diff --git a/trl/data_utils.py b/trl/data_utils.py
index f72c9e5693..7b49132f05 100644
--- a/trl/data_utils.py
+++ b/trl/data_utils.py
@@ -44,7 +44,7 @@ def prepare_multimodal_messages(messages: list[dict[str, Any]], images: list | N
             List of image objects to insert in the messages.
 
     Returns:
-        `list[dict[str, Any]]`: A deep-copied list of messages where every `"content"` value is a list of structured
+        `list[dict[str, Any]]`: A new list of messages where every `"content"` value is a list of structured
         content blocks, and all `"image"` placeholders are populated with the corresponding image objects. If the
         assistant turns contains `"tool_calls"`, then the `"content"` might be empty.
 
@@ -72,36 +72,38 @@ def prepare_multimodal_messages(messages: list[dict[str, Any]], images: list | N
     ```
     """
     images = images or []
-    messages = copy.deepcopy(messages)  # avoid modifying the original messages
 
-    # First, convert all messages to the structured format if needed, and insert image placeholders if needed
+    # First, convert all messages to the structured format if needed, and insert image placeholders if needed.
+    # Build new message dicts only when transforming string content to avoid modifying the originals.
+    new_messages = []
     images_included = False
     for message in messages:
         if message["role"] == "system":
             if isinstance(message["content"], str):  # if already prepared, the content will be a list
-                message["content"] = [{"type": "text", "text": message["content"]}]
+                message = {**message, "content": [{"type": "text", "text": message["content"]}]}
         elif message["role"] == "user":
             if isinstance(message["content"], str) and not images_included:
                 image_entries = [{"type": "image"} for _ in range(len(images))]
-                message["content"] = [*image_entries, {"type": "text", "text": message["content"]}]
+                message = {**message, "content": [*image_entries, {"type": "text", "text": message["content"]}]}
                 images_included = True
-            elif isinstance(message["content"], str) and images_included:
-                message["content"] = [{"type": "text", "text": message["content"]}]
+            elif isinstance(message["content"], str):
+                message = {**message, "content": [{"type": "text", "text": message["content"]}]}
         elif message["role"] == "assistant":
             if message.get("content") and isinstance(message["content"], str):
-                message["content"] = [{"type": "text", "text": message["content"]}]
+                message = {**message, "content": [{"type": "text", "text": message["content"]}]}
         elif message["role"] == "tool":
             if message.get("content") and isinstance(message["content"], str):
-                message["content"] = [{"type": "text", "text": message["content"]}]
+                message = {**message, "content": [{"type": "text", "text": message["content"]}]}
         else:
             raise ValueError(
                 f"Invalid role in message: {message['role']}. Expected 'system', 'user', 'assistant', or 'tool'."
             )
+        new_messages.append(message)
 
     # Then, check that the number of image placeholders matches the number of images provided
     num_placeholders = sum(
         sum(1 for part in message["content"] if part["type"] == "image")
-        for message in messages
+        for message in new_messages
         if message.get("content") and message["role"] != "tool"
     )
     if num_placeholders != len(images):
@@ -111,15 +113,19 @@ def prepare_multimodal_messages(messages: list[dict[str, Any]], images: list | N
 
     # Then, fill in the actual images in the placeholders
     img_idx = 0
-    for message in messages:
+    for i, message in enumerate(new_messages):
         if not message.get("content") or message["role"] == "tool":
             continue
+        new_content = []
         for part in message["content"]:
             if part["type"] == "image":
-                part["image"] = images[img_idx]
+                new_content.append({**part, "image": images[img_idx]})
                 img_idx += 1
+            else:
+                new_content.append(part)
+        new_messages[i] = {**message, "content": new_content}
 
-    return messages
+    return new_messages
 
 
 def prepare_multimodal_messages_vllm(messages: list[dict[str, Any]]) -> list[dict[str, Any]]: