diff --git a/mrpci-core/src/bin/mrpci-headless/main.rs b/mrpci-core/src/bin/mrpci-headless/main.rs
index b54db7a..0b7df36 100644
--- a/mrpci-core/src/bin/mrpci-headless/main.rs
+++ b/mrpci-core/src/bin/mrpci-headless/main.rs
@@ -77,7 +77,17 @@ fn main() {
 
     if let Some(path) = get("--export-bundle") {
         let bundle = gs.world.to_bundle();
-        let j = serde_json::to_string(&bundle).expect("bundle serializes");
+        // Through Value first, so the output is byte-stable.
+        //
+        // A bundle is full of HashMaps — rooms, scripts, sprites, every
+        // prop's msgs — and Rust randomizes their iteration per process, so
+        // exporting the same unchanged game twice produced two different
+        // files. These are committed build outputs; a diff should mean the
+        // game changed. serde_json's Value is BTreeMap-backed by default, so
+        // this one hop sorts every key at every depth.
+        let j = serde_json::to_value(&bundle)
+            .and_then(|v| serde_json::to_string(&v))
+            .expect("bundle serializes");
         std::fs::write(&path, &j).expect("write bundle");
         println!("bundle exported: {} ({} rooms, {} scripts, {} sprites, {} bytes)",
             path, bundle.rooms.len(), bundle.scripts.len(), bundle.sprites.len(), j.len());