fix: raise a clear error when JsonSchemaValidator gets no messages

haystack/components/validators/json_schema.py

@@ -129,9 +129,12 @@ def run(
         :return:  A dictionary with the following keys:
             - "validated": A list of messages if the last message is valid.
             - "validation_error": A list of messages if the last message is invalid.
-        :raises ValueError: If the last message has no text content, or if no JSON schema is provided either in
-            the `run` method or in the component init.
+        :raises ValueError: If `messages` is empty, if the last message has no text content, or if no JSON schema is
+            provided either in the `run` method or in the component init.
         """
+        if not messages:
+            raise ValueError("The provided list of messages is empty.")
+
         last_message = messages[-1]
         if last_message.text is None:
             raise ValueError(f"The provided ChatMessage has no text. ChatMessage: {last_message}")

releasenotes/notes/json-schema-validator-empty-messages-e3fcbf73c284f0f4.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    ``JsonSchemaValidator.run()`` now raises a ``ValueError`` saying the message list is empty when it receives no
+    messages, instead of an ``IndexError`` from reading the last message.

test/components/validators/test_json_schema.py

@@ -85,6 +85,12 @@ def test_validates_message_against_json_schema(self, json_schema_github_compare,
         assert len(result["validated"]) == 1
         assert result["validated"][0] == message
 
+    def test_run_raises_clear_error_for_empty_messages(self, json_schema_github_compare):
+        validator = JsonSchemaValidator(json_schema=json_schema_github_compare)
+
+        with pytest.raises(ValueError, match="The provided list of messages is empty"):
+            validator.run([])
+
     def test_accepts_empty_json_schema(self):
         validator = JsonSchemaValidator(json_schema={})
         message = ChatMessage.from_assistant('{"anything": "is valid"}')