# Error Behavior **Domain:** API Design / Reliability **Doc Type:** Contract Component **Maturity:** Core Design Principle **Related:** [[wiki/Portable Contract|Portable Contract]], [[wiki/Input Rules|Input Rules]], [[wiki/Conformance Testing|Conformance Testing]] --- ## Definition **Error behavior defines how a capability reports invalid input, unavailable host features, conflicts, limits, and unexpected failure.** It includes error categories, return shapes, thrown exceptions, retryability, partial-output rules, and whether failure may mutate state. ## Project Context Cross-language parity requires failure parity as well as success parity. A quote extractor that skips malformed markup in one runtime but aborts in another can create different archives even if both agree on ordinary cases. ## Key Insight **An interface is not fully specified until callers know how it fails.** ## See Also [[wiki/Shared Fixtures|Shared Fixtures]], [[wiki/Permissioned Capability|Permissioned Capability]], [[wiki/Portable Contract|Portable Contract]], [[wiki/Technical Terms|Technical Terms]]