Errors as you type

Published Nov 5, 2026 · Updated Nov 6, 2026 by Claude

The language server checks every template as you type and reports problems in the editor and the Problems panel. The checks follow Erubi, Rails, Haml 7 and Slim 5, and Haml, Slim and Rails errors use those projects’ own messages. No gems are needed.

Every check has a code. The diagnostics reference lists each code with what it means.

ERB

• Ruby syntax errors inside tags, checked with Prism, Ruby’s own parser, for your project’s Ruby version.
• An unclosed <%.
• Blocks without <% end %>.
• Mismatched or unclosed HTML elements.
• <img> without alt.
• Duplicate ids.

In this view the block has no <% end %> and the image has no alt:

app/views/users/index.html.erb

<% @users.each do |user| %>
  <div class="card">
    <img src="<%= user.avatar_url %>">
  </div>

Both are reported: the first as an error from Prism (unexpected end-of-input, assuming it is closing the parent top level context), the second as a warning (`<img>` without `alt`: the alt attribute must be specified (HTML 4.8.4.4.1)).

The HTML checks (tags, alt, duplicate id) apply only to ERB files with an HTML host, such as .html.erb. .js.erb, .yml.erb and the other host types get the ERB and Ruby checks; see File types. At most 20 tag diagnostics are reported per file.

Haml and Slim

The errors Haml and Slim themselves would raise, at the same line and column:

• indentation
• illegal nesting
• unclosed attributes
• unknown filters (Haml) and engines (Slim)
• an explicit end

Ruby syntax errors in code lines, attribute hashes and #{} are reported too.

app/views/users/show.html.haml

%ul
  %li= @user.name
     %span Admin

The third line is reported with Haml’s own message: Inconsistent indentation: 5 spaces used for indentation, but the rest of the document was indented using 2 spaces.

Slim engines your app registers itself (for example mjml:) can be accepted with rubyTemplates.slim.extraEngines.

Rails checks

Two Rails checks run in ERB, Haml and Slim views, and in Jbuilder and Builder views. They report with Rails’ own messages:

• render of a partial that doesn’t exist (unresolved-render), with a quick fix that creates the partial.
• Mismatches with a partial’s strict locals (strict-locals), declared with a comment such as <%# locals: (user:) %>.

app/views/users/_card.html.erb

<%# locals: (user:) %>
<p><%= user.name %></p>

app/views/users/show.html.erb

<%= render "users/card", user: @user %>

Here the render passes the user: the partial declares. A render of users/card that passes something else, or a partial name with no file behind it, is reported.

How partials are looked up in engines, packs, components and gems, and when the missing-partial check stays off, is described in Compatibility.

The rules reference also covers the checks that are planned, such as a render in a tag that outputs nothing, and unsafe output.

Turning checks off

Turn single checks off in a .ruby-templates.toml at the project root, by rule name. The editor and ruby-templates check read it, and it wins over the settings:

[Html.ImgAlt]
enabled = false


[Html.DuplicateId]
exclude = ["app/views/legacy/**"]

Or by code, per language, in your user or workspace settings (the kebab-case codes of earlier versions, such as img-missing-alt, still work):

{
  "rubyTemplates.erb.disabledDiagnostics": ["Html/ImgAlt", "Html/DuplicateId"],
  "rubyTemplates.haml.disabledDiagnostics": ["Syntax/UnknownFilter"],
  "rubyTemplates.slim.disabledDiagnostics": ["Syntax/UnknownEngine"],
  "rubyTemplates.jbuilder.disabledDiagnostics": ["Rails/MissingPartial"]
}

rubyTemplates.jbuilder.disabledDiagnostics covers Jbuilder and Builder views.

Turn a whole language’s diagnostics off:

{
  "rubyTemplates.haml.diagnostics.enabled": false
}

rubyTemplates.slim.diagnostics.enabled also covers Skim files. All settings are listed in Settings.

The same checks run outside the editor with the command line.