diff --git a/README.md b/README.md
index b9d92be..0a5602d 100644
--- a/README.md
+++ b/README.md
@@ -194,9 +194,22 @@ const a: unknown = "Hello";
 // `assert` does nothing or throws an `AssertError`
 assert(a, is.String);
 // a is now narrowed to string
+```
+
+Use [`@core/errorutil/alter`](https://jsr.io/@core/errorutil/doc/alter/~/alter)
+to throw a custom error:
+
+```typescript
+import { alter } from "@core/errorutil/alter";
+import { assert, is } from "@core/unknownutil";
+
+const a: unknown = 0;
 
-// With custom message
-assert(a, is.String, { message: "a must be a string" });
+// The following throws an Error("a is not a string")
+alter(
+  () => assert(a, is.String),
+  new Error("a is not a string"),
+);
 ```
 
 ### ensure
@@ -211,9 +224,22 @@ const a: unknown = "Hello";
 
 // `ensure` returns `string` or throws an `AssertError`
 const _: string = ensure(a, is.String);
+```
+
+Use [`@core/errorutil/alter`](https://jsr.io/@core/errorutil/doc/alter/~/alter)
+to throw a custom error:
+
+```typescript
+import { alter } from "@core/errorutil/alter";
+import { ensure, is } from "@core/unknownutil";
+
+const a: unknown = 0;
 
-// With custom message
-const __: string = ensure(a, is.String, { message: "a must be a string" });
+// The following throws an Error("a is not a string")
+const _: string = alter(
+  () => ensure(a, is.String),
+  new Error("a is not a string"),
+);
 ```
 
 ### maybe
diff --git a/assert.ts b/assert.ts
index c76ad75..9c5415c 100644
--- a/assert.ts
+++ b/assert.ts
@@ -1,81 +1,49 @@
 import type { Predicate } from "./type.ts";
 
-/**
- * A factory function that generates assertion error messages.
- */
-export type AssertMessageFactory = (
-  x: unknown,
-  pred: Predicate<unknown>,
-  name?: string,
-) => string;
-
-/**
- * The default factory function used to generate assertion error messages.
- */
-export const defaultAssertMessageFactory: AssertMessageFactory = (
-  x,
-  pred,
-  name,
-) => {
+const assertMessageFactory = (x: unknown, pred: Predicate<unknown>) => {
   const p = pred.name || "anonymous predicate";
   const t = typeof x;
   const v = JSON.stringify(x, null, 2);
-  return `Expected ${
-    name ?? "a value"
-  } that satisfies the predicate ${p}, got ${t}: ${v}`;
+  return `Expected a value that satisfies the predicate ${p}, got ${t}: ${v}`;
 };
 
-let assertMessageFactory = defaultAssertMessageFactory;
-
 /**
  * Represents an error that occurs when an assertion fails.
  */
 export class AssertError extends Error {
   /**
-   * Constructs a new instance.
-   * @param message The error message.
+   * The value that failed the assertion.
    */
-  constructor(message?: string) {
-    super(message);
+  readonly x: unknown;
 
-    if (Error.captureStackTrace) {
-      Error.captureStackTrace(this, AssertError);
-    }
+  /**
+   * The predicate that the value failed to satisfy.
+   */
+  readonly pred: Predicate<unknown>;
 
+  /**
+   * Constructs a new instance.
+   *
+   * @param x The value that failed the assertion.
+   * @param pred The predicate that the value failed to satisfy.
+   */
+  constructor(x: unknown, pred: Predicate<unknown>) {
+    super(assertMessageFactory(x, pred));
     this.name = this.constructor.name;
+    this.x = x;
+    this.pred = pred;
   }
 }
 
