diff --git a/crates/obscura-cdp/tests/structured_clone_crypto_parity.rs b/crates/obscura-cdp/tests/structured_clone_crypto_parity.rs
index e234b10..a7301ce 100644
--- a/crates/obscura-cdp/tests/structured_clone_crypto_parity.rs
+++ b/crates/obscura-cdp/tests/structured_clone_crypto_parity.rs
@@ -207,3 +207,38 @@ async fn structured_clone_rejects_functions_and_symbols() {
     assert_eq!(val["fn"], "DataCloneError", "functions must not clone");
     assert_eq!(val["sym"], "DataCloneError", "symbols must not clone");
 }
+
+// structuredClone preserves reference identity within one graph, including for
+// platform objects cloned through a hook (issue #423). The CryptoKey hook
+// ignored the `seen` map _structuredClone hands it, so one key referenced twice
+// came back as two distinct objects.
+#[tokio::test(flavor = "current_thread")]
+async fn structured_clone_preserves_cryptokey_identity() {
+    let (mut ctx, sid) = setup().await;
+    let v = eval(
+        &mut ctx,
+        2,
+        r#"(async () => {
+            const key = await crypto.subtle.importKey(
+                "raw", new Uint8Array(32),
+                { name: "HMAC", hash: "SHA-256" }, true, ["sign"]
+            );
+            const clone = structuredClone({ a: key, b: key });
+            // The shared clone must still be usable by crypto.subtle.
+            const sig = await crypto.subtle.sign("HMAC", clone.a, new TextEncoder().encode("abc"));
+            return JSON.stringify({
+                shared: clone.a === clone.b,
+                distinctFromSource: clone.a !== key,
+                tag: clone.a[Symbol.toStringTag],
+                sigLen: new Uint8Array(sig).length,
+            });
+        })()"#,
+        &sid,
+    )
+    .await;
+    let val = serde_json::from_str::<Value>(v["result"]["value"].as_str().unwrap()).unwrap();
+    assert_eq!(val["shared"], true, "one CryptoKey reached twice must clone to one shared object");
+    assert_eq!(val["distinctFromSource"], true, "clone must not alias the source key");
+    assert_eq!(val["tag"], "CryptoKey");
+    assert_eq!(val["sigLen"], 32, "the shared clone must remain usable by crypto.subtle");
+}
diff --git a/crates/obscura-js/js/bootstrap.js b/crates/obscura-js/js/bootstrap.js
index 23cefef..4009bed 100644
--- a/crates/obscura-js/js/bootstrap.js
+++ b/crates/obscura-js/js/bootstrap.js
@@ -7094,8 +7094,13 @@ if (!globalThis.crypto.subtle) {
   // _structuredClone via Symbol.toStringTag ("CryptoKey"); registered lazily
   // because structuredClone is defined before this block (issue #389).
   globalThis.__obscura_clone_hooks = globalThis.__obscura_clone_hooks || {};
-  globalThis.__obscura_clone_hooks["CryptoKey"] = function (src) {
+  // `seen` is the clone memo _structuredClone hands every hook. Populate it so
+  // one key reached twice in a graph clones to one shared object (and its key
+  // material is registered once), matching structuredClone's identity rules.
+  globalThis.__obscura_clone_hooks["CryptoKey"] = function (src, seen) {
+    if (seen && seen.has(src)) return seen.get(src);
     const copy = makeKey(src.type, src.extractable, src.algorithm, src.usages, keyBytes(src));
+    if (seen) seen.set(src, copy);
     return copy;
   };