What has changed between ASDF 1 ASDF 2 and ASDF 3? (ASDF Manual)

13.3 “What has changed between ASDF 1, ASDF 2, and ASDF 3?”

We released ASDF 2.000 on May 31st 2010, ASDF 3.0.0 on May 15th 2013, ASDF 3.1.2 on May 6th 2014. Releases of ASDF 2 and now ASDF 3 have since then been included in all actively maintained CL implementations that used to bundle ASDF 1, plus many implementations that previously did not. ASDF has been made to work with all actively maintained CL implementations and even a few implementations that are not actively maintained.

Furthermore, it is possible to upgrade from ASDF 1 to ASDF 2 or ASDF 3 on the fly (though we recommend instead upgrading your implementation or replacing its ASDF module). For this reason, we have stopped supporting ASDF 1 and ASDF 2. If you are using ASDF 1 or ASDF 2 and are experiencing any kind of issues or limitations, we recommend you upgrade to ASDF 3 — and we explain how to do that. See Loading ASDF.

Note that in the context of compatibility requirements, ASDF 2.27, released on Feb 1st 2013, and further releases up to 2.33, count as pre-releases of ASDF 3, and define the :asdf3 feature, though the first stable release of ASDF 3 was release 3.0.1. Significant new or improved functionality were added in ASDF 3.1; the :asdf3.1 feature is present in recent enough versions to detect this functionality; the first stable release since then was ASDF 3.1.2. New *features* are only added at major milestones, and the next one will probably be :asdf3.2.