diff --git a/src/clip_search.rs b/src/clip_search.rs
index 5e8d5b9..d91e490 100644
--- a/src/clip_search.rs
+++ b/src/clip_search.rs
@@ -48,10 +48,17 @@ pub struct SearchQuery {
     /// CLIP; tunable per call when sweeping. Defaults to 0.20.
     #[serde(default = "default_threshold")]
     pub threshold: f32,
-    /// Optional single-library scope. When omitted, every enabled
-    /// library is searched. Multi-select isn't supported yet — the
-    /// frontend wires through one or all.
+    /// Optional single-library scope. Legacy param — new clients pass
+    /// `library_ids` instead so multi-select scopes (Apollo's HUD library
+    /// chips, FileViewer-React's library picker) actually filter. Kept
+    /// for back-compat; `library_ids` wins when both are supplied.
     pub library: Option<i32>,
+    /// Optional multi-library scope, comma-separated id list
+    /// (`?library_ids=1,3`). Empty / omitted = every enabled library
+    /// (the historical default). Apollo and FileViewer-React both send
+    /// this when 2+ libraries are selected; the single-library case
+    /// works through either param interchangeably.
+    pub library_ids: Option<String>,
     /// Optional model-version filter. Defaults to the live engine's
     /// version (queried lazily). Forces a strict join so mid-flight
     /// model swaps can't mix geometries in a single response.
@@ -172,10 +179,34 @@ pub async fn search_photos(
         }
     };
 
-    // 2. Decide which library scope to search.
-    let library_ids: Vec<i32> = match query.library {
-        Some(id) => vec![id],
-        None => Vec::new(), // empty = all libraries
+    // 2. Decide which library scope to search. `library_ids` (multi)
+    // wins over the legacy `library` (single) when both are present;
+    // either / both empty falls back to "every enabled library".
+    let library_ids: Vec<i32> = if let Some(raw) = query.library_ids.as_deref() {
+        let mut out: Vec<i32> = Vec::new();
+        for piece in raw.split(',') {
+            let trimmed = piece.trim();
+            if trimmed.is_empty() {
+                continue;
+            }
+            match trimmed.parse::<i32>() {
+                Ok(id) => {
+                    if !out.contains(&id) {
+                        out.push(id);
+                    }
+                }
+                Err(_) => {
+                    return Ok(HttpResponse::BadRequest().json(SearchError {
+                        error: format!("invalid library_ids entry: {trimmed:?}"),
+                    }));
+                }
+            }
+        }
+        out
+    } else if let Some(id) = query.library {
+        vec![id]
+    } else {
+        Vec::new()
     };
 
     // 3. Pull the (hash, embedding) matrix. Lock contention here is