diff --git a/.gitignore b/.gitignore
index 584b4c44e..32610f714 100644
--- a/.gitignore
+++ b/.gitignore
@@ -56,7 +56,7 @@ python/*.data
 venv/
 ENV/
 env/
-.python-version
+# .python-version is tracked — pins uv to match the sandbox base image Python
 
 # Installer logs
 pip-log.txt
diff --git a/.python-version b/.python-version
new file mode 100644
index 000000000..44305e2f8
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.13.12