diff --git a/src/dev/entrypoint.rs b/src/dev/entrypoint.rs
index 5266f9c..ba5e35a 100644
--- a/src/dev/entrypoint.rs
+++ b/src/dev/entrypoint.rs
@@ -14,27 +14,66 @@ struct EntrypointParamsResponse {
     entrypoint_params: Value,
 }
 
-pub fn locate_html_entrypoint(upload_dir: &Path) -> Option<PathBuf> {
+pub fn locate_html_entrypoint(upload_dir: &Path) -> Result<Option<PathBuf>> {
+    // `index.html` always wins if present — that's the Web convention and
+    // sidesteps ambiguity when game exporters also emit helper HTML (debug
+    // shells, fullscreen wrappers, etc.).
     let default_index = upload_dir.join("index.html");
     if default_index.is_file() {
-        return Some(default_index);
+        return Ok(Some(default_index));
     }
 
+    // Only look at the upload_dir's top-level — a Godot/Unity/etc. export
+    // puts its entrypoint at the root of the upload folder, and recursing
+    // would flag engine-emitted subdir HTML (e.g. Unity's `TemplateData/`)
+    // as an ambiguity.
+    let mut found: Vec<PathBuf> = Vec::new();
     for entry in WalkDir::new(upload_dir)
         .min_depth(1)
+        .max_depth(1)
         .into_iter()
         .filter_map(|e| e.ok())
     {
-        if entry.file_type().is_file() {
-            if let Some(ext) = entry.path().extension() {
-                if ext.eq_ignore_ascii_case("html") {
-                    return Some(entry.into_path());
-                }
+        if !entry.file_type().is_file() {
+            continue;
+        }
+        let is_html = entry
+            .path()
+            .extension()
+            .is_some_and(|ext| ext.eq_ignore_ascii_case("html") || ext.eq_ignore_ascii_case("htm"));
+        if is_html {
+            found.push(entry.into_path());
+            // Two is enough to know it's ambiguous — but keep collecting so
+            // the error message lists every offender (capped at a handful).
+            if found.len() >= 8 {
+                break;
             }
         }
     }
 
-    None
+    match found.len() {
+        0 => Ok(None),
+        1 => Ok(Some(found.remove(0))),
+        _ => {
+            let mut names: Vec<String> = found
+                .iter()
+                .map(|p| {
+                    p.strip_prefix(upload_dir)
+                        .unwrap_or(p)
+                        .display()
+                        .to_string()
+                })
+                .collect();
+            names.sort();
+            anyhow::bail!(
+                "Multiple HTML files found in upload_dir ({}):\n  - {}\n\n\
+                 Name one of them `index.html`, or remove the extras (often \
+                 stale files from a previous export with a different name).",
+                upload_dir.display(),
+                names.join("\n  - ")
+            )
+        }
+    }
 }
 
 pub async fn fetch_entrypoint_params(engine: &str, engine_version: &str, html_path: &Path) -> Result<Value> {
diff --git a/src/dev/mod.rs b/src/dev/mod.rs
index 47448d8..06ea38e 100644
--- a/src/dev/mod.rs
+++ b/src/dev/mod.rs
@@ -122,7 +122,7 @@ pub async fn handle_dev(config_path: Option<PathBuf>, verbose: bool, no_open: bo
     // Validate required files exist in upload directory
     FileStaging::prepare(&upload_dir, &wavedash_config)?;
 
-    let html_entrypoint = locate_html_entrypoint(&upload_dir);
+    let html_entrypoint = locate_html_entrypoint(&upload_dir)?;
     let engine_version = wavedash_config.engine_version();
     let entrypoint_params = match engine_kind {
         Some(EngineKind::Godot | EngineKind::Unity) => {