diff --git a/src/concurrent_engine.rs b/src/concurrent_engine.rs
index 25e4999..c235bd1 100644
--- a/src/concurrent_engine.rs
+++ b/src/concurrent_engine.rs
@@ -6870,6 +6870,57 @@ impl ConcurrentEngine {
     pub fn unregister_prefilter(&self, name: &str) -> bool {
         self.prefilters.remove(name)
     }
+
+    /// Spawn the prefilter stale-while-revalidate refresh thread.
+    ///
+    /// Ticks every `tick_secs` and, for each registered prefilter past its
+    /// `refresh_interval_secs`, recomputes the bitmap and atomically swaps it
+    /// via ArcSwap. In-flight queries holding the old Arc continue reading
+    /// the old bitmap; the next query after publish sees the new one.
+    ///
+    /// Holds a `Weak<ConcurrentEngine>` so engine drop breaks the cycle.
+    /// Respects `self.shutdown` for cooperative exit.
+    ///
+    /// Call this *after* constructing the engine Arc (e.g. from the server
+    /// during the post-listen phase).
+    pub fn start_prefilter_refresh_thread(
+        self: &Arc<Self>,
+        tick_secs: u64,
+    ) -> std::thread::JoinHandle<()> {
+        let weak = Arc::downgrade(self);
+        let shutdown = Arc::clone(&self.shutdown);
+        let tick = std::time::Duration::from_secs(tick_secs.max(1));
+        std::thread::Builder::new()
+            .name("bitdex-prefilter-swr".into())
+            .spawn(move || {
+                while !shutdown.load(Ordering::Relaxed) {
+                    let engine = match weak.upgrade() {
+                        Some(e) => e,
+                        None => return, // engine dropped
+                    };
+                    let now_unix = std::time::SystemTime::now()
+                        .duration_since(std::time::UNIX_EPOCH)
+                        .map(|d| d.as_secs() as i64)
+                        .unwrap_or(0);
+                    for entry in engine.prefilters.entries() {
+                        if shutdown.load(Ordering::Relaxed) {
+                            return;
+                        }
+                        if entry.is_stale(now_unix) {
+                            if let Err(e) = engine.refresh_prefilter(&entry.name) {
+                                tracing::warn!(
+                                    "prefilter SWR refresh failed for '{}': {}",
+                                    entry.name, e
+                                );
+                            }
+                        }
+                    }
+                    drop(engine);
+                    std::thread::sleep(tick);
+                }
+            })
+            .expect("Failed to spawn prefilter SWR thread")
+    }
     /// Purge the entire BoundStore: disk first, then memory.
     /// Order matters: wipe disk before clearing RAM to prevent stale shard loads.
     /// Safe to call while the server is running — the merge thread will simply
diff --git a/src/server.rs b/src/server.rs
index a889c48..6d4c7ab 100644
--- a/src/server.rs
+++ b/src/server.rs
@@ -1498,6 +1498,12 @@ impl BitdexServer {
                 state.metrics.boot_phase_seconds
                     .with_label_values(&["bound_cache"])
                     .set(phase6_elapsed.as_secs() as i64);
+
+                // Phase 7: prefilter stale-while-revalidate refresh thread.
+                // Ticks every 10s; refreshes entries past their interval via
+                // ArcSwap so in-flight queries never block on compute.
+                let _handle = engine.start_prefilter_refresh_thread(10);
+                eprintln!("  Boot phase: prefilter SWR thread spawned");
             }
         }
 
diff --git a/tests/prefilter_integration.rs b/tests/prefilter_integration.rs
index 28baaaf..415069b 100644
--- a/tests/prefilter_integration.rs
+++ b/tests/prefilter_integration.rs
@@ -224,6 +224,66 @@ fn deleted_docs_excluded_from_stale_prefilter() {
     assert_eq!(ids, vec![1, 2, 4, 6, 8, 9, 10], "deleted slots 3/5/7 must not leak through");
 }
 
+#[test]
+fn swr_thread_refreshes_stale_prefilters() {
+    // Spawn the SWR thread with a 1s tick and a prefilter whose refresh
+    // interval clamps to the 30s minimum — that's too long for a fast
+    // test, so drive staleness manually by rewinding last_refreshed.
+    //
+    // Flow:
+    //   1. Register prefilter covering 10 initial docs.
+    //   2. Insert 5 more matching docs (cached cardinality stays at 10).
+    //   3. Rewind last_refreshed so is_stale() returns true.
+    //   4. SWR thread picks it up on its next tick → cardinality flips to 15.
+    use std::sync::Arc;
+    use std::sync::atomic::Ordering;
+    use std::time::Duration;
+
+    let dir = tempfile::tempdir().unwrap();
+    let engine = Arc::new(ConcurrentEngine::new_with_path(
+        test_config(&dir.path().join("bitmaps")),
+        &dir.path().join("docs"),
+    ).unwrap());
+
+    for i in 1..=10u32 {
+        engine.put(i, &doc(1, true, 100, i as i64)).unwrap();
+    }
+    wait_for_alive(&engine, 10, 2000);
+
+    let entry = engine.register_prefilter(
+        "nsfw1".into(),
+        vec![FilterClause::Eq("nsfwLevel".into(), Value::Integer(1))],
+        30, // clamped to MIN_REFRESH_INTERVAL_SECS
+    ).unwrap();
+    assert_eq!(entry.cardinality(), 10);
+
+    // Boot SWR with a 1-second tick.
+    let _handle = engine.start_prefilter_refresh_thread(1);
+
+    // Add 5 more matching docs.
+    for i in 11..=15u32 {
+        engine.put(i, &doc(1, true, 100, i as i64)).unwrap();
+    }
+    wait_for_alive(&engine, 15, 2000);
+
+    // Cardinality still reflects the pre-refresh snapshot.
+    assert_eq!(entry.cardinality(), 10, "still stale before rewind");
+
+    // Force the entry past its refresh interval: rewind last_refreshed to
+    // the epoch. SWR thread should see this on its next tick.
+    entry.last_refreshed.store(0, Ordering::Relaxed);
+
+    // Wait up to 5s for the SWR thread to pick it up and refresh.
+    let deadline = std::time::Instant::now() + Duration::from_secs(5);
+    while std::time::Instant::now() < deadline {
+        if entry.cardinality() == 15 {
+            break;
+        }
+        std::thread::sleep(Duration::from_millis(50));
+    }
+    assert_eq!(entry.cardinality(), 15, "SWR thread did not refresh within 5s");
+}
+
 #[test]
 fn refresh_updates_cardinality() {
     let dir = tempfile::tempdir().unwrap();