diff --git a/packages/data-model/src/index.ts b/packages/data-model/src/index.ts
--- a/packages/data-model/src/index.ts
+++ b/packages/data-model/src/index.ts
@@ -38,6 +38,11 @@
   toStructuredDebugValue,
 } from "./value-debug.ts";
 
-export { hashOf, hashStringOf, taggedHashStringOf } from "./value-hash.ts";
+export {
+  getFrozenObjectHashCacheHits,
+  hashOf,
+  hashStringOf,
+  taggedHashStringOf,
+} from "./value-hash.ts";
 
 export * from "@/types";
diff --git a/packages/data-model/src/value-hash.ts b/packages/data-model/src/value-hash.ts
--- a/packages/data-model/src/value-hash.ts
+++ b/packages/data-model/src/value-hash.ts
@@ -485,6 +485,13 @@
  */
 const frozenObjectHashCache = new WeakMap<object, FabricHash>();
 
+let frozenObjectHashCacheHits = 0;
+
+/** Returns the number of immutable-object hash lookups served from cache. */
+export function getFrozenObjectHashCacheHits(): number {
+  return frozenObjectHashCacheHits;
+}
+
 /**
  * Returns an already computed immutable hash without reading the value.
  *
@@ -568,6 +575,7 @@
       // checks needed on the fast path.
       const cached = frozenObjectHashCache.get(obj);
       if (cached !== undefined) {
+        frozenObjectHashCacheHits += 1;
         return cached;
       }
 
diff --git a/packages/runner/src/storage/extended-storage-transaction.ts b/packages/runner/src/storage/extended-storage-transaction.ts
--- a/packages/runner/src/storage/extended-storage-transaction.ts
+++ b/packages/runner/src/storage/extended-storage-transaction.ts
@@ -651,9 +651,11 @@
    */
   get accessForTestingOnly(): {
     buildPreparedDigestInput(): PreparedDigestInput;
+    preparedDigest(): string;
   } {
     return {
       buildPreparedDigestInput: () => this.#buildPreparedDigestInput(),
+      preparedDigest: () => preparedDigestFor(this.#buildPreparedDigestInput()),
     };
   }