Fix IndexError in CrewOutput.json when tasks_output is empty

- Add validation to check if tasks_output is empty before accessing [-1]
- Provide meaningful error message for empty tasks case
- Add comprehensive tests covering edge cases
- Fixes #3185

Co-Authored-By: Jo\u00E3o <joao@crewai.com>

src/crewai/crews/crew_output.py

@@ -25,6 +25,11 @@ class CrewOutput(BaseModel):
 
     @property
     def json(self) -> Optional[str]:
+        if not self.tasks_output:
+            raise ValueError(
+                "No tasks found in crew output. Please ensure the crew has completed at least one task before accessing JSON output."
+            )
+        
         if 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."

tests/crew_test.py

@@ -310,6 +310,41 @@ def test_crew_creation(researcher, writer):
     assert result.raw == expected_string_output
 
 
+def test_crew_output_json_empty_tasks():
+    """Test that CrewOutput.json raises ValueError when tasks_output is empty."""
+    from crewai.crews.crew_output import CrewOutput
+    from crewai.types.usage_metrics import UsageMetrics
+    
+    output = CrewOutput(
+        raw="Test output",
+        tasks_output=[],
+        token_usage=UsageMetrics()
+    )
+    
+    with pytest.raises(ValueError) as excinfo:
+        _ = output.json
+    
+    assert "No tasks found in crew output" in str(excinfo.value)
+
+
+def test_crew_output_json_reproduction_case():
+    """Test reproduction case from GitHub issue #3185."""
+    from crewai.crews.crew_output import CrewOutput
+    
+    output = CrewOutput(
+        raw="",
+        pydantic=None,
+        json_dict={"test": "value"},
+        tasks_output=[],
+        token_usage={}
+    )
+    
+    with pytest.raises(ValueError) as excinfo:
+        json_output = output.json
+    
+    assert "No tasks found in crew output" in str(excinfo.value)
+
+
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_sync_task_execution(researcher, writer):
     from unittest.mock import patch