The stdout from ruff format overwrites the file contents when auto-format = true

[jackbashford]

Summary

When using ruff format as the formatter and having auto-format = true, writing the file (via :w or :wq) will first format the file, and then overwrite its contents with the stdout from ruff format, so the file ends up looking like:

1 file reformatted, 12 files left unchanged

Instead of its formatted contents.

(I'm unsure of if this is a bug with regards to Helix or Ruff, but overwriting the file contents with the formatter's stdout seems more likely to be a bug within the editor, since the actual formatting works).

Reproduction Steps

I opened a file in a Python project environment I am working in, changed nothing, and wrote the file contents with :w. The file then contained the following:

1 file reformatted, 12 files left unchanged

As its entire contents. Undoing the change once yielded the file result I expected (the formatted file contents), but this is impossible to write from within Helix. Using ruff format . from the shell produces the same response to stdout, and the file contents are formatted correctly.

My config for ruff is as below:

[[language]]
auto-format = true
file-types = ["py"]
language-servers = ["ruff"]
name = "python"

[language.formatter]
args = ["format"]
command = "/nix/store/v7s30nbh27pvn626q4wmhwmw910rkjxs-ruff-0.11.11/bin/ruff"

Helix log

2025-05-30T22:57:14.305 helix_lsp::transport [ERROR] ruff err <- "2025-05-30 22:57:14.304985988  INFO No workspace settings found for file:///home/jack/Code/project, using default settings\n"
2025-05-30T22:57:14.309 helix_lsp::transport [ERROR] ruff err <- "2025-05-30 22:57:14.309464497  INFO Registering workspace: /home/jack/Code/project\n"
2025-05-30T22:57:14.313 helix_lsp::transport [ERROR] ruff err <- "2025-05-30 22:57:14.312882744  INFO Configuration file watcher successfully registered\n"
2025-05-30T22:57:17.412 helix_lsp::transport [ERROR] haskell-language-server err <- "2025-05-30T12:57:17.402583Z | Info | Live bytes: 167.32MB Heap size: 1744.83MB\n"
2025-05-30T22:57:17.435 helix_lsp::transport [ERROR] ruff err <- "2025-05-30 22:57:17.435571540  INFO Shutdown request received. Waiting for an exit notification...\n"
2025-05-30T22:57:17.435 helix_lsp::transport [ERROR] ruff err <- "2025-05-30 22:57:17.435893299  INFO Exit notification received. Server shutting down...\n"

Platform

Linux (NixOS)

Terminal Emulator

ghostty

Installation Method

flake, from master

Helix Version

helix 25.01.1 (2bd7452)

[the-mikedavis]

External formatters are expected to take the current file in through stdin and print the formatted file to stdout. The command here should be ruff format - instead to have ruff use stdin/stdout instead of directly writing to files.

[jackbashford]

Thanks for the explanation of the expected behaviour, much appreciated.