Is this software written by people or by AI? For Mustang, the answer is both, in different parts. We think you should know which part is which.
The core: written by people
Mustang is the work of a small team that has written mail software since almost 3 decades, several of them on Thunderbird. Since the first commit in January 2021, they have written the parts that need years of experience to get it right: the protocols (JMAP, IMAP, EWS, ActiveSync, Graph and others), the local storage, the encryption, the account setup and the calendar logic. Since a few months, AI tools help with this work, under the same rules as below.
The interface: written mostly by AI
The Mustang interface was written in October 2026, mostly by AI coding agents. Ben Bucksch, who leads Mustang, described what Mustang should do and how the code must look, split the work into tasks, and reviewed the results. A UX designer reviewed the screens. The agents wrote the Svelte components and the tests.
This worked well, because Mustang doesn't change how Mustang handles mail. It calls the same logic that the modern interface of Mustang calls. While building Mustang, the agents were not allowed to change that logic. When they found a bug in it, the fix went through the normal review as a separate change, one bug at a time.
Another reason why it worked well was that the AI agents followed closely the existing Mustang frontend code while writing Mustang, and had specific instructions to imitate the code patterns there. Nonetheless, the first revision did not sufficiently follow that, and on review and instruction of the human developers was rewritten again to precisely mirror the Mustang components, APIs and code patterns. This second rewrite then resulted in a high code quality. This is necessary to maintain the application in the long run.
The rules for AI-written code
AI-written code goes through the same gate as code written by people:
- A person reviews every change before it is merged, often line by line. Changes that don't meet the standards of the project go back.
- The tests run for every change.
- Mustang merely uses the logic of Mustang.
- For Mustang, the agents worked only from public documentation about how Outlook looks and behaves. No Outlook code was used. Outlook was not even launched.