The TLC model checker builds a finite state model of TLA+ specifications for checking invariance properties. TLC generates a set of initial states satisfying the spec, then performs a breadth-first search over all defined state transitions. Execution stops when all state transitions lead to states which have already been discovered. If TLC discovers a state which violates a system invariant, it halts and provides a state trace path to the offending state. TLC provides a method of declaring model symmetries to defend against combinatorial explosion.[14] It also parallelizes the state exploration step, and can run in distributed mode to spread the workload across a large number of computers.[20]
As an alternative to exhaustive breadth-first search, TLC can use depth-first search or generate random behaviours. TLC operates on a subset of TLA+; the model must be finite and enumerable, and some temporal operators are not supported. In distributed mode TLC cannot check liveness properties, nor check random or depth-first behaviours. TLC is available as a command line tool or bundled with the TLA toolbox.