id: AtomicI64::new(1),
             initialized: std::sync::Mutex::new(false),
+            initialize_lock: Mutex::new(()),
             thread_sessions: std::sync::Mutex::new(HashMap::new()),
         }
     }
@@ -1019,14 +1026,37 @@ impl CodexServer {
         self.next_id.fetch_add(1, Ordering::SeqCst)
     }
 
-    fn send_request(&self, id: i64, request: &impl Serialize) -> Option<oneshot::Receiver<Value>> {
+    fn send_request(
+        &self,
+        id: i64,
+        request: &impl Serialize,
+    ) -> Option<oneshot::Receiver<CodexRequestResult>> {
+        self.send_request_with_session(id, request, None)
+    }
+
+    fn send_request_with_session(
+        &self,
+        id: i64,
+        request: &impl Serialize,
+        session_id: Option<String>,
+    ) -> Option<oneshot::Receiver<CodexRequestResult>> {
         let (tx, rx) = oneshot::channel();
         {
             let mut pending = self.pending_requests.lock().unwrap();
             pending.insert(id, tx);
         }
+        if let Some(session_id) = session_id {
+            let mut sessions = self.request_sessions.lock().unwrap();
+            sessions.insert(id, session_id);
+        }
         let line = serde_json::to_string(request).ok()?;
-        self.stdin_sender.send(line).ok()?;
+        if self.stdin_sender.send(line).is_err() {
+            let mut pending = self.pending_requests.lock().unwrap();
+            pending.remove(&id);
+            let mut sessions = self.request_sessions.lock().unwrap();
+            sessions.remove(&id);
+            return None;
+        }
         Some(rx)
     }
 
@@ -1037,7 +1067,7 @@ impl CodexServer {
         self.stdin_sender.send(line).is_ok()
     }
 
-    fn complete_request(&self, id: i64, result: Value) {
+    fn complete_request(&self, id: i64, result: CodexRequestResult) {
         let tx = {
             let mut pending = self.pending_requests.lock().unwrap();
             pending.remove(&id)
@@ -1047,6 +1077,11 @@ impl CodexServer {
         }
     }
 
+    fn take_request_session(&self, id: i64) -> Option<String> {
+        let mut sessions = self.request_sessions.lock().unwrap();
+        sessions.remove(&id)
+    }
+
     fn register_thread(&self, thread_id: String, session_id: String) {
         let mut sessions = self.thread_sessions.lock().unwrap();
         sessions.insert(thread_id, session_id);
@@ -1068,6 +1103,8 @@ impl CodexServer {
     fn clear_pending(&self) {
         let mut pending = self.pending_requests.lock().unwrap();
         pending.clear();
+        let mut sessions = self.request_sessions.lock().unwrap();
+        sessions.clear();
     }
 
     fn clear_threads(&self) {
@@ -1076,6 +1113,12 @@ impl CodexServer {
     }
 }
 
+#[derive(Debug, Clone)]
+enum CodexRequestResult {
+    Response(Value),
+    Error(codex_schema::JsonrpcErrorError),
+}
+
 pub(crate) struct SessionSubscription {
     pub(crate) initial_events: Vec<UniversalEvent>,
     pub(crate) receiver: broadcast::Receiver<UniversalEvent>,
@@ -1885,6 +1928,80 @@ impl SessionManager {
         Ok(())
     }
 
