Can't get texlab to compile LaTeX on save.

[bawjames]

texlab's auto-completion is working, and so is tectonic when run from the shell. Previous fixes for this are somewhat contradictory, and none of them would work for me.

Supposedly, it is possible to compile on the :w command, and in discussion #5002, OP was able to get this to work.

However, the following config (from #5002) in languages.toml

[[language]]
name = "latex"
scope = "source.tex"
injection-regex = "tex"
file-types = ["tex"]
roots = []
comment-token = "%"
language-server = { command = "texlab" }
config = { texlab = { build = { onSave = true } } }
indent = { tab-width = 4, unit = "\t" }

gives the following helix error.

$ helix
Failed to parse language config: unknown field `config`, expected one of `name`, `language-id`, `scope`, `file-types`, `shebangs`, `roots`, `comment-token`, `comment-tokens`, `block-comment-tokens`, `text-width`, `soft-wrap`, `auto-format`, `formatter`, `path-completion`, `diagnostic-severity`, `grammar`, `injection-regex`, `language-servers`, `indent`, `debugger`, `auto-pairs`, `rulers`, `workspace-lsp-roots`, `persistent-diagnostic-sources`
in `language`

Press <ENTER> to continue with default language config

Also, the following config (from #2758) in languages.toml

[[language]]
name = "latex"

[language.config.build]
executable = "tectonic"
args = ["%f"]
onSave = true

gives the same error.

It seems that previous fixes for this contain a languages.config, but this is not a valid field in the toml file.

Also, this is my helix log file, but I am unable to make sense of it.

2025-09-18T17:55:51.998 helix_lsp::transport [ERROR] texlab err: <- StreamClosed
2025-09-18T18:04:02.365 helix_lsp::transport [ERROR] err: <- Other(failed to send a message to server

Caused by:
    channel closed)

From my reading, it seems that this feature is intended to work out-of-the-box, as long as texlab is installed, though perhaps my understanding is incorrect.

Perhaps the intended method to activate this feature has changed?

Any help is appreciated, thanks.