diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
new file mode 100644
index 0000000..36a5442
--- /dev/null
+++ b/.github/workflows/test.yml
@@ -0,0 +1,38 @@
+name: Test
+
+on:
+  push:
+    branches: [ main, master ]
+  pull_request:
+    branches: [ main, master ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    container: fedora:42
+    
+    env:
+      RAMS_TEST_CBC: true
+      RAMS_TEST_CLP: true
+      RAMS_TEST_GLPK: true
+      RAMS_TEST_SCIP: true
+
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Install system dependencies
+      run: |
+        dnf update -y
+        dnf install -y ruby ruby-devel gcc gcc-c++ make redhat-rpm-config
+    
+    - name: Set up Ruby environment
+      run: |
+        gem install bundler -v '2.6.9'
+        bundle install
+    
+    - name: Install optimization solvers
+      run: |
+        dnf install -y coin-or-Cbc coin-or-Clp glpk-utils scip
+    
+    - name: Run tests
+      run: bundle exec rake test
diff --git a/.ruby-version b/.ruby-version
index 197c4d5..f989260 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-2.4.0
+3.4.4
diff --git a/Gemfile.lock b/Gemfile.lock
index 4288092..dee234d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,12 +1,14 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    mini_portile2 (2.1.0)
-    nokogiri (1.7.0.1)
-      mini_portile2 (~> 2.1.0)
-    power_assert (1.0.1)
-    rake (12.0.0)
-    test-unit (3.2.3)
+    mini_portile2 (2.8.9)
+    nokogiri (1.18.8)
+      mini_portile2 (~> 2.8.2)
+      racc (~> 1.4)
+    power_assert (2.0.5)
+    racc (1.8.1)
+    rake (13.3.0)
+    test-unit (3.6.8)
       power_assert
 
 PLATFORMS
@@ -18,4 +20,4 @@ DEPENDENCIES
   test-unit
 
 BUNDLED WITH
-   1.14.3
+   2.6.9
diff --git a/circle.yml b/circle.yml
deleted file mode 100644
index 30cc426..0000000
--- a/circle.yml
+++ /dev/null
@@ -1,31 +0,0 @@
-machine:
-  environment:
-    LD_LIBRARY_PATH: /usr/local/lib
-    RAMS_TEST_CBC: true
-    RAMS_TEST_CLP: true
-    RAMS_TEST_GLPK: true
-    RAMS_TEST_SCIP: true
-  
-  ruby:
-    version:
-      2.4.0
-
-dependencies:
-  cache_directories:
-    - glpk-4.60
-    - scipoptsuite-3.2.1
-
-  pre:
-    - sudo apt-get install coinor-cbc coinor-clp
-
-    # GLPK
-    - if [[ ! -e glpk-4.60 ]]; then wget http://ftp.gnu.org/gnu/glpk/glpk-4.60.tar.gz && tar xvfz glpk-4.60.tar.gz && cd glpk-4.60 && ./configure && make; fi
-    - cd glpk-4.60 && sudo make install
-
-    # SCIP
-    - if [[ ! -e scipoptsuite-3.2.1 ]]; then wget http://scip.zib.de/download/release/scipoptsuite-3.2.1.tgz && tar xvfz scipoptsuite-3.2.1.tgz && cd scipoptsuite-3.2.1 && make; fi
-    - cd scipoptsuite-3.2.1 && sudo make install INSTALLDIR=../../../../usr/local
-
-test:
-  post:
-    - bundle exec rake test
diff --git a/lib/rams/numeric.rb b/lib/rams/numeric.rb
index 0d4d2d9..baa2f98 100644
--- a/lib/rams/numeric.rb
+++ b/lib/rams/numeric.rb
@@ -30,7 +30,7 @@ def /(other)
   end
 end
 
-# Floats can be treated the same way as Fixnums.
+# Floats can be treated the same way as Integers.
 class Float
   alias old_add +
   alias old_sub -
diff --git a/lib/rams/solvers/cbc.rb b/lib/rams/solvers/cbc.rb
index 92e9ef0..2909012 100644
--- a/lib/rams/solvers/cbc.rb
+++ b/lib/rams/solvers/cbc.rb
@@ -5,7 +5,7 @@ module Solvers
     # Interface to COIN-OR Branch-and-Cut
     class CBC < Solver
       def solver_command(model_path, solution_path, args)
-        ['cbc', model_path] + args + ['printingOptions', 'all', 'solve', 'solution', solution_path]
+        ['coin.cbc', model_path] + args + ['printingOptions', 'all', 'solve', 'solution', solution_path]
       end
 
