Open source means the source code needed to build/compile/run Mojo programs is available under a license that permits anyone to freely use/modify/share the code for any purpose, a la https://opensource.org/osd
Modular, the creator of https://en.wikipedia.org/wiki/Mojo_(programming_language) has stated many times that the language would eventually be open source, but they cannot give a timeline, for example:
https://www.modular.com/pricing#:~:text=Will%20Mojo%20ever%20be%20fully%20open%20source%3F
https://www.youtube.com/watch?v=tYujSI_aoqk&lc=Ugy-QQjJ4PYNJyrRc354AaABAg.A7Wv5n61lVtA7j_dibRIk7
Core modules from the Mojo standard library have been open sourced as of March 2024 https://www.modular.com/blog/the-next-big-step-in-mojo-open-source about a year after Mojo was publicly announced. This question is about when the rest of the language implementation needed to run Mojo program is open source.
If Mojo is not open source by the end of last year option, this question will resolve to "other".
Update 2025-06-23 (PST) (AI summary of creator comment): Chris Lattner, a creator of Mojo, has provided a timeline for open-sourcing the compiler:
A commitment to open source the compiler by the end of 2026 at the latest.
He noted this is a conservative target that the team hopes to meet sooner.
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