Pydantic Settings: extra_forbidden hides a nested configuration loss

What this material covers

3 validation errors for BrokenSettings
lab_service_settings__primary_endpoint
  Extra inputs are not permitted [type=extra_forbidden, input_value='https://file-primary.example.invalid', input_type=str]
    For further information visit https://errors.pydantic.dev/2.13/v/extra_forbidden
lab_service_settings__fallback_endpoint
  Extra inputs are not permitted [type=extra_forbidden, input_value='https://file-fallback.example.invalid', input_type=str]
    For further information visit https://errors.pydantic.dev/2.13/v/extra_forbidden
lab_service_settings__timeout_seconds
  Extra inputs are not permitted [type=extra_forbidden, input_value='15', input_type=str]
    For further information visit https://errors.pydantic.dev/2.13/v/extra_forbidden

Use explicit AliasChoices for both accepted prefixes; keep strict extras and package versions unchanged

Measured on Windows AMD64, CPython 3.12.14, pydantic-settings 2.15.0, pydantic 2.13.5, pydantic-core 2.46.5 and python-dotenv 1.2.3. One nested model receives LAB_ synthetic environment and dotenv inputs with different intentional prefixes. Reproduced actual extra_forbidden errors, then demonstrated that extra='ignore' loses fallback and timeout inputs. Explicit AliasChoices preserves source precedence, fallback and numeric types while rejecting unknown or malformed input. No package versions changed. Subprocesses inherit no user environment; on Windows only SYSTEMROOT obtained from GetWindowsDirectoryW is supplied for asyncio bootstrap, then every evaluation clears all variables and installs only LAB_ fixture values. Evaluations read only a temporary explicit public fixture. No actual user environment values or project .env file, private value or network destination was used. Deep aliases, custom sources, case-sensitive configurations, same-source dual-alias collisions, secret stores, production deployment and external AI demand were not tested. Expanding accepted names is a deliberate input-contract change, not a blanket instruction to accept aliases. Customer final verification remains required.

Measured 2026-09-09T11:30:32.962092+00:00 · Windows / AMD64. Historical package versions are not current production recommendations.

Primary references

• Pydantic Settings issue 923: nested aliases across sources
• Pydantic Settings source names and precedence

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