diff --git a/test/annotation.jl b/test/annotation.jl
index 48bea6b5..878cf429 100644
--- a/test/annotation.jl
+++ b/test/annotation.jl
@@ -65,3 +65,49 @@ using PromptingTools: AnnotationMessage, SystemMessage, TracerMessage, UserMessa
         @test all(!isabstractannotationmessage, rendered)
     end
 end
+
+@testset "annotate!" begin
+    # Test basic annotation with single message
+    msg = UserMessage("Hello")
+    annotated = annotate!(msg, "metadata"; tags = [:test])
+    @test length(annotated) == 2
+    @test isabstractannotationmessage(annotated[1])
+    @test annotated[1].content == "metadata"
+    @test annotated[1].tags == [:test]
+    @test annotated[2].content == "Hello"
+
+    # Test annotation with vector of messages
+    messages = [
+        SystemMessage("System"),
+        UserMessage("User"),
+        AIMessage("AI")
+    ]
+    annotated = annotate!(messages, "metadata"; comment = "test comment")
+    @test length(annotated) == 4
+    @test isabstractannotationmessage(annotated[1])
+    @test annotated[1].content == "metadata"
+    @test annotated[1].comment == "test comment"
+    @test annotated[2:end] == messages
+
+    # Test annotation with existing annotations
+    messages = [
+        AnnotationMessage("First annotation"),
+        SystemMessage("System"),
+        UserMessage("User"),
+        AnnotationMessage("Second annotation"),
+        AIMessage("AI")
+    ]
+    annotated = annotate!(messages, "new metadata")
+    @test length(annotated) == 6
+    @test isabstractannotationmessage(annotated[1])
+    @test isabstractannotationmessage(annotated[4])
+    @test annotated[5].content == "new metadata"
+    @test annotated[6].content == "AI"
+
+    # Test annotation with extras
+    extras = Dict{Symbol, Any}(:key => "value")
+    annotated = annotate!(UserMessage("Hello"), "metadata"; extras = extras)
+    @test length(annotated) == 2
+    @test annotated[1].content == "metadata"
+    @test annotated[1].extras == extras
+end
diff --git a/test/messages.jl b/test/messages.jl
index a46d747b..7efb9041 100644
--- a/test/messages.jl
+++ b/test/messages.jl
@@ -7,7 +7,7 @@ using PromptingTools: isusermessage, issystemmessage, isdatamessage, isaimessage
                       istracermessage, isaitoolrequest, istoolmessage,
                       isabstractannotationmessage
 using PromptingTools: TracerMessageLike, TracerMessage, align_tracer!, unwrap,
-                      AbstractTracerMessage, AbstractTracer, pprint
+                      AbstractTracerMessage, AbstractTracer, pprint, annotate!
 using PromptingTools: TracerSchema, SaverSchema
 
 @testset "Message constructors" begin
@@ -79,13 +79,6 @@ end
     @test isabstractannotationmessage(msgs[1])
     @test msgs[1].tags == [:debug]
 
-    # Test single message annotation
-    msg = UserMessage("Test")
-    result = annotate!(msg, "Annotation", comment = "Note")
-    @test length(result) == 2
-    @test isabstractannotationmessage(result[1])
-    @test result[1].comment == "Note"
-
     # Test pretty printing
     io = IOBuffer()
     pprint(io, annotation)