diff --git a/bucket_catch/packages/frontend/src/App.tsx b/bucket_catch/packages/frontend/src/App.tsx
index cf6c327..0e3413a 100644
--- a/bucket_catch/packages/frontend/src/App.tsx
+++ b/bucket_catch/packages/frontend/src/App.tsx
@@ -65,7 +65,7 @@ export default function App(): React.ReactElement {
     if (mode === "puzzle") {
       return (
         <PuzzleCanvas
-          imageFile={files[0]!}
+          imageFile={files[0]}
           gridSize={puzzleGridSize}
           onDone={handlePuzzleDone}
         />
diff --git a/bucket_catch/packages/frontend/src/components/DropZone.test.tsx b/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
index c09ff0d..2ea613f 100644
--- a/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
+++ b/bucket_catch/packages/frontend/src/components/DropZone.test.tsx
@@ -5,6 +5,20 @@ import { DropZone } from "./DropZone";
 const makeFile = (name: string, type = "text/plain"): File =>
   new File(["x"], name, { type });
 
+/**
+ * Query one element and narrow it, failing the test if it is absent.
+ *
+ * Both `as HTMLElement` and `!` are rejected here - the former by
+ * non-nullable-type-assertion-style, the latter by no-non-null-assertion - so
+ * the narrowing is done with a real check that also gives a useful message.
+ */
+const must = (el: Element | null, what: string): HTMLElement => {
+  if (!(el instanceof HTMLElement)) {
+    throw new Error(`expected to find ${what}`);
+  }
+  return el;
+};
+
 describe("DropZone", () => {
   it("renders title and subtitle", () => {
     render(<DropZone onFiles={() => undefined} onPuzzle={() => undefined} />);
@@ -23,7 +37,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={() => undefined} />,
     );
-    const zone = container.firstChild as HTMLElement;
+    const zone = must(
+      container.firstChild as Element | null,
+      "the drop zone",
+    );
     expect(zone.className).not.toContain("dragging");
     fireEvent.dragOver(zone);
     expect(zone.className).toContain("dragging");
@@ -36,7 +53,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const zone = container.firstChild as HTMLElement;
+    const zone = must(
+      container.firstChild as Element | null,
+      "the drop zone",
+    );
     const f1 = makeFile("a.txt");
     const f2 = makeFile("b.txt");
     fireEvent.drop(zone, { dataTransfer: { files: [f1, f2] } });
@@ -48,7 +68,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={() => undefined} />,
     );
-    const zone = container.firstChild as HTMLElement;
+    const zone = must(
+      container.firstChild as Element | null,
+      "the drop zone",
+    );
     fireEvent.drop(zone, { dataTransfer: { files: [makeFile("a.txt")] } });
     expect(screen.getByText("1 file ready")).toBeInTheDocument();
   });
@@ -58,7 +81,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const zone = container.firstChild as HTMLElement;
+    const zone = must(
+      container.firstChild as Element | null,
+      "the drop zone",
+    );
     fireEvent.drop(zone, { dataTransfer: { files: [] } });
     expect(onFiles).not.toHaveBeenCalled();
   });
@@ -68,9 +94,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const fileInput = container.querySelector(
+    const fileInput = must(
+      container.querySelector("input[type='file'][multiple]"),
       "input[type='file'][multiple]",
-    ) as HTMLInputElement;
+    );
     const file = makeFile("c.txt");
     Object.defineProperty(fileInput, "files", {
       value: [file],
@@ -85,9 +112,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const fileInput = container.querySelector(
+    const fileInput = must(
+      container.querySelector("input[type='file'][multiple]"),
       "input[type='file'][multiple]",
-    ) as HTMLInputElement;
+    );
     Object.defineProperty(fileInput, "files", {
       value: [],
       configurable: true,
