diff --git a/crates/ui-egui/src/menubar.rs b/crates/ui-egui/src/menubar.rs
index 04eea2439..70c878731 100644
--- a/crates/ui-egui/src/menubar.rs
+++ b/crates/ui-egui/src/menubar.rs
@@ -1286,7 +1286,168 @@ mod tests {
         let _ = std::fs::remove_dir_all(&dir);
     }
 
+    /// #1528: `file.findMissing` with `wait: true` must propagate folder validation and search
+    /// failures as errors instead of returning `Ok(null)`.
     #[test]
+    fn find_missing_waited_invalid_folder_returns_error_with_zero_mutations() {
+        let dir = std::env::temp_dir().join(format!("lc-ui-missing-err-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).unwrap();
+        let dummy = dir.join("dummy.png");
+        let img = lightcraft_raster::Rgba8::from_fn(8, 8, |_, _| [10, 20, 30, 255]);
+        let o = lightcraft_engine::export::ExportOptions { format: lightcraft_engine::export::ExportFormat::Png, ..Default::default() };
+        std::fs::write(&dummy, lightcraft_engine::export::encode_image(&img, &o).unwrap()).unwrap();
+
+        let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Default::default());
+        app.session.execute("library.import", &json!({"paths": [dummy.to_string_lossy()]})).unwrap();
+        // remove the file on disk so the photo is missing
+        std::fs::remove_file(&dummy).unwrap();
+        let id = app.session.catalog.photos().next().unwrap().id;
+        let original_path = match &app.session.catalog.photo(id).unwrap().source {
+            lightcraft_catalog::Source::File { path } => path.clone(),
+            _ => panic!("expected file source"),
+        };
+        let undo_count_before = app.session.undo.len();
+
+        // 1. Point at a nonexistent directory
+        let bad_dir = dir.join("nonexistent_subdir");
+        let err = run_item(&mut app, "file.findMissing", json!({"folder": bad_dir.to_string_lossy(), "wait": true})).unwrap_err();
+        assert!(err.contains("not a folder"), "expected 'not a folder' error, got: {err}");
+        assert!(app.ui.toast.as_ref().map(|t| t.0.contains("not a folder")).unwrap_or(false), "toast should reflect failure");
+        assert!(app.ui.last_find_missing.is_none(), "last_find_missing must remain None on failure");
+
+        // 2. Invalid timeout parameters are rejected cleanly
+        let err = run_item(&mut app, "file.findMissing", json!({"folder": dir.to_string_lossy(), "wait": true, "timeout": -1.0})).unwrap_err();
+        assert!(err.contains("non-negative number of seconds"), "{err}");
+        let err = run_item(&mut app, "file.findMissing", json!({"folder": dir.to_string_lossy(), "wait": true, "timeout": "invalid"})).unwrap_err();
+        assert!(err.contains("number of seconds"), "{err}");
+        let err = run_item(&mut app, "file.findMissing", json!({"folder": dir.to_string_lossy(), "wait": true, "timeout": 1e300})).unwrap_err();
+        assert!(err.contains("too large"), "{err}");
+
+        // Catalog and photo path must not have been mutated
+        assert_eq!(app.session.undo.len(), undo_count_before, "no undo steps added on failed search");
+        let path_after = match &app.session.catalog.photo(id).unwrap().source {
+            lightcraft_catalog::Source::File { path } => path.clone(),
+            _ => panic!("expected file source"),
+        };
+        assert_eq!(path_after, original_path, "missing photo path must not change on failure");
+
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    /// #1528: `file.findMissing` with `wait: true` returns structured results on success,
+    /// and background mode (`wait: false` or omitted) continues returning `{"background": true}`.
