diff --git a/src/dayamlchecker/__init__.py b/src/dayamlchecker/__init__.py
index c077a01..fdc61a6 100644
--- a/src/dayamlchecker/__init__.py
+++ b/src/dayamlchecker/__init__.py
@@ -1,4 +1,7 @@
-from dayamlchecker.messages import Finding, FindingClass
+from dayamlchecker.messages import (
+    Finding,
+    FindingClass,
+)
 from dayamlchecker.yaml_structure import (
     RuntimeOptions,
     find_errors,
diff --git a/src/dayamlchecker/messages.py b/src/dayamlchecker/messages.py
index a5a598a..7489cfe 100644
--- a/src/dayamlchecker/messages.py
+++ b/src/dayamlchecker/messages.py
@@ -32,6 +32,7 @@ class MessageId(StrEnum):
 
     PYTHON_CODE_TYPE = "python_code_type"
     PYTHON_SYNTAX_ERROR = "python_syntax_error"
+    PYTHON_CODE_FUNCTION_DEF = "python_code_function_def"
     VALIDATION_CODE_MISSING_VALIDATION_ERROR = (
         "validation_code_missing_validation_error"
     )
@@ -273,6 +274,16 @@ class MessageDefinition:
         summary="Python syntax error",
         template="Python syntax error: {error}",
     ),
+    MessageId.PYTHON_CODE_FUNCTION_DEF: MessageDefinition(
+        code="WG123",
+        severity=Severity.WARNING,
+        finding_class=FindingClass.GENERAL,
+        summary="Code block defines a Python function",
+        template=(
+            "code block defines function `{function_name}`; move reusable "
+            "helper functions to a Python module instead, or use inline code for small snippets that are only used once"
+        ),
+    ),
     MessageId.VALIDATION_CODE_MISSING_VALIDATION_ERROR: MessageDefinition(
         code="WG101",
         severity=Severity.WARNING,
diff --git a/src/dayamlchecker/yaml_structure.py b/src/dayamlchecker/yaml_structure.py
index 8399077..382526f 100644
--- a/src/dayamlchecker/yaml_structure.py
+++ b/src/dayamlchecker/yaml_structure.py
@@ -185,7 +185,7 @@ def __init__(self, x):
             ]
             return
         try:
-            ast.parse(x)
+            tree = ast.parse(x)
         except SyntaxError as ex:
             # ex.lineno gives line number within the code block
             lineno = ex.lineno or 1
@@ -193,6 +193,18 @@ def __init__(self, x):
             self.errors = [
                 draft(MessageId.PYTHON_SYNTAX_ERROR, line_number=lineno, error=msg)
             ]
+            return
+
+        for node in ast.walk(tree):
+            if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef)):
+                self.errors.append(
+                    draft(
+                        MessageId.PYTHON_CODE_FUNCTION_DEF,
+                        line_number=getattr(node, "lineno", 1) or 1,
+                        function_name=node.name,
+                    )
+                )
+                break
 
 
 class ValidationCode(PythonText):
diff --git a/tests/test_yaml_structure.py b/tests/test_yaml_structure.py
index cdc5e8c..67e0d6c 100644
--- a/tests/test_yaml_structure.py
+++ b/tests/test_yaml_structure.py
@@ -19,6 +19,35 @@ def test_valid_question_no_errors(self):
         errs = find_errors_from_string(valid, input_file="<string_valid>")
         self.assertEqual(len(errs), 0, f"Expected no errors, got: {errs}")
 
+    def test_code_block_function_def_warns(self):
+        yaml_content = """code: |
+  answer = 1
+  def helper():
+    return answer
+"""
+        errs = find_errors_from_string(yaml_content, input_file="<string_invalid>")
+
+        warning = next(
+            err for err in errs if err.message_id == "python_code_function_def"
+        )
+        self.assertEqual(warning.code, "WG123")
+        self.assertEqual(warning.line_number, 4)
+        self.assertEqual(warning.context["function_name"], "helper")
+        self.assertIn("Python module", warning.message)
+
+    def test_code_block_without_function_def_does_not_warn(self):
+        yaml_content = """code: |
+  answer = 1
+  if answer:
+    result = answer
+"""
+        errs = find_errors_from_string(yaml_content, input_file="<string_valid>")
+
+        self.assertFalse(
+            any(err.message_id == "python_code_function_def" for err in errs),
+            f"Did not expect function-def warning, got: {errs}",
+        )
+
     def test_question_and_template_exclusive_error(self):
         invalid = """
 question: |
@@ -513,8 +542,14 @@ def test_accessibility_this_website_link_text_fails(self):
             input_file="<string_invalid>",
             lint_mode="accessibility",
         )
-        generic_errs = [e for e in errs if "link text" in e.err_str.lower() and "too generic" in e.err_str.lower()]
-        self.assertEqual(len(generic_errs), 3, f"Expected 3 generic link-text errors, got: {errs}")
+        generic_errs = [
+            e
+            for e in errs
+            if "link text" in e.err_str.lower() and "too generic" in e.err_str.lower()
+        ]
+        self.assertEqual(
+            len(generic_errs), 3, f"Expected 3 generic link-text errors, got: {errs}"
+        )
         self.assertTrue(all(e.severity.value == "error" for e in generic_errs))
 
     def test_accessibility_sitio_web_link_text_fails(self):
@@ -528,8 +563,14 @@ def test_accessibility_sitio_web_link_text_fails(self):
             input_file="<string_invalid>",
             lint_mode="accessibility",
         )
-        generic_errs = [e for e in errs if "link text" in e.err_str.lower() and "too generic" in e.err_str.lower()]
-        self.assertEqual(len(generic_errs), 3, f"Expected 3 generic link-text errors, got: {errs}")
+        generic_errs = [
+            e
+            for e in errs
+            if "link text" in e.err_str.lower() and "too generic" in e.err_str.lower()
+        ]
+        self.assertEqual(
+            len(generic_errs), 3, f"Expected 3 generic link-text errors, got: {errs}"
+        )
         self.assertTrue(all(e.severity.value == "error" for e in generic_errs))
 
     def test_accessibility_empty_markdown_link_fails(self):