diff --git a/build.sbt b/build.sbt
index 2e35672..01b9035 100644
--- a/build.sbt
+++ b/build.sbt
@@ -81,6 +81,15 @@ lazy val sttp = (project in file("sttp"))
   )
   .dependsOn(core % "compile->compile;test->test")
 
+lazy val testkit = (project in file("testkit"))
+  .settings(
+    commonSettings ++ Seq(
+      name := "oidc4s-testkit",
+      mimaPreviousArtifacts := Set.empty // TODO remove after release
+    )
+  )
+  .dependsOn(core)
+
 lazy val quickSttpCirce = (project in file("quick-sttp-circe"))
   .settings(
     commonSettings ++ Seq(
@@ -94,4 +103,4 @@ lazy val root = (project in file("."))
     publish / skip := true,
     mimaFailOnNoPrevious := false
   )
-  .aggregate(core, circe, sttp, quickSttpCirce)
+  .aggregate(core, circe, sttp, quickSttpCirce, testkit)
diff --git a/testkit/src/main/scala/IdTokenVerifierMock.scala b/testkit/src/main/scala/IdTokenVerifierMock.scala
new file mode 100644
index 0000000..1e0f166
--- /dev/null
+++ b/testkit/src/main/scala/IdTokenVerifierMock.scala
@@ -0,0 +1,129 @@
+import cats.Applicative
+import cats.Traverse
+import cats.data.NonEmptySet
+import cats.effect.Clock
+import cats.implicits._
+import me.wojnowski.oidc4s.IdTokenClaims.Audience
+import me.wojnowski.oidc4s.json.JsonDecoder.ClaimsDecoder
+import me.wojnowski.oidc4s.ClientId
+import me.wojnowski.oidc4s.IdTokenClaims
+import me.wojnowski.oidc4s.IdTokenVerifier
+import me.wojnowski.oidc4s.Issuer
+import me.wojnowski.oidc4s.IdTokenVerifier.Error.CouldNotDecodeClaim
+import me.wojnowski.oidc4s.json.JsonDecoder
+import me.wojnowski.oidc4s.json.JsonSupport
+
+object IdTokenVerifierMock {
+
+  def constRawClaims[F[_]: Applicative](rawClaims: String)(implicit jsonSupport: JsonSupport): IdTokenVerifier[F] =
+    constRawClaimsEither(Right(rawClaims))
+
+  def constRawClaimsEither[F[_]: Applicative](rawClaimsEither: Either[IdTokenVerifier.Error, String])(implicit jsonSupport: JsonSupport)
+    : IdTokenVerifier[F] = constRawClaimsEitherPF(_ => rawClaimsEither)
+
+  def constRawClaimsEitherPF[F[_]: Applicative](
+    rawTokenToRawClaimsEither: PartialFunction[String, Either[IdTokenVerifier.Error, String]]
+  )(implicit jsonSupport: JsonSupport
+  ): IdTokenVerifier[F] = new IdTokenVerifier[F] {
+    import jsonSupport._
+
+    override def verify(rawToken: String, expectedClientId: ClientId): F[Either[IdTokenVerifier.Error, IdTokenClaims.Subject]] =
+      verifyAndDecode(rawToken).map(_.map(_.subject))
+
+    override def verifyAndDecode(rawToken: String): F[Either[IdTokenVerifier.Error, IdTokenClaims]] =
+      rawTokenToRawClaimsEither
+        .lift(rawToken)
+        .toRight(IdTokenVerifier.Error.MalformedToken: IdTokenVerifier.Error)
+        .flatten
+        .flatMap { rawClaims =>
+          JsonDecoder[IdTokenClaims]
+            .decode(rawClaims)
+            .leftMap(IdTokenVerifier.Error.CouldNotDecodeClaim(_): IdTokenVerifier.Error)
+        }
+        .pure[F]
+
+    override def verifyAndDecodeCustom[A](rawToken: String)(implicit decoder: ClaimsDecoder[A]): F[Either[IdTokenVerifier.Error, A]] =
+      rawTokenToRawClaimsEither
+        .lift(rawToken)
+        .toRight(IdTokenVerifier.Error.MalformedToken: IdTokenVerifier.Error)
+        .flatten
