After some research, I've come up with a shortlist of contenders:

1. MIT
It's the one currently on the project and requires preservation of copyright and license notices (i.e., copies or substantial ports of the software must also carry the MIT license). Licensed works, modifications, and larger works may be distributed under different terms and without source code.

2. GNU GPLv3
This is what is termed a “copyleft” license, which means that any derivative work must be distributed under the same or equivalent terms. (Basically, this license attempts to enforce free access to the work and derivative works; the source code must remain open to the public.)

3. Creative Commons CC0
The license being used by Toolhub itself. All rights, including the copyright, are waived and the material is dedicated to the public domain. The most permissive option.

In T324643#8451953, @NicoleLBee wrote:

3. Creative Commons CC0
The license being used by Toolhub itself. All rights, including the copyright, are waived and the material is dedicated to the public domain. The most permissive option.

Minor clarification, the CC0 license only applies to the structured data hosted by Toolhub. The Toolhub source code is licensed GPL-3.0-or-later.

In T324643#8452970, @bd808 wrote:

In T324643#8451953, @NicoleLBee wrote:

3. Creative Commons CC0
The license being used by Toolhub itself. All rights, including the copyright, are waived and the material is dedicated to the public domain. The most permissive option.

Minor clarification, the CC0 license only applies to the structured data hosted by Toolhub. The Toolhub source code is licensed GPL-3.0-or-later.

Thank you for the clarification!

Given that, my inclination would be to follow suit and adopt the GPL license.

In T324643#8453202, @NicoleLBee wrote:

Given that, my inclination would be to follow suit and adopt the GPL license.

I'm very much not an expert on these matters, but I think Toolhunt can be considered a derivative work of Toolhub, and as such, has to be distributed under the same license (or a more permissive one?). If that is the case, MIT is not even an option, and I agree we should use the same license as Toolhub.

In T324643#8453392, @Slst2020 wrote:

I'm very much not an expert on these matters, but I think Toolhunt can be considered a derivative work of Toolhub, and as such, has to be distributed under the same license (or a more permissive one?).

(Obligatory "I am not a lawyer, and even if I was I am not your lawyer." disclaimer is in effect for the following statements. This is not legal advice offered by myself or my employer to any readers including but not limited to the author of the post I am replying to.)

Interacting with the Toolhub API would not be considered a derivative work. Toolhub API clients can be offered under any license (or even no license) without any copyright repercussions.

If you copy code directly from Toolhub into Toolhunt then that would be creating a derivative work. The GPL-3.0-or-later license of Toolhub demands that derivative works also be GPL 3.0 or a future GPL license that supersedes GPL 3.0 (meaning a potential 3.1 or 4.0 version of the license). This is probably the most fundamental difference between the GPL family of licenses and the "permissive" licenses. The GPL demands derivative works also be GPL licensed. "Permissive" licenses such as the MIT license merely demand that the copied code continues to state its original author and license terms. The embedding project can choose other licensing for it's original content.

@bd808 Thanks for the clarification.

Let's go ahead with the GPL-3.0-or-later license. You can find instructions on how to apply it here: https://choosealicense.com/licenses/gpl-3.0/

Thank you, @bd808 and @Slst2020. I'll check the information and open a pull request.

Pull request opened at https://github.com/wikimedia/toolhunt/pull/2