From a77cbf4a3c3ac0905b25cb78f046b32ca7c79f84 Mon Sep 17 00:00:00 2001
From: Jared Stowell <jared@jhs.dev>
Date: Sat, 7 Mar 2026 20:23:23 -0600
Subject: [PATCH] Fix unstable_cache undefined cache

---
 packages/vinext/src/shims/cache.ts | 19 +++++++++++++++++--
 tests/shims.test.ts                | 21 +++++++++++++++++++++
 2 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/packages/vinext/src/shims/cache.ts b/packages/vinext/src/shims/cache.ts
index 34e48ead..5b7f73e4 100644
--- a/packages/vinext/src/shims/cache.ts
+++ b/packages/vinext/src/shims/cache.ts
@@ -581,6 +581,21 @@ const _UNSTABLE_CACHE_ALS_KEY = Symbol.for("vinext.unstableCache.als");
 const _unstableCacheAls = (
   (_g[_UNSTABLE_CACHE_ALS_KEY] ??= new AsyncLocalStorage<boolean>()) as AsyncLocalStorage<boolean>
 );
+const UNSTABLE_CACHE_UNDEFINED_SENTINEL = "__vinext_unstable_cache_undefined__";
+
+function serializeUnstableCacheResult(value: unknown): string {
+  return value === undefined
+    ? UNSTABLE_CACHE_UNDEFINED_SENTINEL
+    : JSON.stringify(value);
+}
+
+function deserializeUnstableCacheResult(body: string): unknown {
+  if (body === UNSTABLE_CACHE_UNDEFINED_SENTINEL) {
+    return undefined;
+  }
+
+  return JSON.parse(body);
+}
 
 /**
  * Check if the current execution context is inside an unstable_cache() callback.
@@ -625,7 +640,7 @@ export function unstable_cache<T extends (...args: any[]) => Promise<any>>(
     });
     if (existing?.value && existing.value.kind === "FETCH" && existing.cacheState !== "stale") {
       try {
-        return JSON.parse(existing.value.data.body);
+        return deserializeUnstableCacheResult(existing.value.data.body);
       } catch {
         // Corrupted entry, fall through to re-fetch
       }
@@ -641,7 +656,7 @@ export function unstable_cache<T extends (...args: any[]) => Promise<any>>(
       kind: "FETCH",
       data: {
         headers: {},
-        body: JSON.stringify(result),
+        body: serializeUnstableCacheResult(result),
         url: cacheKey,
       },
       tags,
diff --git a/tests/shims.test.ts b/tests/shims.test.ts
index eb6368e5..7584e8a4 100644
--- a/tests/shims.test.ts
+++ b/tests/shims.test.ts
@@ -733,6 +733,27 @@ describe("next/cache shim", () => {
     setCacheHandler(new MemoryCacheHandler());
   });
 
+  it("unstable_cache caches undefined results", async () => {
+    const { unstable_cache, setCacheHandler, MemoryCacheHandler } =
+      await import("../packages/vinext/src/shims/cache.js");
+
+    setCacheHandler(new MemoryCacheHandler());
+
+    let callCount = 0;
+    const cached = unstable_cache(async () => {
+      callCount++;
+      return undefined;
+    }, ["undefined-result-test"]);
+
+    await expect(cached()).resolves.toBeUndefined();
+    expect(callCount).toBe(1);
+
+    await expect(cached()).resolves.toBeUndefined();
+    expect(callCount).toBe(1);
+
+    setCacheHandler(new MemoryCacheHandler());
+  });
+
   it("revalidateTag invalidates cached entries", async () => {
     const {
       unstable_cache,