+        .flatMap { rawClaims =>
+          ClaimsDecoder[A]
+            .decode(rawClaims)
+            .map(_._1)
+            .leftMap(IdTokenVerifier.Error.CouldNotDecodeClaim(_): IdTokenVerifier.Error)
+        }
+        .pure[F]
+
+    override def verifyAndDecodeCustom[A](rawToken: String, expectedClientId: ClientId)(implicit decoder: ClaimsDecoder[A])
+      : F[Either[IdTokenVerifier.Error, A]] =
+      verifyAndDecodeCustom(rawToken)
+
+  }
+
+  def constSubject[F[_]: Applicative: Traverse: Clock](subject: IdTokenClaims.Subject): IdTokenVerifier[F] =
+    constSubjectEither[F](Right(subject))
+
+  def constSubjectEither[F[_]: Applicative: Traverse: Clock](errorOrSubject: Either[IdTokenVerifier.Error, IdTokenClaims.Subject])
+    : IdTokenVerifier[F] =
+    constSubjectPF[F]((_: String) => errorOrSubject)
+
+  def constSubjectPF[F[_]: Applicative: Traverse: Clock](
+    rawTokenToSubjectPF: PartialFunction[String, Either[IdTokenVerifier.Error, IdTokenClaims.Subject]]
+  ): IdTokenVerifier[F] =
+    constStandardClaimsPF(
+      rawTokenToSubjectPF.map { errorOrSubject =>
+        Applicative[F].map(Clock[F].realTimeInstant) { now =>
+          errorOrSubject.map(subject =>
+            IdTokenClaims(
+              Issuer("https://example.com"),
+              subject,
+              NonEmptySet.of(Audience("https://example.com")),
+              expiration = now.plusSeconds(600),
+              issuedAt = now
+            )
+          )
+        }
+      }
+    )
+
+  def constStandardClaims[F[_]: Applicative: Traverse](claims: IdTokenClaims): IdTokenVerifier[F] = constStandardClaimsEither(Right(claims))
+
+  def constStandardClaimsEither[F[_]: Applicative: Traverse](claimsEither: Either[IdTokenVerifier.Error, IdTokenClaims])
+    : IdTokenVerifier[F] =
+    constStandardClaimsPF[F](_ => claimsEither.pure[F])
+
+  def constStandardClaimsPF[F[_]: Applicative: Traverse](
+    rawTokenToClaimsPF: PartialFunction[String, F[Either[IdTokenVerifier.Error, IdTokenClaims]]]
+  ): IdTokenVerifier[F] = new IdTokenVerifier[F] {
+
+    override def verifyAndDecode(rawToken: String): F[Either[IdTokenVerifier.Error, IdTokenClaims]] =
+      Applicative[F].map(
+        rawTokenToClaimsPF
+          .lift(rawToken)
+          .toRight(IdTokenVerifier.Error.MalformedToken: IdTokenVerifier.Error)
+          .sequence
+      )(_.flatten)
+
+    override def verify(rawToken: String, expectedClientId: ClientId): F[Either[IdTokenVerifier.Error, IdTokenClaims.Subject]] =
+      Applicative[F].map(verifyAndDecode(rawToken))(_.map(_.subject))
+
+    override def verifyAndDecodeCustom[A](rawToken: String)(implicit decoder: ClaimsDecoder[A]): F[Either[IdTokenVerifier.Error, A]] =
+      Applicative[F].map(
+        rawTokenToClaimsPF
+          .lift(rawToken)
+          .toRight(IdTokenVerifier.Error.MalformedToken: IdTokenVerifier.Error)
+          .sequence
+      ) {
+        _.flatten.flatMap { _ =>
+          CouldNotDecodeClaim("mock").asLeft[A]
+        }
+      }
+
+    override def verifyAndDecodeCustom[A](rawToken: String, expectedClientId: ClientId)(implicit decoder: ClaimsDecoder[A])
+      : F[Either[IdTokenVerifier.Error, A]] =
+      verifyAndDecodeCustom(rawToken)
+
+  }
+
+}