diff --git a/front_matter/deno.json b/front_matter/deno.json
index ff70d0f7c4c1..443fda982885 100644
--- a/front_matter/deno.json
+++ b/front_matter/deno.json
@@ -8,6 +8,8 @@
     "./test": "./test.ts",
     "./toml": "./toml.ts",
     "./yaml": "./yaml.ts",
+    "./unstable-json": "./unstable_json.ts",
+    "./unstable-toml": "./unstable_toml.ts",
     "./unstable-yaml": "./unstable_yaml.ts",
     "./types": "./types.ts"
   }
diff --git a/front_matter/json_test.ts b/front_matter/json_test.ts
index 70cf511cbc54..76265c1d75a9 100644
--- a/front_matter/json_test.ts
+++ b/front_matter/json_test.ts
@@ -1,9 +1,11 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
+import { assert } from "@std/assert";
 import { assertThrows } from "../assert/throws.ts";
 import { extract } from "./json.ts";
 
 import { assertEquals } from "@std/assert/equals";
+import { test as unstableTest } from "./unstable_json.ts";
 
 Deno.test("json() extracts type error on invalid input", () => {
   assertThrows(() => extract(""));
@@ -80,3 +82,20 @@ Deno.test("extractJson() throws at missing newline before body", () => {
     "Unexpected end of input",
   );
 });
+
+Deno.test("(unstable)test() handles valid json input", () => {
+  assert(unstableTest("---json\nname = 'deno'\n---\n"));
+  assert(unstableTest("= json =\nname = 'deno'\n= json =\n"));
+  assert(unstableTest("= json =\nname = 'deno'\n= json =\ndeno is awesome\n"));
+});
+
+Deno.test("(unstable)test() handles invalid json input", () => {
+  assert(!unstableTest(""));
+  assert(!unstableTest("---"));
+  assert(!unstableTest("---json"));
+  assert(!unstableTest("= json ="));
+  assert(!unstableTest("---\n"));
+  assert(!unstableTest("---json\n"));
+  assert(!unstableTest("= json =\n"));
+  assert(!unstableTest("---\nasdasdasd"));
+});
diff --git a/front_matter/toml_test.ts b/front_matter/toml_test.ts
index 9e4770ca15b8..bb2bafb96530 100644
--- a/front_matter/toml_test.ts
+++ b/front_matter/toml_test.ts
@@ -1,8 +1,10 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
+import { assert } from "@std/assert";
 import { assertThrows } from "../assert/throws.ts";
 import { extract } from "./toml.ts";
 import { assertEquals } from "@std/assert/equals";
+import { test as unstableTest } from "./unstable_toml.ts";
 
 Deno.test("toml() extracts type error on invalid input", () => {
   assertThrows(() => extract(""));
@@ -101,3 +103,20 @@ Deno.test("extractToml() throws at missing newline before body", () => {
     "Unexpected end of input",
   );
 });
+
+Deno.test("(unstable)test() handles valid toml input", () => {
+  assert(unstableTest("---toml\nname = 'deno'\n---\n"));
+  assert(unstableTest("= toml =\nname = 'deno'\n= toml =\n"));
+  assert(unstableTest("= toml =\nname = 'deno'\n= toml =\ndeno is awesome\n"));
+});
+
+Deno.test("(unstable)test() handles invalid toml input", () => {
+  assert(!unstableTest(""));
+  assert(!unstableTest("---"));
+  assert(!unstableTest("---toml"));
+  assert(!unstableTest("= toml ="));
+  assert(!unstableTest("---\n"));
+  assert(!unstableTest("---toml\n"));
+  assert(!unstableTest("= toml =\n"));
+  assert(!unstableTest("---\nasdasdasd"));
+});
diff --git a/front_matter/unstable_json.ts b/front_matter/unstable_json.ts
new file mode 100644
index 000000000000..45c7a2bf42f4
--- /dev/null
+++ b/front_matter/unstable_json.ts
@@ -0,0 +1,43 @@
+// Copyright 2018-2026 the Deno authors. MIT license.
+// This module is browser compatible.
+
+import { test as _test } from "./test.ts";
+
+/**
+ * Tests if a string has valid front matter.
+ * Supports {@link https://www.json.org/ | JSON}.
+ *
+ * @experimental **UNSTABLE**: New API, yet to be vetted.
+ *
+ * @param str String to test.
+ * @returns `true` if the string has valid JSON front matter, otherwise `false`.
+ *
+ * @example Test for valid JSON front matter
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-json";
+ * import { assert } from "@std/assert";
+ *
+ * const result = test(
+ * `---json
+ * {"title": "Three dashes followed by format marks the spot"}
+ * ---
+ * `);
+ * assert(result);
+ * ```
+ *
+ * @example TOML front matter is not valid as JSON
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-json";
+ * import { assertFalse } from "@std/assert";
+ *
+ * const result = test(
+ * `---toml
+ * title = 'Three dashes followed by format marks the spot'
+ * ---
+ * `);
+ * assertFalse(result);
+ * ```
+ */
+export function test(str: string): boolean {
+  return _test(str, ["json"]);
+}
diff --git a/front_matter/unstable_toml.ts b/front_matter/unstable_toml.ts
new file mode 100644
index 000000000000..69299ba5aa0f
--- /dev/null
+++ b/front_matter/unstable_toml.ts
@@ -0,0 +1,43 @@
+// Copyright 2018-2026 the Deno authors. MIT license.
+// This module is browser compatible.
+
+import { test as _test } from "./test.ts";
+
+/**
+ * Tests if a string has valid TOML front matter.
+ * Supports {@link https://toml.io | TOML}.
+ *
+ * @experimental **UNSTABLE**: New API, yet to be vetted.
+ *
+ * @param str String to test.
+ * @returns `true` if the string has valid TOML front matter, otherwise `false`.
+ *
+ * @example Test for valid TOML front matter
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-toml";
+ * import { assert } from "@std/assert";
+ *
+ * const result = test(
+ * `---toml
+ * title = 'Three dashes followed by format marks the spot'
+ * ---
+ * `);
+ * assert(result);
+ * ```
+ *
+ * @example JSON front matter is not valid as TOML
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-toml";
+ * import { assertFalse } from "@std/assert";
+ *
+ * const result = test(
+ * `---json
+ * {"title": "Three dashes followed by format marks the spot"}
+ * ---
+ * `);
+ * assertFalse(result);
+ * ```
+ */
+export function test(str: string): boolean {
+  return _test(str, ["toml"]);
+}
diff --git a/front_matter/unstable_yaml.ts b/front_matter/unstable_yaml.ts
index 10532b9dbf78..71dfb697993e 100644
--- a/front_matter/unstable_yaml.ts
+++ b/front_matter/unstable_yaml.ts
@@ -5,6 +5,7 @@ import { extractFrontMatter } from "./_shared.ts";
 import { parse, type ParseOptions } from "@std/yaml/parse";
 import type { Extract } from "./types.ts";
 import { EXTRACT_YAML_REGEXP } from "./_formats.ts";
+import { test as _test } from "./test.ts";
 
