diff --git a/vscode/package.json b/vscode/package.json
index 073402434..0d114432b 100644
--- a/vscode/package.json
+++ b/vscode/package.json
@@ -551,6 +551,14 @@
           "type": "boolean",
           "default": true
         },
+        "rubyLsp.addonLanguageIds": {
+          "description": "Language IDs of other files to send to the server, for languages that an installed add-on supports (for example, `slim`). Without such an add-on, the server treats these files as Ruby",
+          "type": "array",
+          "items": {
+            "type": "string"
+          },
+          "default": []
+        },
         "rubyLsp.featureFlags": {
           "description": "Allows opting in or out of feature flags",
           "type": "object",
diff --git a/vscode/src/client.ts b/vscode/src/client.ts
index c71737937..2b8b1dde0 100644
--- a/vscode/src/client.ts
+++ b/vscode/src/client.ts
@@ -30,7 +30,7 @@ import {
   ErrorCodes,
 } from "vscode-languageclient/node";
 
-import { LSP_NAME, ClientInterface, Addon, SUPPORTED_LANGUAGE_IDS, FEATURE_FLAGS, featureEnabled } from "./common";
+import { LSP_NAME, ClientInterface, Addon, supportedLanguageIds, FEATURE_FLAGS, featureEnabled } from "./common";
 import { Ruby } from "./ruby";
 import { WorkspaceChannel } from "./workspaceChannel";
 
@@ -169,14 +169,15 @@ function collectClientOptions(
   // 1. Files inside of the workspace itself
   // 2. Bundled gems
   // 3. Default gems
-  let documentSelector: DocumentSelector = SUPPORTED_LANGUAGE_IDS.flatMap((language) => {
+  const languageIds = supportedLanguageIds();
+  let documentSelector: DocumentSelector = languageIds.flatMap((language) => {
     return { scheme: "file", language, pattern: `${fsPath}/**/*` };
   });
 
   // Only the first language server we spawn should handle unsaved files, otherwise requests will be duplicated across
   // all workspaces
   if (isMainWorkspace) {
-    SUPPORTED_LANGUAGE_IDS.forEach((language) => {
+    languageIds.forEach((language) => {
       documentSelector.push({
         language,
         scheme: "untitled",
diff --git a/vscode/src/common.ts b/vscode/src/common.ts
index e27ba06fc..39dfe67b2 100644
--- a/vscode/src/common.ts
+++ b/vscode/src/common.ts
@@ -75,6 +75,13 @@ export const LOG_CHANNEL = vscode.window.createOutputChannel(LSP_NAME, {
 });
 export const SUPPORTED_LANGUAGE_IDS = ["ruby", "erb"];
 
+// Add-ons can make the server understand other languages, such as Slim templates. Users opt into sending those files to
+// the server, since without a matching add-on it would treat them as Ruby
+export function supportedLanguageIds(): string[] {
+  const addonLanguageIds = vscode.workspace.getConfiguration("rubyLsp").get<string[]>("addonLanguageIds") ?? [];
+  return [...SUPPORTED_LANGUAGE_IDS, ...addonLanguageIds];
+}
+
 // A list of feature flags where the key is the name and the value is the rollout percentage.
 //
 // Note: names added here should also be added to the `rubyLsp.optedOutFeatureFlags` enum in the `package.json` file
diff --git a/vscode/src/rubyLsp.ts b/vscode/src/rubyLsp.ts
index e6336fbba..09ff582e1 100644
--- a/vscode/src/rubyLsp.ts
+++ b/vscode/src/rubyLsp.ts
@@ -6,7 +6,7 @@ import { Range } from "vscode-languageclient/node";
 
