diff --git a/app/src-tauri/build.rs b/app/src-tauri/build.rs
index d860e1e..76e7375 100644
--- a/app/src-tauri/build.rs
+++ b/app/src-tauri/build.rs
@@ -1,3 +1,49 @@
+use std::{env, fs, path::PathBuf};
+
 fn main() {
-    tauri_build::build()
+    tauri_build::build();
+    copy_sidecar_deps();
+}
+
+/// Tauri's `externalBin` copies only the sidecar **.exe** to the Cargo target
+/// directory.  The .NET apphost (`sim-bridge.exe`) also needs its managed
+/// assembly (`sim-bridge.dll`), runtime config, deps manifest, and all
+/// framework / third-party DLLs next to it.
+///
+/// This function copies every non-exe file from `src-tauri/binaries/` into the
+/// same target directory so the sidecar can boot at `cargo tauri dev` time.
+fn copy_sidecar_deps() {
+    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
+    let binaries_dir = manifest_dir.join("binaries");
+
+    if !binaries_dir.exists() {
+        return;
+    }
+
+    // OUT_DIR is e.g. target/debug/build/<crate-hash>/out — go up 3 levels to
+    // reach the profile directory (target/debug/ or target/release/).
+    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
+    let target_dir = out_dir
+        .ancestors()
+        .nth(3)
+        .expect("could not derive target dir from OUT_DIR");
+
+    println!("cargo:rerun-if-changed=binaries");
+
+    let Ok(entries) = fs::read_dir(&binaries_dir) else {
+        return;
+    };
+
+    for entry in entries.flatten() {
+        let path = entry.path();
+        if !path.is_file() {
+            continue;
+        }
+        // Skip .exe files — Tauri already handles copying the sidecar executable.
+        if path.extension().and_then(|e| e.to_str()) == Some("exe") {
+            continue;
+        }
+        let dest = target_dir.join(path.file_name().unwrap());
+        let _ = fs::copy(&path, &dest);
+    }
 }