+    #[test]
+    fn find_missing_waited_structured_results_and_background_mode() {
+        let dir = std::env::temp_dir().join(format!("lc-ui-missing-bg-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(dir.join("orig")).unwrap();
+        std::fs::create_dir_all(dir.join("found")).unwrap();
+        let img = lightcraft_raster::Rgba8::from_fn(8, 8, |_, _| [10, 20, 30, 255]);
+        let o = lightcraft_engine::export::ExportOptions { format: lightcraft_engine::export::ExportFormat::Png, ..Default::default() };
+        std::fs::write(dir.join("orig/pic.png"), lightcraft_engine::export::encode_image(&img, &o).unwrap()).unwrap();
+
+        let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Default::default());
+        app.session.execute("library.import", &json!({"paths": [dir.join("orig").to_string_lossy()]})).unwrap();
+        std::fs::rename(dir.join("orig/pic.png"), dir.join("found/pic.png")).unwrap();
+
+        // Background mode (wait omitted): returns {"background": true} immediately
+        let bg = run_item(&mut app, "file.findMissing", json!({"folder": dir.join("found").to_string_lossy()})).unwrap();
+        assert_eq!(bg, json!({"background": true}));
+
+        // Wait for background task to complete
+        let ctx = app.tasks.repaint.clone().unwrap_or_default();
+        assert!(crate::tasks::wait(&mut app, &ctx, std::time::Duration::from_secs(5)));
+        let last = app.ui.last_find_missing.as_ref().unwrap();
+        assert_eq!(last["found"].as_array().map(Vec::len), Some(1));
+
+        // Now move again and run with wait: true -> returns structured results directly
+        std::fs::create_dir_all(dir.join("found2")).unwrap();
+        std::fs::rename(dir.join("found/pic.png"), dir.join("found2/pic.png")).unwrap();
+        let r = run_item(&mut app, "file.findMissing", json!({"folder": dir.join("found2").to_string_lossy(), "wait": true})).unwrap();
+        assert!(r.get("found").is_some(), "must have 'found' array: {r}");
+        assert!(r.get("missing").is_some(), "must have 'missing' count: {r}");
+        assert!(r.get("ambiguous").is_some(), "must have 'ambiguous' array: {r}");
+        assert_eq!(r["found"].as_array().map(Vec::len), Some(1));
+
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    /// #1528: `file.findMissing` with `wait: true` when timed out returns an actionable error
+    /// indicating timeout and that the search continues in the background, rather than returning `Ok(null)`.
+    #[test]
+    fn find_missing_waited_timeout_behavior() {
+        let dir = std::env::temp_dir().join(format!("lc-ui-missing-to-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).unwrap();
+
+        let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Default::default());
+        // Injected test-only worker delay so wait times out deterministically
+        app.ui.find_missing_test_delay = Some(std::time::Duration::from_millis(100));
+        let err = run_item(&mut app, "file.findMissing", json!({"folder": dir.to_string_lossy(), "wait": true, "timeout": 0.005})).unwrap_err();
+
+        assert!(err.contains("search timed out"), "error should mention timeout: {err}");
+        assert!(err.contains("search continues in the background"), "error should document continuation: {err}");
+
+        // The background task continues running in app.tasks
+        assert!(app.tasks.is_running("Find Missing Photos"), "task should continue running in background");
+
+        // Clean up the running task so no worker hangs
+        let ctx = app.tasks.repaint.clone().unwrap_or_default();
+        assert!(crate::tasks::wait_for(&mut app, &ctx, "Find Missing Photos", std::time::Duration::from_secs(5)));
+        assert!(!app.tasks.is_running("Find Missing Photos"));
+
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    /// #1528: `file.findMissing` with `wait: true` must return when its own search finishes,
+    /// even if unrelated background tasks remain active in `app.tasks`.