@@ -101,9 +129,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={onFiles} onPuzzle={() => undefined} />,
     );
-    const fileInput = container.querySelector(
+    const fileInput = must(
+      container.querySelector("input[type='file'][multiple]"),
       "input[type='file'][multiple]",
-    ) as HTMLInputElement;
+    );
     Object.defineProperty(fileInput, "files", {
       value: null,
       configurable: true,
@@ -117,9 +146,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={onPuzzle} />,
     );
-    const puzzleInput = container.querySelector(
+    const puzzleInput = must(
+      container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
-    ) as HTMLInputElement;
+    );
     const imgFile = makeFile("photo.png", "image/png");
     Object.defineProperty(puzzleInput, "files", {
       value: [imgFile],
@@ -134,9 +164,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={onPuzzle} />,
     );
-    const puzzleInput = container.querySelector(
+    const puzzleInput = must(
+      container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
-    ) as HTMLInputElement;
+    );
     Object.defineProperty(puzzleInput, "files", {
       value: null,
       configurable: true,
@@ -150,9 +181,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={onPuzzle} />,
     );
-    const puzzleInput = container.querySelector(
+    const puzzleInput = must(
+      container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
-    ) as HTMLInputElement;
+    );
     Object.defineProperty(puzzleInput, "files", {
       value: [],
       configurable: true,
@@ -165,9 +197,10 @@ describe("DropZone", () => {
     const { container } = render(
       <DropZone onFiles={() => undefined} onPuzzle={() => undefined} />,
     );
-    const puzzleInput = container.querySelector(
+    const puzzleInput = must(
+      container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
-    ) as HTMLInputElement;
+    );
     const clickSpy = vi.spyOn(puzzleInput, "click").mockImplementation(
       () => undefined,
     );
@@ -208,9 +241,10 @@ describe("DropZone", () => {
       <DropZone onFiles={() => undefined} onPuzzle={onPuzzle} />,
     );
     fireEvent.click(screen.getByText("3×3")); // change gridSize to 3
-    const puzzleInput = container.querySelector(
+    const puzzleInput = must(
+      container.querySelector("input[accept='image/*']"),
       "input[accept='image/*']",
-    ) as HTMLInputElement;
+    );
     const imgFile = makeFile("img.png", "image/png");
     Object.defineProperty(puzzleInput, "files", {
       value: [imgFile],
diff --git a/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx b/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
index 58aafbe..7b8a44f 100644
--- a/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
+++ b/bucket_catch/packages/frontend/src/components/PuzzleResult.test.tsx
@@ -129,9 +129,10 @@ describe("PuzzleResult", () => {
     const imgs = container.querySelectorAll("img");
     expect(imgs).toHaveLength(2);
     // The grid div has inline style with gridTemplateColumns; its non-img children are holes
-    const gridEl = container.querySelector(
-      '[style*="repeat"]',
-    ) as HTMLElement;
+    const gridEl = container.querySelector('[style*="repeat"]');
+    if (!(gridEl instanceof HTMLElement)) {
+      throw new Error("expected to find the puzzle grid element");
+    }
     const holeDivs = Array.from(gridEl.children).filter(
       (c) => c.tagName === "DIV",
     );
diff --git a/bucket_catch/packages/frontend/src/components/ScoreScreen.test.tsx b/bucket_catch/packages/frontend/src/components/ScoreScreen.test.tsx
index 6d39c40..fcc22a4 100644
--- a/bucket_catch/packages/frontend/src/components/ScoreScreen.test.tsx
+++ b/bucket_catch/packages/frontend/src/components/ScoreScreen.test.tsx
@@ -39,7 +39,7 @@ describe("ScoreScreen", () => {
     expect(screen.getByText("Missed")).toBeInTheDocument();
   });
 
