diff --git a/src/Lean/Meta/DiscrTree.lean b/src/Lean/Meta/DiscrTree.lean
index 4b6b882141..45a60f5264 100644
--- a/src/Lean/Meta/DiscrTree.lean
+++ b/src/Lean/Meta/DiscrTree.lean
@@ -80,6 +80,51 @@ def Key.format : Key → Format
 
 instance : ToFormat Key := ⟨Key.format⟩
 
+/--
+Helper function for converting an entry (i.e., `Array Key`) to the discrimination tree into
+`MessageData` that is more user-friendly. We use this function to implement diagnostic information.
+-/
+partial def keysAsPattern (keys : Array Key) : CoreM MessageData := do
+  go (parenIfNonAtomic := false) |>.run' keys.toList
+where
+  next? : StateRefT (List Key) CoreM (Option Key) := do
+    let key :: keys ← get | return none
+    set keys
+    return some key
+
+  mkApp (f : MessageData) (args : Array MessageData) (parenIfNonAtomic : Bool) : CoreM MessageData := do
+    if args.isEmpty then
+      return f
+    else
+      let mut r := f
+      for arg in args do
+        r := r ++ m!" {arg}"
+      if parenIfNonAtomic then
+        return m!"({r})"
+      else
+        return r
+
+  go (parenIfNonAtomic := true) : StateRefT (List Key) CoreM MessageData := do
+    let some key ← next? | return .nil
+    match key with
+    | .const declName nargs =>
+      mkApp m!"{← mkConstWithLevelParams declName}" (← goN nargs) parenIfNonAtomic
+    | .fvar fvarId nargs =>
+      mkApp m!"{mkFVar fvarId}" (← goN nargs) parenIfNonAtomic
+    | .proj _ i nargs =>
+      mkApp m!"{← go}.{i+1}" (← goN nargs) parenIfNonAtomic
+    | .arrow => return "<arrow>"
+    | .star => return "_"
+    | .other => return "<other>"
+    | .lit (.natVal v) => return m!"{v}"
+    | .lit (.strVal v) => return m!"{v}"
+
+  goN (num : Nat) : StateRefT (List Key) CoreM (Array MessageData) := do
+    let mut r := #[]
+    for _ in [: num] do
+      r := r.push (← go)
+    return r
+
 def Key.arity : Key → Nat
   | .const _ a  => a
   | .fvar _ a   => a
diff --git a/src/Lean/Meta/Tactic/Simp/Diagnostics.lean b/src/Lean/Meta/Tactic/Simp/Diagnostics.lean
index 3a07e5c172..23a8dba8ab 100644
--- a/src/Lean/Meta/Tactic/Simp/Diagnostics.lean
+++ b/src/Lean/Meta/Tactic/Simp/Diagnostics.lean
@@ -41,7 +41,7 @@ private def mkTheoremsWithBadKeySummary (thms : PArray SimpTheorem) : MetaM Diag
   else
     let mut data := #[]
     for thm in thms do
-      data := data.push m!"{if data.isEmpty then "  " else "\n"}{← originToKey thm.origin}, key: {thm.keys.map (·.format)}"
+      data := data.push m!"{if data.isEmpty then "  " else "\n"}{← originToKey thm.origin}, key: {← DiscrTree.keysAsPattern thm.keys}"
       pure ()
     return { data }
 
diff --git a/tests/lean/run/4171.lean b/tests/lean/run/4171.lean
index e387e573e3..4dec00f314 100644
--- a/tests/lean/run/4171.lean
+++ b/tests/lean/run/4171.lean
@@ -717,20 +717,7 @@ example (M : Comon_ (Mon_ C)) : Mon_ (Comon_ C) where
 
 /--
 info: [simp] theorems with bad keys
-    foo, key: [Quiver.Hom.unop,
-   *,
-   *,
-   *,
-   *,
-   Opposite.op,
-   Quiver.Hom,
-   *,
-   *,
-   Opposite.0,
-   *,
-   Opposite.0,
-   *,
-   *]use `set_option diagnostics.threshold <num>` to control threshold for reporting counters
+    foo, key: @Quiver.Hom.unop _ _ _ _ (@Opposite.op (@Quiver.Hom _ _ _.1 _.1) _)use `set_option diagnostics.threshold <num>` to control threshold for reporting counters
 -/
 #guard_msgs in
 example (M : Comon_ (Mon_ C)) : Mon_ (Comon_ C) where
@@ -749,20 +736,7 @@ attribute [simp] foo
 
 /--
 info: [simp] theorems with bad keys
-    foo, key: [Quiver.Hom.unop,
-   *,
-   *,
-   *,
-   *,
-   Opposite.op,
-   Quiver.Hom,
-   *,
-   *,
-   Opposite.0,
-   *,
-   Opposite.0,
-   *,
-   *]use `set_option diagnostics.threshold <num>` to control threshold for reporting counters
+    foo, key: @Quiver.Hom.unop _ _ _ _ (@Opposite.op (@Quiver.Hom _ _ _.1 _.1) _)use `set_option diagnostics.threshold <num>` to control threshold for reporting counters
 -/
 #guard_msgs in
 example (M : Comon_ (Mon_ C)) : Mon_ (Comon_ C) where
diff --git a/tests/lean/run/simpIndexDiag.lean b/tests/lean/run/simpIndexDiag.lean
new file mode 100644
index 0000000000..a106207b85
--- /dev/null
+++ b/tests/lean/run/simpIndexDiag.lean
@@ -0,0 +1,21 @@
+namespace Ex1
+opaque f : Nat → Nat → Nat
+
+@[simp] theorem foo : f x (x, y).2 = y := by sorry
+
+example : f a b ≤ b := by
+  fail_if_success simp -- should fail
+  set_option diagnostics true in
+  simp (config := { index := false })
+
+end Ex1
+
+namespace Ex2
+opaque f : Nat → Nat → Nat
+
+@[simp] theorem foo : f x (no_index (x, y).2) = y := by sorry
+
+example : f a b ≤ b := by
+  simp -- should work since `foo` is using `no_index`
+
+end Ex2