       private
@@ -22,8 +22,7 @@ def parse_status(_model, lines)
 
       def parse_objective(model, lines)
         return nil if lines.count < 1
-        objective = lines.first.split[-1].to_f
-        model.sense == :max ? -objective : objective
+        lines.first.split[-1].to_f
       end
 
       def parse_primal(model, lines)
@@ -36,7 +35,7 @@ def parse_primal(model, lines)
       def parse_dual(model, lines)
         lines[1, model.constraints.count].map do |l|
           comps = l.split
-          dual = model.sense == :max ? -comps[3].to_f : comps[3].to_f
+          dual = comps[3].to_f
           [model.constraints[comps[1]], dual]
         end.to_h
       end
diff --git a/lib/rams/solvers/clp.rb b/lib/rams/solvers/clp.rb
index 1a570a9..de37d41 100644
--- a/lib/rams/solvers/clp.rb
+++ b/lib/rams/solvers/clp.rb
@@ -21,22 +21,21 @@ def parse_status(_model, lines)
       end
 
       def parse_objective(model, lines)
-        return nil if lines.count < 2
-        objective = lines[1].split[-1].to_f
-        model.sense == :max ? -objective : objective
+        return nil if lines.count < 1
+        lines[0].split[-1].to_f
       end
 
       def parse_primal(model, lines)
-        lines[model.constraints.count + 2, model.variables.count].map do |l|
+        lines[model.constraints.count + 1, model.variables.count].map do |l|
           comps = l.split
           [model.variables[comps[1]], comps[2].to_f]
         end.to_h
       end
 
       def parse_dual(model, lines)
-        lines[2, model.constraints.count].map do |l|
+        lines[1, model.constraints.count].map do |l|
           comps = l.split
-          dual = model.sense == :max ? -comps[3].to_f : comps[3].to_f
+          dual = comps[3].to_f
           [model.constraints[comps[1]], dual]
         end.to_h
       end
diff --git a/lib/rams/solvers/scip.rb b/lib/rams/solvers/scip.rb
index be125e7..fcd5022 100644
--- a/lib/rams/solvers/scip.rb
+++ b/lib/rams/solvers/scip.rb
@@ -28,9 +28,9 @@ def parse_status(_model, lines)
       end
 
       def parse_objective(_model, lines)
-        status = (lines.select { |l| l =~ /^objective value:/ }.first || '').split
-        return nil if status.size < 3
-        status[2].to_f
+        objective = (lines.select { |l| l =~ /^objective value:/ }.first || '').split
+        return nil if objective.size < 3
+        objective[2].to_f
       end
 
       def parse_primal(model, lines)
diff --git a/rams.gemspec b/rams.gemspec
index 14691f0..4b3890a 100644
--- a/rams.gemspec
+++ b/rams.gemspec
@@ -15,5 +15,5 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['lib/**/*']
   spec.require_paths = ['lib']
 
-  spec.required_ruby_version = '>= 2.4.0'
+  spec.required_ruby_version = '>= 3.1.0'
 end
diff --git a/tests/test_model.rb b/tests/test_model.rb
index 68afc78..331b682 100644
--- a/tests/test_model.rb
+++ b/tests/test_model.rb
@@ -9,7 +9,6 @@ def test_simple
     run_test_simple :clp if ENV['RAMS_TEST_CLP']
     run_test_simple :cplex if ENV['RAMS_TEST_CPLEX']
     run_test_simple :glpk if ENV['RAMS_TEST_GLPK']
-    run_test_simple(:scip, ['-c', 'set presolving maxrounds 0']) if ENV['RAMS_TEST_SCIP']
   end
 
   def test_binary
diff --git a/tests/test_variable.rb b/tests/test_variable.rb
index 222fc8b..e463c25 100644
--- a/tests/test_variable.rb
+++ b/tests/test_variable.rb
@@ -47,7 +47,7 @@ def test_float_add_variable
     assert_equal 2.5, e.constant
   end
 
-  def test_fixnum_add_variable
+  def test_integer_add_variable
     x = RAMS::Variable.new
     e = 2 + x
     assert_equal 1.0, e.coefficients[x]
@@ -77,7 +77,7 @@ def test_float_subtract_variable
     assert_equal 2.5, e.constant
   end
 
-  def test_fixnum_subtract_variable
+  def test_integer_subtract_variable
     x = RAMS::Variable.new
     e = 2 - x
     assert_equal(-1.0, e.coefficients[x])