# Language versions

> Pin the Python, Node, R or Go version your app builds and runs on.

Source: /toft-docs/latest/user/build/versions/

With no pin, your app gets the install's default version. To choose, declare a
range in `toft.toml` — or, for Go, in `go.mod`.

```toml
# toft.toml
[runtime]
python = ">=3.12,<3.14"
node   = "22"
r      = ">=4.5"
```

Toft picks the **highest** installed version that satisfies the range.

## How to write a range

| Language | Compared on | Examples |
|---|---|---|
| Python, R | major.minor | `3.13`, `>=3.12,<3.14` |
| Node | major | `22`, `>=20` |

- Operators are `>=`, `<=`, `>`, `<` and `==`. A bare version means exactly
  that version. A comma means AND.
- One wildcard is accepted: a trailing `.*` after `==`, as in
  `python = "==3.13.*"`, which means the 3.13 line — the same as `3.13`.
- `^`, `~`, `~=`, `||`, `!=` and any other `*` are refused, each with the range
  to write instead.
- Extra components are refused, not truncated: `3.12.4` for Python, `22.4` for
  Node.

To stay on an older version, give an upper bound or an exact version
(`python = "3.13"`). A bare `>=` floor always takes the newest.

## Node's `engines` and `.nvmrc`

Toft does not read `engines.node` in `package.json`, `.nvmrc` or
`.node-version`. A tree that states its Node version only there builds on the
install's default, and `toft check` and the deploy say so with a
`node_version_unread` warning naming the version it got. Pin it in `toft.toml`
instead: `[runtime] node = "22"`.

## Python's `requires-python`

With no `[runtime] python` pin, Toft uses the install's default Python unless
`requires-python` in `pyproject.toml` rules it out. In that case it builds with
the newest installed version that `requires-python` allows, and the build log and
`toft check` say so. If no installed version is allowed, the deploy is refused
and the error lists the versions the install has.

A lower bound such as `>=3.8` never changes the version, because the default
already satisfies it. To choose a version yourself, pin it with `[runtime] python`.

## Go

Go reads the `go` line in `go.mod` as a minimum — `go 1.25` means 1.25 or
newer. If no installed toolchain meets it, the `go` command either downloads
one or fails, depending on your install. See [Go](/toft-docs/latest/user/build/go.md).

## When no version matches

The deploy is refused before anything builds, and the error lists the versions
the install has. Toft never substitutes a different version.

## Which versions exist

Your administrator decides. The published Toft image ships three of each
language, Go included, with the newest as the default:

| Language | Versions |
|---|---|
| Python | 3.15, 3.14, 3.13 |
| Node | 24, 22, 20 |
| R | 4.6, 4.5, 4.4 |

Your install may differ.

## What's running

Your app's **Settings** tab, under **Runtime**, shows the exact version the
serving deployment runs on. A restart keeps it.

If your administrator removes a version, running apps keep it. Your next deploy
re-resolves and is refused if the pin no longer matches — that's your cue to
update the pin.