diff --git a/src/builds.rs b/src/builds.rs
index 9ba86ca..c109d20 100644
--- a/src/builds.rs
+++ b/src/builds.rs
@@ -52,6 +52,7 @@ async fn get_temp_credentials(
     engine: &str,
     engine_version: &str,
     entrypoint: Option<&str>,
+    entrypoint_params: Option<serde_json::Value>,
     message: Option<&str>,
     api_key: &str,
 ) -> Result<TempCredsResponse> {
@@ -68,11 +69,16 @@ async fn get_temp_credentials(
         "engineVersion": engine_version
     });
 
-    // Add entrypoint if provided
+    // Add entrypoint if provided (for custom engine)
     if let Some(ep) = entrypoint {
         request_body["entrypoint"] = serde_json::json!(ep);
     }
 
+    // Add entrypointParams if provided (for JSDOS/Ruffle)
+    if let Some(ep_params) = entrypoint_params {
+        request_body["entrypointParams"] = ep_params;
+    }
+
     // Add build message if provided
     if let Some(msg) = message {
         request_body["buildMessage"] = serde_json::json!(msg);
@@ -176,6 +182,7 @@ pub async fn handle_build_push(config_path: PathBuf, verbose: bool, message: Opt
         engine_kind.as_config_key(),
         wavedash_config.engine_version()?,
         wavedash_config.entrypoint(),
+        wavedash_config.executable_entrypoint_params(),
         message.as_deref(),
         &api_key,
     )
diff --git a/src/config.rs b/src/config.rs
index 93d1c66..3d11f64 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -105,13 +105,15 @@ pub struct CustomSection {
 #[derive(Debug, Deserialize)]
 pub struct JsDosSection {
     pub version: String,
-    pub entrypoint: String,
+    pub executable: String,
+    pub loader_url: Option<String>,
 }
 
 #[derive(Debug, Deserialize)]
 pub struct RuffleSection {
     pub version: String,
-    pub entrypoint: String,
+    pub executable: String,
+    pub loader_url: Option<String>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -226,7 +228,41 @@ impl WavedashConfig {
         self.custom
             .as_ref()
             .map(|c| c.entrypoint.as_str())
-            .or_else(|| self.jsdos.as_ref().map(|j| j.entrypoint.as_str()))
-            .or_else(|| self.ruffle.as_ref().map(|r| r.entrypoint.as_str()))
+    }
+
+    /// For JSDOS/Ruffle engines, returns the entrypointParams (executable + optional loader_url)
+    pub fn executable_entrypoint_params(&self) -> Option<serde_json::Value> {
+        if let Some(ref jsdos) = self.jsdos {
+            let mut params = serde_json::json!({ "executable": jsdos.executable });
+            if let Some(ref loader_url) = jsdos.loader_url {
+                params["loaderUrl"] = serde_json::json!(loader_url);
+            }
+            Some(params)
+        } else if let Some(ref ruffle) = self.ruffle {
+            let mut params = serde_json::json!({ "executable": ruffle.executable });
+            if let Some(ref loader_url) = ruffle.loader_url {
+                params["loaderUrl"] = serde_json::json!(loader_url);
+            }
+            Some(params)
+        } else {
+            None
+        }
+    }
+
+    /// For JSDOS/Ruffle engines, returns all files that must exist in upload_dir
+    pub fn executable_files_to_validate(&self) -> Vec<&str> {
+        let mut files = Vec::new();
+        if let Some(ref jsdos) = self.jsdos {
+            files.push(jsdos.executable.as_str());
+            if let Some(ref loader_url) = jsdos.loader_url {
+                files.push(loader_url.as_str());
+            }
+        } else if let Some(ref ruffle) = self.ruffle {
+            files.push(ruffle.executable.as_str());
+            if let Some(ref loader_url) = ruffle.loader_url {
+                files.push(loader_url.as_str());
+            }
+        }
+        files
     }
 }
diff --git a/src/file_staging.rs b/src/file_staging.rs
index 3dab468..e79d897 100644
--- a/src/file_staging.rs
+++ b/src/file_staging.rs
@@ -32,7 +32,7 @@ impl FileStaging {
             }
         };
 
-        // Validate entrypoint exists in the upload directory
+        // Validate entrypoint exists in the upload directory (for custom engine)
         if let Some(entrypoint_str) = wavedash_config.entrypoint() {
             let entrypoint_path = upload_dir.join(entrypoint_str);
             if !entrypoint_path.exists() {
@@ -44,6 +44,18 @@ impl FileStaging {
             }
         }
 
+        // Validate executable and loader_url files exist (for JSDOS/Ruffle)
+        for file in wavedash_config.executable_files_to_validate() {
+            let file_path = upload_dir.join(file);
+            if !file_path.exists() {
+                anyhow::bail!(
+                    "'{}' not found in upload_dir ({}). The file must exist inside your upload_dir.",
+                    file,
+                    upload_dir.display()
+                );
+            }
+        }
+
         Ok(Self {
             config_dest,
         })