diff --git a/periscope/src/frontend/src/app/favicon.ico b/periscope/src/frontend/src/app/favicon.ico
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Binary files /dev/null and b/periscope/src/frontend/src/app/favicon.ico differ
diff --git a/tests/test_periscope_favicon_overlay.py b/tests/test_periscope_favicon_overlay.py
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--- /dev/null
+++ b/tests/test_periscope_favicon_overlay.py
@@ -0,0 +1,17 @@
+"""App favicon overlay is the Periscope mark, not the leftover OSS ico."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+SRC = ROOT / "periscope" / "src" / "frontend"
+
+
+def test_app_favicon_is_src_periscope_mark():
+    overlay = SRC / "src" / "app" / "favicon.ico"
+    mark = SRC / "public" / "favicon_io" / "favicon.ico"
+    assert overlay.is_file()
+    assert mark.is_file()
+    assert overlay.stat().st_size == mark.stat().st_size
+    assert overlay.read_bytes() == mark.read_bytes()