-/**
- * Sets the factory function used to generate assertion error messages.
- *
- * ```ts
- * import { is, setAssertMessageFactory } from "@core/unknownutil";
- *
- * setAssertMessageFactory((x, pred) => {
- *   if (pred === is.String) {
- *     return `Expected a string, got ${typeof x}`;
- *   } else if (pred === is.Number) {
- *     return `Expected a number, got ${typeof x}`;
- *   } else if (pred === is.Boolean) {
- *     return `Expected a boolean, got ${typeof x}`;
- *   } else {
- *     return `Expected a value that satisfies the predicate, got ${typeof x}`;
- *   }
- * });
- * ```
- *
- * @param factory The factory function.
- */
-export function setAssertMessageFactory(factory: AssertMessageFactory): void {
-  assertMessageFactory = factory;
-}
-
 /**
  * Asserts that the given value satisfies the provided predicate.
  *
  * It throws {@linkcode AssertError} if the value does not satisfy the predicate.
  *
+ * @param x The value to be asserted.
+ * @param pred The predicate function to test the value against.
+ * @returns The function has a return type of `asserts x is T` to help TypeScript narrow down the type of `x` after the assertion.
+ *
  * ```ts
  * import { assert, is } from "@core/unknownutil";
  *
@@ -84,19 +52,22 @@ export function setAssertMessageFactory(factory: AssertMessageFactory): void {
  * // a is now narrowed to string
  * ```
  *
- * @param x The value to be asserted.
- * @param pred The predicate function to test the value against.
- * @param options Optional configuration for the assertion.
- * @returns The function has a return type of `asserts x is T` to help TypeScript narrow down the type of `x` after the assertion.
+ * Use {@linkcode https://jsr.io/@core/errorutil/doc/alter/~/alter|@core/errorutil/alter.alter} to alter error.
+ *
+ * ```ts
+ * import { alter } from "@core/errorutil/alter";
+ * import { assert, is } from "@core/unknownutil";
+ *
+ * const a: unknown = 42;
+ * alter(() => assert(a, is.String), new Error("a is not a string"));
+ * // Error: a is not a string
+ * ```
  */
 export function assert<T>(
   x: unknown,
   pred: Predicate<T>,
-  options: { message?: string; name?: string } = {},
 ): asserts x is T {
   if (!pred(x)) {
-    throw new AssertError(
-      options.message ?? assertMessageFactory(x, pred, options.name),
-    );
+    throw new AssertError(x, pred);
   }
 }
diff --git a/assert_test.ts b/assert_test.ts
index ee96eb2..f5bb7e6 100644
--- a/assert_test.ts
+++ b/assert_test.ts
@@ -1,10 +1,5 @@
 import { assertThrows } from "@std/assert";
-import {
-  assert,
-  AssertError,
-  defaultAssertMessageFactory,
-  setAssertMessageFactory,
-} from "./assert.ts";
+import { assert, AssertError } from "./assert.ts";
 
 const x: unknown = Symbol("x");
 
