diff --git a/src/lib/guardrails/videoBridgeContactSheet.ts b/src/lib/guardrails/videoBridgeContactSheet.ts
index 7ef2c58e69..4fdf18e258 100644
--- a/src/lib/guardrails/videoBridgeContactSheet.ts
+++ b/src/lib/guardrails/videoBridgeContactSheet.ts
@@ -1,4 +1,5 @@
-import { decodeJpegFrameDataUri } from "./videoBridgeFrameContract";
+import { decodeJpegFrameDataUri, estimateJpegFrameBytes } from "./videoBridgeFrameContract";
+import { VIDEO_FRAME_MAX_BYTES } from "./videoBridgeRuntime";
 
 export interface ContactSheetFrame {
   dataUri: string;
@@ -85,13 +86,18 @@ export async function buildVideoContactSheet(
     const { default: sharp } = await import("sharp");
     if (signal.aborted) throw new Error("Video contact sheet was aborted");
     const tiles = await Promise.all(
-      frames.map(async (frame) =>
-        sharp(decodeJpegFrameDataUri(frame.dataUri))
+      frames.map(async (frame) => {
+        // Reject before decoding: an oversized frame must never reach sharp() just to be
+        // discovered later — estimateJpegFrameBytes reads the encoded length only.
+        if (estimateJpegFrameBytes(frame.dataUri) > VIDEO_FRAME_MAX_BYTES) {
+          throw new Error("Contact sheet frame exceeds the maximum per-frame size");
+        }
+        return sharp(decodeJpegFrameDataUri(frame.dataUri))
           .resize(TILE_SIZE, TILE_SIZE, { fit: "contain", background: "#000000" })
           .composite([{ input: buildTimestampLabel(frame.timestampSeconds), left: 0, top: 0 }])
           .jpeg({ quality: 80 })
-          .toBuffer()
-      )
+          .toBuffer();
+      })
     );
     if (signal.aborted) throw new Error("Video contact sheet was aborted");
     const output = await sharp({
diff --git a/tests/unit/guardrails/videoBridgeContactSheet.test.ts b/tests/unit/guardrails/videoBridgeContactSheet.test.ts
index b8c41c83a0..d466da2836 100644
--- a/tests/unit/guardrails/videoBridgeContactSheet.test.ts
+++ b/tests/unit/guardrails/videoBridgeContactSheet.test.ts
@@ -97,6 +97,29 @@ test("contact sheet falls back to individual frames when decoding fails", async
   assert.deepEqual(result.frames, frames);
 });
 
+test("contact sheet falls back to individual frames when a frame exceeds the per-frame byte cap", async () => {
+  const validJpegBytes = await sharp({
+    create: { background: "red", channels: 3, height: 24, width: 32 },
+  })
+    .jpeg()
+    .toBuffer();
+  // A technically-decodable JPEG prefix followed by zero-filled padding past
+  // VIDEO_FRAME_MAX_BYTES (4 MiB): without a pre-decode size guard, sharp decodes the
+  // leading valid JPEG and ignores the trailing bytes after EOI, so an oversized frame
+  // would otherwise sail through the contact-sheet path undetected (used: true).
+  const oversizedBytes = Buffer.concat([validJpegBytes, Buffer.alloc(5 * 1024 * 1024, 0)]);
+  const frames = [
+    {
+      dataUri: `data:image/jpeg;base64,${oversizedBytes.toString("base64")}`,
+      timestampSeconds: 2,
+    },
+  ];
+  const result = await buildVideoContactSheet(frames);
+  assert.equal(result.used, false);
+  assert.equal(result.fallbackReason, "CONTACT_SHEET_UNAVAILABLE");
+  assert.deepEqual(result.frames, frames);
+});
+
 test("contact sheet respects the parent abort signal", async () => {
   const controller = new AbortController();
   controller.abort();