diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 976c845..e458a43 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,3 +23,13 @@ jobs:
           bundler-cache: true
       - run: bundle exec rspec
       - run: gem build millionsend.gemspec
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
+      - uses: ruby/setup-ruby@95ef2b042f9d7a56d8268cba8559e2842e2ad01b # v1.321.0
+        with:
+          ruby-version: "3.3"
+          bundler-cache: true
+      - run: bundle exec rubocop
diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 0000000..8dc7bee
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,52 @@
+AllCops:
+  TargetRubyVersion: 3.0
+  NewCops: enable
+  SuggestExtensions: false
+
+Gemspec/DevelopmentDependencies:
+  EnforcedStyle: gemspec
+
+Layout/FirstArgumentIndentation:
+  EnforcedStyle: consistent
+
+Layout/FirstHashElementIndentation:
+  EnforcedStyle: consistent
+
+Layout/HashAlignment:
+  EnforcedHashRocketStyle: [key, table]
+
+# Specs inline whole URLs and JSON bodies; wrapping them hurts more than it helps.
+Layout/LineLength:
+  Enabled: false
+
+# Size thresholds would demand refactors rather than catch bugs.
+Metrics:
+  Enabled: false
+
+Style/Alias:
+  EnforcedStyle: prefer_alias_method
+
+# ENV[] returning nil for an unset variable is the intended fallback.
+Style/FetchEnvVar:
+  Enabled: false
+
+Style/StringLiterals:
+  EnforcedStyle: double_quotes
+
+Style/TrailingCommaInArrayLiteral:
+  EnforcedStyleForMultiline: diff_comma
+
+Style/TrailingCommaInHashLiteral:
+  EnforcedStyleForMultiline: diff_comma
+
+# These rewrites change behavior for some caller inputs: Hash#except also drops the string
+# keys of a HashWithIndifferentAccess, compact keeps [key, nil] pairs in an array, and
+# freezing USER_AGENT breaks callers that append to it.
+Style/CollectionCompact:
+  Enabled: false
+
+Style/HashExcept:
+  Enabled: false
+
+Style/MutableConstant:
+  Enabled: false
diff --git a/lib/millionsend/contacts.rb b/lib/millionsend/contacts.rb
index 481c76f..54c2a5c 100644
--- a/lib/millionsend/contacts.rb
+++ b/lib/millionsend/contacts.rb
@@ -20,7 +20,7 @@ def get(id_or_email)
       # everything else is the body. A nil value clears a field; omit a key to
       # leave it unchanged.
       def update(params)
-        body = params.reject { |k, _| [:id, :email].include?(k) }
+        body = params.reject { |k, _| %i[id email].include?(k) }
         Millionsend::Request.new(method: :patch, path: member_path(params), body: body).perform
       end
 
diff --git a/lib/millionsend/request.rb b/lib/millionsend/request.rb
index bbe5a5c..7654f40 100644
--- a/lib/millionsend/request.rb
+++ b/lib/millionsend/request.rb
@@ -23,7 +23,7 @@ class Request
     # (WebMock::NetConnectNotAllowedError) is not swallowed as a transport error.
     TRANSPORT_ERRORS = [
       Timeout::Error, SocketError, SystemCallError, IOError,
-      Net::OpenTimeout, Net::ReadTimeout, OpenSSL::SSL::SSLError
+      Net::OpenTimeout, Net::ReadTimeout, OpenSSL::SSL::SSLError,
     ].freeze
 