 export type { Extract };
 
@@ -42,3 +43,42 @@ export function extract<T>(text: string, options?: ParseOptions): Extract<T> {
   const attrs = parse(frontMatter, options) as T;
   return { frontMatter, body, attrs };
 }
+
+/**
+ * Tests if a string has valid YAML front matter.
+ * Supports {@link https://yaml.org | YAML}.
+ *
+ * @experimental **UNSTABLE**: New API, yet to be vetted.
+ *
+ * @param str String to test.
+ * @returns `true` if the string has valid YAML front matter, otherwise `false`.
+ *
+ * @example Test for valid YAML front matter
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-yaml";
+ * import { assert } from "@std/assert";
+ *
+ * const result = test(
+ * `---
+ * title: Three dashes marks the spot
+ * ---
+ * `);
+ * assert(result);
+ * ```
+ *
+ * @example JSON front matter is not valid as YAML
+ * ```ts
+ * import { test } from "@std/front-matter/unstable-yaml";
+ * import { assertFalse } from "@std/assert";
+ *
+ * const result = test(
+ * `---json
+ * {"title": "Three dashes followed by format marks the spot"}
+ * ---
+ * `);
+ * assertFalse(result);
+ * ```
+ */
+export function test(str: string): boolean {
+  return _test(str, ["yaml"]);
+}
diff --git a/front_matter/yaml_test.ts b/front_matter/yaml_test.ts
index d8d584d295b6..1a9ecfb0973d 100644
--- a/front_matter/yaml_test.ts
+++ b/front_matter/yaml_test.ts
@@ -1,7 +1,11 @@
 // Copyright 2018-2026 the Deno authors. MIT license.
 
 import { extract } from "./yaml.ts";
-import { extract as unstableExtract } from "./unstable_yaml.ts";
+import {
+  extract as unstableExtract,
+  test as unstableTest,
+} from "./unstable_yaml.ts";
+import { assert } from "../assert/mod.ts";
 import { assertEquals } from "@std/assert/equals";
 import { assertThrows } from "../assert/throws.ts";
 
@@ -126,3 +130,21 @@ Deno.test("extractYaml() throws at missing newline before body", () => {
     "Unexpected end of input",
   );
 });
+
+Deno.test("(unstable)test() handles valid yaml input", () => {
+  assert(unstableTest("---yaml\nname = 'deno'\n---\n"));
+  assert(unstableTest("= yaml =\nname = 'deno'\n= yaml =\n"));
+  assert(unstableTest("= yaml =\nname = 'deno'\n= yaml =\ndeno is awesome\n"));
+  assert(unstableTest("---\nname: deno\n---\n"));
+});
+
+Deno.test("(unstable)test() handles invalid yaml input", () => {
+  assert(!unstableTest(""));
+  assert(!unstableTest("---"));
+  assert(!unstableTest("---yaml"));
+  assert(!unstableTest("= yaml ="));
+  assert(!unstableTest("---\n"));
+  assert(!unstableTest("---yaml\n"));
+  assert(!unstableTest("= yaml =\n"));
+  assert(!unstableTest("---\nasdasdasd"));
+});