Validate prompt_template type for clearer user-facing errors

Author: cafferychen777

python/mllmcelltype/prompts.py

@@ -129,8 +129,14 @@ def create_prompt(
     """
     write_log(f"Creating prompt for {len(marker_genes)} clusters")
 
+    # Validate custom template type and use default template when empty.
+    if prompt_template is not None and not isinstance(prompt_template, str):
+        raise ValueError(
+            f"prompt_template must be a string or None, got {type(prompt_template).__name__}"
+        )
+
     # Use default template if not provided
-    if not prompt_template:
+    if not prompt_template or not prompt_template.strip():
         prompt_template = DEFAULT_PROMPT_TEMPLATE
 
     # Default tissue if none provided

python/tests/test_prompts.py

@@ -88,6 +88,16 @@ def test_create_prompt_with_malformed_template_raises_clear_error(self):
                 prompt_template=bad_template,
             )
 
+    def test_create_prompt_with_non_string_template_raises_clear_error(self):
+        """Test non-string custom templates fail with explicit type guidance."""
+        with pytest.raises(ValueError, match="prompt_template must be a string or None"):
+            create_prompt(
+                marker_genes=self.marker_genes,
+                species="human",
+                tissue="blood",
+                prompt_template=123,  # type: ignore[arg-type]
+            )
+
     def test_create_initial_discussion_prompt(self):
         """Test initial discussion prompt creation."""
         cluster_id = "1"