+    #[test]
+    fn find_missing_waited_completes_while_unrelated_task_remains_active() {
+        let dir = std::env::temp_dir().join(format!("lc-ui-missing-multi-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(dir.join("orig")).unwrap();
+        std::fs::create_dir_all(dir.join("found")).unwrap();
+        let img = lightcraft_raster::Rgba8::from_fn(8, 8, |_, _| [10, 20, 30, 255]);
+        let o = lightcraft_engine::export::ExportOptions { format: lightcraft_engine::export::ExportFormat::Png, ..Default::default() };
+        std::fs::write(dir.join("orig/pic.png"), lightcraft_engine::export::encode_image(&img, &o).unwrap()).unwrap();
+
+        let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Default::default());
+        app.session.execute("library.import", &json!({"paths": [dir.join("orig").to_string_lossy()]})).unwrap();
+        std::fs::rename(dir.join("orig/pic.png"), dir.join("found/pic.png")).unwrap();
+
+        // Spawn an unrelated background task that blocks on a channel until released
+        let (tx_unblock, rx_unblock) = std::sync::mpsc::channel::<()>();
+        crate::tasks::spawn(
+            &mut app,
+            "Unrelated Long Background Task",
+            None,
+            move || {
+                let _ = rx_unblock.recv();
+            },
+            |_app, _ctx, _| {},
+        )
+        .unwrap();
+        assert!(app.tasks.is_running("Unrelated Long Background Task"));
+
+        // Run file.findMissing with wait: true: must complete without waiting for the unrelated task
+        let r = run_item(&mut app, "file.findMissing", json!({"folder": dir.join("found").to_string_lossy(), "wait": true, "timeout": 5.0})).unwrap();
+        assert_eq!(r["found"].as_array().map(Vec::len), Some(1), "Find Missing must complete successfully: {r}");
+
+        // The unrelated task must STILL be active
+        assert!(app.tasks.is_running("Unrelated Long Background Task"), "unrelated task must still be running");
+
+        // Clean up the blocked background task
+        let _ = tx_unblock.send(());
+        let ctx = app.tasks.repaint.clone().unwrap_or_default();
+        assert!(crate::tasks::wait(&mut app, &ctx, std::time::Duration::from_secs(5)));
+        assert!(app.tasks.is_empty());
+
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+
     fn auto_tag_from_tracklog() {
         let dir = std::env::temp_dir().join(format!("lc-ui-tracklog-{}", std::process::id()));
         std::fs::create_dir_all(&dir).unwrap();
diff --git a/crates/ui-egui/src/menus.rs b/crates/ui-egui/src/menus.rs
index 141d6d782..4dec68ad3 100644
--- a/crates/ui-egui/src/menus.rs
+++ b/crates/ui-egui/src/menus.rs
@@ -1378,6 +1378,25 @@ pub fn run_ui_command(app: &mut LightcraftApp, id: &str, p: &Value) -> Option<Re
                 Some(Value::String(_)) => return Some(Err("file.findMissing: `within` must not be empty".into())),
                 Some(value) => return Some(Err(format!("file.findMissing: `within` must be a folder path, not {value}"))),
             };
+            let wait = p.get("wait").and_then(Value::as_bool).unwrap_or(false);
+            let timeout = match p.get("timeout") {
+                None | Some(Value::Null) => std::time::Duration::from_secs(600),
+                Some(Value::Number(n)) => {
+                    if let Some(s) = n.as_f64() {
+                        if s.is_finite() && s >= 0.0 {
+                            match std::time::Duration::try_from_secs_f64(s) {
+                                Ok(d) => d,
+                                Err(_) => return Some(Err("file.findMissing: `timeout` is too large".into())),
+                            }
+                        } else {
+                            return Some(Err("file.findMissing: `timeout` must be a non-negative number of seconds".into()));
+                        }
+                    } else {
+                        return Some(Err("file.findMissing: `timeout` must be a number of seconds".into()));
+                    }
+                }
+                Some(other) => return Some(Err(format!("file.findMissing: `timeout` must be a number of seconds, not {other}"))),
+            };
             let title = if within.is_some() { "Locate Missing Files…" } else { "Find Missing Photos…" };
             let folder = match p.get("folder").and_then(Value::as_str) {
                 Some(f) => Some(f.to_string()),
@@ -1399,7 +1418,15 @@ pub fn run_ui_command(app: &mut LightcraftApp, id: &str, p: &Value) -> Option<Re
             if let Some(within) = within {
                 candidates.retain(|candidate| lightcraft_catalog::query::folder_within(&candidate.path, &within));
             }
-            let work = move || lightcraft_engine::cmd::missing::plan_find_missing(&candidates, &folder);
+            #[cfg(test)]