-  it("shows download button for download mode with multiple files", async () => {
+  it("shows download button for download mode with multiple files", () => {
     render(
       <ScoreScreen result={result1} mode="download" onRestart={() => undefined} />,
     );
diff --git a/bucket_catch/packages/frontend/src/hooks/useBasketControl.test.ts b/bucket_catch/packages/frontend/src/hooks/useBasketControl.test.ts
index 917943c..8803f79 100644
--- a/bucket_catch/packages/frontend/src/hooks/useBasketControl.test.ts
+++ b/bucket_catch/packages/frontend/src/hooks/useBasketControl.test.ts
@@ -4,8 +4,9 @@ import { fireEvent } from "@testing-library/react";
 import type React from "react";
 import { useBasketControl } from "./useBasketControl";
 
-const makeRef = <T>(val: T): React.RefObject<T> =>
-  ({ current: val }) as React.RefObject<T>;
+const makeRef = <T>(val: T): React.RefObject<T> => ({
+  current: val,
+});
 
 describe("useBasketControl", () => {
   it("initialises basket x to half of window.innerWidth", () => {
diff --git a/bucket_catch/packages/frontend/src/hooks/useGameLoop.test.ts b/bucket_catch/packages/frontend/src/hooks/useGameLoop.test.ts
index 2c07b3f..5bdc477 100644
--- a/bucket_catch/packages/frontend/src/hooks/useGameLoop.test.ts
+++ b/bucket_catch/packages/frontend/src/hooks/useGameLoop.test.ts
@@ -3,8 +3,7 @@ import { renderHook, act, waitFor } from "@testing-library/react";
 import type React from "react";
 import { useGameLoop } from "./useGameLoop";
 
-const makeRef = <T>(val: T): React.RefObject<T> =>
-  ({ current: val }) as React.RefObject<T>;
+const makeRef = <T>(val: T): React.RefObject<T> => ({ current: val });
 
 describe("useGameLoop", () => {
   let rafCallbacks: FrameRequestCallback[];
@@ -88,8 +87,12 @@ describe("useGameLoop", () => {
 
     flushTick(); // allDone=true on first frame → setResult
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caught).toHaveLength(0);
-    expect(result.current!.missed).toHaveLength(0);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caught).toHaveLength(0);
+    expect(settled.missed).toHaveLength(0);
   });
 
   it("catches file and resolves result (caught branch + caught-draw branch)", async () => {
@@ -114,8 +117,12 @@ describe("useGameLoop", () => {
     flushTick(); // tick 1: draw caught + allDone=true → setResult
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caught).toHaveLength(1);
-    expect(result.current!.missed).toHaveLength(0);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caught).toHaveLength(1);
+    expect(settled.missed).toHaveLength(0);
   });
 
   it("misses file (still-falling + missed branches)", async () => {
@@ -142,8 +149,12 @@ describe("useGameLoop", () => {
     }
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caught).toHaveLength(0);
-    expect(result.current!.missed).toHaveLength(1);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caught).toHaveLength(0);
+    expect(settled.missed).toHaveLength(1);
   });
 
   it("does not start RAF when canvas.getContext returns null", () => {
@@ -187,7 +198,11 @@ describe("useGameLoop", () => {
     }
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caught).toHaveLength(2);
-    expect(result.current!.missed).toHaveLength(0);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caught).toHaveLength(2);
+    expect(settled.missed).toHaveLength(0);
   });
 });
diff --git a/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoop.ts b/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoop.ts
index 6696e20..b6f2197 100644
--- a/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoop.ts
+++ b/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoop.ts
@@ -85,8 +85,10 @@ export function usePuzzleGameLoop(
           scheduleRef.current.length > 0 &&
           frame >= scheduleRef.current[0].spawnFrame
         ) {
-          // while condition guarantees length > 0, so shift() is always defined
-          const s = scheduleRef.current.shift()!;
+          // The while condition already indexes [0] unasserted, so this is
+          // the same non-null value; shift() only advances the queue.
+          const s = scheduleRef.current[0];
+          scheduleRef.current.shift();
           activeItemsRef.current.push({
             kind: "puzzle",
             id: `puzzle-${s.piece.row}-${s.piece.col}`,
diff --git a/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoopPlay.test.ts b/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoopPlay.test.ts
index 057471c..80457ad 100644
--- a/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoopPlay.test.ts
+++ b/bucket_catch/packages/frontend/src/hooks/usePuzzleGameLoopPlay.test.ts
@@ -51,8 +51,12 @@ describe("usePuzzleGameLoop play", () => {
     flushTick(); // tick 0: spawn, move, caught, flash, allDone → setResult
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caughtPieces).toHaveLength(1);
-    expect(result.current!.missedPieces).toHaveLength(0);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caughtPieces).toHaveLength(1);
+    expect(settled.missedPieces).toHaveLength(0);
   });
 
   it("misses piece (still-falling + missed branches)", async () => {
@@ -77,8 +81,12 @@ describe("usePuzzleGameLoop play", () => {
     }
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caughtPieces).toHaveLength(0);
-    expect(result.current!.missedPieces).toHaveLength(1);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caughtPieces).toHaveLength(0);
+    expect(settled.missedPieces).toHaveLength(1);
   });
 
   it("tests flash-during and flash-expired branches with 2 pieces", async () => {
@@ -114,8 +122,12 @@ describe("usePuzzleGameLoop play", () => {
     }
 
