diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 31147a3..af77ccb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -8,7 +8,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
-        with: { ruby-version: "3.3", bundler-cache: true }
+        with: { bundler-cache: true }
       # Node present ONLY to build the bundle and run conformance reference
       - uses: actions/setup-node@v4
         with: { node-version: "22" }
diff --git a/.ruby-version b/.ruby-version
new file mode 100644
index 0000000..d13e837
--- /dev/null
+++ b/.ruby-version
@@ -0,0 +1 @@
+4.0.6
diff --git a/Gemfile b/Gemfile
index b78f19c..29e4898 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,6 +2,8 @@
 
 source "https://rubygems.org"
 
+ruby file: ".ruby-version"
+
 gemspec
 
 group :development do
diff --git a/Gemfile.lock b/Gemfile.lock
index d05cf5b..fa84a5e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -96,6 +96,7 @@ DEPENDENCIES
 
 CHECKSUMS
   ast (2.4.3) sha256=954615157c1d6a382bc27d690d973195e79db7f55e9765ac7c481c60bdb4d383
+  bundler (4.0.16) sha256=d6ca5dd440c24f9abce9844cf44cc8e18c6a553de65a47efb4544137af92c47d
   diff-lcs (1.6.2) sha256=9ae0d2cba7d4df3075fe8cd8602a8604993efc0dfa934cff568969efb1909962
   herb (0.10.3) sha256=da76e8bfbb302586ee51a4fb608624f48218fbfea198a9493540b42f7654024b
   herb (0.10.3-aarch64-linux-gnu) sha256=1eaea87e6577557492a1b0ddaae989fba8a8ddd382e888ee5f939ce6ded06eb8
@@ -136,5 +137,8 @@ CHECKSUMS
   unicode-display_width (3.2.0) sha256=0cdd96b5681a5949cdbc2c55e7b420facae74c4aaf9a9815eee1087cb1853c42
   unicode-emoji (4.2.0) sha256=519e69150f75652e40bf736106cfbc8f0f73aa3fb6a65afe62fefa7f80b0f80f
 
+RUBY VERSION
+  ruby 4.0.6
+
 BUNDLED WITH
   4.0.16