diff --git a/build.gradle b/build.gradle
index b0eaaa9b..9b9107ed 100644
--- a/build.gradle
+++ b/build.gradle
@@ -16,7 +16,9 @@ compileKotlin {
 	kotlinOptions {
 		jvmTarget = '1.8'
 		freeCompilerArgs += [
+				'-opt-in=kotlin.RequiresOptIn',
 				'-opt-in=kotlin.contracts.ExperimentalContracts',
+				'-opt-in=org.koitharu.kotatsu.parsers.InternalParsersApi',
 		]
 	}
 }
@@ -25,7 +27,9 @@ compileTestKotlin {
 	kotlinOptions {
 		jvmTarget = '1.8'
 		freeCompilerArgs += [
+				'-opt-in=kotlin.RequiresOptIn',
 				'-opt-in=kotlinx.coroutines.ExperimentalCoroutinesApi',
+				'-opt-in=org.koitharu.kotatsu.parsers.InternalParsersApi',
 		]
 	}
 }
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/InternalParsersApi.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/InternalParsersApi.kt
new file mode 100644
index 00000000..676dae33
--- /dev/null
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/InternalParsersApi.kt
@@ -0,0 +1,14 @@
+package org.koitharu.kotatsu.parsers
+
+/**
+ * This marker distinguishes the internal API and is used to opt-in for that feature when parsers developing.
+ *
+ * Any usage of a declaration annotated with `@InternalParsersApi` must be accepted either by
+ * annotating that usage with the [OptIn] annotation, e.g. `@OptIn(InternalParsersApi::class)`,
+ * or by using the compiler argument `-opt-in=org.koitharu.kotatsu.parsers.InternalParsersApi`.
+ */
+@Retention(AnnotationRetention.BINARY)
+@SinceKotlin("1.3")
+@RequiresOptIn
+@MustBeDocumented
+annotation class InternalParsersApi()
\ No newline at end of file
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/MangaParser.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/MangaParser.kt
index c50aa3ea..c2453568 100644
--- a/src/main/kotlin/org/koitharu/kotatsu/parsers/MangaParser.kt
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/MangaParser.kt
@@ -89,6 +89,7 @@ abstract class MangaParser(val source: MangaSource) {
 	 * @see [MangaChapter.id]
 	 * @see [MangaPage.id]
 	 */
+	@InternalParsersApi
 	protected fun generateUid(url: String): Long {
 		var h = 1125899906842597L
 		source.name.forEach { c ->
@@ -107,6 +108,7 @@ abstract class MangaParser(val source: MangaSource) {
 	 * @see [MangaChapter.id]
 	 * @see [MangaPage.id]
 	 */
+	@InternalParsersApi
 	protected fun generateUid(id: Long): Long {
 		var h = 1125899906842597L
 		source.name.forEach { c ->
@@ -127,6 +129,7 @@ abstract class MangaParser(val source: MangaSource) {
 		return toAbsoluteUrl(domain)
 	}
 
+	@InternalParsersApi
 	@Suppress("NOTHING_TO_INLINE")
 	protected inline fun parseFailed(message: String? = null): Nothing {
 		throw ParseException(message, null)
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/AuthRequiredException.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/AuthRequiredException.kt
index 9a9f1a36..d94770e4 100644
--- a/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/AuthRequiredException.kt
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/AuthRequiredException.kt
@@ -1,10 +1,11 @@
 package org.koitharu.kotatsu.parsers.exception
 
+import org.koitharu.kotatsu.parsers.InternalParsersApi
 import org.koitharu.kotatsu.parsers.model.MangaSource
 
 /**
  * Authorization is required for access to the requested content
  */
-class AuthRequiredException(
+class AuthRequiredException @InternalParsersApi constructor(
 	val source: MangaSource,
 ) : RuntimeException("Authorization required")
\ No newline at end of file
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/ParseException.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/ParseException.kt
index 5499fdfb..c0415ac4 100644
--- a/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/ParseException.kt
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/exception/ParseException.kt
@@ -1,6 +1,8 @@
 package org.koitharu.kotatsu.parsers.exception
 
-class ParseException @JvmOverloads constructor(
+import org.koitharu.kotatsu.parsers.InternalParsersApi
+
+class ParseException @InternalParsersApi @JvmOverloads constructor(
 	message: String?,
 	cause: Throwable? = null,
 ) : RuntimeException(message, cause)
\ No newline at end of file
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/model/Manga.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/model/Manga.kt
index 49066162..c346518e 100644
--- a/src/main/kotlin/org/koitharu/kotatsu/parsers/model/Manga.kt
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/model/Manga.kt
@@ -1,5 +1,7 @@
 package org.koitharu.kotatsu.parsers.model
 
+import org.koitharu.kotatsu.parsers.InternalParsersApi
+
 class Manga(
 	/**
 	 * Unique identifier for manga
@@ -74,7 +76,8 @@ class Manga(
 	val hasRating: Boolean
 		get() = rating in 0f..1f
 
-	internal fun copy(
+	@InternalParsersApi
+	fun copy(
 		title: String = this.title,
 		altTitle: String? = this.altTitle,
 		publicUrl: String = this.publicUrl,
diff --git a/src/main/kotlin/org/koitharu/kotatsu/parsers/model/WordSet.kt b/src/main/kotlin/org/koitharu/kotatsu/parsers/model/WordSet.kt
index 3f2c0c68..c2e91358 100644
--- a/src/main/kotlin/org/koitharu/kotatsu/parsers/model/WordSet.kt
+++ b/src/main/kotlin/org/koitharu/kotatsu/parsers/model/WordSet.kt
@@ -1,6 +1,9 @@
 package org.koitharu.kotatsu.parsers.model
 
-internal class WordSet(private vararg val words: String) {
+import org.koitharu.kotatsu.parsers.InternalParsersApi
+
+@InternalParsersApi
+class WordSet(private vararg val words: String) {
 
 	fun anyWordIn(dateString: String): Boolean = words.any {
 		dateString.contains(it, ignoreCase = true)