Great question! The default DiagnosticCategory only locks you into 4 built-in severity colors, but there are two solid ways to add more underline options to your custom Monaco Editor IDE. Let’s break them down in detail:

If you want to stick with the Diagnostics system (since it plays nicely with hover tooltips, the Problems panel, and other built-in Monaco features), you can attach custom CSS classes to your diagnostics and define entirely new underline styles for them.

Step 1: Add a custom class to your Diagnostic #

When creating your diagnostic object, include a className property (Monaco lets you extend marker data with custom attributes):

const customDiagnostic: monaco.languages.Diagnostic = {
  severity: monaco.languages.DiagnosticCategory.Error, // Use any base category—we'll override its style
  range: new monaco.Range(1, 1, 1, 15),
  message: "Custom purple underline example",
  source: "my-custom-ide",
  className: "custom-purple-wavy-underline" // Your unique class name
};

// Register the diagnostic with your language's provider
monaco.languages.registerDiagnosticProvider("my-language", {
  provideDiagnostics: (model) => {
    return [customDiagnostic];
  }
});

Step 2: Inject custom CSS to style the class #

Monaco renders diagnostics as DOM elements, so you can target your custom class with CSS to override the default underline. Add this to your page styles:

/* Target Monaco's editor content area specifically */
.monaco-editor .custom-purple-wavy-underline {
  text-decoration: underline wavy #9d00ff !important;
  text-decoration-thickness: 2px;
}

This will replace the default error underline with a thick purple wavy line for your custom diagnostic.

If you don’t need to tie your underlines to the Diagnostics ecosystem (e.g., you just want visual highlighting without integrating with the Problems panel), Text Decorations are the way to go. They let you define fully custom styles without any limits from DiagnosticCategory.

Step 1: Create custom decoration types #

First, define your desired styles with createTextEditorDecorationType:

// Teal dashed underline
const tealDashedUnderline = monaco.editor.createTextEditorDecorationType({
  textDecoration: "underline dashed #00bfa5",
  textDecorationThickness: "1.5px"
});

// Orange double underline
const orangeDoubleUnderline = monaco.editor.createTextEditorDecorationType({
  textDecoration: "underline double #ff9800"
});

Step 2: Apply decorations to specific text ranges #

Next, apply these decorations to target parts of your editor (you can trigger this on model changes, or after running your code analysis):

const editor = monaco.editor.create(document.getElementById("editor-container"), {
  value: "Hello custom underline world!",
  language: "my-language"
});

// Define which ranges get which decorations
const decorationRanges = [
  {
    range: new monaco.Range(1, 1, 1, 5),
    decorationType: tealDashedUnderline
  },
  {
    range: new monaco.Range(1, 20, 1, 25),
    decorationType: orangeDoubleUnderline
  }
];

// Apply each decoration type to its ranges
editor.setDecorations(tealDashedUnderline, decorationRanges.filter(d => d.decorationType === tealDashedUnderline));
editor.setDecorations(orangeDoubleUnderline, decorationRanges.filter(d => d.decorationType === orangeDoubleUnderline));

Bonus: Clean up to avoid memory leaks #

When your editor is destroyed or you no longer need the decorations, dispose of the types:

tealDashedUnderline.dispose();
orangeDoubleUnderline.dispose();

Which approach should you pick? #

• Use Diagnostics + custom CSS if you want your underlines to integrate with Monaco’s built-in tools (hover tooltips, Problems panel, severity filtering).
• Use Text Decorations if you need full styling control and don’t require Diagnostics ecosystem integration.

内容的提问来源于stack exchange，提问作者SoftTimur