diff --git a/mobile/src/mobile-web-shell/bridge-host-screencast.test.ts b/mobile/src/mobile-web-shell/bridge-host-screencast.test.ts
index 5494afc72de..75b13ae66e5 100644
--- a/mobile/src/mobile-web-shell/bridge-host-screencast.test.ts
+++ b/mobile/src/mobile-web-shell/bridge-host-screencast.test.ts
@@ -90,7 +90,11 @@ describe('a binary frame crosses as the event the page decodes', () => {
     expect(event).toMatchObject({ v: 1, type: 'event', id: ID, seq: 1 })
     const binary = 'binary' in event ? event.binary : null
     expect(binary).not.toBeNull()
-    expect(binary === null ? null : decodeBridgeScreencastFrame(binary)).toEqual(frame(41, IMAGE))
+    // The decoded frame carries the wire's own base64 beside the bytes (C6.2's data-URI reuse).
+    expect(binary === null ? null : decodeBridgeScreencastFrame(binary)).toEqual({
+      ...frame(41, IMAGE),
+      b64: binary?.b64
+    })
   })
 
   /** One ledger, not two: the page acks by the event seq, so a binary frame that restarted or
diff --git a/mobile/src/mobile-web-shell/bridge/bridge-screencast-encoder.test.ts b/mobile/src/mobile-web-shell/bridge/bridge-screencast-encoder.test.ts
index 3c37b47b6f2..cfafd4b6fd7 100644
--- a/mobile/src/mobile-web-shell/bridge/bridge-screencast-encoder.test.ts
+++ b/mobile/src/mobile-web-shell/bridge/bridge-screencast-encoder.test.ts
@@ -44,12 +44,15 @@ describe('the shell encodes a screencast frame the page can decode', () => {
       frameOf(Uint8Array.of(1, 2, 3), { format: 'png', seq: 77, metadata })
     )
     expect(event).toEqual({ b64: expect.any(String), format: 'png', frameSeq: 77, metadata })
+    // `b64` rides on the decoded frame so the page's data URI can reuse it (C6.2); exact, so a
+    // re-encode on either side cannot hide behind a wildcard.
     expect(decodeBridgeScreencastFrame(event)).toEqual({
       opcode: BrowserScreencastOpcode.Frame,
       seq: 77,
       format: 'png',
       metadata,
-      image: Uint8Array.of(1, 2, 3)
+      image: Uint8Array.of(1, 2, 3),
+      b64: 'AQID'
     })
   })