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All content by Kyle E. Mitchell, who is not your lawyer.

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API Copyleft 2.0.0-pre.1bringing the best of Parity to API Copyleft

Thanks to renewed interest from others, I’ve put some attention back on the API Copyleft License, a software license that’s copyleft for improvements and extensions, but permissive for applications. In particular, I’ve been able to “backport” some of the big substance and style improvements that made it into Parity 7.0.0. The text of the new API Copyleft License 2.0.0-pre.1 follows.

If that sounds interesting, have a quick read through and feed back on the GitHub PR, send me an e-mail, or Heck, mail me an open letter. If you’re familiar with the Blue Oak Model License or Parity, you can skip right to the meat of the license, the Copyleft and Applications sections.

I’ll give this a little time, field any feedback, and proceed to 2.0.0 in short order.

As always, this is not just my work. API Copyleft never could have come this far without a lot of great thought and attention from others. The THANKS file in the repo mentions a few of them.

API Copyleft License 2.0.0-pre.1


This license allows you to use and share this software for free, but you have to share improvements, but not applications, alike.


In order to receive this license, you have to agree to its rules. Those rules are both obligations under that agreement and conditions to your license. Don’t do anything with this software that triggers a rule you can’t or won’t follow.


Make sure everyone who gets a copy of any part of this software from you, with or without changes, also gets the text of this license or a link to


Contribute all changes and additions to this software, as well as all software that invokes this software’s functionality. When in doubt, contribute.

This this rule encourages cooperative development of this software, minimizing duplication of effort across competing substitutes.


You don’t have to contribute any software that only invokes this software’s functionality through the interfaces this software exposes, without exposing this software’s interfaces or functionality to users or other software to such an extent that it becomes a practical substitute for this software.

Interfaces exposed by this software include all the interfaces this software provides users or other software to invoke its functionality, such as command line, graphical, application programming, remote procedure call, and inter-process communication interfaces.

This exception to Copyleft allows building, sharing, and licensing applications as you like.


You don’t have to contribute any change, addition, or other software that meets all these criteria:

  1. You don’t use it for more than thirty days.

  2. You don’t share it outside the team developing it, other than for non-production user testing.

  3. You don’t use it to provide any service to anyone outside the team developing it.

This exception to Copyleft allows private experimentation.


To contribute software:

  1. Publish all source code for the software in the preferred form for making changes through a freely accessible distribution system widely used for similar source code so contributors and others can find and copy it.

  2. Make sure every part of the source code is available under this license or another license that allows everything this license does, such as the Blue Oak Model License 1.0.0, the Apache License 2.0, the MIT license, or the two-clause BSD license.

  3. Take these steps within thirty days.

  4. Note that this license does not allow you to change the license terms for this software. You must follow Notices.


You’re excused for unknowingly breaking Copyleft if you contribute as required, or stop doing anything requiring this license, within thirty days of learning you broke the rule. You’re excused for unknowingly breaking Notices if you take all practical steps to comply within thirty days of learning you broke the rule.

Each contributor licenses you to do everything with this software that would otherwise infringe that contributor’s copyright in it.


Each contributor licenses you to do everything with this software that would otherwise infringe any patent claims they can license or become able to license.


No contributor can revoke this license.

No Liability

As far as the law allows, this software comes as is, without any warranty or condition, and no contributor will be liable to anyone for any damages related to this software or this license, under any kind of legal claim.

The permanent, canonical link for this version of the license text is The headings have html id attributes, so you can link right to specific sections, like Copyleft.

Your thoughts and feedback are always welcome by e-mail.

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