diff --git a/.changelog/121053e34770402bb72d4cf2c1057b2e.json b/.changelog/121053e34770402bb72d4cf2c1057b2e.json
new file mode 100644
index 00000000000..6c73f9c1aa2
--- /dev/null
+++ b/.changelog/121053e34770402bb72d4cf2c1057b2e.json
@@ -0,0 +1,8 @@
+{
+    "id": "121053e3-4770-402b-b72d-4cf2c1057b2e",
+    "type": "feature",
+    "description": "Add `middleware.WithHeaderComment` API, which explicitly re-adds behavior that was previously unintentially possible through `middleware.AddUserAgentKey`.",
+    "modules": [
+        "aws/middleware"
+    ]
+}
diff --git a/aws/middleware/header.go b/aws/middleware/header.go
new file mode 100644
index 00000000000..c826cfe0870
--- /dev/null
+++ b/aws/middleware/header.go
@@ -0,0 +1,82 @@
+package middleware
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/aws/smithy-go/middleware"
+	smithyhttp "github.com/aws/smithy-go/transport/http"
+)
+
+// WithHeaderComment instruments a middleware stack to append an HTTP field
+// comment to the given header as specified in RFC 9110
+// (https://www.rfc-editor.org/rfc/rfc9110#name-comments).
+//
+// The header is case-insensitive. If the provided header exists when the
+// middleware runs, the content will be inserted as-is enclosed in parentheses.
+//
+// Note that per the HTTP specification, comments are only allowed in fields
+// containing "comment" as part of their field value definition, but this API
+// will NOT verify whether the provided header is one of them.
+//
+// WithHeaderComment MAY be applied more than once to a middleware stack and/or
+// more than once per header.
+func WithHeaderComment(header, content string) func(*middleware.Stack) error {
+	return func(s *middleware.Stack) error {
+		m, err := getOrAddHeaderComment(s)
+		if err != nil {
+			return err
+		}
+
+		m.values.Add(header, content)
+		return nil
+	}
+}
+
+type headerCommentMiddleware struct {
+	values http.Header // hijack case-insensitive access APIs
+}
+
+func (*headerCommentMiddleware) ID() string {
+	return "headerComment"
+}
+
+func (m *headerCommentMiddleware) HandleBuild(ctx context.Context, in middleware.BuildInput, next middleware.BuildHandler) (
+	out middleware.BuildOutput, metadata middleware.Metadata, err error,
+) {
+	r, ok := in.Request.(*smithyhttp.Request)
+	if !ok {
+		return out, metadata, fmt.Errorf("unknown transport type %T", in.Request)
+	}
+
+	for h, contents := range m.values {
+		for _, c := range contents {
+			if existing := r.Header.Get(h); existing != "" {
+				r.Header.Set(h, fmt.Sprintf("%s (%s)", existing, c))
+			}
+		}
+	}
+
+	return next.HandleBuild(ctx, in)
+}
+
+func getOrAddHeaderComment(s *middleware.Stack) (*headerCommentMiddleware, error) {
+	id := (*headerCommentMiddleware)(nil).ID()
+	m, ok := s.Build.Get(id)
+	if !ok {
+		m := &headerCommentMiddleware{values: http.Header{}}
+		if err := s.Build.Add(m, middleware.After); err != nil {
+			return nil, err
+		}
+
+		return m, nil
+	}
+
+	hc, ok := m.(*headerCommentMiddleware)
+	if !ok {
+		return nil, fmt.Errorf("existing middleware w/ id %s is not *headerCommentMiddleware", id)
+	}
+
+	return hc, nil
+}
diff --git a/aws/middleware/header_test.go b/aws/middleware/header_test.go
new file mode 100644
index 00000000000..c7c4257e689
--- /dev/null
+++ b/aws/middleware/header_test.go
@@ -0,0 +1,114 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/aws/smithy-go/middleware"
+	smithyhttp "github.com/aws/smithy-go/transport/http"
+)
+
+func TestWithHeaderComment_CaseInsensitive(t *testing.T) {
+	stack, err := newTestStack(
+		WithHeaderComment("foo", "bar"),
+	)
+	if err != nil {
+		t.Errorf("expected no error on new stack, got %v", err)
+	}
+
+	r := injectBuildRequest(stack)
+	r.Header.Set("Foo", "baz")
+
+	if err := handle(stack); err != nil {
+		t.Errorf("expected no error on handle, got %v", err)
+	}
+
+	expectHeader(t, r.Header, "Foo", "baz (bar)")
+}
+
+func TestWithHeaderComment_Noop(t *testing.T) {
+	stack, err := newTestStack(
+		WithHeaderComment("foo", "bar"),
+	)
+	if err != nil {
+		t.Errorf("expected no error on new stack, got %v", err)
+	}
+
+	r := injectBuildRequest(stack)
+
+	if err := handle(stack); err != nil {
+		t.Errorf("expected no error on handle, got %v", err)
+	}
+
+	expectHeader(t, r.Header, "Foo", "")
+}
+
+func TestWithHeaderComment_MultiCaseInsensitive(t *testing.T) {
+	stack, err := newTestStack(
+		WithHeaderComment("foo", "c1"),
+		WithHeaderComment("Foo", "c2"),
+		WithHeaderComment("baz", "c3"),
+		WithHeaderComment("Baz", "c4"),
+	)
+	if err != nil {
+		t.Errorf("expected no error on new stack, got %v", err)
+	}
+
+	r := injectBuildRequest(stack)
+	r.Header.Set("Foo", "1")
+	r.Header.Set("Baz", "2")
+
+	if err := handle(stack); err != nil {
+		t.Errorf("expected no error on handle, got %v", err)
+	}
+
+	expectHeader(t, r.Header, "Foo", "1 (c1) (c2)")
+	expectHeader(t, r.Header, "Baz", "2 (c3) (c4)")
+}
+
+func newTestStack(fns ...func(*middleware.Stack) error) (*middleware.Stack, error) {
+	s := middleware.NewStack("", smithyhttp.NewStackRequest)
+	for _, fn := range fns {
+		if err := fn(s); err != nil {
+			return nil, err
+		}
+	}
+	return s, nil
+}
+
+func handle(stack *middleware.Stack) error {
+	_, _, err := middleware.DecorateHandler(
+		middleware.HandlerFunc(
+			func(ctx context.Context, input interface{}) (
+				interface{}, middleware.Metadata, error,
+			) {
+				return nil, middleware.Metadata{}, nil
+			},
+		),
+		stack,
+	).Handle(context.Background(), nil)
+	return err
+}
+
+func injectBuildRequest(s *middleware.Stack) *smithyhttp.Request {
+	r := smithyhttp.NewStackRequest()
+	s.Build.Add(
+		middleware.BuildMiddlewareFunc(
+			"injectBuildRequest",
+			func(ctx context.Context, in middleware.BuildInput, next middleware.BuildHandler) (
+				middleware.BuildOutput, middleware.Metadata, error,
+			) {
+				return next.HandleBuild(ctx, middleware.BuildInput{Request: r})
+			},
+		),
+		middleware.Before,
+	)
+	return r.(*smithyhttp.Request)
+}
+
+func expectHeader(t *testing.T, header http.Header, h, ev string) {
+	if av := header.Get(h); ev != av {
+		t.Errorf("expected header '%s: %s', got '%s'", h, ev, av)
+	}
+}