@@ -39,40 +34,4 @@ Deno.test("assert", async (t) => {
       );
     },
   );
-
-  await t.step(
-    "throws an `AssertError` on false predicate with a custom name",
-    () => {
-      assertThrows(
-        () => assert(x, falsePredicate, { name: "hello world" }),
-        AssertError,
-        `Expected hello world that satisfies the predicate falsePredicate, got symbol: undefined`,
-      );
-    },
-  );
-
-  await t.step(
-    "throws an `AssertError` with a custom message on false predicate",
-    () => {
-      assertThrows(
-        () => assert(x, falsePredicate, { message: "Hello" }),
-        AssertError,
-        "Hello",
-      );
-    },
-  );
-});
-
-Deno.test("setAssertMessageFactory", async (t) => {
-  setAssertMessageFactory((x, pred) => `Hello ${typeof x} ${pred.name}`);
-
-  await t.step("change `AssertError` message on `assert` failure", () => {
-    assertThrows(
-      () => assert(x, falsePredicate),
-      AssertError,
-      "Hello symbol falsePredicate",
-    );
-  });
-
-  setAssertMessageFactory(defaultAssertMessageFactory);
 });
diff --git a/deno.jsonc b/deno.jsonc
index 7d37baf..3064849 100644
--- a/deno.jsonc
+++ b/deno.jsonc
@@ -69,6 +69,7 @@
     ]
   },
   "imports": {
+    "@core/errorutil": "jsr:@core/errorutil@^1.2.0",
     "@core/iterutil": "jsr:@core/iterutil@^0.3.0",
     "@core/unknownutil": "./mod.ts",
     "@deno/dnt": "jsr:@deno/dnt@^0.41.1",
diff --git a/ensure.ts b/ensure.ts
index 6de3e7a..1adbc4e 100644
--- a/ensure.ts
+++ b/ensure.ts
@@ -6,6 +6,10 @@ import { assert } from "./assert.ts";
  *
  * It throws {@linkcode [assert].AssertError|AssertError} if the value does not satisfy the predicate.
  *
+ * @param x The value to be ensured.
+ * @param pred The predicate function to test the value against.
+ * @returns The input value `x`.
+ *
  * ```ts
  * import { ensure, is } from "@core/unknownutil";
  *
@@ -13,16 +17,21 @@ import { assert } from "./assert.ts";
  * const _: string = ensure(a, is.String);
  * ```
  *
- * @param x The value to be ensured.
- * @param pred The predicate function to test the value against.
- * @param options Optional configuration for the assertion.
- * @returns The input value `x`.
+ * Use {@linkcode https://jsr.io/@core/errorutil/doc/alter/~/alter|@core/errorutil/alter.alter} to alter error.
+ *
+ * ```ts
+ * import { alter } from "@core/errorutil/alter";
+ * import { ensure, is } from "@core/unknownutil";
+ *
+ * const a: unknown = 42;
+ * const _: string = alter(() => ensure(a, is.String), new Error("a is not a string"));
+ * // Error: a is not a string
+ * ```
  */
 export function ensure<T>(
   x: unknown,
   pred: Predicate<T>,
-  options: { message?: string; name?: string } = {},
 ): T {
-  assert(x, pred, options);
+  assert(x, pred);
   return x;
 }
diff --git a/ensure_test.ts b/ensure_test.ts
index 8659fa0..a836ad8 100644
--- a/ensure_test.ts
+++ b/ensure_test.ts
@@ -1,9 +1,5 @@
 import { assertStrictEquals, assertThrows } from "@std/assert";
-import {
-  AssertError,
-  defaultAssertMessageFactory,
-  setAssertMessageFactory,
-} from "./assert.ts";
+import { AssertError } from "./assert.ts";
 import { ensure } from "./ensure.ts";
 
 const x: unknown = Symbol("x");
@@ -28,40 +24,4 @@ Deno.test("ensure", async (t) => {
       `Expected a value that satisfies the predicate falsePredicate, got symbol: undefined`,
     );
   });
-
-  await t.step(
-    "throws an `AssertError` on false predicate with a custom name",
-    () => {
-      assertThrows(
-        () => ensure(x, falsePredicate, { name: "hello world" }),
-        AssertError,
-        `Expected hello world that satisfies the predicate falsePredicate, got symbol: undefined`,
-      );
-    },
-  );
-
-  await t.step(
-    "throws an `AssertError` with a custom message on false predicate",
-    () => {
-      assertThrows(
-        () => ensure(x, falsePredicate, { message: "Hello" }),
-        AssertError,
-        "Hello",
-      );
-    },
-  );
-});
-
-Deno.test("setAssertMessageFactory", async (t) => {
-  setAssertMessageFactory((x, pred) => `Hello ${typeof x} ${pred.name}`);
-
-  await t.step("change `AssertError` message on `ensure` failure", () => {
-    assertThrows(
-      () => ensure(x, falsePredicate),
-      AssertError,
-      "Hello symbol falsePredicate",
-    );
-  });
-
-  setAssertMessageFactory(defaultAssertMessageFactory);
 });