     def initialize(method:, path:, body: nil, query: nil, idempotency_key: nil, batch_validation: nil)
diff --git a/millionsend.gemspec b/millionsend.gemspec
index 396c189..b7c5f47 100644
--- a/millionsend.gemspec
+++ b/millionsend.gemspec
@@ -25,5 +25,6 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.12"
+  spec.add_development_dependency "rubocop", "1.91.0"
   spec.add_development_dependency "webmock", "~> 3.19"
 end
diff --git a/spec/resources_spec.rb b/spec/resources_spec.rb
index c099cc2..dba2ee8 100644
--- a/spec/resources_spec.rb
+++ b/spec/resources_spec.rb
@@ -242,7 +242,7 @@ def list_ok
     it "list joins include: into one comma-separated query value and omits it when unset" do
       stub_request(:get, "https://api.test/contacts")
         .with(query: { "limit" => "100", "include" => "properties,topics" }).to_return(list_ok)
-      Millionsend::Contacts.list(limit: 100, include: ["properties", "topics"])
+      Millionsend::Contacts.list(limit: 100, include: %w[properties topics])
       expect(WebMock).to have_requested(:get, "https://api.test/contacts")
         .with(query: { "limit" => "100", "include" => "properties,topics" })
 
@@ -361,11 +361,11 @@ def list_ok
     it "Batch.remove posts ids or emails to /contacts/batch/remove" do
       stub_request(:post, "https://api.test/contacts/batch/remove")
         .to_return(ok('{"data":[{"object":"contact","contact":"c1","deleted":true}]}'))
-      res = Millionsend::Contacts::Batch.remove({ ids: ["c1", "c2"] })
+      res = Millionsend::Contacts::Batch.remove({ ids: %w[c1 c2] })
       Millionsend::Contacts::Batch.remove({ emails: ["a@x.dev"] })
       Millionsend::Contacts::Batch.remove(emails: ["b@x.dev"], erase: true)
       expect(WebMock).to have_requested(:post, "https://api.test/contacts/batch/remove")
-        .with(body: { "ids" => ["c1", "c2"] })
+        .with(body: { "ids" => %w[c1 c2] })
       expect(WebMock).to have_requested(:post, "https://api.test/contacts/batch/remove")
         .with(body: { "emails" => ["a@x.dev"] })
       expect(WebMock).to have_requested(:post, "https://api.test/contacts/batch/remove")
@@ -383,12 +383,12 @@ def list_ok
       ))
       res = Millionsend::Contacts::Batch.get(
         ["c1", "ghost@x.dev", { email: "b@x.dev" }, { contact_id: "c4" }, { id: "c5" }],
-        include: ["properties", "topics"]
+        include: %w[properties topics]
       )
       expect(WebMock).to have_requested(:post, "https://api.test/contacts/batch/get").with(
         body: {
           "contacts" => [{ "id" => "c1" }, { "email" => "ghost@x.dev" }, { "email" => "b@x.dev" }, { "id" => "c4" }, { "id" => "c5" }],
-          "include" => ["properties", "topics"]
+          "include" => %w[properties topics],
         }
       )
       expect(res[:data].first).to include(object: "contact", id: "c1", email: "a@x.dev", unsubscribed: false)
@@ -537,7 +537,7 @@ def list_ok
       expect(WebMock).to have_requested(:post, "https://api.test/segments").with(
         body: {
           "name" => "Active",
-          "filter" => { "match" => "all", "conditions" => [{ "field" => "email", "op" => "is_set", "value" => nil }] }
+          "filter" => { "match" => "all", "conditions" => [{ "field" => "email", "op" => "is_set", "value" => nil }] },
         }
       )
 
@@ -607,11 +607,11 @@ def list_ok
 
       stub_request(:post, "https://api.test/suppressions/batch/remove").to_return(ok('{"data":[]}'))
       Millionsend::Suppressions::Batch.remove({ emails: ["a@x.dev"] })
-      Millionsend::Suppressions::Batch.remove({ ids: ["sup1", "sup2"] })
+      Millionsend::Suppressions::Batch.remove({ ids: %w[sup1 sup2] })
       expect(WebMock).to have_requested(:post, "https://api.test/suppressions/batch/remove")
         .with(body: { "emails" => ["a@x.dev"] })
       expect(WebMock).to have_requested(:post, "https://api.test/suppressions/batch/remove")
-        .with(body: { "ids" => ["sup1", "sup2"] })
+        .with(body: { "ids" => %w[sup1 sup2] })
     end
   end