diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 23c105a71..60ed2a3b1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -87,6 +87,10 @@ jobs:
               run: npm install
             - name: Check Types
               run: npm run test:types
+            - name: Check Types (TypeScript 5.x)
+              run: npm run test:types:5.x
+            - name: Check Types (TypeScript 5.3)
+              run: npm run test:types:5.3
 
     are_the_types_wrong:
         name: Are the types wrong?
diff --git a/eslint.config-content.js b/eslint.config-content.js
index 506ab8af5..a1bcc887e 100644
--- a/eslint.config-content.js
+++ b/eslint.config-content.js
@@ -3,7 +3,7 @@ import markdown from "./src/index.js";
 
 export default defineConfig([
 	globalIgnores(
-		["**/*.js", "**/.cjs", "**/.mjs"],
+		["**/*.js", "**/.cjs", "**/.mjs", "tests/fixtures/"],
 		"markdown/content/ignores",
 	),
 
diff --git a/eslint.config.js b/eslint.config.js
index 858a4e7eb..19cf1c39f 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -134,9 +134,18 @@ export default defineConfig([
 		},
 	},
 	{
+		name: "json/json",
 		plugins: { json },
 		files: ["**/*.json", ".c8rc"],
 		language: "json/json",
 		extends: ["json/recommended"],
 	},
+	{
+		name: "json/jsonc",
+		plugins: { json },
+		files: ["**/*.jsonc", "**/tsconfig*.json"],
+		language: "json/jsonc",
+		languageOptions: { allowTrailingCommas: true },
+		extends: ["json/recommended"],
+	},
 ]);
diff --git a/examples/typescript/package.json b/examples/typescript/package.json
index fe1d19147..c0e27186d 100644
--- a/examples/typescript/package.json
+++ b/examples/typescript/package.json
@@ -7,7 +7,7 @@
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "eslint": "^10.0.3",
-    "typescript": "^5.9.3",
-    "typescript-eslint": "^8.57.0"
+    "typescript": "^6.0.3",
+    "typescript-eslint": "^8.59.1"
   }
 }
diff --git a/package.json b/package.json
index 51abe5127..0a7e45a7e 100644
--- a/package.json
+++ b/package.json
@@ -70,7 +70,10 @@
     "test": "mocha \"tests/**/*.test.js\" --timeout 30000",
     "test:coverage": "c8 npm test",
     "test:jsr": "npx -y jsr@latest publish --dry-run",
-    "test:types": "tsc -p tests/types/tsconfig.json"
+    "test:types": "tsc -p tests/types/tsconfig.json",
+    "test:types:5.x": "npx -p typescript@5.x -y -- tsc -p tests/types/tsconfig.json",
+    "test:types:5.3": "npx -p typescript@5.3 -y -- tsc -p tests/types/tsconfig.legacy.json",
+    "test:types:all": "npm run test:types && npm run test:types:5.x && npm run test:types:5.3"
   },
   "devDependencies": {
     "@arethetypeswrong/cli": "^0.18.2",
@@ -90,7 +93,7 @@
     "mocha": "^11.7.5",
     "prettier": "3.8.3",
     "semver": "^7.7.3",
-    "typescript": "^5.9.3",
+    "typescript": "^6.0.3",
     "yorkie": "^2.0.0"
   },
   "dependencies": {
diff --git a/src/index.js b/src/index.js
index 1a827a031..cd90e053b 100644
--- a/src/index.js
+++ b/src/index.js
@@ -129,8 +129,8 @@ const plugin = {
 	},
 };
 
-// @ts-expect-error
-recommendedPlugins.markdown = processorPlugins.markdown = plugin;
+Object.assign(recommendedPlugins, { markdown: plugin });
+Object.assign(processorPlugins, { markdown: plugin });
 
 export default plugin;
 export { MarkdownSourceCode };
diff --git a/src/language/markdown-source-code.js b/src/language/markdown-source-code.js
index ddb8f84c0..97c54aacf 100644
--- a/src/language/markdown-source-code.js
+++ b/src/language/markdown-source-code.js
@@ -21,7 +21,7 @@ import { lineEndingPattern } from "../util.js";
 
 /**
  * @import { Position } from "unist";
- * @import { Root, Node, Html } from "mdast";
+ * @import { Parent, Root, Node, Html } from "mdast";
  * @import { TraversalStep, FileProblem, DirectiveType, RulesConfig } from "@eslint/core";
  * @import { MarkdownLanguageOptions } from "../types.js";
  */
@@ -77,7 +77,7 @@ function extractInlineConfigCommentsFromHTML(node, sourceCode) {
 	/** @type {Array<InlineConfigComment>} */
 	const comments = [];
 
-	/** @type {RegExpExecArray} */
+	/** @type {RegExpExecArray | null} */
 	let match;
 
 	while ((match = htmlComment.exec(node.value))) {
@@ -124,7 +124,7 @@ export class MarkdownSourceCode extends TextSourceCodeBase {
 
 	/**
 	 * Cache of parent nodes.
-	 * @type {WeakMap<Node, Node>}
+	 * @type {WeakMap<Node, Parent|undefined>}
 	 */
 	#parents = new WeakMap();
 
@@ -163,7 +163,7 @@ export class MarkdownSourceCode extends TextSourceCodeBase {
 	/**
 	 * Returns the parent of the given node.
 	 * @param {Node} node The node to get the parent of.
-	 * @returns {Node|undefined} The parent of the node.
+	 * @returns {Parent|undefined} The parent of the node.
 	 */
 	getParent(node) {
 		return this.#parents.get(node);
@@ -303,6 +303,12 @@ export class MarkdownSourceCode extends TextSourceCodeBase {
 		/** @type {Array<VisitNodeStep>} */
 		const steps = (this.#steps = []);
 