 import DocumentProvider from "./documentProvider";
 import { Workspace } from "./workspace";
-import { Command, featureEnabled, LOG_CHANNEL, STATUS_EMITTER, SUPPORTED_LANGUAGE_IDS } from "./common";
+import { Command, featureEnabled, LOG_CHANNEL, STATUS_EMITTER, supportedLanguageIds } from "./common";
 import { ManagerIdentifier, ManagerConfiguration } from "./ruby";
 import { StatusItems } from "./status";
 import { TestController } from "./testController";
@@ -72,7 +72,7 @@ export class RubyLsp {
       }),
       // Lazily activate workspaces that do not contain a lockfile
       vscode.workspace.onDidOpenTextDocument(async (document) => {
-        if (!SUPPORTED_LANGUAGE_IDS.includes(document.languageId)) {
+        if (!supportedLanguageIds().includes(document.languageId)) {
           return;
         }
 
@@ -127,7 +127,7 @@ export class RubyLsp {
     // eagerly activate the workspace for that file too
     const activeDocument = vscode.window.activeTextEditor?.document;
 
-    if (activeDocument && SUPPORTED_LANGUAGE_IDS.includes(activeDocument.languageId)) {
+    if (activeDocument && supportedLanguageIds().includes(activeDocument.languageId)) {
       const workspaceFolder = vscode.workspace.getWorkspaceFolder(activeDocument.uri);
 
       if (workspaceFolder) {
diff --git a/vscode/src/status.ts b/vscode/src/status.ts
index 3492b9120..3fe6177ec 100644
--- a/vscode/src/status.ts
+++ b/vscode/src/status.ts
@@ -1,7 +1,7 @@
 import * as vscode from "vscode";
 import { State } from "vscode-languageclient";
 
-import { Command, STATUS_EMITTER, WorkspaceInterface, SUPPORTED_LANGUAGE_IDS } from "./common";
+import { Command, STATUS_EMITTER, WorkspaceInterface, supportedLanguageIds } from "./common";
 
 const STOPPED_SERVER_OPTIONS = [
   { label: "Ruby LSP: Start", description: Command.Start },
@@ -21,7 +21,7 @@ export abstract class StatusItem {
   public item: vscode.LanguageStatusItem;
 
   constructor(id: string) {
-    const documentSelector = SUPPORTED_LANGUAGE_IDS.map((language) => ({
+    const documentSelector = supportedLanguageIds().map((language) => ({
       language,
     }));
 
diff --git a/vscode/src/test/suite/common.test.ts b/vscode/src/test/suite/common.test.ts
index 954797fe4..b8b7c1ff6 100644
--- a/vscode/src/test/suite/common.test.ts
+++ b/vscode/src/test/suite/common.test.ts
@@ -3,7 +3,7 @@ import * as assert from "assert";
 import * as vscode from "vscode";
 import sinon from "sinon";
 
-import { featureEnabled, FEATURE_FLAGS } from "../../common";
+import { featureEnabled, FEATURE_FLAGS, supportedLanguageIds } from "../../common";
 
 suite("featureEnabled", () => {
   let sandbox: sinon.SinonSandbox;
@@ -131,3 +131,31 @@ suite("featureEnabled", () => {
     secondStub.restore();
   });
 });
+
+suite("supportedLanguageIds", () => {
+  let sandbox: sinon.SinonSandbox;
+
+  setup(() => {
+    sandbox = sinon.createSandbox();
+  });
+
+  teardown(() => {
+    sandbox.restore();
+  });
+
+  test("returns Ruby and ERB by default", () => {
+    sandbox.stub(vscode.workspace, "getConfiguration").returns({
+      get: () => undefined,
+    } as any);
+
+    assert.deepStrictEqual(supportedLanguageIds(), ["ruby", "erb"]);
+  });
+
+  test("includes the language IDs configured for add-ons", () => {
+    sandbox.stub(vscode.workspace, "getConfiguration").returns({
+      get: (key: string) => (key === "addonLanguageIds" ? ["slim"] : undefined),
+    } as any);
+
+    assert.deepStrictEqual(supportedLanguageIds(), ["ruby", "erb", "slim"]);
+  });
+});