diff --git a/backend/src/utils.test.ts b/backend/src/utils.test.ts
index 345c32a3..9d72ce0f 100644
--- a/backend/src/utils.test.ts
+++ b/backend/src/utils.test.ts
@@ -1,6 +1,10 @@
-import { convertDelayStringToMS, convertMSToDelayString, getUrlsInString } from "./utils";
+import * as ioTs from "io-ts";
+import { convertDelayStringToMS, convertMSToDelayString, getUrlsInString, tAllowedMentions } from "./utils";
 
 import test from "ava";
+import { AllowedMentions as ErisAllowedMentions } from "eris";
+
+type AssertEquals<TActual, TExpected> = TActual extends TExpected ? true : false;
 
 test("getUrlsInString(): detects full links", t => {
   const urls = getUrlsInString("foo https://google.com/ bar");
@@ -45,3 +49,9 @@ test("delay strings: reverse conversion (conservative)", t => {
   const expected = "2w";
   t.is(convertMSToDelayString(ms), expected);
 });
+
+test("tAllowedMentions matches Eris's AllowedMentions", t => {
+  type TAllowedMentions = ioTs.TypeOf<typeof tAllowedMentions>;
+  const typeTest: AssertEquals<TAllowedMentions, ErisAllowedMentions> = true;
+  t.pass();
+});
diff --git a/backend/src/utils.ts b/backend/src/utils.ts
index 26889b3d..22fabce5 100644
--- a/backend/src/utils.ts
+++ b/backend/src/utils.ts
@@ -282,6 +282,16 @@ export const tStrictMessageContent = t.type({
 
 export const tMessageContent = t.union([t.string, tStrictMessageContent]);
 
+/**
+ * Mirrors AllowedMentions from Eris
+ */
+export const tAllowedMentions = t.type({
+  everyone: tNormalizedNullOptional(t.boolean),
+  users: tNormalizedNullOptional(t.union([t.boolean, t.array(t.string)])),
+  roles: tNormalizedNullOptional(t.union([t.boolean, t.array(t.string)])),
+  repliedUser: tNormalizedNullOptional(t.boolean),
+});
+
 export function dropPropertiesByName(obj, propName) {
   if (obj.hasOwnProperty(propName)) delete obj[propName];
   for (const value of Object.values(obj)) {