+		/**
+		 * Recursively visits a node and its children.
+		 * @param {Node} node The node to visit.
+		 * @param {Parent} [parent] The parent of the node.
+		 * @returns {void}
+		 */
 		const visit = (node, parent) => {
 			// first set the parent
 			this.#parents.set(node, parent);
@@ -318,13 +324,15 @@ export class MarkdownSourceCode extends TextSourceCodeBase {
 
 			// save HTML nodes
 			if (node.type === "html") {
-				this.#htmlNodes.push(node);
+				this.#htmlNodes.push(/** @type {Html} */ (node));
 			}
 
 			// then visit the children
-			if (node.children) {
-				node.children.forEach(child => {
-					visit(child, node);
+			if ("children" in node) {
+				const parentNode = /** @type {Parent} */ (node);
+
+				parentNode.children.forEach(child => {
+					visit(child, parentNode);
 				});
 			}
 
diff --git a/src/processor.js b/src/processor.js
index 3b75fb32e..7b1e48504 100644
--- a/src/processor.js
+++ b/src/processor.js
@@ -42,14 +42,14 @@ const blocksCache = new Map();
 /**
  * Performs a depth-first traversal of the Markdown AST.
  * @param {Node} node A Markdown AST node.
- * @param {{[key: string]: (node?: Node) => void}} callbacks A map of node types to callbacks.
+ * @param {{[key: string]: (() => void) | ((node: Code) => void) | ((node: Html) => void)}} callbacks A map of node types to callbacks.
  * @returns {void}
  */
 function traverse(node, callbacks) {
 	if (callbacks[node.type]) {
-		callbacks[node.type](node);
+		/** @type {(node: Node) => void} */ (callbacks[node.type])(node);
 	} else {
-		callbacks["*"]();
+		/** @type {() => void} */ (callbacks["*"])();
 	}
 
 	const parent = /** @type {Parent} */ (node);
@@ -339,7 +339,11 @@ function preprocess(sourceText, filename) {
 	return blocks.map((block, index) => {
 		const [language] = block.lang.trim().split(" ");
 		const fileExtension = Object.hasOwn(languageToFileExtension, language)
-			? languageToFileExtension[language]
+			? languageToFileExtension[
+					/** @type {keyof typeof languageToFileExtension} */ (
+						language
+					)
+				]
 			: language;
 
 		return {
@@ -412,6 +416,7 @@ function adjustBlock(block) {
 			return null;
 		}
 
+		/** @type {Pick<Message, "line" | "column" | "endLine" | "suggestions">} */
 		const out = {
 			line: lineInCode + blockStart,
 			column: message.column + block.rangeMap[lineInCode].indent,
diff --git a/tests/types/tsconfig.json b/tests/types/tsconfig.json
index 8387c00c5..94376dbd1 100644
--- a/tests/types/tsconfig.json
+++ b/tests/types/tsconfig.json
@@ -1,12 +1,17 @@
 {
   "extends": "../../tsconfig.json",
   "compilerOptions": {
-    "emitDeclarationOnly": false,
+    "allowJs": false,
+    "checkJs": false,
     "noEmit": true,
     "rootDir": "../..",
     "strict": true,
-    "exactOptionalPropertyTypes": true
+    "strictNullChecks": true,
+    "useUnknownInCatchVariables": true,
+    "exactOptionalPropertyTypes": true,
+    "verbatimModuleSyntax": true,
+    "erasableSyntaxOnly": true
   },
   "files": [],
-  "include": ["**/*.test.ts", "**/*.test.cts", "../../dist"]
+  "include": [".", "../../dist"]
 }
diff --git a/tests/types/tsconfig.legacy.json b/tests/types/tsconfig.legacy.json
new file mode 100644
index 000000000..11f712e18
--- /dev/null
+++ b/tests/types/tsconfig.legacy.json
@@ -0,0 +1,17 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "allowJs": false,
+    "checkJs": false,
+    "noEmit": true,
+    "rootDir": "../..",
+    "strict": true,
+    "strictNullChecks": true,
+    "useUnknownInCatchVariables": true,
+    "exactOptionalPropertyTypes": true,
+    "verbatimModuleSyntax": true
+    // "erasableSyntaxOnly" is not supported in TypeScript < 5.8.
+  },
+  "files": [],
+  "include": [".", "../../dist"]
+}
diff --git a/tests/types/types.test.ts b/tests/types/types.test.ts
index e0a00094f..2603e5281 100644
--- a/tests/types/types.test.ts
+++ b/tests/types/types.test.ts
@@ -194,6 +194,7 @@ const invalidLanguageOptions2: MarkdownLanguageOptions = {
 			sourceCode.getIndexFromLoc({ line: 1, column: 1 }) satisfies number;
 			sourceCode.getRange(node) satisfies SourceRange;
 			sourceCode.getParent(node) satisfies Node | undefined;
+			sourceCode.getParent(node) satisfies Parent | undefined;
 			sourceCode.getAncestors(node) satisfies Node[];
 			sourceCode.getText(node) satisfies string;
 			sourceCode.applyInlineConfig().configs[0].loc.start
diff --git a/tsconfig.json b/tsconfig.json
index 2c2f76737..db9fdfac5 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -7,6 +7,10 @@
     "outDir": "dist",
     "target": "ES2022",
     "moduleResolution": "NodeNext",
-    "module": "NodeNext"
+    "module": "NodeNext",
+    "rootDir": "./src",
+    "strictNullChecks": false,
+    "useUnknownInCatchVariables": false,
+    "types": []
   }
 }