diff --git a/.jules/bolt.md b/.jules/bolt.md
index d54cf10fc..0936ce84b 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -61,3 +61,7 @@
 ## 2026-07-13 - Array.from mapping optimization
 **Learning:** Using `Array.from({ length: N }).map(...)` creates an intermediate array of `undefined` values which requires memory allocation and garbage collection, adding O(N) unnecessary overhead in frequently re-rendered UI components.
 **Action:** Use `Array.from({ length: N }, (_, index) => ...)` to map elements directly during array creation, avoiding intermediate allocations.
+
+## 2026-08-01 - O(1) early exit for validating byte arrays
+**Learning:** Using `.every()` on large byte arrays creates O(N) intermediate callback allocations which degrades performance significantly.
+**Action:** Use a standard `for` loop with an early return to achieve O(1) memory and significantly faster execution.
diff --git a/apps/desktop/src/features/score/scoreStorage.test.ts b/apps/desktop/src/features/score/scoreStorage.test.ts
index 0feec199e..34a8c80bb 100644
--- a/apps/desktop/src/features/score/scoreStorage.test.ts
+++ b/apps/desktop/src/features/score/scoreStorage.test.ts
@@ -32,4 +32,16 @@ describe("scoreStorage bridge resolution", () => {
       BRIDGE_UNAVAILABLE_MESSAGE
     );
   });
+
+  it("returns invalid response if byte array contains non-numbers to trigger break path", async () => {
+    // Stub window to bypass getInvoke null check
+    const mockInvoke = vi.fn().mockResolvedValue([1, 2, "not-a-number", 4]);
+    vi.stubGlobal("window", {
+      __TAURI_INVOKE__: mockInvoke
+    });
+
+    await expect(readScorePdf("project-1", "score-1")).rejects.toThrow(
+      "Invalid score bridge response"
+    );
+  });
 });
diff --git a/apps/desktop/src/features/score/scoreStorage.ts b/apps/desktop/src/features/score/scoreStorage.ts
index 492f12591..e7ca1068f 100644
--- a/apps/desktop/src/features/score/scoreStorage.ts
+++ b/apps/desktop/src/features/score/scoreStorage.ts
@@ -91,8 +91,19 @@ export async function readScorePdf(projectId: string, scoreId: string): Promise<
   if (response instanceof ArrayBuffer) {
     return new Uint8Array(response);
   }
-  if (Array.isArray(response) && response.every((byte) => typeof byte === "number")) {
-    return Uint8Array.from(response as number[]);
+  if (Array.isArray(response)) {
+    // Performance: Avoid O(N) intermediate callback allocations from .every() on large byte arrays.
+    // Use a standard for loop with early return for O(1) memory and significantly faster execution.
+    let isValid = true;
+    for (let i = 0; i < response.length; i++) {
+      if (typeof response[i] !== "number") {
+        isValid = false;
+        break;
+      }
+    }
+    if (isValid) {
+      return Uint8Array.from(response as number[]);
+    }
   }
 
   throw new Error(INVALID_RESPONSE_MESSAGE);