+    async fn mark_codex_model_unavailable(&self, model_id: &str) -> bool {
+        let mut catalog = self.model_catalog.lock().await;
+        let inserted = catalog
+            .codex_unavailable_models
+            .insert(model_id.to_string());
+        if inserted {
+            // Force a fresh fetch so provider/model lists drop unavailable models.
+            catalog.models.remove(&AgentId::Codex);
+        }
+        inserted
+    }
+
+    async fn clear_codex_session_model_if_unavailable(
+        &self,
+        session_id: &str,
+        model_id: &str,
+    ) -> bool {
+        let mut sessions = self.sessions.lock().await;
+        let Some(session) = SessionManager::session_mut(&mut sessions, session_id) else {
+            return false;
+        };
+        if session.agent == AgentId::Codex && session.model.as_deref() == Some(model_id) {
+            session.model = None;
+            session.updated_at = std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .map(|d| d.as_millis() as i64)
+                .unwrap_or(session.updated_at);
+            return true;
+        }
+        false
+    }
+
+    async fn codex_native_session_id(&self, session_id: &str) -> Option<String> {
+        let sessions = self.sessions.lock().await;
+        let session = SessionManager::session_ref(&sessions, session_id)?;
+        if session.agent != AgentId::Codex {
+            return None;
+        }
+        session.native_session_id.clone()
+    }
+
+    async fn handle_codex_model_unavailable(
+        &self,
+        session_id: &str,
+        model_id: &str,
+        native_session_id: Option<String>,
+    ) {
+        let newly_marked = self.mark_codex_model_unavailable(model_id).await;
+        if newly_marked {
+            tracing::warn!(
+                model_id = %model_id,
+                "codex model marked unavailable after runtime error"
+            );
+        }
+        if self
+            .clear_codex_session_model_if_unavailable(session_id, model_id)
+            .await
+        {
+            let native_session_id = match native_session_id {
+                Some(native_session_id) => Some(native_session_id),
+                None => self.codex_native_session_id(session_id).await,
+            };
+            let _ = self
+                .record_conversions(
+                    session_id,
+                    vec![codex_model_unavailable_status_event(
+                        native_session_id,
+                        model_id,
+                    )],
+                )
+                .await;
+        }
+    }
+
     pub(crate) async fn delete_session(&self, session_id: &str) -> Result<(), SandboxError> {
         let (agent, native_session_id) = {
             let mut sessions = self.sessions.lock().await;
@@ -3548,7 +3665,8 @@ impl SessionManager {
                 codex_schema::JsonrpcMessage::Response(response) => {
                     // Route response to waiting request
                     if let Some(id) = codex_request_id_to_i64(&response.id) {
-                        server.complete_request(id, response.result.clone());
+                        server.take_request_session(id);
+                        server.complete_request(id, CodexRequestResult::Response(response.result));
                     }
                 }
                 codex_schema::JsonrpcMessage::Notification(_) => {
@@ -3560,6 +3678,20 @@ impl SessionManager {
                             codex_thread_id_from_server_notification(&notification)
                         {
                             if let Some(session_id) = server.session_for_thread(&thread_id) {
+                                if let codex_schema::ServerNotification::Error(params) =
+                                    &notification
+                                {
+                                    if let Some(model_id) =
+                                        codex_unavailable_model_from_message(&params.error.message)
+                                    {
+                                        self.handle_codex_model_unavailable(
+                                            &session_id,
+                                            &model_id,
+                                            Some(thread_id.clone()),
+                                        )
+                                        .await;
+                                    }
+                                }
                                 let conversions =
                                     match convert_codex::notification_to_universal(&notification) {
                                         Ok(c) => c,
@@ -3601,8 +3733,28 @@ impl SessionManager {
                     }
                 }
                 codex_schema::JsonrpcMessage::Error(error) => {
-                    // Log error but don't have a session to route to
-                    eprintln!("Codex server error: {:?}", error);
+                    if let Some(id) = codex_request_id_to_i64(&error.id) {
+                        let session_id = server.take_request_session(id);
+                        server.complete_request(id, CodexRequestResult::Error(error.error.clone()));
+                        if let Some(session_id) = session_id {
+                            if let Some(model_id) =
+                                codex_unavailable_model_from_rpc_error(&error.error)
+                            {
+                                self.handle_codex_model_unavailable(&session_id, &model_id, None)
+                                    .await;
+                            }
+                            let _ = self
+                                .record_conversions(
+                                    &session_id,
+                                    vec![codex_rpc_error_to_universal(&error)],
+                                )
+                                .await;
+                        } else {
+                            eprintln!("Codex server error: {:?}", error);
+                        }
+                    } else {
+                        eprintln!("Codex server error: {:?}", error);
+                    }
                 }
             }
         }
@@ -3610,6 +3762,7 @@ impl SessionManager {
 
     /// Performs the initialize/initialized handshake with the Codex server.
     async fn codex_server_initialize(&self, server: &CodexServer) -> Result<(), SandboxError> {
+        let _initialize_guard = server.initialize_lock.lock().await;
         if server.is_initialized() {
             return Ok(());
         }
@@ -3635,7 +3788,7 @@ impl SessionManager {
         // Wait for initialize response with timeout
         let result = tokio::time::timeout(Duration::from_secs(30), rx).await;
         match result {
-            Ok(Ok(_)) => {
+            Ok(Ok(CodexRequestResult::Response(_))) => {
                 // Send initialized notification
                 let notification = codex_schema::JsonrpcNotification {
                     method: "initialized".to_string(),
@@ -3645,6 +3798,10 @@ impl SessionManager {
                 server.set_initialized();
                 Ok(())
             }
+            Ok(Ok(CodexRequestResult::Error(error))) => Err(codex_request_error_to_sandbox(
+                "initialize request failed",
+                &error,
+            )),
             Ok(Err(_)) => Err(SandboxError::StreamError {
                 message: "initialize request cancelled".to_string(),
             }),
@@ -3682,7 +3839,7 @@ impl SessionManager {
         // Wait for thread/start response
         let result = tokio::time::timeout(Duration::from_secs(30), rx).await;
         match result {
-            Ok(Ok(response)) => {
+            Ok(Ok(CodexRequestResult::Response(response))) => {
                 // Extract thread_id from response
                 let thread_id = response
                     .get("thread")
@@ -3699,6 +3856,10 @@ impl SessionManager {
 
