diff --git a/src/Lean/Meta/Tactic/Cbv/Main.lean b/src/Lean/Meta/Tactic/Cbv/Main.lean
index 5e3f26ad1d..23b562155f 100644
--- a/src/Lean/Meta/Tactic/Cbv/Main.lean
+++ b/src/Lean/Meta/Tactic/Cbv/Main.lean
@@ -8,6 +8,7 @@ module
 
 prelude
 public import Lean.Meta.Sym.Simp.SimpM
+public import Lean.Meta.Tactic.Cbv.Opaque
 import Lean.Meta.Tactic.Cbv.Util
 import Lean.Meta.Tactic.Cbv.TheoremsLookup
 import Lean.Meta.Sym
@@ -78,6 +79,14 @@ def handleApp : Simproc := fun e => do
   | .lam .. => betaReduce e
   | _ => return .rfl
 
+def isOpaqueApp : Simproc := fun e => do
+  let some fnName := e.getAppFn.constName? | return .rfl
+  return .rfl (← isCbvOpaque fnName)
+
+def isOpaqueConst : Simproc := fun e => do
+  let .const constName _ := e | return .rfl
+  return .rfl (← isCbvOpaque  constName)
+
 def foldLit : Simproc := fun e => do
  let some n := e.rawNatLit? | return .rfl
  -- TODO: check performance of sharing
@@ -139,7 +148,9 @@ def handleConst : Simproc := fun e => do
 
 def cbvPre : Simproc :=
       isBuiltinValue <|> isProofTerm <|> skipBinders
-  >>  (tryMatcher >> simpControl) <|> (handleConst <|> simplifyAppFn <|> handleProj)
+  >>  isOpaqueApp
+  >>  (tryMatcher >> simpControl)
+    <|> ((isOpaqueConst >> handleConst) <|> simplifyAppFn <|> handleProj)
 
 def cbvPost : Simproc :=
       evalGround
diff --git a/src/Lean/Meta/Tactic/Cbv/Opaque.lean b/src/Lean/Meta/Tactic/Cbv/Opaque.lean
new file mode 100644
index 0000000000..f85246d1ce
--- /dev/null
+++ b/src/Lean/Meta/Tactic/Cbv/Opaque.lean
@@ -0,0 +1,43 @@
+/-
+Copyright (c) 2026 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Wojciech Różowski
+-/
+module
+
+prelude
+public import Lean.ScopedEnvExtension
+
+public section
+
+namespace Lean.Meta.Tactic.Cbv
+
+abbrev CbvOpaqueExtension := SimpleScopedEnvExtension Name (Std.HashSet Name)
+
+builtin_initialize cbvOpaqueExt : CbvOpaqueExtension ←
+  registerSimpleScopedEnvExtension {
+    name     := `cbvOpaqueExt
+    initial  := {}
+    addEntry := fun s n => s.insert n
+  }
+
+builtin_initialize
+  registerBuiltinAttribute {
+    ref   := `cbvOpaqueAttr
+    name  := `cbv_opaque
+    descr := "Mark declarations that should not be unfolded by the `cbv` tactic"
+    applicationTime := AttributeApplicationTime.afterCompilation
+    add   := fun declName _ kind =>
+      cbvOpaqueExt.add declName kind
+    erase := fun declName => do
+      let s := cbvOpaqueExt.getState (← getEnv)
+      modifyEnv fun env => cbvOpaqueExt.modifyState env fun _ => s.erase declName
+  }
+
+def cbvOpaque : CoreM (Std.HashSet Name) := do
+  return cbvOpaqueExt.getState (← getEnv)
+
+def isCbvOpaque (name : Name) : CoreM Bool := do
+  return cbvOpaqueExt.getState (← getEnv) |>.contains name
+
+end Lean.Meta.Tactic.Cbv
diff --git a/tests/lean/run/cbv1.lean b/tests/lean/run/cbv1.lean
index 6e53445e70..5af12ef451 100644
--- a/tests/lean/run/cbv1.lean
+++ b/tests/lean/run/cbv1.lean
@@ -171,3 +171,18 @@ example : Nat.brazilianFactorial 7 = 125411328000 := by
   conv =>
     lhs
     cbv
+
+attribute [cbv_opaque] Std.DHashMap.emptyWithCapacity
+attribute [cbv_opaque] Std.DHashMap.insert
+attribute [cbv_opaque] Std.DHashMap.contains
+attribute [cbv_opaque] Std.DHashMap.getEntry
+
+/--
+error: unsolved goals
+⊢ (Std.DHashMap.emptyWithCapacity.insert 5 3).contains 5 = true
+-/
+#guard_msgs in
+example : ((Std.DHashMap.emptyWithCapacity : Std.DHashMap Nat (fun _ => Nat)).insert 5 3).contains 5 = true := by
+  conv =>
+    lhs
+    cbv