diff --git a/.gitignore b/.gitignore
index b957bb5..a136532 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ node_modules/
 /lib/
 /index.js
 yarn.lock
+!dev/index.d.ts
diff --git a/dev/index.d.ts b/dev/index.d.ts
new file mode 100644
index 0000000..b52a5c2
--- /dev/null
+++ b/dev/index.d.ts
@@ -0,0 +1,100 @@
+export {frontmatterHtml} from './lib/html.js'
+export {frontmatter} from './lib/syntax.js'
+export {toMatters} from './lib/to-matters.js'
+
+/**
+ * Sequence.
+ *
+ * Depending on how this structure is used, it reflects a marker or a fence.
+ */
+export interface Info {
+  /**
+   * Closing.
+   */
+  close: string
+  /**
+   * Opening.
+   */
+  open: string
+}
+
+/**
+ * Fence configuration.
+ */
+interface FenceFields {
+  /**
+   * Complete fences.
+   *
+   * This can be used when fences contain different characters or lengths
+   * other than 3.
+   * Pass `open` and `close` to interface to specify different characters for opening and
+   * closing fences.
+   */
+  fence: Info | string
+  /**
+   * If `fence` is set, `marker` must not be set.
+   */
+  marker?: never
+}
+
+/**
+ * Marker configuration.
+ */
+interface MarkerFields {
+  /**
+   * Character repeated 3 times, used as complete fences.
+   *
+   * For example the character `'-'` will result in `'---'` being used as the
+   * fence
+   * Pass `open` and `close` to specify different characters for opening and
+   * closing fences.
+   */
+  marker: Info | string
+  /**
+   * If `marker` is set, `fence` must not be set.
+   */
+  fence?: never
+}
+
+/**
+ * Fields describing a kind of matter.
+ */
+interface MatterFields {
+  /**
+   * Node type to tokenize as.
+   */
+  type: string
+  /**
+   * Whether matter can be found anywhere in the document, normally, only matter
+   * at the start of the document is recognized.
+   *
+   * > 👉 **Note**: using this is a terrible idea.
+   * > It’s called frontmatter, not matter-in-the-middle or so.
+   * > This makes your markdown less portable.
+   */
+  anywhere?: boolean | null | undefined
+}
+
+/**
+ * Fields describing a kind of matter.
+ *
+ * > 👉 **Note**: using `anywhere` is a terrible idea.
+ * > It’s called frontmatter, not matter-in-the-middle or so.
+ * > This makes your markdown less portable.
+ *
+ * > 👉 **Note**: `marker` and `fence` are mutually exclusive.
+ * > If `marker` is set, `fence` must not be set, and vice versa.
+ */
+export type Matter =
+  | (MatterFields & FenceFields)
+  | (MatterFields & MarkerFields)
+
+/**
+ * Configuration.
+ */
+export type Options = Matter | Preset | Array<Matter | Preset>
+
+/**
+ * Known name of a frontmatter style.
+ */
+export type Preset = 'toml' | 'yaml'
diff --git a/dev/index.js b/dev/index.js
index c5ecadd..f34cad4 100644
--- a/dev/index.js
+++ b/dev/index.js
@@ -1,13 +1,4 @@
-/**
- * @typedef {import('./lib/to-matters.js').Info} Info
- * @typedef {import('./lib/to-matters.js').Matter} Matter
- * @typedef {import('./lib/to-matters.js').Options} Options
- * @typedef {import('./lib/to-matters.js').Preset} Preset
- */
-
+// Note: types exposed from `index.d.ts`.
 export {frontmatter} from './lib/syntax.js'
 export {frontmatterHtml} from './lib/html.js'
 export {toMatters} from './lib/to-matters.js'
-
-// Note: we don’t have an `index.d.ts` in this extension because all token
-// types are dynamic in JS
diff --git a/dev/lib/html.js b/dev/lib/html.js
index b52a8fa..abc3b0b 100644
--- a/dev/lib/html.js
+++ b/dev/lib/html.js
@@ -1,9 +1,6 @@
 /**
- * @typedef {import('micromark-util-types').CompileContext} CompileContext
- * @typedef {import('micromark-util-types').Handle} Handle
- * @typedef {import('micromark-util-types').HtmlExtension} HtmlExtension
- * @typedef {import('micromark-util-types').TokenType} TokenType
- * @typedef {import('./to-matters.js').Options} Options
+ * @import {Options} from 'micromark-extension-frontmatter'
+ * @import {CompileContext, Handle, HtmlExtension, TokenType} from 'micromark-util-types'
  */
 
