Skip to content
Architecture

ADR 0003: no GPL code; clean-room implementation from papers

Accepted · 2026-10-31 · anatomy/docs/adr/0003-no-gpl-clean-room-policy.md

Status: Accepted Date: 2026-10-31 Source: the stack’s no-GPL policy, set in cortex’s ADR 0002, which cmake/deps.cmake already cites Related: ADR 0002

Context

Section titled “Context”

cortex’s ADR 0002 rules out GPL code anywhere in cortex, for any purpose. The reason applies to anatomy unchanged: partners redistribute the stack inside their own certified products, and any GPL obligation, even one confined to a test, is something their legal reviews would have to clear.

anatomy simulates vehicles and adapts flight controllers, and in that domain several of the most widely used open flight stacks are GPL-licensed:

  • Betaflight, INAV and Cleanflight (GPL-3.0)
  • ArduPilot (GPL-3.0)
  • Paparazzi (GPL-2.0)

PX4 (BSD-3-Clause) is the permissive flight stack, and reflex, which anatomy builds on, already takes its conventions from PX4.

Decision

Section titled “Decision”

No GPL-licensed code anywhere in anatomy, for any purpose. Not as a dependency, not in a test target, not as a build-time tool in what anatomy builds, and not as a reference oracle. The scope is ADR 0002’s: what anatomy ships, the code that builds it and the tests, and not the docs site’s toolchain.

  • Everything is implemented clean-room from papers, textbooks and specifications, plus permissively licensed (MIT / BSD / Apache-2.0) references.
  • Check a reference’s license before opening its code. Reading GPL source and then writing the same thing “from scratch” is not clean-room. A GPL project’s structure may be consulted, meaning its directory layout and how it splits the problem into modules, and nothing else: never its code, its algorithms’ details, or its data, which includes tuning tables, gains, parameters and trained weights.
  • Test oracles are in-house or permissive: anatomy’s own simulator and replay logs, reflex’s models, closed-form results from the literature, and permissive stacks such as PX4. Never compare against the output of a GPL program (Betaflight, INAV, ArduPilot, …), its simulator or its logs.
  • Conventions are not code. Following a convention that a GPL project also uses (a frame, a rotor order, a unit) is fine, when it is taken from a permissive source or a specification.
  • LGPL, MPL, AGPL and SSPL code is rejected, as a dependency and as a reference, as ADR 0002 rejects it. An exception would need an ADR of its own that supersedes this one.

Consequences

Section titled “Consequences”
  • Each component costs more to build: the design comes from the literature, and validation uses anatomy’s own simulation and permissive references.
  • A paper’s results can be compared against; its GPL code cannot be read or run.
  • Partners can redistribute anatomy under MIT terms with no copyleft review.
  • Current state: anatomy’s framework, simulator and adapters are written in-house. No GPL code or oracle is used.