diff --git a/.github/workflows/test.yml b/.github/workflows/ci.yml
similarity index 55%
rename from .github/workflows/test.yml
rename to .github/workflows/ci.yml
index 55ce66d..15d752d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/ci.yml
@@ -1,10 +1,10 @@
-name: Test
+name: CI
 
 on:
   push:
 
 jobs:
-  rspec:
+  test:
     runs-on: ubuntu-latest
 
     strategy:
@@ -23,3 +23,17 @@ jobs:
           bundler-cache: true
 
       - run: bundle exec rake test
+
+  steep:
+    name: Type check (Steep)
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.4.4'
+          bundler-cache: true
+
+      - run: bundle exec steep check
diff --git a/Steepfile b/Steepfile
new file mode 100644
index 0000000..db0fc84
--- /dev/null
+++ b/Steepfile
@@ -0,0 +1,5 @@
+target :lib do
+  signature "sig"
+
+  check "lib"
+end
\ No newline at end of file
diff --git a/lib/predicateable.rb b/lib/predicateable.rb
index 0b846cd..f630f60 100644
--- a/lib/predicateable.rb
+++ b/lib/predicateable.rb
@@ -13,7 +13,8 @@ def predicate(name, values, prefix: false, strict: false)
         method = prefix ? "#{name}_#{value}?" : "#{value}?"
 
         define_method(method) do
-          actual = send(name)
+          # @type self: Object
+          actual = public_send(name)
 
           if strict
             actual == value
diff --git a/predicateable.gemspec b/predicateable.gemspec
index 6ee0b7a..b033aad 100644
--- a/predicateable.gemspec
+++ b/predicateable.gemspec
@@ -31,4 +31,6 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "minitest", "~> 5.22"
+  spec.add_development_dependency "rbs",   "~> 3.9"
+  spec.add_development_dependency "steep", "~> 1.10"
 end
diff --git a/sig/predicateable.rbs b/sig/predicateable.rbs
index 7e62b0e..29a9d24 100644
--- a/sig/predicateable.rbs
+++ b/sig/predicateable.rbs
@@ -1,4 +1,12 @@
 module Predicateable
-  VERSION: String
-  # See the writing guide of rbs: https://github.com/ruby/rbs#guides
+  def self.included: (Module base) -> void
+
+  module ClassMethods : Module
+    def predicate: (
+      Symbol | String name,
+      Array[Symbol | String] values,
+      ?prefix: bool,
+      ?strict: bool
+    ) -> Array[Symbol | String]
+  end
 end
diff --git a/sig/predicateable/version.rbs b/sig/predicateable/version.rbs
new file mode 100644
index 0000000..b9039f2
--- /dev/null
+++ b/sig/predicateable/version.rbs
@@ -0,0 +1,3 @@
+module Predicateable
+  VERSION: String
+end