diff --git a/crates/cord-gui/src/app.rs b/crates/cord-gui/src/app.rs
index 8e98086..a554860 100644
--- a/crates/cord-gui/src/app.rs
+++ b/crates/cord-gui/src/app.rs
@@ -6,7 +6,7 @@ use iced::widget::{
 use iced::{Background, Border, Color, Element, Fill, Length, Padding, Shadow, Subscription};
 use iced::{mouse, keyboard, window, Point, Vector};
 use std::path::PathBuf;
-use std::sync::{Arc, Mutex, OnceLock};
+use std::sync::{Arc, Mutex, OnceLock, mpsc};
 use std::time::Duration;
 
 static OPEN_QUEUE: OnceLock<Mutex<Vec<PathBuf>>> = OnceLock::new();
@@ -45,6 +45,7 @@ pub struct App {
     cursor_line: usize,
     line_eval_text: Option<String>,
     mesh_path: Option<PathBuf>,
+    mesh_import_rx: Option<mpsc::Receiver<Result<(String, PathBuf), String>>>,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -87,6 +88,7 @@ pub enum Message {
     RenderAll,
     DecomposeMesh,
     ConvertToCordial,
+    MeshImportComplete(Result<(String, PathBuf), String>),
     Tick,
 }
 
@@ -154,6 +156,7 @@ impl App {
             cursor_line: 0,
             line_eval_text: None,
             mesh_path: None,
+            mesh_import_rx: None,
         };
         app.reparse();
 
@@ -335,6 +338,26 @@ impl App {
             }
             Message::DecomposeMesh => self.decompose_mesh(),
             Message::ConvertToCordial => self.convert_to_cordial(),
+            Message::MeshImportComplete(result) => {
+                match result {
+                    Ok((source, path)) => {
+                        self.source = text_editor::Content::with_text(&source);
+                        self.undo_stack = vec![source];
+                        self.redo_stack.clear();
+                        self.mesh_path = Some(path.clone());
+                        self.current_path = Some(path.clone());
+                        self.dirty = false;
+                        self.detect_mode();
+                        self.reparse();
+                        self.update_markdown();
+                        self.push_recent(&path);
+                        self.status = Some(format!("opened: {}", path.display()));
+                    }
+                    Err(e) => {
+                        self.status = Some(format!("mesh import error: {e}"));
+                    }
+                }
+            }
             Message::Tick => {
                 if !self.menu_ready {
                     setup_native_menu();
@@ -347,6 +370,13 @@ impl App {
                         self.open_path(&path);
                     }
                 }
+                if let Some(rx) = &self.mesh_import_rx {
+                    if let Ok(result) = rx.try_recv() {
+                        self.mesh_import_rx = None;
+                        self.update(Message::MeshImportComplete(result));
+                        return;
+                    }
+                }
                 let new_line = self.source.cursor().position.line;
                 if new_line != self.cursor_line {
                     self.cursor_line = new_line;
@@ -696,8 +726,18 @@ impl App {
             .unwrap_or("")
             .to_lowercase();
 
-        let is_mesh = matches!(ext.as_str(), "obj" | "stl" | "3mf");
-        let mut fallback_note: Option<&str> = None;
+        if matches!(ext.as_str(), "obj" | "stl" | "3mf") {
+            self.status = Some(format!("decomposing mesh: {}…", path.display()));
+            let (tx, rx) = mpsc::channel();
+            let path_owned = path.to_path_buf();
+            std::thread::spawn(move || {
+                let result = import_mesh(&path_owned)
+                    .map(|source| (source, path_owned));
+                let _ = tx.send(result);
+            });
+            self.mesh_import_rx = Some(rx);
+            return;
+        }
 
         let result = match ext.as_str() {
             "zcd" => load_zcd(path),
@@ -705,17 +745,6 @@ impl App {
             "cord" | "bin" => {
                 Err("binary format (.cord) — no source to edit".into())
             }
-            "obj" | "stl" | "3mf" => {
-                match import_mesh(path) {
-                    Ok(imp) => {
-                        if imp.is_fallback {
-                            fallback_note = Some(" (bounding box — full decompose failed)");
-                        }
-                        Ok(imp.source)
-                    }
-                    Err(e) => Err(e),
-                }
-            }
             "step" | "stp" => {
                 Err(format!("import for .{ext} not yet implemented"))
             }
@@ -728,14 +757,13 @@ impl App {
                 self.undo_stack = vec![source];
                 self.redo_stack.clear();
                 self.current_path = Some(path.to_path_buf());
-                self.mesh_path = if is_mesh { Some(path.to_path_buf()) } else { None };
+                self.mesh_path = None;
                 self.dirty = false;
                 self.detect_mode();
                 self.reparse();
                 self.update_markdown();
                 self.push_recent(path);
-                let note = fallback_note.unwrap_or("");
-                self.status = Some(format!("opened: {}{note}", path.display()));
+                self.status = Some(format!("opened: {}", path.display()));
             }
             Err(e) => {
                 self.status = Some(format!("error: {e}"));
@@ -1766,26 +1794,15 @@ fn load_zcd(path: &std::path::Path) -> Result<String, String> {
     Err("no readable layers in .zcd".into())
 }
 
-struct MeshImport {
-    source: String,
-    is_fallback: bool,
-}
-
-fn import_mesh(path: &std::path::Path) -> Result<MeshImport, String> {
+fn import_mesh(path: &std::path::Path) -> Result<String, String> {
     use cord_decompile::mesh::TriangleMesh;
     use cord_decompile::{decompile, DecompileConfig};
 
     let mesh = TriangleMesh::load(path).map_err(|e| e.to_string())?;
     let config = DecompileConfig::default();
     match decompile(&mesh, &config) {
-        Ok(result) => Ok(MeshImport {
-            source: cord_sdf::sdf_to_cordial(&result.sdf),
-            is_fallback: false,
-        }),
-        Err(_) => Ok(MeshImport {
-            source: mesh_bounding_source(&mesh),
-            is_fallback: true,
-        }),
+        Ok(result) => Ok(cord_sdf::sdf_to_cordial(&result.sdf)),
+        Err(_) => Ok(mesh_bounding_source(&mesh)),
     }
 }