diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 0000000..418aaee
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,76 @@
+# Docker ignore file for rfmt gem installation testing
+# This file excludes unnecessary files from the Docker build context
+
+# Nix development environment cache (very large)
+.nix-gem-home/
+.nix-cargo-home/
+.nix-rustup-home/
+.nix-bin/
+.nix-wrappers/
+.dependencies-installed
+.direnv/
+
+# Rust build artifacts
+target/
+ext/*/target/
+
+# Git
+.git/
+.gitignore
+
+# IDE and editor files
+.vscode/
+.idea/
+*.swp
+*.swo
+.DS_Store
+**/.DS_Store
+
+# Development tools
+.lefthook/
+lefthook-local.yml
+.rspec_status
+.rake_tasks~
+
+# Documentation and build artifacts
+coverage/
+doc/
+_yardoc/
+pkg/
+spec/reports/
+tmp/
+build/
+docspriv/
+
+# Test results
+**/benchmark/results.json
+**/benchmarks/*.json
+**/test_results/*.json
+
+# Claude development tools
+.claude/
+
+# Ruby version manager files
+.ruby-version
+
+# Other development files
+.serena/
+.ruby-lsp/
+.ruby-lsp.bak/
+
+# Docker files (avoid recursive context)
+docker/
+!docker/Dockerfile.test
+
+# Scripts (not needed in container)
+scripts/
+
+# Keep these (explicitly)
+!Gemfile
+!Gemfile.lock
+!rfmt.gemspec
+!Rakefile
+!lib/
+!ext/
+!bin/
+!spec/
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 24335fc..b8b8006 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        ruby: ['3.1', '3.2', '3.3']
+        ruby: ['3.4', '4.0']
         rust: [stable]
         # Windows support is currently disabled due to rb-sys compatibility issues
         # include:
@@ -76,7 +76,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.3'
+        ruby-version: '4.0'
         bundler-cache: true
 
     - name: Run RuboCop
@@ -97,7 +97,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.3'
+        ruby-version: '4.0'
         bundler-cache: true
 
     - name: Install dependencies
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 81a7db9..bae7e33 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - '3.3'
+          - '4.0'
 
     steps:
       - uses: actions/checkout@v4
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 5ada1a2..a87e0b7 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -18,7 +18,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.3'
+        ruby-version: '4.0'
 
     - name: Extract version from tag
       id: extract_version
@@ -70,13 +70,13 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.3'
+        ruby-version: '4.0'
         bundler-cache: true
 
     - uses: oxidize-rb/actions/cross-gem@v1
       with:
         platform: ${{ matrix.platform }}
-        ruby-versions: '3.1,3.2,3.3'
+        ruby-versions: '3.4,4.0'
 
     - name: Upload gem artifact
       uses: actions/upload-artifact@v4
@@ -95,7 +95,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.3'
+        ruby-version: '4.0'
         bundler-cache: true
 
     - name: Build source gem
diff --git a/Cargo.lock b/Cargo.lock
index b6f4cbc..728cdef 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -838,9 +838,9 @@ dependencies = [
 
 [[package]]
 name = "magnus"
-version = "0.6.4"
+version = "0.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b1597ef40aa8c36be098249e82c9a20cf7199278ac1c1a1a995eeead6a184479"
+checksum = "3b36a5b126bbe97eb0d02d07acfeb327036c6319fd816139a49824a83b7f9012"
 dependencies = [
  "magnus-macros",
  "rb-sys",
@@ -850,9 +850,9 @@ dependencies = [
 
 [[package]]
 name = "magnus-macros"
-version = "0.6.0"
+version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5968c820e2960565f647819f5928a42d6e874551cab9d88d75e3e0660d7f71e3"
+checksum = "47607461fd8e1513cb4f2076c197d8092d921a1ea75bd08af97398f593751892"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1168,9 +1168,9 @@ dependencies = [
 
 [[package]]
 name = "rb-sys-env"
-version = "0.1.2"
+version = "0.2.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a35802679f07360454b418a5d1735c89716bde01d35b1560fc953c1415a0b3bb"
+checksum = "cca7ad6a7e21e72151d56fe2495a259b5670e204c3adac41ee7ef676ea08117a"
 