                 Ok(thread_id)
             }
+            Ok(Ok(CodexRequestResult::Error(error))) => Err(codex_request_error_to_sandbox(
+                "thread/start request failed",
+                &error,
+            )),
             Ok(Err(_)) => Err(SandboxError::StreamError {
                 message: "thread/start request cancelled".to_string(),
             }),
@@ -3726,6 +3887,22 @@ impl SessionManager {
 
         let id = server.next_request_id();
         let prompt_text = codex_prompt_for_mode(prompt, Some(&session.agent_mode));
+        let mut model = session.model.clone();
+        if let Some(model_id) = model.clone() {
+            let is_unavailable = {
+                let catalog = self.model_catalog.lock().await;
+                catalog.codex_unavailable_models.contains(&model_id)
+            };
+            if is_unavailable {
+                self.handle_codex_model_unavailable(
+                    &session.session_id,
+                    &model_id,
+                    session.native_session_id.clone(),
+                )
+                .await;
+                model = None;
+            }
+        }
         let params = codex_schema::TurnStartParams {
             approval_policy: codex_approval_policy(Some(&session.permission_mode)),
             collaboration_mode: None,
@@ -3735,7 +3912,7 @@ impl SessionManager {
                 text: prompt_text,
                 text_elements: Vec::new(),
             }],
-            model: session.model.clone(),
+            model,
             output_schema: None,
             sandbox_policy: codex_sandbox_policy(Some(&session.permission_mode)),
             summary: None,
@@ -3749,7 +3926,7 @@ impl SessionManager {
 
         // Send but don't wait for response - notifications will stream back
         server
-            .send_request(id, &request)
+            .send_request_with_session(id, &request, Some(session.session_id.clone()))
             .ok_or_else(|| SandboxError::StreamError {
                 message: "failed to send turn/start request".to_string(),
             })?;
@@ -3899,6 +4076,10 @@ impl SessionManager {
 
     async fn fetch_codex_models(self: &Arc<Self>) -> Result<AgentModelsResponse, SandboxError> {
         let started = Instant::now();
+        let unavailable_models = {
+            let catalog = self.model_catalog.lock().await;
+            catalog.codex_unavailable_models.clone()
+        };
         let server = self.ensure_codex_server().await?;
         tracing::info!(
             elapsed_ms = started.elapsed().as_millis() as u64,
@@ -3932,7 +4113,19 @@ impl SessionManager {
             let result =
                 tokio::time::timeout(Duration::from_secs(CODEX_MODEL_LIST_TIMEOUT_SECS), rx).await;
             let value = match result {
-                Ok(Ok(value)) => value,
+                Ok(Ok(CodexRequestResult::Response(value))) => value,
+                Ok(Ok(CodexRequestResult::Error(error))) => {
+                    tracing::warn!(
+                        elapsed_ms = started.elapsed().as_millis() as u64,
+                        page = pages + 1,
+                        error = %error.message,
+                        "codex model/list request failed"
+                    );
+                    return Err(codex_request_error_to_sandbox(
+                        "model/list request failed",
+                        &error,
+                    ));
+                }
                 Ok(Err(_)) => {
                     tracing::warn!(
                         elapsed_ms = started.elapsed().as_millis() as u64,
@@ -3977,6 +4170,9 @@ impl SessionManager {
                 let Some(model_id) = model_id else {
                     continue;
                 };
+                if unavailable_models.contains(model_id) {
+                    continue;
+                }
                 if !seen.insert(model_id.to_string()) {
                     continue;
                 }
@@ -4039,6 +4235,12 @@ impl SessionManager {
         }
 