     await waitFor(() => { expect(result.current).not.toBeNull(); });
-    expect(result.current!.caughtPieces).toHaveLength(1);
-    expect(result.current!.missedPieces).toHaveLength(1);
+    const settled = result.current;
+    if (settled === null) {
+      throw new Error('result never settled');
+    }
+    expect(settled.caughtPieces).toHaveLength(1);
+    expect(settled.missedPieces).toHaveLength(1);
   });
 
   it("uses drawImage when piece image is loaded (img.complete=true branch)", async () => {
diff --git a/bucket_catch/packages/frontend/src/lib/sliceImage.test.ts b/bucket_catch/packages/frontend/src/lib/sliceImage.test.ts
index 295af36..87e0fe4 100644
--- a/bucket_catch/packages/frontend/src/lib/sliceImage.test.ts
+++ b/bucket_catch/packages/frontend/src/lib/sliceImage.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import type { MockInstance } from "vitest";
 import { sliceImage } from "./sliceImage";
 
 // Captures the last Image instance created so tests can trigger onload/onerror.
@@ -22,10 +23,14 @@ class FakeImage {
 }
 
 describe("sliceImage", () => {
+  let revokeUrlSpy: MockInstance<typeof URL.revokeObjectURL>;
+
   beforeEach(() => {
     vi.stubGlobal("Image", FakeImage);
     vi.spyOn(URL, "createObjectURL").mockReturnValue("blob:mock");
-    vi.spyOn(URL, "revokeObjectURL").mockImplementation(() => undefined);
+    revokeUrlSpy = vi
+      .spyOn(URL, "revokeObjectURL")
+      .mockImplementation(() => undefined);
   });
 
   afterEach(() => {
@@ -44,7 +49,7 @@ describe("sliceImage", () => {
     expect(pieces).toHaveLength(4); // 2×2 grid
     expect(pieces[0]).toMatchObject({ row: 0, col: 0, gridSize: 2 });
     expect(pieces[3]).toMatchObject({ row: 1, col: 1, gridSize: 2 });
-    expect(URL.revokeObjectURL).toHaveBeenCalledWith("blob:mock");
+    expect(revokeUrlSpy).toHaveBeenCalledWith("blob:mock");
   });
 
   it("includes pieceWidth and pieceHeight from image dimensions", async () => {
@@ -74,7 +79,7 @@ describe("sliceImage", () => {
     const promise = sliceImage(file, 1);
     capturedImg.onload?.();
     await expect(promise).rejects.toThrow("Canvas 2D context not available");
-    expect(URL.revokeObjectURL).toHaveBeenCalledWith("blob:mock");
+    expect(revokeUrlSpy).toHaveBeenCalledWith("blob:mock");
 
     // Restore so subsequent tests in this file have the mock context
     HTMLCanvasElement.prototype.getContext = savedImpl;
@@ -85,6 +90,6 @@ describe("sliceImage", () => {
     const promise = sliceImage(file, 2);
     capturedImg.onerror?.();
     await expect(promise).rejects.toThrow("Failed to load image for slicing");
-    expect(URL.revokeObjectURL).toHaveBeenCalledWith("blob:mock");
+    expect(revokeUrlSpy).toHaveBeenCalledWith("blob:mock");
   });
 });
diff --git a/bucket_catch/packages/frontend/src/lib/zipDownload.test.ts b/bucket_catch/packages/frontend/src/lib/zipDownload.test.ts
index 23a8881..9d2568b 100644
--- a/bucket_catch/packages/frontend/src/lib/zipDownload.test.ts
+++ b/bucket_catch/packages/frontend/src/lib/zipDownload.test.ts
@@ -1,13 +1,22 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import type { MockInstance } from "vitest";
 import { zipDownload } from "./zipDownload";
 
 describe("zipDownload", () => {
+  let createUrlSpy: MockInstance<typeof URL.createObjectURL>;
+  let revokeUrlSpy: MockInstance<typeof URL.revokeObjectURL>;
+  let clickSpy: MockInstance<typeof HTMLAnchorElement.prototype.click>;
+
   beforeEach(() => {
-    vi.spyOn(URL, "createObjectURL").mockReturnValue("blob:mock-url");
-    vi.spyOn(URL, "revokeObjectURL").mockImplementation(() => undefined);
-    vi.spyOn(HTMLAnchorElement.prototype, "click").mockImplementation(
-      () => undefined,
-    );
+    createUrlSpy = vi
+      .spyOn(URL, "createObjectURL")
+      .mockReturnValue("blob:mock-url");
+    revokeUrlSpy = vi
+      .spyOn(URL, "revokeObjectURL")
+      .mockImplementation(() => undefined);
+    clickSpy = vi
+      .spyOn(HTMLAnchorElement.prototype, "click")
+      .mockImplementation(() => undefined);
   });
 
   afterEach(() => {
@@ -21,30 +30,38 @@ describe("zipDownload", () => {
     const file = new File(["hello"], "hello.txt");
     await zipDownload([file]);
 
