diff --git a/internal/server/request_id_internal_test.go b/internal/server/request_id_internal_test.go
new file mode 100644
index 0000000..d81a05b
--- /dev/null
+++ b/internal/server/request_id_internal_test.go
@@ -0,0 +1,58 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// requestIDHeader is the header that carries a request's ID.
+const requestIDHeader = "X-Request-Id"
+
+// TestResponsesCarryRequestID verifies that every response, whatever its route
+// and status, carries the request's ID as X-Request-Id, so a client can quote
+// it when reporting a problem: one pixa made up when the request brought none,
+// and the request's own X-Request-Id when it brought one.
+func TestResponsesCarryRequestID(t *testing.T) {
+	t.Parallel()
+
+	const clientRequestID = "client-request-id"
+
+	s := newTestServer(t)
+
+	paths := []string{
+		"/robots.txt",
+		"/no-such-path",
+		unsignedImagePath,
+		encryptedImagePath,
+	}
+
+	for _, path := range paths {
+		t.Run(path, func(t *testing.T) {
+			t.Parallel()
+
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, path, nil))
+			t.Logf("status %d, %s %q",
+				rec.Code, requestIDHeader, rec.Header().Get(requestIDHeader))
+
+			if rec.Header().Get(requestIDHeader) == "" {
+				t.Errorf("response has no %s", requestIDHeader)
+			}
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, path, nil)
+			req.Header.Set(requestIDHeader, clientRequestID)
+
+			rec = httptest.NewRecorder()
+			s.ServeHTTP(rec, req)
+
+			got := rec.Header().Get(requestIDHeader)
+			if got != clientRequestID {
+				t.Errorf("%s = %q, want the request's own %q",
+					requestIDHeader, got, clientRequestID)
+			}
+		})
+	}
+}