+            let test_delay = app.ui.find_missing_test_delay;
+            let work = move || {
+                #[cfg(test)]
+                if let Some(d) = test_delay {
+                    std::thread::sleep(d);
+                }
+                lightcraft_engine::cmd::missing::plan_find_missing(&candidates, &folder)
+            };
             let done = |app: &mut LightcraftApp, ctx: &egui::Context, plan: Result<lightcraft_engine::cmd::missing::FindPlan, String>| {
                 // relinked under the session as it is now: photos relinked meanwhile and files
                 // now in use are skipped
@@ -1424,19 +1451,31 @@ pub fn run_ui_command(app: &mut LightcraftApp, id: &str, p: &Value) -> Option<Re
                                 unsure = unsure
                             ),
                         );
-                        app.ui.last_find_missing = Some(v);
+                        app.ui.last_find_missing = Some(v.clone());
+                        app.ui.last_find_missing_result = Some(Ok(v));
+                    }
+                    Err(e) => {
+                        app.toast(ctx, e.clone());
+                        app.ui.last_find_missing_result = Some(Err(e));
                     }
-                    Err(e) => app.toast(ctx, e),
                 }
             };
             app.ui.last_find_missing = None;
+            app.ui.last_find_missing_result = None;
             if let Err(e) = crate::tasks::spawn(app, LABEL, Some("findMissing"), work, done) {
                 return Some(Err(e));
             }
-            if p.get("wait").and_then(Value::as_bool).unwrap_or(false) {
+            if wait {
                 let ctx = app.tasks.repaint.clone().unwrap_or_default();
-                crate::tasks::wait(app, &ctx, std::time::Duration::from_secs(600));
-                return Some(Ok(app.ui.last_find_missing.clone().unwrap_or(Value::Null)));
+                let completed = crate::tasks::wait_for(app, &ctx, LABEL, timeout);
+                if !completed {
+                    return Some(Err("Find Missing Photos: search timed out (search continues in the background)".into()));
+                }
+                return match app.ui.last_find_missing_result.take() {
+                    Some(Ok(v)) => Some(Ok(v)),
+                    Some(Err(e)) => Some(Err(e)),
+                    None => Some(Err("Find Missing Photos: worker terminated unexpectedly".into())),
+                };
             }
             Ok(json!({"background": true}))
         }
diff --git a/crates/ui-egui/src/state.rs b/crates/ui-egui/src/state.rs
index 6c4dd3f6a..8e5021b5a 100644
--- a/crates/ui-egui/src/state.rs
+++ b/crates/ui-egui/src/state.rs
@@ -736,6 +736,13 @@ pub struct UiState {
     /// The result of the last Find Missing Photos (it searches in the background).
     #[serde(skip)]
     pub last_find_missing: Option<serde_json::Value>,
+    /// The outcome (success or error) of the last Find Missing Photos task.
+    #[serde(skip)]
+    pub last_find_missing_result: Option<Result<serde_json::Value, String>>,
+    /// Injected delay for Find Missing Photos worker (test-only, for timeout testing).
+    #[cfg(test)]
+    #[serde(skip)]
+    pub find_missing_test_delay: Option<std::time::Duration>,
     #[serde(skip)]
     pub status: String,
     #[serde(skip)]
@@ -1140,6 +1147,9 @@ impl Default for UiState {
             reference: None,
             toast: None,
             last_find_missing: None,
+            last_find_missing_result: None,
+            #[cfg(test)]
+            find_missing_test_delay: None,
             status: String::new(),
             dialog: None,
         }
diff --git a/crates/ui-egui/src/tasks.rs b/crates/ui-egui/src/tasks.rs
index fb5a1f031..70abd629a 100644
--- a/crates/ui-egui/src/tasks.rs
+++ b/crates/ui-egui/src/tasks.rs
@@ -116,3 +116,28 @@ pub fn wait(app: &mut LightcraftApp, ctx: &egui::Context, timeout: std::time::Du
         app.tasks.is_empty()
     }
 }
+
+/// Wait for a specific task by label and apply finished tasks. Returns `true` once the labeled
+/// task finishes (or is removed), or `false` on timeout. Other background tasks may continue running.
+pub fn wait_for(app: &mut LightcraftApp, ctx: &egui::Context, label: &str, timeout: std::time::Duration) -> bool {
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        let t0 = std::time::Instant::now();
+        loop {
+            poll(app, ctx);
+            if !app.tasks.is_running(label) {
+                return true;
+            }
+            if t0.elapsed() > timeout {
+                return false;
+            }
+            std::thread::sleep(std::time::Duration::from_millis(2));
+        }
+    }
+    #[cfg(target_arch = "wasm32")]
+    {
+        let _ = timeout;
+        poll(app, ctx);
+        !app.tasks.is_running(label)
+    }
+}