diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 00000000..17e5a705
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,51 @@
+name: FHIR Model Tests
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - 'master'
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    if: github.repository == 'incendilabs/fhir_models'
+    name: FHIR ${{ matrix.fhir-version }}
+    runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - fhir-version: R4
+            suite: r4
+          - fhir-version: R4B
+            suite: r4b
+          - fhir-version: R5
+            suite: r5
+
+    steps:
+      - name: Checkout fhir_models
+        uses: actions/checkout@v7
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.4.9'
+
+      - name: Install test dependencies
+        run: |
+          gem install --no-document \
+            bcp47:0.3.3 \
+            date_time_precision:0.8.1 \
+            mime-types:3.7.0 \
+            nokogiri:1.19.4 \
+            nokogiri-diff:0.3.0 \
+            pry:0.15.2 \
+            simplecov:0.22.0 \
+            test-unit:3.7.7
+
+      - name: Run FHIR ${{ matrix.fhir-version }} test suite
+        run: ruby -Ilib -Itest test/run_version_suite.rb ${{ matrix.suite }}
diff --git a/lib/fhir_models/bootstrap/model.rb b/lib/fhir_models/bootstrap/model.rb
index 36227c79..f69bf31e 100644
--- a/lib/fhir_models/bootstrap/model.rb
+++ b/lib/fhir_models/bootstrap/model.rb
@@ -33,8 +33,8 @@ def ==(other)
     def method_missing(method, *_args, &_block)
       if defined?(self.class::MULTIPLE_TYPES) && self.class::MULTIPLE_TYPES[method.to_s]
         self.class::MULTIPLE_TYPES[method.to_s].each do |type|
-          type[0] = type[0].upcase
-          value = send("#{method}#{type}".to_sym)
+          type_name = type[0].upcase + type[1..]
+          value = send("#{method}#{type_name}".to_sym)
           return value unless value.nil?
         end
         return nil
diff --git a/spec/lib/fhir_models/bootstrap/model_spec.rb b/spec/lib/fhir_models/bootstrap/model_spec.rb
index d70c1198..1c29b09d 100644
--- a/spec/lib/fhir_models/bootstrap/model_spec.rb
+++ b/spec/lib/fhir_models/bootstrap/model_spec.rb
@@ -96,4 +96,12 @@
       expect(range.low.value).to equal(18)
     end
   end
+
+  describe '#method_missing' do
+    it 'resolves polymorphic fields without mutating type metadata' do
+      observation = FHIR::Observation.new(valueQuantity: { value: 1 })
+
+      expect(observation.value).to be_a(FHIR::Quantity)
+    end
+  end
 end
diff --git a/test/run_version_suite.rb b/test/run_version_suite.rb
new file mode 100644
index 00000000..4af0277e
--- /dev/null
+++ b/test/run_version_suite.rb
@@ -0,0 +1,34 @@
+VERSION_SUITES = {
+  'r4' => lambda {
+    Dir['test/unit/**/*_test.rb'].reject do |file|
+      File.basename(file).match?(/\A(?:r4b|r5)_/)
+    end
+  },
+  'r4b' => lambda {
+    %w[
+      test/unit/r4b_models_test.rb
+      test/unit/r4b_definitions_test.rb
+      test/unit/r4b_xml_schema_validation_test.rb
+    ]
+  },
+  'r5' => lambda {
+    %w[
+      test/unit/r5_models_test.rb
+      test/unit/r5_definitions_test.rb
+      test/unit/r5_xml_schema_validation_test.rb
+      test/acceptance/r5_serialization_acceptance_test.rb
+      test/acceptance/r5_runtime_definitions_acceptance_test.rb
+      test/acceptance/cross_version_model_isolation_test.rb
+    ]
+  }
+}.freeze
+
+version = ARGV.fetch(0) do
+  abort "Usage: ruby -Ilib -Itest #{__FILE__} <#{VERSION_SUITES.keys.join('|')}>"
+end.downcase
+
+suite = VERSION_SUITES[version]
+abort "Unsupported FHIR version: #{version}" unless suite
+
+ARGV.clear
+suite.call.sort.each { |file| require File.expand_path(file) }