Problem

CustomSubagentConfig.model accepts an empty string. Later, the executor treats a present model value as an override and passes it to model creation, which can fail at runtime with a confusing provider/model error.

Impact

A simple config typo produces a late runtime failure instead of a clear validation error.

Suggested Fix

Validate model as either absent/None, a reserved inherit value if supported, or a non-blank string. Use Pydantic validation such as min_length=1 plus trimming.

Tests

• Blank model string fails config validation with a clear error.
• Missing model continues to inherit parent behavior.
• Valid model string continues to work.

References

• backend/packages/harness/deerflow/config/subagents_config.py:55
• backend/packages/harness/deerflow/subagents/executor.py:123
• backend/packages/harness/deerflow/models/factory.py:61