diff --git a/compiler/src/dotty/tools/repl/Rendering.scala b/compiler/src/dotty/tools/repl/Rendering.scala
index 17c876661e18..b246b75dd5c1 100644
--- a/compiler/src/dotty/tools/repl/Rendering.scala
+++ b/compiler/src/dotty/tools/repl/Rendering.scala
@@ -167,11 +167,14 @@ private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None) {
     val cause = ite.getCause match
       case e: ExceptionInInitializerError => e.getCause
       case e => e
-    def isWrapperCode(ste: StackTraceElement) =
-      ste.getClassName == d.symbol.owner.name.show
+    // detect
+    //at repl$.rs$line$2$.<clinit>(rs$line$2:1)
+    //at repl$.rs$line$2.res1(rs$line$2)
+    def isWrapperInitialization(ste: StackTraceElement) =
+      ste.getClassName.startsWith(nme.REPL_PACKAGE.toString + ".")  // d.symbol.owner.name.show is simple name
       && (ste.getMethodName == nme.STATIC_CONSTRUCTOR.show || ste.getMethodName == nme.CONSTRUCTOR.show)
 
-    cause.formatStackTracePrefix(!isWrapperCode(_))
+    cause.formatStackTracePrefix(!isWrapperInitialization(_))
   end renderError
 
   private def infoDiagnostic(msg: String, d: Denotation)(using Context): Diagnostic =
diff --git a/compiler/test/dotty/tools/repl/ReplCompilerTests.scala b/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
index bc06727f2ea4..3a050a998964 100644
--- a/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
+++ b/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
@@ -219,6 +219,17 @@ class ReplCompilerTests extends ReplTest {
     run("val (x: 1) = 2")
     assertEquals("scala.MatchError: 2 (of class java.lang.Integer)", storedOutput().linesIterator.next())
   }
+  @Test def `i12920 must truncate stack trace to user code` = fromInitialState { implicit state =>
+    run("???")
+    val all = lines()
+    assertEquals(3, all.length)
+    assertEquals("scala.NotImplementedError: an implementation is missing", all.head)
+    /* avoid asserting much about line number or elided count
+    scala.NotImplementedError: an implementation is missing
+      at scala.Predef$.$qmark$qmark$qmark(Predef.scala:344)
+      ... 28 elided
+     */
+  }
 }
 
 object ReplCompilerTests {