diff --git a/packages/media-understanding-common/src/video.test.ts b/packages/media-understanding-common/src/video.test.ts
new file mode 100644
index 00000000000..f775ae79caa
--- /dev/null
+++ b/packages/media-understanding-common/src/video.test.ts
@@ -0,0 +1,28 @@
+// Media Understanding Common tests cover video payload sizing behavior.
+import { describe, expect, it } from "vitest";
+import { DEFAULT_VIDEO_MAX_BASE64_BYTES } from "./defaults.js";
+import { estimateBase64Size, resolveVideoMaxBase64Bytes } from "./video.js";
+
+describe("estimateBase64Size", () => {
+  it("rounds byte counts to base64 quanta", () => {
+    expect(estimateBase64Size(1)).toBe(4);
+    expect(estimateBase64Size(2)).toBe(4);
+    expect(estimateBase64Size(3)).toBe(4);
+    expect(estimateBase64Size(4)).toBe(8);
+  });
+});
+
+describe("resolveVideoMaxBase64Bytes", () => {
+  it("allows raw byte limits that expand to valid base64 boundaries", () => {
+    expect(resolveVideoMaxBase64Bytes(1)).toBe(4);
+    expect(resolveVideoMaxBase64Bytes(2)).toBe(4);
+    expect(resolveVideoMaxBase64Bytes(3)).toBe(4);
+    expect(resolveVideoMaxBase64Bytes(4)).toBe(8);
+  });
+
+  it("keeps the shared maximum base64 payload cap", () => {
+    expect(resolveVideoMaxBase64Bytes(DEFAULT_VIDEO_MAX_BASE64_BYTES)).toBe(
+      DEFAULT_VIDEO_MAX_BASE64_BYTES,
+    );
+  });
+});
diff --git a/packages/media-understanding-common/src/video.ts b/packages/media-understanding-common/src/video.ts
index 501a9fc9536..515aa86b21b 100644
--- a/packages/media-understanding-common/src/video.ts
+++ b/packages/media-understanding-common/src/video.ts
@@ -10,6 +10,6 @@ export function estimateBase64Size(bytes: number): number {
 
 /** Resolve video base64 byte limit from raw byte limit and global cap. */
 export function resolveVideoMaxBase64Bytes(maxBytes: number): number {
-  const expanded = Math.floor(maxBytes * (4 / 3));
+  const expanded = estimateBase64Size(maxBytes);
   return Math.min(expanded, DEFAULT_VIDEO_MAX_BASE64_BYTES);
 }