diff --git a/kani-compiler/src/kani_middle/attributes.rs b/kani-compiler/src/kani_middle/attributes.rs
index 31cc7bd18f24..0b69b856940a 100644
--- a/kani-compiler/src/kani_middle/attributes.rs
+++ b/kani-compiler/src/kani_middle/attributes.rs
@@ -26,6 +26,8 @@ enum KaniAttributeKind {
     ShouldPanic,
     Solver,
     Stub,
+    /// Attribute used to mark unstable APIs.
+    Unstable,
     Unwind,
 }
 
@@ -113,6 +115,10 @@ pub fn extract_harness_attributes(tcx: TyCtxt, def_id: DefId) -> Option<HarnessA
                             parse_unwind(tcx, expect_single(tcx, kind, &attributes))
                     }
                     KaniAttributeKind::Proof => harness.proof = true,
+                    KaniAttributeKind::Unstable => {
+                        // Internal attribute which shouldn't exist here.
+                        unreachable!()
+                    }
                 };
                 harness
             },
diff --git a/library/kani/src/lib.rs b/library/kani/src/lib.rs
index 5acd07d657ee..e82b57d35457 100644
--- a/library/kani/src/lib.rs
+++ b/library/kani/src/lib.rs
@@ -1,6 +1,9 @@
 // Copyright Kani Contributors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+// Required so we can use kani_macros attributes.
+#![feature(register_tool)]
+#![register_tool(kanitool)]
 // Used for rustc_diagnostic_item.
 #![feature(rustc_attrs)]
 // This is required for the optimized version of `any_array()`
diff --git a/library/kani_macros/src/lib.rs b/library/kani_macros/src/lib.rs
index c3749daabd18..d54684cbff87 100644
--- a/library/kani_macros/src/lib.rs
+++ b/library/kani_macros/src/lib.rs
@@ -73,6 +73,15 @@ pub fn stub(attr: TokenStream, item: TokenStream) -> TokenStream {
 pub fn solver(attr: TokenStream, item: TokenStream) -> TokenStream {
     attr_impl::solver(attr, item)
 }
+
+/// Mark an API as unstable. This should only be used inside the Kani sysroot.
+/// See https://model-checking.github.io/kani/rfc/rfcs/0006-unstable-api.html for more details.
+#[doc(hidden)]
+#[proc_macro_attribute]
+pub fn unstable(attr: TokenStream, item: TokenStream) -> TokenStream {
+    attr_impl::unstable(attr, item)
+}
+
 /// Allow users to auto generate Arbitrary implementations by using `#[derive(Arbitrary)]` macro.
 #[proc_macro_error]
 #[proc_macro_derive(Arbitrary)]
@@ -176,9 +185,10 @@ mod sysroot {
     }
 
     kani_attribute!(should_panic, no_args);
-    kani_attribute!(unwind);
-    kani_attribute!(stub);
     kani_attribute!(solver);
+    kani_attribute!(stub);
+    kani_attribute!(unstable);
+    kani_attribute!(unwind);
 }
 
 /// This module provides dummy implementations of Kani attributes which cannot be interpreted by
@@ -207,7 +217,8 @@ mod regular {
     }
 
     no_op!(should_panic);
-    no_op!(unwind);
-    no_op!(stub);
     no_op!(solver);
+    no_op!(stub);
+    no_op!(unstable);
+    no_op!(unwind);
 }