feat: implement background API connectivity checks and update state based on connectivity status

src/app/runtime/actions/mod.rs

@@ -300,6 +300,13 @@ pub fn apply_action(state: &mut AppStateRest, action: Action) {
             state.misc.tick_count = state.misc.tick_count.wrapping_add(1);
             let now_ms = chrono::Utc::now().timestamp_millis();
             state.misc.drain_expired_toasts(now_ms);
+            // Background API connectivity check — runs on a background thread
+            // every ~1s while disconnected, every ~30s while connected, so the
+            // status bar reflects real API availability without user input.
+            let check_interval = if state.misc.api_connected { 600 } else { 20 };
+            if state.misc.tick_count % check_interval == 0 {
+                spawn_api_connectivity_check(state);
+            }
     crate::app::review::maybe_run_staleness_sweep(state);
     if let Some(ref rt) = state.session_runtime {
         let _ = crate::app::review::process_pending_lessons(&rt.session_dir, &state.memory_dir);
@@ -392,6 +399,8 @@ pub fn apply_action(state: &mut AppStateRest, action: Action) {
                             if let Some(ref mut rt) = state.session_runtime {
                                 rt.push_message(crate::dto::chat::message::ChatMessage::system(message.clone()));
                             }
+                        } else if kind == "connectivity" {
+                            state.misc.api_connected = message == "connected";
                         } else {
                             state.push_toast(Toast::new(ToastKind::Info, message));
                         }
@@ -1142,6 +1151,54 @@ fn run_oauth_flow(provider: &str) -> anyhow::Result<String> {
     Ok(format!("Successfully authenticated with {}.", provider))
 }
 
+/// Spawn a background thread that checks API reachability via a lightweight HEAD
+/// request to `<base_url>/models`, pushing the result as a `SystemNote` so the
+/// next `Tick` handler updates `api_connected`.
+///
+/// Flow: resolve the provider's base URL → build a short-lived reqwest client
+/// with 3s connect / 5s total timeout → HEAD the `/models` endpoint → push
+/// a `connectivity` SystemNote with the result.
+///
+/// Why: runs off the event loop so a slow/TIMEOUT network does not block the TUI.
+fn spawn_api_connectivity_check(state: &AppStateRest) {
+    let base_url = state
+        .app_config
+        .providers
+        .get(&state.settings.provider)
+        .map(|p| p.api_base.clone())
+        .unwrap_or_else(|| crate::service::provider::DEFAULT_BASE_URL.to_string());
+    let turn_events = state.turn_events.clone();
+
+    std::thread::spawn(move || {
+        let url = format!("{}/models", base_url.trim_end_matches('/'));
+        let connected = match reqwest::blocking::Client::builder()
+            .timeout(std::time::Duration::from_secs(5))
+            .connect_timeout(std::time::Duration::from_secs(3))
+            .build()
+        {
+            Ok(client) => match client.head(&url).send() {
+                Ok(resp) => {
+                    let s = resp.status();
+                    // 401/403 means the server is reachable (just auth is wrong)
+                    s.is_success() || s.as_u16() == 401 || s.as_u16() == 403
+                }
+                Err(_) => false,
+            },
+            Err(_) => false,
+        };
+        if let Ok(mut q) = turn_events.lock() {
+            q.push_back(TurnEvent::SystemNote {
+                kind: "connectivity".to_string(),
+                message: if connected {
+                    "connected".to_string()
+                } else {
+                    "disconnected".to_string()
+                },
+            });
+        }
+    });
+}
+
 /// Generate `n` pseudo-random bytes from the system clock mixed with a monotonic
 /// counter, providing sufficient unpredictability for a per-flow OAuth state
 /// token without a `rand` dependency.

src/service/provider.rs

@@ -9,7 +9,7 @@ use crate::app::runtime::stream::turn::StreamedTurn;
 use crate::dto::chat::message::ChatMessage;
 use crate::dto::provider::request::{ChatRequest, StreamOptions, ToolDef};
 
-const DEFAULT_BASE_URL: &str = "https://opencode.ai/zen/v1";
+pub(crate) const DEFAULT_BASE_URL: &str = "https://opencode.ai/zen/v1";
 const DEFAULT_MODEL: &str = "deepseek-v4-flash-free";
 pub const DEFAULT_API_KEY: &str = "sk-5dd268d88adb496b-818beb-6bc7498e";
 const CONNECT_TIMEOUT: Duration = Duration::from_secs(10);