Just a week after releasing Mojo 1.0, Modular has delivered on a major promise by making the Mojo compiler and its development tools fully open source.
You can now find the source code in Modular’s main GitHub repository, released under the Apache 2.0 license with LLVM exceptions. This includes the Mojo compiler, tools, and everything needed to build the language from scratch.
This is a big shift for Mojo. Although the language has been developed openly with community input for four years, the compiler stayed closed. Modular had already open-sourced the standard library, much of the kernel code, and supporting tools, but the compiler was the last major part to go public.
This changed soon after Mojo reached version 1.0 a week ago. That release gave the Python-inspired language its first stable foundation, adding source stability, Python-style lambdas, better Language Server Protocol support, and improved memory-safety checks.
Now that the compiler is open, developers can look at and build the entire Mojo language stack on their own, instead of depending only on Modular’s binaries.
To build the compiler locally, developers use Bazel. After cloning the main Modular repository, they can build Mojo from source with the --config=build-mojo option. According to Modular, Bazel will then download or build everything needed for both the compiler and the standard library.
If you are not working directly on the compiler, you can use the --config=prebuilt-mojo option. This downloads the latest nightly compiler binary, so you do not have to build it yourself. Modular points out that you still need a prebuilt Mojo compiler when customizing MAX kernels or models.
It is also important to note the license. Mojo is now under Apache 2.0 with LLVM exceptions, which is a permissive license. This means the language and its binaries can be used widely, even in commercial projects.
However, one important limitation remains. Even though the source code is public, Modular is not yet accepting outside contributions to the Mojo compiler or tools.
The standard library has accepted community contributions since 2024, but Modular wants to be more careful with changes to the compiler. The company plans to start accepting contributions to the compiler and tools by the end of 2026.
For additional details, see the official announcement.
Image credits: Modular
