diff --git a/backend/cpp/llama-cpp/grpc-server.cpp b/backend/cpp/llama-cpp/grpc-server.cpp
index 72ed1b09b568..a26d995a458c 100644
--- a/backend/cpp/llama-cpp/grpc-server.cpp
+++ b/backend/cpp/llama-cpp/grpc-server.cpp
@@ -621,6 +621,11 @@ class BackendServiceImpl final : public backend::Backend::Service {
                             content_val = msg.content();
                         }
                         
+                        // If content is an object (e.g., from tool call failures), convert to string
+                        if (content_val.is_object()) {
+                            content_val = content_val.dump();
+                        }
+                        
                         // If content is a string and this is the last user message with images/audio, combine them
                         if (content_val.is_string() && is_last_user_msg && has_images_or_audio) {
                             json content_array = json::array();
@@ -1046,6 +1051,11 @@ class BackendServiceImpl final : public backend::Backend::Service {
                             content_val = msg.content();
                         }
                         
+                        // If content is an object (e.g., from tool call failures), convert to string
+                        if (content_val.is_object()) {
+                            content_val = content_val.dump();
+                        }
+                        
                         // If content is a string and this is the last user message with images/audio, combine them
                         if (content_val.is_string() && is_last_user_msg && has_images_or_audio) {
                             json content_array = json::array();