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All SDK errors extend UnlinkError with a code discriminator. Four subclasses ship: ApiError, CapabilityError, ValidationError, and TimeoutError.
Use CapabilityError for missing local setup, such as a missing EVM provider or an account that cannot execute. Use ValidationError for invalid SDK inputs. The ApiError class also covers transport failures. If a connection drops mid-response, the SDK throws ApiError with code: "CONNECTION_LOST" instead of leaking a raw TypeError. It is distinct from an Engine rejection, transient, and safe to retry. The underlying error is attached as err.cause.