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ADR 0004 — License

  • Status: Accepted
  • Date: 2026-09-26
  • Decides: D5 in roadmap.md

Context

rainbow-fmt is a developer tool intended to be adopted in both open-source and commercial codebases, embedded in editors and CI systems, and extended by third-party language packs. The license should minimize adoption friction and be compatible with the licenses of its dependencies: tree-sitter and most tree-sitter grammars (MIT), tomlkit (MIT), and ruamel.yaml (MIT).

Decision

rainbow-fmt is licensed under the MIT License. The full text is in LICENSE at the repository root. The package metadata declares it as the SPDX expression MIT.

Third-party language packs may choose their own license.

Consequences

  • Anyone may use, modify, embed, and redistribute rainbow-fmt, including in proprietary products, provided the copyright notice and license text are retained.
  • Compatible with the MIT-licensed core dependencies; no copyleft obligations flow to users or plugin authors.
  • MIT includes no explicit patent grant. This is accepted as the norm for comparable tools (Prettier, Black, and Ruff are all MIT-licensed).
  • Contributions are accepted under the same license (inbound = outbound); no contributor license agreement is required.

Alternatives considered

  • Apache-2.0. Adds an explicit patent grant, but is longer, more complex to comply with (NOTICE file handling), and not GPLv2-compatible.
  • MPL-2.0. File-level copyleft would require modifications to rainbow-fmt's own files to be shared, which adds friction for companies embedding it, with little benefit for a formatter.