Copying config.example.yaml to config.yaml and starting DeerFlow crashed with `pydantic ValidationError: models — Input should be a valid list [input_value=None]`, because the example ships every entry under `models:` commented out, so PyYAML parses the key as null. Reported in #1444.
Add a field_validator(mode="before") on AppConfig that coerces null models/tools/tool_groups to [] (matching their default_factory=list), and emit an actionable warning from from_file when no models are configured (pointing to config.example.yaml / make setup). Adds regression tests.
Closes#1444
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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