diff --git a/packages/backend/src/misc/is-error.ts b/packages/backend/src/misc/is-error.ts
new file mode 100644
index 0000000000..1e1f00e48c
--- /dev/null
+++ b/packages/backend/src/misc/is-error.ts
@@ -0,0 +1,75 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { types } from 'node:util';
+
+const hasDOMException = Reflect.has(globalThis, 'DOMException');
+const hasErrorIsError = Reflect.has(globalThis.Error, 'isError');
+
+/**
+ * Polyfill for https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/isError
+ */
+export const isError: IsErrorFunc = hasErrorIsError ? Error.isError : isErrorPolyfill;
+
+/**
+ * Returns true if error is an instance of Error, false otherwise.
+ * More robust than instanceof.
+ */
+export type IsErrorFunc = (error: unknown) => error is Error;
+
+export function isErrorPolyfill(error: unknown): error is Error {
+	// These are the fastest checks, so run them first
+	if (isErrorByInstance(error)) {
+		return true;
+	}
+
+	// Errors must be a non-null object
+	if (typeof(error) !== 'object' || error == null) {
+		return false;
+	}
+
+	// jest, and maybe a few other edge cases
+	// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/species
+	if ('constructor' in error && isErrorByInstance(error.constructor[Symbol.species])) {
+		return true;
+	}
+
+	// If it looks like a duck and quacks like a duck...
+	if ('name' in error && typeof(error.name) === 'string') {
+		if ('message' in error && typeof(error.message) === 'string') {
+			if (!('stack' in error) || typeof(error.stack) === 'string' || typeof(error.stack) === 'undefined') {
+				return true;
+			}
+		}
+	}
+
+	// Guess it's not :(
+	return false;
+}
+
+function isErrorByInstance(error: unknown): boolean {
+	// It must be a non-null object
+	if (typeof(error) !== 'object' || error == null) {
+		return false;
+	}
+
+	// An actual instance, nice
+	if (error instanceof Error) {
+		return true;
+	}
+
+	// DOMException is an Error, just without the prototype chain
+	if (hasDOMException && error instanceof DOMException) {
+		return true;
+	}
+
+	// Realm fuckery
+	if (types.isNativeError(error)) {
+		return true;
+	}
+
+	return false;
+}
+