 import {toMatters} from './to-matters.js'
diff --git a/dev/lib/syntax.js b/dev/lib/syntax.js
index 9a6411f..bfbcd53 100644
--- a/dev/lib/syntax.js
+++ b/dev/lib/syntax.js
@@ -1,15 +1,6 @@
 /**
- * @typedef {import('micromark-util-types').Construct} Construct
- * @typedef {import('micromark-util-types').ConstructRecord} ConstructRecord
- * @typedef {import('micromark-util-types').Extension} Extension
- * @typedef {import('micromark-util-types').State} State
- * @typedef {import('micromark-util-types').TokenType} TokenType
- * @typedef {import('micromark-util-types').TokenizeContext} TokenizeContext
- * @typedef {import('micromark-util-types').Tokenizer} Tokenizer
- *
- * @typedef {import('./to-matters.js').Info} Info
- * @typedef {import('./to-matters.js').Matter} Matter
- * @typedef {import('./to-matters.js').Options} Options
+ * @import {Info, Matter, Options} from 'micromark-extension-frontmatter'
+ * @import {Construct, ConstructRecord, Extension, State, TokenType, TokenizeContext, Tokenizer} from 'micromark-util-types'
  */
 
 import {markdownLineEnding, markdownSpace} from 'micromark-util-character'
diff --git a/dev/lib/to-matters.js b/dev/lib/to-matters.js
index 55f795f..44ddab5 100644
--- a/dev/lib/to-matters.js
+++ b/dev/lib/to-matters.js
@@ -1,64 +1,5 @@
 /**
- * @typedef {'toml' | 'yaml'} Preset
- *   Known name of a frontmatter style.
- *
- * @typedef Info
- *   Sequence.
- *
- *   Depending on how this structure is used, it reflects a marker or a fence.
- * @property {string} close
- *   Closing.
- * @property {string} open
- *   Opening.
- *
- * @typedef MatterProps
- *   Fields describing a kind of matter.
- * @property {string} type
- *   Node type to tokenize as.
- * @property {boolean | null | undefined} [anywhere=false]
- *   Whether matter can be found anywhere in the document, normally, only matter
- *   at the start of the document is recognized.
- *
- *   > 👉 **Note**: using this is a terrible idea.
- *   > It’s called frontmatter, not matter-in-the-middle or so.
- *   > This makes your markdown less portable.
- *
- * @typedef MarkerProps
- *   Marker configuration.
- * @property {Info | string} marker
- *   Character repeated 3 times, used as complete fences.
- *
- *   For example the character `'-'` will result in `'---'` being used as the
- *   fence
- *   Pass `open` and `close` to specify different characters for opening and
- *   closing fences.
- * @property {never} [fence]
- *   If `marker` is set, `fence` must not be set.
- *
- * @typedef FenceProps
- *   Fence configuration.
- * @property {Info | string} fence
- *   Complete fences.
- *
- *   This can be used when fences contain different characters or lengths
- *   other than 3.
- *   Pass `open` and `close` to interface to specify different characters for opening and
- *   closing fences.
- * @property {never} [marker]
- *   If `fence` is set, `marker` must not be set.
- *
- * @typedef {(MatterProps & FenceProps) | (MatterProps & MarkerProps)} Matter
- *   Fields describing a kind of matter.
- *
- *   > 👉 **Note**: using `anywhere` is a terrible idea.
- *   > It’s called frontmatter, not matter-in-the-middle or so.
- *   > This makes your markdown less portable.
- *
- *   > 👉 **Note**: `marker` and `fence` are mutually exclusive.
- *   > If `marker` is set, `fence` must not be set, and vice versa.
- *
- * @typedef {Matter | Preset | Array<Matter | Preset>} Options
- *   Configuration.
+ * @import {Matter, Options, Preset} from 'micromark-extension-frontmatter'
  */
 
 import {fault} from 'fault'
diff --git a/package.json b/package.json
index cff9558..dab8b81 100644
--- a/package.json
+++ b/package.json
@@ -83,6 +83,31 @@
     "strict": true
   },
   "xo": {
+    "overrides": [
+      {
+        "files": [
+          "**/*.d.ts"
+        ],
+        "rules": {
+          "@typescript-eslint/array-type": [
+            "error",
+            {
+              "default": "generic"
+            }
+          ],
+          "@typescript-eslint/ban-types": [
+            "error",
+            {
+              "extendDefaults": true
+            }
+          ],
+          "@typescript-eslint/consistent-type-definitions": [
+            "error",
+            "interface"
+          ]
+        }
+      }
+    ],
     "prettier": true,
     "rules": {
       "unicorn/no-this-assignment": "off",
diff --git a/test/index.js b/test/index.js
index 06af15b..92b2498 100644
--- a/test/index.js
+++ b/test/index.js
@@ -1,5 +1,5 @@
 /**
- * @typedef {import('micromark-extension-frontmatter').Options} Options
+ * @import {Options} from 'micromark-extension-frontmatter'
  */
 
 import assert from 'node:assert/strict'
diff --git a/tsconfig.json b/tsconfig.json
index 241c4e6..e82ed5a 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -11,5 +11,5 @@
     "target": "es2022"
   },
   "exclude": ["coverage/", "lib/", "node_modules/", "index.js"],
-  "include": ["**/*.js"]
+  "include": ["**/*.js", "dev/index.d.ts"]
 }