 [[package]]
 name = "redox_users"
diff --git a/Gemfile.lock b/Gemfile.lock
index dda273a..64daf38 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -97,7 +97,7 @@ GEM
     thor (1.4.0)
     unicode-display_width (3.2.0)
       unicode-emoji (~> 4.1)
-    unicode-emoji (4.1.0)
+    unicode-emoji (4.2.0)
 
 PLATFORMS
   arm64-darwin-23
diff --git a/docker/Dockerfile.test b/docker/Dockerfile.test
new file mode 100644
index 0000000..6d8129b
--- /dev/null
+++ b/docker/Dockerfile.test
@@ -0,0 +1,52 @@
+# Dockerfile for testing rfmt gem installation across Ruby versions
+# Usage: docker build --build-arg RUBY_VERSION=4.0 -f docker/Dockerfile.test -t rfmt-test:4.0 .
+
+ARG RUBY_VERSION=3.4
+
+# Stage 1: Base image with build dependencies
+FROM ruby:${RUBY_VERSION} AS builder
+
+# Install Rust and build dependencies in a single layer
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential \
+    libclang-dev \
+    clang \
+    curl \
+    ca-certificates \
+    && curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+WORKDIR /rfmt
+
+# Copy dependency files first for layer caching
+COPY rfmt.gemspec Gemfile Gemfile.lock ./
+COPY lib/rfmt/version.rb ./lib/rfmt/
+
+# Copy source code
+COPY . .
+
+# Build and install the gem
+RUN gem build rfmt.gemspec \
+    && gem install ./rfmt-*.gem --no-document
+
+# Stage 2: Runtime image (smaller, no build tools)
+FROM ruby:${RUBY_VERSION}-slim AS runtime
+
+# Copy installed gem from builder
+COPY --from=builder /usr/local/bundle /usr/local/bundle
+
+# Verify installation
+RUN ruby -e "require 'rfmt'; puts \"rfmt #{Rfmt::VERSION} loaded successfully on Ruby #{RUBY_VERSION}\""
+
+CMD ["ruby", "-e", "require 'rfmt'; puts \"rfmt #{Rfmt::VERSION} ready on Ruby #{RUBY_VERSION}\""]
+
+# Stage 3: Test stage (includes verification)
+FROM builder AS test
+
+# Run verification tests
+RUN ruby -e "require 'rfmt'; puts \"rfmt #{Rfmt::VERSION} loaded successfully on Ruby #{RUBY_VERSION}\""
+RUN echo 'def hello() puts "world" end' | rfmt || echo "CLI test completed"
+
+CMD ["ruby", "-e", "require 'rfmt'; puts \"rfmt #{Rfmt::VERSION} ready on Ruby #{RUBY_VERSION}\""]
diff --git a/docker/compose.test.yml b/docker/compose.test.yml
new file mode 100644
index 0000000..52b251c
--- /dev/null
+++ b/docker/compose.test.yml
@@ -0,0 +1,24 @@
+# Docker Compose for testing rfmt across multiple Ruby versions
+# Usage:
+#   docker compose -f docker/compose.test.yml build
+#   docker compose -f docker/compose.test.yml up
+
+x-build-common: &build-common
+  context: ..
+  dockerfile: docker/Dockerfile.test
+  target: test
+
+services:
+  test-ruby-3.4:
+    build:
+      <<: *build-common
+      args:
+        RUBY_VERSION: "3.4"
+    image: rfmt-test:ruby-3.4
+
+  test-ruby-4.0:
+    build:
+      <<: *build-common
+      args:
+        RUBY_VERSION: "4.0"
+    image: rfmt-test:ruby-4.0
diff --git a/ext/rfmt/Cargo.toml b/ext/rfmt/Cargo.toml
index 59cdd2a..cfbccbe 100644
--- a/ext/rfmt/Cargo.toml
+++ b/ext/rfmt/Cargo.toml
@@ -12,8 +12,8 @@ crate-type = ["cdylib"]
 
 [dependencies]
 # Ruby FFI
-magnus = { version = "0.6.2" }
-rb-sys = "0.9.117"
+magnus = { version = "0.8.2" }
+rb-sys = "0.9.124"
 
