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ADR 003 — Python 3.14 runtime

Status: Accepted
Date: 2026-09-04
Deciders: Developer
Supersedes: PLAN.md original backend runtime of Python 3.12

Context

The initial plan pinned Python 3.12 as a conservative, widely supported FastAPI baseline. Inspection of the implementation machine found python3 3.14.4 and no python on PATH.

The developer approved using the newest Python runtime available on this machine rather than installing or containerising 3.12 only to match the first draft.

Decision

The project runtime is Python 3.14.

  • Local backend work uses the host 3.14 interpreter (observed 3.14.4).
  • Phase 1 containers, requires-python and CI must pin Python 3.14, not 3.12.
  • Patch-level drift (3.14.x) is acceptable; a major/minor change is not.

Alternatives considered

Alternative Why not now
Keep Python 3.12 via pyenv or a 3.12 image only Rejected by the developer in favour of the machine's current runtime.
Python 3.13 as a middle ground Still not the approved host runtime; would add another version without a compatibility investigation.

Consequences

  • Newer standard-library and typing behaviour is available without a later upgrade.
  • Some third-party wheels, mypy plugin combinations or Docker tags may lag 3.14. Phase 1 must verify install and test on 3.14 before locking dependencies.
  • Documentation that said 3.12 is updated through the PLAN.md change-control record.

Follow-up

During Phase 1, confirm that FastAPI, Pydantic, SQLAlchemy, pytest, Ruff and mypy install and run on Python 3.14. If a required package has no viable 3.14 build, stop and propose a documented rollback rather than silently switching versions.