ZPAQ development began on Feb. 15, 2009 with a series of mutually incompatible experimental programs (v0.01 through v0.09, now obsolete). Archives created with these versions could only be read by the same version. The level 1 standard became fixed on Mar. 12, 2009 with version 1.00. All versions from 1.00 onward are level 1 compliant and can extract older archives. With two exceptions, all programs can also read archives produced by newer versions. This forward compatibility was broken twice:
Feb. 1, 2012: libzpaq 5.00: Level 2 standard allowed high speed compression without a context model (pre/post processing only).
Sept. 26, 2012: zpaq 6.00: Journaling format to support deduplication, fast indexing, update recovery, and storing multiple versions of files and directories.