Regression: DAG import fails with HITLOperator when params have no explicit default (ParamValidationError)

[jgoedeke]

Apache Airflow version

3.1.5

If "Other Airflow 3 version" selected, which one?

No response

What happened?

When using the HITLOperator, if a DAG param does not have an explicit default value, DAG import fails with a ParamValidationError (previously this worked). See error in logs:

[2025-12-16 16:44:52] ERROR - Failed to import: /opt/airflow/dags/legacy_data_processing/mydag.py
ParamValidationError: Invalid input for param my_param: No value passed and Param has no default value
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/models/dagbag.py", line 405 in parse
  File "<frozen importlib._bootstrap_external>", line 940 in exec_module
  File "<frozen importlib._bootstrap>", line 241 in _call_with_frames_removed
  File "/opt/airflow/dags/legacy_data_processing/mydag.py", line 438 in <module>
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/definitions/dag.py", line 1557 in factory
  File "/opt/airflow/dags/legacy_data_processing/mydag.py", line 228 in mydag
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/bases/operator.py", line 522 in apply_defaults
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/providers/standard/operators/hitl.py", line 101 in __init__
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/providers/standard/operators/hitl.py", line 121 in validate_params
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/definitions/param.py", line 242 in validate

What you think should happen instead?

Airflow should allow DAGs with params lacking explicit defaults to import, as in earlier versions. This validation should not prevent import for HITLOperator and similar uses.

How to reproduce

1. Create a DAG with a HITLOperator and a param without a default (e.g. params={"my_param": Param(type="string")})
2. Place the DAG in the dags folder for Airflow 3.1.5
3. Import will fail with ParamValidationError on the missing default value
4. This does not occur in older Airflow versions

Operating System

Debian based, official docker image

Versions of Apache Airflow Providers

No response

Deployment

Other Docker-based deployment

Deployment details

No response

Anything else?

No response

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[jgoedeke]

To make the DAG parse we set default=None in the Param:

        'start_time': Param(
            default=None,
            title='Start time of the test',
            type=['string', 'null'],
            format='date-time',
        ),

But then when using the HITL action the following error occurs:

[2025-12-17 13:10:55] ERROR - Task failed with exception
HITLTriggerEventError: {'error': "Invalid input for param start_time: {'value': None, 'schema': {'title': 'Start time of the test', 'type': ['string', 'null'], 'format': 'date-time'}} is not of type 'string', 'null'\n\nFailed validating 'type' in schema:\n    {'title': 'Start time of the test',\n     'type': ['string', 'null'],\n     'format': 'date-time'}\n\nOn instance:\n    {'value': None,\n     'schema': {'title': 'Start time of the test',\n                'type': ['string', 'null'],\n                'format': 'date-time'}}", 'error_type': 'validation'}
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/execution_time/task_runner.py", line 934 in run
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/execution_time/task_runner.py", line 1325 in _execute_task
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/sdk/bases/operator.py", line 1633 in resume_execution
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/providers/standard/operators/hitl.py", line 184 in execute_complete
  File "/home/airflow/.local/lib/python3.11/site-packages/airflow/providers/standard/operators/hitl.py", line 201 in process_trigger_event_error