[model-gateway] bugfix: backward compatibility for GET endpoints (#15413)

sgl-model-gateway/src/core/steps/worker/local/discover_metadata.rs

@@ -58,6 +58,48 @@ fn strip_protocol(url: &str) -> String {
         .to_string()
 }
 
+/// Fallback function to GET JSON from old endpoint (with "get_" prefix) for backward compatibility.
+async fn get_json_fallback(
+    base_url: &str,
+    endpoint: &str,
+    api_key: Option<&str>,
+) -> Result<Value, String> {
+    // FIXME: This fallback logic should be removed together with /get_server_info
+    // and /get_model_info endpoints in http_server.py
+    warn!(
+        concat!(
+            "Endpoint '/{}' returned 404, falling back to '/get_{}' for backward compatibility. ",
+            "The '/get_{}' endpoint is deprecated and will be removed in a future version. ",
+            "Please use '/{}' instead."
+        ),
+        endpoint, endpoint, endpoint, endpoint
+    );
+
+    let old_url = format!("{}/get_{}", base_url, endpoint);
+    let mut req = HTTP_CLIENT.get(&old_url);
+    if let Some(key) = api_key {
+        req = req.bearer_auth(key);
+    }
+
+    let response = req
+        .send()
+        .await
+        .map_err(|e| format!("Failed to connect to {}: {}", old_url, e))?;
+
+    if !response.status().is_success() {
+        return Err(format!(
+            "Server returned status {} from {}",
+            response.status(),
+            old_url
+        ));
+    }
+
+    response
+        .json::<Value>()
+        .await
+        .map_err(|e| format!("Failed to parse response from {}: {}", old_url, e))
+}
+
 /// Get server info from /server_info endpoint.
 pub async fn get_server_info(url: &str, api_key: Option<&str>) -> Result<ServerInfo, String> {
     let base_url = url.trim_end_matches('/');
@@ -73,6 +115,13 @@ pub async fn get_server_info(url: &str, api_key: Option<&str>) -> Result<ServerI
         .await
         .map_err(|e| format!("Failed to connect to {}: {}", server_info_url, e))?;
 
+    // If /server_info returns 404, fallback to /get_server_info for backward compatibility
+    if response.status() == reqwest::StatusCode::NOT_FOUND {
+        let json = get_json_fallback(base_url, "server_info", api_key).await?;
+        return serde_json::from_value(json)
+            .map_err(|e| format!("Failed to parse server info: {}", e));
+    }
+
     if !response.status().is_success() {
         return Err(format!(
             "Server returned status {} from {}",
@@ -104,6 +153,13 @@ pub async fn get_model_info(url: &str, api_key: Option<&str>) -> Result<ModelInf
         .await
         .map_err(|e| format!("Failed to connect to {}: {}", model_info_url, e))?;
 
+    // If /model_info returns 404, fallback to /get_model_info for backward compatibility
+    if response.status() == reqwest::StatusCode::NOT_FOUND {
+        let json = get_json_fallback(base_url, "model_info", api_key).await?;
+        return serde_json::from_value(json)
+            .map_err(|e| format!("Failed to parse model info: {}", e));
+    }
+
     if !response.status().is_success() {
         return Err(format!(
             "Server returned status {} from {}",