diff --git a/.gitignore b/.gitignore
index b67e3d6..1c4933b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -138,8 +138,10 @@ tools/cartridge-minter/target/
 panic-attack-report*.json
 /json
 
-# asdf version manager
-.tool-versions
+# asdf version manager — tracked in this repo to pin reproducible
+# toolchain versions (zig 0.15.1 not 0.16.0; elixir 1.18+ not apt's 1.14;
+# etc.). asdf reads .tool-versions on `cd` into the repo and automatically
+# uses the pinned versions.
 
 # wokelangiser generated i18n locale files (too large for GitHub)
 generated/wokelangiser/i18n/
diff --git a/.tool-versions b/.tool-versions
new file mode 100644
index 0000000..3f22a19
--- /dev/null
+++ b/.tool-versions
@@ -0,0 +1,4 @@
+zig 0.15.1
+elixir 1.18.4-otp-25
+idris2 0.8.0
+deno 2.7.14