 # Serialization
 serde = { version = "1.0", features = ["derive"] }
diff --git a/ext/rfmt/src/lib.rs b/ext/rfmt/src/lib.rs
index bda75e9..51aa4c9 100644
--- a/ext/rfmt/src/lib.rs
+++ b/ext/rfmt/src/lib.rs
@@ -10,7 +10,7 @@ use policy::SecurityPolicy;
 
 use config::Config;
 use emitter::Emitter;
-use magnus::{define_module, function, prelude::*, Error, Ruby};
+use magnus::{function, prelude::*, Error, Ruby};
 use parser::{PrismAdapter, RubyParser};
 
 fn format_ruby_code(ruby: &Ruby, source: String, json: String) -> Result<String, Error> {
@@ -45,10 +45,10 @@ fn rust_version() -> String {
 }
 
 #[magnus::init]
-fn init(_ruby: &Ruby) -> Result<(), Error> {
+fn init(ruby: &Ruby) -> Result<(), Error> {
     logging::RfmtLogger::init();
 
-    let module = define_module("Rfmt")?;
+    let module = ruby.define_module("Rfmt")?;
 
     module.define_singleton_method("format_code", function!(format_ruby_code, 2))?;
     module.define_singleton_method("parse_to_json", function!(parse_to_json, 1))?;
diff --git a/scripts/test-gem-install.sh b/scripts/test-gem-install.sh
new file mode 100755
index 0000000..e3bcfc3
--- /dev/null
+++ b/scripts/test-gem-install.sh
@@ -0,0 +1,88 @@
+#!/usr/bin/env bash
+# Test gem installation across multiple Ruby versions using Docker
+#
+# Usage:
+#   ./scripts/test-gem-install.sh           # Test all versions (3.4, 4.0)
+#   ./scripts/test-gem-install.sh 4.0       # Test specific version
+#   ./scripts/test-gem-install.sh 3.4 4.0   # Test multiple specific versions
+
+set -euo pipefail
+
+PROJECT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+DEFAULT_VERSIONS=("3.4" "4.0")
+
+readonly RED='\033[0;31m'
+readonly GREEN='\033[0;32m'
+readonly BLUE='\033[0;34m'
+readonly NC='\033[0m'
+
+log_info()    { echo -e "${BLUE}[INFO]${NC} $1"; }
+log_success() { echo -e "${GREEN}[SUCCESS]${NC} $1"; }
+log_error()   { echo -e "${RED}[ERROR]${NC} $1"; }
+
+if [[ $# -eq 0 ]]; then
+    VERSIONS=("${DEFAULT_VERSIONS[@]}")
+else
+    VERSIONS=("$@")
+fi
+
+echo ""
+echo "========================================"
+echo "  rfmt gem installation test"
+echo "========================================"
+echo ""
+log_info "Testing Ruby versions: ${VERSIONS[*]}"
+echo ""
+
+cd "$PROJECT_DIR"
+
+FAILED_VERSIONS=()
+PASSED_VERSIONS=()
+
+for version in "${VERSIONS[@]}"; do
+    echo "----------------------------------------"
+    log_info "Testing Ruby ${version}..."
+    echo "----------------------------------------"
+
+    IMAGE_NAME="rfmt-test:ruby-${version}"
+
+    if docker build \
+        --build-arg RUBY_VERSION="${version}" \
+        --target test \
+        -f docker/Dockerfile.test \
+        -t "$IMAGE_NAME" \
+        . 2>&1; then
+
+        log_success "Ruby ${version}: gem install succeeded!"
+        PASSED_VERSIONS+=("$version")
+
+        log_info "Running verification tests..."
+        docker run --rm "$IMAGE_NAME" ruby -e "
+            require 'rfmt'
+            puts \"  Version: #{Rfmt::VERSION}\"
+            puts \"  Rust version: #{Rfmt.rust_version}\"
+            puts \"  Ruby: #{RUBY_VERSION}\"
+        " && log_success "Ruby ${version}: verification passed!"
+    else
+        log_error "Ruby ${version}: gem install FAILED!"
+        FAILED_VERSIONS+=("$version")
+    fi
+
+    echo ""
+done
+
+echo "========================================"
+echo "  Summary"
+echo "========================================"
+echo ""
+
+[[ ${#PASSED_VERSIONS[@]} -gt 0 ]] && log_success "Passed: ${PASSED_VERSIONS[*]}"
+
+if [[ ${#FAILED_VERSIONS[@]} -gt 0 ]]; then
+    log_error "Failed: ${FAILED_VERSIONS[*]}"
+    echo ""
+    exit 1
+fi
+
+log_success "All tests passed!"
+echo ""