-    expect(URL.createObjectURL).toHaveBeenCalled();
-    expect(HTMLAnchorElement.prototype.click).toHaveBeenCalled();
-    expect(URL.revokeObjectURL).toHaveBeenCalledWith("blob:mock-url");
+    expect(createUrlSpy).toHaveBeenCalled();
+    expect(clickSpy).toHaveBeenCalled();
+    expect(revokeUrlSpy).toHaveBeenCalledWith("blob:mock-url");
     expect(appendSpy).toHaveBeenCalled();
     expect(removeSpy).toHaveBeenCalled();
   });
 
   it("sets download attribute and href on the anchor", async () => {
     const anchors: HTMLAnchorElement[] = [];
+    // Bound before the spy replaces it, so the implementation below can build
+    // a real element without recursing into its own mock.
+    const realCreateElement = document.createElement.bind(document);
     vi.spyOn(document, "createElement").mockImplementation((tag: string) => {
-      const el = document.createElement.bind(document)(tag) as HTMLElement;
-      if (tag === "a") anchors.push(el as HTMLAnchorElement);
+      const el = realCreateElement(tag);
+      if (el instanceof HTMLAnchorElement) anchors.push(el);
       return el;
     });
-    // Restore after spy above since we used bind trick
+    // Restore after spy above since we used bind trick. That discards the
+    // handles from beforeEach, so re-spy and keep the new ones.
     vi.restoreAllMocks();
     vi.spyOn(URL, "createObjectURL").mockReturnValue("blob:mock-url");
-    vi.spyOn(URL, "revokeObjectURL").mockImplementation(() => undefined);
-    vi.spyOn(HTMLAnchorElement.prototype, "click").mockImplementation(() => undefined);
+    const localRevokeSpy = vi
+      .spyOn(URL, "revokeObjectURL")
+      .mockImplementation(() => undefined);
+    vi.spyOn(HTMLAnchorElement.prototype, "click").mockImplementation(
+      () => undefined,
+    );
 
     const file = new File(["x"], "data.bin");
     await zipDownload([file]);
 
-    expect(URL.revokeObjectURL).toHaveBeenCalledWith("blob:mock-url");
+    expect(localRevokeSpy).toHaveBeenCalledWith("blob:mock-url");
   });
 
   it("works with multiple files", async () => {
@@ -54,7 +71,7 @@ describe("zipDownload", () => {
       new File(["c"], "c.png", { type: "image/png" }),
     ];
     await expect(zipDownload(files)).resolves.toBeUndefined();
-    expect(URL.createObjectURL).toHaveBeenCalledTimes(1);
-    expect(URL.revokeObjectURL).toHaveBeenCalledTimes(1);
+    expect(createUrlSpy).toHaveBeenCalledTimes(1);
+    expect(revokeUrlSpy).toHaveBeenCalledTimes(1);
   });
 });
diff --git a/bucket_catch/packages/frontend/src/test/puzzleLoopFixtures.ts b/bucket_catch/packages/frontend/src/test/puzzleLoopFixtures.ts
index 98e8746..d58b76b 100644
--- a/bucket_catch/packages/frontend/src/test/puzzleLoopFixtures.ts
+++ b/bucket_catch/packages/frontend/src/test/puzzleLoopFixtures.ts
@@ -10,8 +10,9 @@ import type { PuzzlePiece } from "../types";
  * the 100% gate.
  */
 
-export const makeRef = <T>(val: T): React.RefObject<T> =>
-  ({ current: val }) as React.RefObject<T>;
+export const makeRef = <T>(val: T): React.RefObject<T> => ({
+  current: val,
+});
 
 export const makePiece = (row: number, col: number): PuzzlePiece => ({
   row,