Open source software is software whose source code is published under a license that lets anyone use, study, modify, and redistribute it.
What makes software open source
Publishing code is not enough. The license must grant the rights. The Open Source Initiative's Open Source Definition lists the criteria, including free redistribution, access to source, permission to make derived works, and no discrimination against people or fields of use.
Licenses fall into two broad families:
- Permissive, such as MIT and Apache 2.0: you can reuse the code in almost any project, including closed ones.
- Copyleft, such as GPL and AGPL: if you distribute a modified version, you must share its source under the same license.
"Source-available" or "fair-code" licenses that restrict commercial use, as used by some tools, are not open source under that definition.
An example
You find a bug in an open-source scheduler. You read the code, fix it in your copy, and send the fix back as a pull request. You did not need anyone's permission to look.
Why it matters for AI-agent publishing
A tool that holds your social account tokens and decides what agents may post is a security boundary. Open source lets you check how it stores tokens, enforces permissions, and writes the audit log, rather than taking it on trust.
SocialHandoff is open source under AGPL-3.0. You can read the code at https://github.com/socialhandoff/socialhandoff, self-host it for free, or use the hosted version. See the open source page and the open-source maintainers use case.