Agents window doesn't highlight Kotlin in diffs with JetBrains' official extension (jetbrains.kotlin-server is missing from the allowlist)

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1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

With JetBrains’ official Kotlin extension installed, Kotlin files show as plain text in the agents window’s diff view. Markdown files in the same view are highlighted. Kotlin is highlighted correctly in the regular editor.

That list includes jetbrains.intellij-server, fwcd.kotlin and mathiasfrohlich.kotlin, but not jetbrains.kotlin-server. JetBrains renamed its Kotlin extension from jetbrains.kotlin to jetbrains.kotlin-server (see the kotlin-lsp README). It’s the ID the Kotlin team now recommends, and it’s on Open VSX.

Steps to Reproduce

Install jetbrains.kotlin-server (v0.0.12) and no other Kotlin extension.
Have an agent create or edit a .kt file.
Open the diff in the agents window. The Kotlin code isn’t highlighted, while .md diffs are.
Workaround Install mathiasfrohlich.kotlin, which is on the allowlist, and reload the window. Kotlin diffs are then highlighted. This confirms the allowlist is the cause.

Expected behavior Kotlin diffs are highlighted when the official JetBrains extension is installed.

Expected Behavior

Add jetbrains.kotlin-server to the GlassWorkspaceLsp extension allowlist.

Operating System

MacOS

Version Information

Version: 3.21.18
VSCode Version: 1.128.0
Commit: c4730f7d93d787d9ab120af715999f0345ee5bc0
Date: 2026-09-22T02:04:34.641Z
OS: macOS 26.5.1 (25F80), arm64

Does this stop you from using Cursor

No - Cursor works, but with this issue

5

Hey @ezeugo_basic, thanks for the report and for tracking down the cause.

You’re right: the Agents Window only loads language extensions from a fixed list, and “Kotlin by JetBrains” (jetbrains.kotlin-server) isn’t on it yet, so its Kotlin grammar never gets loaded there. I’ve flagged this to the team so it can be added.

In the meantime, either of these will give you Kotlin highlighting in Agents Window diffs:

  1. Keep the Kotlin Language extension (mathiasfrohlich.kotlin) installed alongside it, as you found.
  2. Or use “Java and Kotlin by IntelliJ IDEA” (jetbrains.intellij-server), which is already supported in the Agents Window and includes Kotlin highlighting.

Reload the window after installing either one.

7

Thanks for the quick reply!