diff --git a/src-tauri/src/launcher.rs b/src-tauri/src/launcher.rs
index 7e893c9..c1c1d1c 100644
--- a/src-tauri/src/launcher.rs
+++ b/src-tauri/src/launcher.rs
@@ -199,6 +199,19 @@ struct Inner {
 }
 
 impl Inner {
+  /// Take `url` as the DHP's Engine API, and keep it in launcher.json so a launcher started
+  /// offline next time still arms live mode with it. Err is only the save failing: the
+  /// address is in use either way.
+  fn adopt_engine(&mut self, url: String) -> Result<(), String> {
+    let site = self.settings.site_url.clone();
+    let changed = self.state.remember_engine(&site, &url);
+    self.engine_url = Some(url);
+    if changed {
+      self.state.save(&self.settings.install_path())?;
+    }
+    Ok(())
+  }
+
   fn status(&self) -> Status {
     let installed = self.state.installed_version.clone();
     // Online means "update before launch"; offline launches whatever is installed.
@@ -343,6 +356,8 @@ impl Launcher {
   pub fn load(app: &AppHandle) -> Result<Launcher, String> {
     let settings = Settings::load(app)?;
     let state = LauncherState::load(&settings.install_path())?;
+    // Where live mode forwards, known before the first probe: offline, there is no probe.
+    let engine_url = state.engine_url(&settings.site_url);
     let token = tokens::load(&settings.site_url);
     let http = reqwest::Client::builder()
       .user_agent(concat!("ODIAN-Launcher/", env!("CARGO_PKG_VERSION")))
@@ -350,7 +365,7 @@ impl Launcher {
       .map_err(|e| format!("cannot start the HTTP client: {e}"))?;
     Ok(Launcher {
       http,
-      inner: Mutex::new(Inner { settings, state, online: false, available: None, token, phase: Phase::Checking, error: None, sync: None, engine_url: None, live: None, map_from_site: None, map_problem: None, tiles_waiting: None, unfinished: None, dev: None, dev_refusal: None, market_checked: None, portable: crate::settings::portable_drive().is_some(), updating_self: false, offline_reason: None, drive_extras: None }),
+      inner: Mutex::new(Inner { settings, state, online: false, available: None, token, phase: Phase::Checking, error: None, sync: None, engine_url, live: None, map_from_site: None, map_problem: None, tiles_waiting: None, unfinished: None, dev: None, dev_refusal: None, market_checked: None, portable: crate::settings::portable_drive().is_some(), updating_self: false, offline_reason: None, drive_extras: None }),
       stack: Mutex::new(None),
       stack_states: std::sync::Mutex::new(Vec::new()),
       sign_in_cancel: Notify::new(),
@@ -518,14 +533,15 @@ impl Launcher {
         false => self.log(app, "warn", format!("Your DHP did not answer. {why}")),
       }
     }
-    {
+    let kept = {
       let mut inner = self.inner.lock().await;
       inner.online = probe.online;
       inner.available = probe.version;
       // Keep the last known address while offline: it is where live mode resumes.
-      if engine.is_some() {
-        inner.engine_url = engine;
-      }
+      engine.map(|url| inner.adopt_engine(url)).unwrap_or(Ok(()))
+    };
+    if let Err(e) = kept {
+      self.log(app, "warn", format!("Could not remember the DHP's Engine API address: {e}"));
     }
     // Offline nothing is checked or forgotten: the remembered operator still launches.
     if let (true, Some(token)) = (probe.online, token) {
@@ -856,6 +872,9 @@ impl Launcher {
       if next.site_url != inner.settings.site_url {
         inner.token = tokens::load(&next.site_url);
         inner.available = None;
+        // Another DHP's Engine API is not this one's: only what this PC learned from the
+        // new site, if it was ever on it.
+        inner.engine_url = inner.state.engine_url(&next.site_url);
       }
       inner.settings = next;
     }
@@ -1745,6 +1764,12 @@ impl Launcher {
 
