Hi — hoping someone has seen this. I’m on a monorepo (Vite frontend + FastAPI under api/index.py ) on the Hobby plan. The Python builder always picks CPython 3.14.3 regardless of which of the three documented pin mechanisms I use, at any location. The docs say 3.12 is the default; on my project it effectively isn’t. What I’ve tried (all on a fresh push, build cache skipped) | Config | Result | |—|—| | No Python pin anywhere | Using CPython 3.14.3 | | .python-version = 3.12 at repo root | Using CPython 3.14.3 | | .python-version = 3.13 at repo root | Using CPython 3.14.3 | | .python-version inside api/ | Using CPython 3.14.3 | | pyproject.toml at root with requires-python = ">=3.12" | Using CPython 3.14.3 | | pyproject.toml inside api/ with requires-python = "==3.12.*" + .python-version + uv.lock (exact mirror of vercel/examples/python/next-fastapi-monorepo/backend ) | Using CPython 3.14.3 | | Pipfile + Pipfile.lock at root with python_version = "3.12" | Using CPython 3.14.3 | | PYTHON_VERSION=3.12 env var on the project (all three scopes) | Using CPython 3.14.3 | | Project Settings → Node.js: 24.x → 22.x (confirmed by Skipping build cache since Node.js version changed from "24.x" to "22.x" ) | Using CPython 3.14.3 | | package.json engines.node = "22.x" | Using CPython 3.14.3 | Representative build log (no pin at all) Cloning github.com/…/… (Branch: dev, Commit: …) Using CPython 3.14.3 Building psycopg2-binary==2.9.10 × Failed to build `psycopg2-binary==2.9.10` Error: pg_config executable not found. Error: Command "uv pip install" exited with 1 Note: older community posts (e.g. Deploy start to fail with uvicorn: command not found , Nov 2025) show builders that emitted No Python version specified in pyproject.toml or Pipfile.lock. Using latest installed version: 3.12 . My builds no longer emit any “Python version specified” line — uv just declares Using CPython 3.14.3 directly. vercel.json { "$schema": "https://openapi.vercel.sh/vercel.json", "version": 2, "buildCommand": "cd frontend && npm install && npm run build", "outputDirectory": "frontend/dist", "rewrites": [ { "source": "/api/(.*)", "destination": "/api/index.py" }, { "source": "/auth/(.*)", "destination": "/api/index.py" }, { "source": "/health", "destination": "/api/index.py" }, { "source": "/(.*)", "destination": "/index.html" } ] } api/index.py import sys, os sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "backend")) from app.main import app # noqa: F401 Project context Created 2026-04-14 (recent) Framework Preset: Other Node.js: 22.x (recently switched from 24.x, no effect on Python) Region: iad1 Plan: Hobby package.json lives in frontend/ Python source lives in api/index.py + a shared package in backend/ Questions Is the project silently enrolled in an early 3.14 default rollout? If yes, what’s the opt-out path on Hobby — any project setting, env var, or file I can commit? If no, why are the three documented pin files being ignored? Is there an additional prerequisite (a specific file name, a specific location, a specific vercel.json shape)? Happy to share more of the build log on request. Thanks!