diff --git a/marimo/_server/ai/prompts.py b/marimo/_server/ai/prompts.py
index cc8da16e471..c2d7d83d78c 100644
--- a/marimo/_server/ai/prompts.py
+++ b/marimo/_server/ai/prompts.py
@@ -72,20 +72,22 @@ def _format_variables(
     if not variables:
         return ""
 
-    variable_info = "\n\n## Available variables from other cells:\n"
+    # Use a list for efficient string concatenation
+    lines = ["\n\n## Available variables from other cells:\n"]
+    append = lines.append  # local variable for faster access
+
     for variable in variables:
         if isinstance(variable, VariableContext):
-            if _is_private_variable := variable.name.startswith("_"):
+            if variable.name.startswith("_"):
                 continue
-            variable_info += f"- variable: `{variable.name}`\n"
-            variable_info += f"  - value_type: {variable.value_type}\n"
-            variable_info += f"  - value_preview: {variable.preview_value}\n"
+            append(f"- variable: `{variable.name}`\n")
+            append(f"  - value_type: {variable.value_type}\n")
+            append(f"  - value_preview: {variable.preview_value}\n")
         else:
-            if _is_private_variable := variable.startswith("_"):
+            if variable.startswith("_"):
                 continue
-            variable_info += f"- variable: `{variable}`"
-
-    return variable_info
+            append(f"- variable: `{variable}`")
+    return "".join(lines)
 
 
 def _rules(rules: list[str]) -> str: