diff --git a/internal/requestid/requestid.go b/internal/requestid/requestid.go
new file mode 100644
index 0000000..a6bc8ef
--- /dev/null
+++ b/internal/requestid/requestid.go
@@ -0,0 +1,19 @@
+// Package requestid provides a shared context key for request IDs,
+// allowing both client and server packages to access request IDs
+// without circular imports.
+package requestid
+
+import "context"
+
+type key struct{}
+
+// NewContext returns a context with the given request ID.
+func NewContext(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, key{}, id)
+}
+
+// FromContext returns the request ID from ctx, or empty string if not set.
+func FromContext(ctx context.Context) string {
+	id, _ := ctx.Value(key{}).(string)
+	return id
+}
diff --git a/middleware/requestid.go b/middleware/requestid.go
new file mode 100644
index 0000000..67d41be
--- /dev/null
+++ b/middleware/requestid.go
@@ -0,0 +1,23 @@
+package middleware
+
+import (
+	"net/http"
+
+	"git.codelab.vc/pkg/httpx/internal/requestid"
+)
+
+// RequestID returns a middleware that propagates the request ID from the
+// request context to the outgoing X-Request-Id header. This pairs with
+// the server.RequestID middleware: the server stores the ID in the context,
+// and the client middleware forwards it to downstream services.
+func RequestID() Middleware {
+	return func(next http.RoundTripper) http.RoundTripper {
+		return RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if id := requestid.FromContext(req.Context()); id != "" {
+				req = req.Clone(req.Context())
+				req.Header.Set("X-Request-Id", id)
+			}
+			return next.RoundTrip(req)
+		})
+	}
+}
diff --git a/middleware/requestid_test.go b/middleware/requestid_test.go
new file mode 100644
index 0000000..4229592
--- /dev/null
+++ b/middleware/requestid_test.go
@@ -0,0 +1,69 @@
+package middleware_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx/internal/requestid"
+	"git.codelab.vc/pkg/httpx/middleware"
+)
+
+func TestRequestID(t *testing.T) {
+	t.Run("propagates ID from context", func(t *testing.T) {
+		var gotHeader string
+		base := middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			gotHeader = req.Header.Get("X-Request-Id")
+			return &http.Response{StatusCode: http.StatusOK, Body: http.NoBody}, nil
+		})
+
+		mw := middleware.RequestID()(base)
+
+		ctx := requestid.NewContext(context.Background(), "test-id-123")
+		req, _ := http.NewRequestWithContext(ctx, http.MethodGet, "http://example.com", nil)
+		_, err := mw.RoundTrip(req)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if gotHeader != "test-id-123" {
+			t.Fatalf("X-Request-Id = %q, want %q", gotHeader, "test-id-123")
+		}
+	})
+
+	t.Run("no ID in context skips header", func(t *testing.T) {
+		var gotHeader string
+		base := middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			gotHeader = req.Header.Get("X-Request-Id")
+			return &http.Response{StatusCode: http.StatusOK, Body: http.NoBody}, nil
+		})
+
+		mw := middleware.RequestID()(base)
+
+		req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, "http://example.com", nil)
+		_, err := mw.RoundTrip(req)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if gotHeader != "" {
+			t.Fatalf("expected no X-Request-Id header, got %q", gotHeader)
+		}
+	})
+
+	t.Run("does not mutate original request", func(t *testing.T) {
+		base := middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			return &http.Response{StatusCode: http.StatusOK, Body: http.NoBody}, nil
+		})
+
+		mw := middleware.RequestID()(base)
+
+		ctx := requestid.NewContext(context.Background(), "test-id")
+		req, _ := http.NewRequestWithContext(ctx, http.MethodGet, "http://example.com", nil)
+		_, _ = mw.RoundTrip(req)
+
+		if req.Header.Get("X-Request-Id") != "" {
+			t.Fatal("original request was mutated")
+		}
+	})
+}
diff --git a/server/middleware_requestid.go b/server/middleware_requestid.go
index 081a5a7..5cfdb26 100644
--- a/server/middleware_requestid.go
+++ b/server/middleware_requestid.go
@@ -5,9 +5,9 @@ import (
 	"crypto/rand"
 	"fmt"
 	"net/http"
-)
 
-type requestIDKey struct{}
+	"git.codelab.vc/pkg/httpx/internal/requestid"
+)
 
 // RequestID returns a middleware that assigns a unique request ID to each
 // request. If the incoming request already has an X-Request-Id header, that
@@ -23,7 +23,7 @@ func RequestID() Middleware {
 				id = newUUID()
 			}
 
-			ctx := context.WithValue(r.Context(), requestIDKey{}, id)
+			ctx := requestid.NewContext(r.Context(), id)
 			w.Header().Set("X-Request-Id", id)
 			next.ServeHTTP(w, r.WithContext(ctx))
 		})
@@ -33,8 +33,7 @@ func RequestID() Middleware {
 // RequestIDFromContext returns the request ID from the context, or an empty
 // string if none is set.
 func RequestIDFromContext(ctx context.Context) string {
-	id, _ := ctx.Value(requestIDKey{}).(string)
-	return id
+	return requestid.FromContext(ctx)
 }
 
 // newUUID generates a UUID v4 string using crypto/rand.