= Cutdown Compatibility Fallback Policy

== Principle

Unknown or unsupported constructs must degrade deterministically with diagnostics.

Parsers must not silently reinterpret unknown constructs into different known semantics.

== Required Behavior

When input contains syntax or extension behavior not supported by the active profile/version:

- Continue parsing and return AST.
- Apply documented fallback (typically literal-text or core fallback path).
- Emit structured diagnostics with stable code and span.

== Forbidden Behavior

- Silent reinterpretation into a different construct.
- Non-deterministic fallback choices across runs/implementations.
- Fail-fast rejection as the default mode.

== Conformance Requirements

Conformance tests must include:

- unknown-extension under disabled profile,
- unknown future syntax under current version,
- expected fallback AST and diagnostics for both.

== Related

- [``./parser-profile-policy.md``](/latest/policies/parser-profile-policy)
- [``./diagnostics-policy.md``](/latest/policies/diagnostics-policy)