diff --git a/crates/ide-host/src/project.rs b/crates/ide-host/src/project.rs
index 1fb6617a0..e63e8db8b 100644
--- a/crates/ide-host/src/project.rs
+++ b/crates/ide-host/src/project.rs
@@ -305,6 +305,7 @@ impl Host {
                     index.update(&changed, &batch.removed);
                     kotlin_bridge.files_changed(changed.iter().chain(&batch.removed));
                     lsp.project_files_changed(batch.created.iter().chain(&batch.removed));
+                    lsp.watched_files_changed(&batch.created, &batch.modified, &batch.removed);
                     // Atomic saves by external tools surface as `created`.
                     documents.on_disk_changed(&changed);
                     // A framework's root marker (`angular.json`) created or deleted: its
diff --git a/crates/ide-lsp/src/lib.rs b/crates/ide-lsp/src/lib.rs
index 97a94fc2d..252b66bf3 100644
--- a/crates/ide-lsp/src/lib.rs
+++ b/crates/ide-lsp/src/lib.rs
@@ -1973,6 +1973,29 @@ impl LspManager {
         }
     }
 
+    /// Files that changed on disk, outside the editor too (a checkout, another tool): each running
+    /// server hears of those of its languages under its root through `workspace/didChangeWatchedFiles`.
+    /// Without it jdtls's incremental build never sees them, so a launch runs stale classes.
+    pub fn watched_files_changed(&self, created: &[PathBuf], modified: &[PathBuf], removed: &[PathBuf]) {
+        let servers: Vec<Arc<Server>> = self.servers.lock().clone();
+        for s in servers {
+            let Some(run) = s.running() else { continue };
+            let languages = s.languages();
+            let changes: Vec<Value> = [(created, 1), (modified, 2), (removed, 3)]
+                .into_iter()
+                .flat_map(|(paths, kind)| paths.iter().map(move |p| (p, kind)))
+                .filter(|(p, _)| p.starts_with(&s.root) && languages.contains(&self.languages.detect(p, None)))
+                .map(|(p, kind)| json!({ "uri": uri::path_to_uri(p), "type": kind }))
+                .collect();
+            if changes.is_empty() {
+                continue;
+            }
+            if let Err(e) = run.peer.notify("workspace/didChangeWatchedFiles", json!({ "changes": changes })) {
+                tracing::debug!("{}: didChangeWatchedFiles: {e}", s.name);
+            }
+        }
+    }
+
     /// Checks initialized capabilities, including dynamic registrations, before choosing a formatter.
     pub async fn can_format(&self, doc: DocId) -> IdeResult<bool> {
         let snap = self.snapshot(doc)?;
diff --git a/crates/ide-lsp/tests/mock_server.rs b/crates/ide-lsp/tests/mock_server.rs
index c9889ae7c..93d9d3708 100644
--- a/crates/ide-lsp/tests/mock_server.rs
+++ b/crates/ide-lsp/tests/mock_server.rs
@@ -405,6 +405,37 @@ async fn linked_projects_initialize_separate_language_server_roots() {
     h.mgr.shutdown_all().await;
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn files_changed_on_disk_reach_the_servers_of_their_language() {
+    let h = harness(&[("mock", incremental_caps())]);
+    h.open("a.mk", "x");
+    h.wait_status("mock", ServerStatus::Ready).await;
+    let s = h.launcher.state("mock");
+    let (created, modified, removed) = (h.root.join("new.mk"), h.root.join("a.mk"), h.root.join("old.mk"));
+    // Other languages and files outside the server's root are left out.
+    let other = h.root.join("notes.txt");
+    let outside = tempfile::tempdir().unwrap().path().join("far.mk");
+    h.mgr.watched_files_changed(
+        &[created.clone(), other],
+        &[modified.clone(), outside],
+        std::slice::from_ref(&removed),
+    );
+    eventually(|| !s.notified("workspace/didChangeWatchedFiles").is_empty()).await;
+    assert_eq!(
+        s.notified("workspace/didChangeWatchedFiles")[0]["changes"],
+        json!([
+            { "uri": uri_of(&created), "type": 1 },
+            { "uri": uri_of(&modified), "type": 2 },
+            { "uri": uri_of(&removed), "type": 3 },
+        ])
+    );
+    // Nothing of its languages: no notification.
+    h.mgr.watched_files_changed(&[h.root.join("b.txt")], &[], &[]);
+    tokio::time::sleep(std::time::Duration::from_millis(100)).await;
+    assert_eq!(s.notified("workspace/didChangeWatchedFiles").len(), 1);
+    h.mgr.shutdown_all().await;
+}
+
 #[tokio::test(flavor = "multi_thread")]
 async fn initialize_and_incremental_sync() {
     let h = harness(&[("mock", incremental_caps())]);