curl started out GPL licensed, as direct inheritance from its httpget days, but in October 1998 we switched to the Mozilla Public License (MPL).

In 2001, we added the MIT license as an additional license to optionally use in addition to MPL. We made curl dual licensed because clearly the MPL license caused problems for some users – primarily because it was deemed not compatible with GPL.

In 2002 we dropped the MPL option and curl was from then on licensed only under the MIT license. Kind of.

The twist

Because of reasons we cannot recall anymore, I decided to ever so slightly edit the language in the MIT license text around that time. (I strongly discourage anyone else from doing this, and if my time machine was functional I would go back and correct this decision.)

The license text we then kept referring to and thinking about as an MIT license, was in fact not exactly MIT anymore. No one objected at the time. I can’t remember anyone really considering the implications at the time. Maybe because not that many people noticed or cared. It was in essence still the MIT license.

Much later

Decades later, the SPDX project quite correctly and ruthlessly made the distinction between a proper MIT and a modified MIT matter: they created a separate and unique identified to the license curl uses. It may be almost like MIT but they deemed the differences big enough to say that the curl license is a special beast. It is since then known as the curl license.

SPDX-License-Identifier: curl

Sort of MIT but not

I have over the years casually kept referring to the license as MIT since it helps users understand even though it is technically incorrect. Open Source people in general tend to know the MIT license and the implications of using it.

A drawback with having our own license is that it was not immediately recognized and known as Open Source or Free Software; not acknowledged and verified by FSF nor OSI.

OSI approved

On April 17, 2026. The Open Source Initiative approved the curl license. It is since then officially arguably slightly more Open Source than before.

I don’t think anyone ever doubted or that it was actually much of a drawback in adoption or usage, but it feels good to finally have this checkmark.

Why now?

I write about this now (many months after the fact) because I totally forgot that this was happening and I only recently realized OSI approved it back in April.

The curl license verbatim

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