What are license compatibility issues in open source?

License compatibility issues in open source occur when combining software components under different licenses creates legal conflicts. Open source licenses define how code can be used, modified, and distributed, but their requirements often differ. When code under one license is combined with code under another, the resulting project must comply with all applicable license terms. If the licenses impose conflicting obligations, the combination becomes legally impossible, leading to incompatibility. For example, a license requiring derivative works to adopt the same license (like the GPL) can clash with a license that prohibits such requirements (like some proprietary licenses).

A common example is the GNU General Public License (GPL) and permissive licenses like MIT or Apache. The GPL requires that any derivative work must also be licensed under the GPL, while MIT and Apache allow more flexibility. If you combine GPL-licensed code with MIT-licensed code, the entire project must follow the GPL’s terms, which the MIT license permits. However, issues arise when combining GPL code with licenses that impose incompatible restrictions. For instance, the original BSD license included an advertising clause requiring attribution in documentation, which conflicted with the GPL’s prohibition on additional restrictions. Similarly, code under the Mozilla Public License (MPL) can be combined with GPL code only if specific conditions are met, such as explicitly declaring compatibility in the MPL 2.0.

To avoid compatibility issues, developers must carefully review license terms before integrating third-party code. Tools like SPDX identifiers and compatibility matrices from organizations like the Free Software Foundation (FSF) and Open Source Initiative (OSI) help clarify which licenses can coexist. For example, the FSF lists which licenses are compatible with the GPL, and the Apache 2.0 license is explicitly compatible with GPLv3. When starting a project, choosing a permissive license (MIT, Apache) reduces compatibility risks. For projects requiring copyleft, using the LGPL (Lesser GPL) instead of the GPL allows linking with proprietary code. Understanding these nuances ensures legal compliance and prevents costly rework.