diff --git a/__tests__/draft.ts b/__tests__/draft.ts
index 52d15996..b8155df7 100644
--- a/__tests__/draft.ts
+++ b/__tests__/draft.ts
@@ -1,4 +1,4 @@
-import {assert, _} from "spec.ts"
+import {assert, _} from "./spec_ts"
 import {produce, Draft, castDraft, original} from "../src/immer"
 
 // For checking if a type is assignable to its draft type (and vice versa)
diff --git a/__tests__/immutable.ts b/__tests__/immutable.ts
index 1e8ec9be..a0941638 100644
--- a/__tests__/immutable.ts
+++ b/__tests__/immutable.ts
@@ -1,4 +1,4 @@
-import {assert, _} from "spec.ts"
+import {assert, _} from "./spec_ts"
 import {produce, Immutable, castImmutable} from "../src/immer"
 
 test("types are ok", () => {
diff --git a/__tests__/produce.ts b/__tests__/produce.ts
index c5074216..22f0a318 100644
--- a/__tests__/produce.ts
+++ b/__tests__/produce.ts
@@ -1,4 +1,4 @@
-import {assert, _} from "spec.ts"
+import {assert, _} from "./spec_ts"
 import {
 	produce,
 	applyPatches,
diff --git a/__tests__/redux.ts b/__tests__/redux.ts
index 0f51f52c..d5dc2343 100644
--- a/__tests__/redux.ts
+++ b/__tests__/redux.ts
@@ -1,4 +1,4 @@
-import {assert, _} from "spec.ts"
+import {assert, _} from "./spec_ts"
 import {produce, Draft} from "../src/immer"
 import * as redux from "redux"
 
diff --git a/__tests__/spec_ts.ts b/__tests__/spec_ts.ts
new file mode 100644
index 00000000..b8debafa
--- /dev/null
+++ b/__tests__/spec_ts.ts
@@ -0,0 +1,56 @@
+/**
+ * This file is literally copied from https://github.com/aleclarson/spec.ts/blob/master/index.d.ts.
+ * For the sole reason, that the package somehow fails to install in our GitHub workflow.
+ * It is unclear why, but all credits to @aleclarson!
+ */
+
+// Give "any" its own class
+export class Any {
+	// @ts-ignore
+	private _: true
+}
+
+// Conditional returns can enforce identical types.
+// See here: https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-421529650
+// prettier-ignore
+type TestExact<Left, Right> =
+    (<U>() => U extends Left ? 1 : 0) extends (<U>() => U extends Right ? 1 : 0) ? Any : never;
+
+type IsAny<T> = Any extends T ? ([T] extends [Any] ? 1 : 0) : 0
+
+export type Test<Left, Right> = IsAny<Left> extends 1
+	? IsAny<Right> extends 1
+		? 1
+		: "❌ Left type is 'any' but right type is not"
+	: IsAny<Right> extends 1
+	? "❌ Right type is 'any' but left type is not"
+	: [Left] extends [Right]
+	? [Right] extends [Left]
+		? Any extends TestExact<Left, Right>
+			? 1
+			: "❌ Unexpected or missing 'readonly' property"
+		: "❌ Right type is not assignable to left type"
+	: "❌ Left type is not assignable to right type"
+
+type Assert<T, U> = U extends 1
+	? T // No error.
+	: IsAny<T> extends 1
+	? never // Ensure "any" is refused.
+	: U // Return the error message.
+
+/**
+ * Raise a compiler error when both argument types are not identical.
+ */
+export const assert: <Left, Right>(
+	left: Assert<Left, Test<Left, Right>>,
+	right: Assert<Right, Test<Left, Right>>
+) => Right = x => x as any
+
+/**
+ * Placeholder value followed by "as T"
+ */
+export const _: any = Symbol("spec.ts placeholder")
+
+test("empty test to silence jest", () => {
+	expect(true).toBeTruthy()
+})
diff --git a/package.json b/package.json
index 445c6641..ee4cd40f 100644
--- a/package.json
+++ b/package.json
@@ -79,7 +79,6 @@
     "rimraf": "^2.6.2",
     "seamless-immutable": "^7.1.3",
     "semantic-release": "^17.0.2",
-    "spec.ts": "^1.1.0",
     "ts-jest": "^29.0.0",
     "tsup": "^6.7.0",
     "typescript": "^5.0.2"
diff --git a/yarn.lock b/yarn.lock
index a1752d03..9814b566 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -8301,11 +8301,6 @@ spdx-license-ids@^3.0.0:
   resolved "https://registry.yarnpkg.com/spdx-license-ids/-/spdx-license-ids-3.0.13.tgz#7189a474c46f8d47c7b0da4b987bb45e908bd2d5"
   integrity sha512-XkD+zwiqXHikFZm4AX/7JSCXA98U5Db4AFd5XUg/+9UNtnH75+Z9KxtpYiJZx36mUDVOwH83pl7yvCer6ewM3w==
 
-spec.ts@^1.1.0:
-  version "1.1.3"
-  resolved "https://registry.yarnpkg.com/spec.ts/-/spec.ts-1.1.3.tgz#2951ed40a984386fdd5bb287719e67a1ceaafff1"
-  integrity sha512-xDzHAwbHqe9OIHT1c+pnVpVuYSHNC5vk51aFKKoql2aH1RCzHplkd7MoSV7uLfzgm3GmlfpAfwLtfH2T3lQMmw==
-
 split-string@^3.0.1, split-string@^3.0.2:
   version "3.1.0"
   resolved "https://registry.yarnpkg.com/split-string/-/split-string-3.1.0.tgz#7cb09dda3a86585705c64b39a6466038682e8fe2"