We investigate optional predicate agreement in Santiago Tz’utujil (Mayan). Several generalizations emerge: (i) inanimate
arguments base-generated as complements control agreement optionally; (ii) some animate arguments base-generated as complements control
agreement optionally; (iii) all arguments base-generated as specifiers control full agreement obligatorily. We propose that two conditions
must be met for the operation Agree to succeed, resulting in the exponence of all the features of the agreement controller. First,
a goal must be visible (bear the right feature). Second, a goal must be accessible (be in the right structural position). If one or both
conditions are not met, Agree fails, but the derivation converges and 3sg agreement is exponed. While Agree is
deterministic, surface optionality arises when the operation fails. We use optional agreement to diagnose the syntactic structure of
understudied constructions in Mayan (nominalizations, Agent Focus). We discuss microvariation, highlighting methodological considerations
that arise when assuming an I-language approach.