diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
index c4e5e5dc..3de5f682 100644
--- a/.devcontainer/devcontainer.json
+++ b/.devcontainer/devcontainer.json
@@ -1,6 +1,6 @@
 {
 	"name": "thor",
-	"image": "ghcr.io/rails/devcontainer/images/ruby:3.4.5",
+	"image": "ghcr.io/rails/devcontainer/images/ruby:4.0.0",
 
 	"features": {
     "ghcr.io/devcontainers/features/github-cli:1": {}
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index cf2567e5..5f970454 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -5,7 +5,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4','head']
+        ruby: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0', 'head']
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v6
diff --git a/thor.gemspec b/thor.gemspec
index 1284f438..3da4ec23 100644
--- a/thor.gemspec
+++ b/thor.gemspec
@@ -29,5 +29,5 @@ Gem::Specification.new do |spec|
   spec.executables = %w(thor)
   spec.require_paths = %w(lib)
 
-  spec.add_development_dependency "bundler", ">= 1.0", "< 3"
+  spec.add_development_dependency "bundler", ">= 1.0"
 end