diff --git a/.python-version b/.python-version
new file mode 100644
index 0000000..e4fba21
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.12
diff --git a/docs/environment.md b/docs/environment.md
index 8b24d00..f4fd92e 100644
--- a/docs/environment.md
+++ b/docs/environment.md
@@ -2,7 +2,10 @@
 
 START supports Python 3.10, 3.11, and 3.12. Use the locked `uv` environment
 for local commands, CI, and documentation builds so dependency resolution does
-not vary between surfaces.
+not vary between surfaces. The repo pins the interpreter with
+[`.python-version`](../.python-version) (`3.12`) because key dependencies (e.g.
+`spacy`) publish no wheel for newer CPythons; without the pin a cold `uv run`
+on a 3.14-default machine fails at install time.
 
 ```bash
 uv sync --all-extras --dev