fix(crews): raise guided ValueError instead of IndexError from CrewOutput.json on empty tasks

lib/crewai/src/crewai/crews/crew_output.py

@@ -62,7 +62,10 @@ def json(self) -> str | None:  # type: ignore[override]
 
         None when that output could not be converted to its ``output_json`` model.
         """
-        if self.tasks_output[-1].output_format != OutputFormat.JSON:
+        if (
+            not self.tasks_output
+            or self.tasks_output[-1].output_format != OutputFormat.JSON
+        ):
             raise ValueError(
                 "No JSON output found in the final task. Please make sure to set the output_json property in the final task in your crew."
             )

lib/crewai/tests/test_crew_output.py

@@ -0,0 +1,12 @@
+import pytest
+
+from crewai.crews.crew_output import CrewOutput
+
+
+def test_json_raises_value_error_when_tasks_output_is_empty():
+    """A CrewOutput built outside a kickoff (empty tasks_output) must raise the
+    guided ValueError instead of a bare IndexError."""
+    output = CrewOutput(raw="no tasks")
+
+    with pytest.raises(ValueError, match="No JSON output"):
+        _ = output.json