         models.sort_by(|a, b| a.id.cmp(&b.id));
+        if default_model
+            .as_ref()
+            .is_some_and(|model_id| unavailable_models.contains(model_id))
+        {
+            default_model = None;
+        }
         if default_model.is_none() {
             default_model = models.first().map(|model| model.id.clone());
         }
@@ -5713,6 +5915,46 @@ mod tests {
         assert!(!options.env.contains_key("ANTHROPIC_API_KEY"));
         assert!(!options.env.contains_key("CLAUDE_API_KEY"));
     }
+
+    #[test]
+    fn codex_unavailable_model_parser_handles_requested_model_message() {
+        let message = "The requested model 'gpt-5.3-codex' does not exist.";
+        assert_eq!(
+            codex_unavailable_model_from_message(message),
+            Some("gpt-5.3-codex".to_string())
+        );
+    }
+
+    #[test]
+    fn codex_unavailable_model_parser_handles_chatgpt_account_message() {
+        let message = "The 'gpt-5.3-codex-NOTREAL' model is not supported when using Codex with a ChatGPT account.";
+        assert_eq!(
+            codex_unavailable_model_from_message(message),
+            Some("gpt-5.3-codex-NOTREAL".to_string())
+        );
+    }
+
+    #[test]
+    fn codex_unavailable_model_parser_ignores_non_model_messages() {
+        let message = "Network error while contacting provider.";
+        assert_eq!(codex_unavailable_model_from_message(message), None);
+    }
+
+    #[test]
+    fn codex_unavailable_model_parser_ignores_non_unavailable_model_messages() {
+        let message = "using model 'gpt-5.3-codex' for this turn";
+        assert_eq!(codex_unavailable_model_from_message(message), None);
+    }
+
+    #[test]
+    fn codex_unavailable_model_parser_handles_embedded_json_detail_message() {
+        let message = "http 400 Bad Request: Some(\"{\\\"detail\\\":\\\"The 'gpt-5.3-codex-NOTREAL' model is not supported when using Codex with a ChatGPT account.\\\"}\")";
+        assert_eq!(
+            codex_unavailable_model_from_message(message),
+            Some("gpt-5.3-codex-NOTREAL".to_string())
+        );
+    }
+
 }
 
 fn claude_input_session_id(session: &SessionSnapshot) -> String {
@@ -6403,6 +6645,111 @@ fn codex_rpc_error_to_universal(error: &codex_schema::JsonrpcError) -> EventConv
     EventConversion::new(UniversalEventType::Error, UniversalEventData::Error(data))
 }
 
+fn codex_request_error_to_sandbox(
+    context: &str,
+    error: &codex_schema::JsonrpcErrorError,
+) -> SandboxError {
+    SandboxError::StreamError {
+        message: format!("{context}: {} (code {})", error.message, error.code),
+    }
+}
+
+fn codex_model_unavailable_status_event(
+    native_session_id: Option<String>,
+    model_id: &str,
+) -> EventConversion {
+    EventConversion::new(
+        UniversalEventType::ItemCompleted,
+        UniversalEventData::Item(ItemEventData {
+            item: UniversalItem {
+                item_id: String::new(),
+                native_item_id: None,
+                parent_id: None,
+                kind: ItemKind::Status,
+                role: Some(ItemRole::System),
+                content: vec![ContentPart::Status {
+                    label: "codex.model.unavailable".to_string(),
+                    detail: Some(format!(
+                        "Model '{}' was rejected by provider; falling back to default for this session.",
+                        model_id
+                    )),
+                }],
+                status: ItemStatus::Completed,
+            },
+        }),
+    )
+    .synthetic()
+    .with_native_session(native_session_id)
+}
+
+fn codex_unavailable_model_from_message(message: &str) -> Option<String> {
+    let normalized = message.to_ascii_lowercase();
+    if !normalized.contains("model") {
+        return None;
+    }
+    let is_known_unavailable_shape = normalized.contains("does not exist")
+        || normalized.contains("model_not_found")
+        || normalized.contains("requested model")
+        || normalized.contains("not supported when using codex with a chatgpt account");
+    if !is_known_unavailable_shape {
+        return None;
+    }
+    for token in extract_quoted_tokens(message, '\'')
+        .into_iter()
+        .chain(extract_quoted_tokens(message, '"').into_iter())
+    {
+        if is_likely_model_id(token) {
+            return Some(token.to_string());
+        }
+    }
+    None
+}
+
+fn codex_unavailable_model_from_rpc_error(
+    error: &codex_schema::JsonrpcErrorError,
+) -> Option<String> {
+    codex_unavailable_model_from_message(&error.message).or_else(|| {
+        error
+            .data
+            .as_ref()
+            .and_then(|data| codex_unavailable_model_from_message(&data.to_string()))
+    })
+}
+
+fn extract_quoted_tokens<'a>(message: &'a str, quote: char) -> Vec<&'a str> {
+    let mut out = Vec::new();
+    let mut start: Option<usize> = None;
+    for (idx, ch) in message.char_indices() {
+        if ch != quote {
+            continue;
+        }
+        if let Some(open) = start.take() {
+            if open < idx {
+                out.push(&message[open..idx]);
+            }
+        } else {
+            start = Some(idx + ch.len_utf8());
+        }
+    }
+    out
+}
+
+fn is_likely_model_id(candidate: &str) -> bool {
+    if candidate.len() < 3 || candidate.len() > 128 {
+        return false;
+    }
+    if candidate.chars().any(|ch| ch.is_whitespace()) {
+        return false;
+    }
+    if !candidate
+        .chars()
+        .all(|ch| ch.is_ascii_alphanumeric() || matches!(ch, '-' | '_' | '.'))
+    {
+        return false;
+    }
+    candidate.contains('-')
+}
+
 fn codex_permission_response_line(
     permission_id: &str,
     pending: &PendingPermission,