   /// Start the local stack. Returns once launching has begun; `stack` and `status` events follow.
   pub async fn launch(&self, app: &AppHandle) -> Result<(), String> {
+    // The DHP is looked at on start, on focus and on request. A PC that was offline then and
+    // has connected since would otherwise launch on that stale answer - no live mode for the
+    // whole session, and no update check. Offline it costs one probe (2 s at most).
+    if !self.inner.lock().await.online {
+      self.refresh(app).await;
+    }
     let config = {
       let mut inner = self.inner.lock().await;
       let status = inner.status();
@@ -2314,12 +2339,13 @@ impl Launcher {
       }
       let probe = site::probe(&self.http, &site_url).await;
       let engine = self.pick_engine(app, &probe).await;
-      {
+      let kept = {
         let mut inner = self.inner.lock().await;
         inner.online = probe.online;
-        if engine.is_some() {
-          inner.engine_url = engine;
-        }
+        engine.map(|url| inner.adopt_engine(url)).unwrap_or(Ok(()))
+      };
+      if let Err(e) = kept {
+        self.log(app, "warn", format!("Could not remember the DHP's Engine API address: {e}"));
       }
       if !probe.online {
         if was_online {
diff --git a/src-tauri/src/state.rs b/src-tauri/src/state.rs
index c964155..38d964a 100644
--- a/src-tauri/src/state.rs
+++ b/src-tauri/src/state.rs
@@ -42,6 +42,20 @@ pub struct LauncherState {
   /// while it was away. None until the first sync.
   #[serde(default)]
   pub tile_sync: Option<TileSync>,
+  /// The DHP's Engine API this PC last reached, so a launcher started OFFLINE still arms
+  /// live mode: the local API then switches to the DHP by itself once the network comes.
+  /// Held only in memory before, so a PC that started the launcher offline ran the whole
+  /// session on its own data even after it connected.
+  #[serde(default, skip_serializing_if = "Option::is_none")]
+  pub engine: Option<KnownEngine>,
+}
+
+/// Whose Engine API, and where. The site is named for the same reason as `TileSync`: an
+/// address learned from one DHP means nothing to a PC pointed at another.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct KnownEngine {
+  pub site: String,
+  pub url: String,
 }
 
 /// Whose journal, and how far through it. The site is named because a PC moved to another
@@ -60,6 +74,21 @@ impl LauncherState {
       _ => 0,
     }
   }
+
+  /// The Engine API this PC last reached on `site`, None for a site it never reached.
+  pub fn engine_url(&self, site: &str) -> Option<String> {
+    self.engine.as_ref().filter(|known| known.site == site && !site.is_empty()).map(|known| known.url.clone())
+  }
+
+  /// Remember `url` as `site`'s Engine API. True when that changed what is on disk.
+  pub fn remember_engine(&mut self, site: &str, url: &str) -> bool {
+    let known = KnownEngine { site: site.to_string(), url: url.to_string() };
+    if site.is_empty() || self.engine.as_ref() == Some(&known) {
+      return false;
+    }
+    self.engine = Some(known);
+    true
+  }
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
@@ -105,3 +134,42 @@ pub fn cached_notes(install_dir: &Path, version: &str) -> Option<String> {
 pub fn cache_notes(install_dir: &Path, version: &str, markdown: &str) -> Result<(), String> {
   write_atomic(&state_dir(install_dir).join("notes").join(format!("{version}.md")), markdown.as_bytes())
 }
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  const SITE: &str = "https://engine.odian.fr/dhp";
+  const ENGINE: &str = "https://engine-api.odian.fr";
+
+  #[test]
+  fn the_engine_api_is_remembered_per_site() {
+    let mut state = LauncherState::default();
+    assert_eq!(state.engine_url(SITE), None, "a PC that never reached the DHP knows no address");
+    assert!(state.remember_engine(SITE, ENGINE));
+    assert_eq!(state.engine_url(SITE).as_deref(), Some(ENGINE));
+    // Another DHP's address is not this one's.
+    assert_eq!(state.engine_url("http://192.168.7.2:30097"), None);
+    // Unchanged is not a change: no write for every probe that finds the same address.
+    assert!(!state.remember_engine(SITE, ENGINE));
+    // And a PC set offline on purpose (no DHP address) remembers nothing.
+    assert!(!state.remember_engine("", ENGINE));
+    assert_eq!(state.engine_url(""), None);
+  }
+
+  #[test]
+  fn the_engine_api_survives_a_restart_and_old_files_still_load() {
+    let dir = std::env::temp_dir().join(format!("odian-state-{}", uuid::Uuid::new_v4()));
+    let mut state = LauncherState::default();
+    state.remember_engine(SITE, ENGINE);
+    state.save(&dir).unwrap();
+    // What a launcher started offline reads: the address it had before it was closed.
+    assert_eq!(LauncherState::load(&dir).unwrap().engine_url(SITE).as_deref(), Some(ENGINE));
+
+    // A launcher.json from before this field loads, with no address.
+    std::fs::write(dir.join("state").join("launcher.json"), br#"{"install_id":"abc","operator":"jimmy"}"#).unwrap();
+    let old = LauncherState::load(&dir).unwrap();
+    assert_eq!((old.install_id.as_str(), old.engine_url(SITE)), ("abc", None));
+    let _ = std::fs::remove_dir_all(&dir);
+  }
+}