diff --git a/requirements.txt b/requirements.txt
index 5d45ba0..6bd1803 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -14,7 +14,12 @@ jupyter==1.0.0
 streamlit==1.28.1
 plotly==5.18.0
 altair==5.1.2
-protobuf==5.24.1
+# protobuf: was pinned to ==5.24.1, which PyPI later yanked. The 5.24.x and
+# 5.25.x lines are unreachable on a fresh install; available 5.x runs jump
+# from 5.23.4 to 5.26.0. Relax to a range so pip's resolver picks whatever
+# streamlit / mlflow / plotly mutually agree on. Day-7 CI exposed this on
+# the first clean install.
+protobuf>=4.25,<6
 
 # Day 1 -- MLflow tracking + registry (sqlite-backed locally)
 mlflow>=2.18,<3