diff --git a/lib/mcp/annotations.rb b/lib/mcp/annotations.rb
index 540588a6..f8562f22 100644
--- a/lib/mcp/annotations.rb
+++ b/lib/mcp/annotations.rb
@@ -2,9 +2,14 @@
 
 module MCP
   class Annotations
+    SUPPORTED_AUDIENCES = ["user", "assistant"].freeze
+
     attr_reader :audience, :priority, :last_modified
 
     def initialize(audience: nil, priority: nil, last_modified: nil)
+      if audience && !(audience.is_a?(Array) && audience.all? { |role| SUPPORTED_AUDIENCES.include?(role) })
+        raise ArgumentError, 'The value of audience must be an array of "user" or "assistant".'
+      end
       raise ArgumentError, "The value of priority must be between 0 and 1." if priority && !priority.between?(0, 1)
 
       @audience = audience
diff --git a/test/mcp/annotations_test.rb b/test/mcp/annotations_test.rb
index a82b9552..1e5d3df2 100644
--- a/test/mcp/annotations_test.rb
+++ b/test/mcp/annotations_test.rb
@@ -5,24 +5,24 @@
 module MCP
   class AnnotationsTest < ActiveSupport::TestCase
     def test_initialization
-      annotations = Annotations.new(audience: ["developers"], priority: 0.8)
+      annotations = Annotations.new(audience: ["user"], priority: 0.8)
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_equal(0.8, annotations.priority)
       assert_nil(annotations.last_modified)
 
-      assert_equal({ audience: ["developers"], priority: 0.8 }, annotations.to_h)
+      assert_equal({ audience: ["user"], priority: 0.8 }, annotations.to_h)
     end
 
     def test_initialization_with_all_attributes
       timestamp = Time.utc(2025, 1, 12, 15, 0, 58).iso8601
-      annotations = Annotations.new(audience: ["developers"], priority: 0.8, last_modified: timestamp)
+      annotations = Annotations.new(audience: ["user"], priority: 0.8, last_modified: timestamp)
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_equal(0.8, annotations.priority)
       assert_equal(timestamp, annotations.last_modified)
 
-      assert_equal({ audience: ["developers"], priority: 0.8, lastModified: timestamp }, annotations.to_h)
+      assert_equal({ audience: ["user"], priority: 0.8, lastModified: timestamp }, annotations.to_h)
     end
 
     def test_initialization_by_default
@@ -36,13 +36,13 @@ def test_initialization_by_default
     end
 
     def test_initialization_with_partial_attributes
-      annotations = Annotations.new(audience: ["developers"])
+      annotations = Annotations.new(audience: ["user"])
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_nil(annotations.priority)
       assert_nil(annotations.last_modified)
 
-      assert_equal({ audience: ["developers"] }, annotations.to_h)
+      assert_equal({ audience: ["user"] }, annotations.to_h)
     end
 
     def test_initialization_with_last_modified_only
@@ -81,5 +81,50 @@ def test_invalid_priority_below_lower_bound
       end
       assert_equal("The value of priority must be between 0 and 1.", exception.message)
     end
+
+    def test_valid_audience_with_user
+      assert_nothing_raised do
+        Annotations.new(audience: ["user"])
+      end
+    end
+
+    def test_valid_audience_with_assistant
+      assert_nothing_raised do
+        Annotations.new(audience: ["assistant"])
+      end
+    end
+
+    def test_valid_audience_with_multiple_roles
+      assert_nothing_raised do
+        Annotations.new(audience: ["user", "assistant"])
+      end
+    end
+
+    def test_valid_audience_with_empty_array
+      assert_nothing_raised do
+        Annotations.new(audience: [])
+      end
+    end
+
+    def test_invalid_audience_with_unknown_role
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: ["developers"])
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
+
+    def test_invalid_audience_with_mixed_roles
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: ["user", "developers"])
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
+
+    def test_invalid_audience_when_not_array
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: "user")
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
   end
 end