diff --git a/src/providers/helpers.rs b/src/providers/helpers.rs
index 2bedaae4..54dc208b 100644
--- a/src/providers/helpers.rs
+++ b/src/providers/helpers.rs
@@ -72,7 +72,7 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         if let Some(tool_calls) = &m.tool_calls {
             // Sanitize tool call IDs for OpenAI compatibility (max 40 chars)
             let sanitized_calls: Vec<_> = tool_calls.iter().map(|tc| {
@@ -84,7 +84,7 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
             }).collect();
 
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
@@ -165,7 +165,7 @@ pub async fn messages_to_openai_json_text_only(
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         if let Some(tool_calls) = &m.tool_calls {
             // Sanitize tool call IDs for OpenAI compatibility (max 40 chars)
             let sanitized_calls: Vec<_> = tool_calls.iter().map(|tc| {
@@ -177,7 +177,7 @@ pub async fn messages_to_openai_json_text_only(
             }).collect();
 
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }