diff --git a/src/compiler/scala/tools/nsc/tasty/TreeUnpickler.scala b/src/compiler/scala/tools/nsc/tasty/TreeUnpickler.scala
index 99a1df8bbe47..680949260515 100644
--- a/src/compiler/scala/tools/nsc/tasty/TreeUnpickler.scala
+++ b/src/compiler/scala/tools/nsc/tasty/TreeUnpickler.scala
@@ -734,24 +734,21 @@ class TreeUnpickler[Tasty <: TastyUniverse](
             val isMacro = repr.originalFlagSet.is(Erased | Macro)
             checkUnsupportedFlags(repr.tastyOnlyFlags &~ (Extension | Exported | Infix | optFlag(isMacro)(Erased)))
             val isCtor = sym.isConstructor
-            val paramDefss = readParamss(skipTypeParams=isCtor)(localCtx) // TODO: this can mix type and term parameters, must skip type params if ctor
+            val paramDefss = readParamss(skipTypeParams=isCtor)(localCtx)
             val typeParams: List[Symbol] = {
               if (isCtor) {
-                // skipTypeParams()
                 sym.owner.typeParams
               }
               else {
-                // readParams[NoCycle](TYPEPARAM)(localCtx).map(symFromNoCycle)
-
-                // TODO: Handle extension methods with multiple param lists
                 val first = paramDefss.take(1).flatten.map(symFromNoCycle)
                 if (first.headOption.exists(_.isType)) first else Nil
               }
             }
             val vparamss: List[List[NoCycle]] = {
-              // TODO: Handle extension methods with multiple param lists
-              val droppedClauses = if (typeParams.isEmpty) 0 else 1
-              paramDefss.drop(droppedClauses).ensuring(_.flatten.forall(nc => symFromNoCycle(nc).isTerm))
+              val valueClauses = paramDefss.drop(if (typeParams.isEmpty) 0 else 1)
+              val hasTypeParams = valueClauses.exists(_.exists(nc => symFromNoCycle(nc).isType))
+              unsupportedWhen(hasTypeParams, s"extension method with secondary type parameter list: $tname")
+              valueClauses
             }
             val tpt = readTpt()(localCtx)
             if (isMacro) {
diff --git a/test/tasty/neg/src-2/TestDependentExtension.check b/test/tasty/neg/src-2/TestDependentExtension.check
new file mode 100644
index 000000000000..4a8c391801cf
--- /dev/null
+++ b/test/tasty/neg/src-2/TestDependentExtension.check
@@ -0,0 +1,4 @@
+TestDependentExtension_fail.scala:11: error: Unsupported Scala 3 extension method with secondary type parameter list: extract; found in trait tastytest.DependentExtension.
+    val res = implicitly[DependentExtension].extract(box)(_ + 1)
+                                             ^
+1 error
diff --git a/test/tasty/neg/src-2/TestDependentExtension_fail.scala b/test/tasty/neg/src-2/TestDependentExtension_fail.scala
new file mode 100644
index 000000000000..cfd4bdee8a84
--- /dev/null
+++ b/test/tasty/neg/src-2/TestDependentExtension_fail.scala
@@ -0,0 +1,15 @@
+package tastytest
+
+object TestDependentExtension {
+  import DependentExtension._
+
+  def test = {
+    val box = new Box {
+      type Repr = Int
+      val value: Int = 23
+    }
+    val res = implicitly[DependentExtension].extract(box)(_ + 1)
+    assert(res == 24)
+  }
+
+}
diff --git a/test/tasty/neg/src-2/TestRealFunctor.check b/test/tasty/neg/src-2/TestRealFunctor.check
new file mode 100644
index 000000000000..f992b382783d
--- /dev/null
+++ b/test/tasty/neg/src-2/TestRealFunctor.check
@@ -0,0 +1,4 @@
+TestRealFunctor_fail.scala:6: error: Unsupported Scala 3 extension method with secondary type parameter list: map; found in trait tastytest.RealFunctor.
+    def map[A, B](fa: List[A])(f: A => B): List[B] = fa.map(f)
+        ^
+1 error
diff --git a/test/tasty/neg/src-2/TestRealFunctor_fail.scala b/test/tasty/neg/src-2/TestRealFunctor_fail.scala
new file mode 100644
index 000000000000..75e868da2d67
--- /dev/null
+++ b/test/tasty/neg/src-2/TestRealFunctor_fail.scala
@@ -0,0 +1,9 @@
+package tastytest
+
+object TestRealFunctor {
+  // in this test, we try to implement an extension method that has two type-parameter lists
+  implicit object ListRealFunctor extends RealFunctor[List] {
+    def map[A, B](fa: List[A])(f: A => B): List[B] = fa.map(f)
+  }
+
+}
diff --git a/test/tasty/neg/src-3/DependentExtension.scala b/test/tasty/neg/src-3/DependentExtension.scala
new file mode 100644
index 000000000000..f076c9197246
--- /dev/null
+++ b/test/tasty/neg/src-3/DependentExtension.scala
@@ -0,0 +1,20 @@
+package tastytest
+
+import DependentExtension.Box
+
+// `DependentExtension` uses two type parameter lists in its `extract` method. the second type parameter list is
+// dependent on the first term parameter list. It is unclear how to interpret this.
+trait DependentExtension {
+  extension [B <: Box](b: B) def extract[R >: b.Repr, O](f: R => O): O = f(b.value)
+}
+
+object DependentExtension {
+
+  given DependentExtension with {}
+
+  trait Box {
+    type Repr
+    val value: Repr
+  }
+
+}
diff --git a/test/tasty/neg/src-3/RealFunctor.scala b/test/tasty/neg/src-3/RealFunctor.scala
new file mode 100644
index 000000000000..ec2b3977ca56
--- /dev/null
+++ b/test/tasty/neg/src-3/RealFunctor.scala
@@ -0,0 +1,7 @@
+package tastytest
+
+// This `Functor` uses two type parameter lists in its `map` method.
+// In this example, it may be safe to merge the type parameter lists.
+trait RealFunctor[F[_]] {
+  extension [A](fa: F[A]) def map[B](f: A => B): F[B]
+}