diff --git a/internal/middleware/request_id_internal_test.go b/internal/middleware/request_id_internal_test.go
new file mode 100644
index 0000000..5b381d9
--- /dev/null
+++ b/internal/middleware/request_id_internal_test.go
@@ -0,0 +1,118 @@
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5/middleware"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// sendRequestID sends a request through the RequestID middleware, carrying
+// incoming as its X-Request-Id unless that is empty. It returns the ID the
+// next handler found in the request context, which the upstream fetch sends
+// and the log lines carry, and the X-Request-Id of the response.
+func sendRequestID(t *testing.T, incoming string) (string, string) {
+	t.Helper()
+
+	mw := &Middleware{log: slog.Default(), config: &config.Config{}}
+
+	var inContext string
+
+	handler := mw.RequestID()(http.HandlerFunc(
+		func(_ http.ResponseWriter, r *http.Request) {
+			inContext = middleware.GetReqID(r.Context())
+		}))
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
+	if incoming != "" {
+		req.Header.Set("X-Request-Id", incoming)
+	}
+
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	return inContext, rec.Header().Get("X-Request-Id")
+}
+
+// TestRequestIDKeepsShortPlainID verifies that a request's own X-Request-Id of
+// at most 64 letters, digits, '-', '_' or '.' is kept as its ID.
+func TestRequestIDKeepsShortPlainID(t *testing.T) {
+	t.Parallel()
+
+	for _, incoming := range []string{
+		"client-request-id",
+		"A1_b2.c3-d4",
+		strings.Repeat("a", 64),
+	} {
+		inContext, inResponse := sendRequestID(t, incoming)
+
+		if inContext != incoming || inResponse != incoming {
+			t.Errorf("incoming %q: context has %q, response %q, want both %q",
+				incoming, inContext, inResponse, incoming)
+		}
+	}
+}
+
+// TestRequestIDReplacesLongOrUnusualID verifies that a request's own
+// X-Request-Id that is over 64 characters or holds anything but letters,
+// digits, '-', '_' or '.' is neither sent back nor sent upstream: the request
+// gets a fresh ID instead.
+func TestRequestIDReplacesLongOrUnusualID(t *testing.T) {
+	t.Parallel()
+
+	for _, incoming := range []string{
+		strings.Repeat("a", 65),
+		strings.Repeat("a", 9000),
+		"has space",
+		"a/b",
+		"a,b",
+		"<script>",
+		"ünicode",
+	} {
+		inContext, inResponse := sendRequestID(t, incoming)
+		t.Logf("incoming %.20q: made up %q", incoming, inResponse)
+
+		if inResponse == "" || inResponse == incoming {
+			t.Errorf("incoming %.20q: response has %q, want a fresh ID",
+				incoming, inResponse)
+		}
+
+		if inContext != inResponse {
+			t.Errorf("incoming %.20q: context has %q, want the response's %q",
+				incoming, inContext, inResponse)
+		}
+	}
+}
+
+// TestRequestIDMadeUpTellsNothing verifies that the ID made up for a request
+// that sent none differs for every request and does not hold the host name.
+func TestRequestIDMadeUpTellsNothing(t *testing.T) {
+	t.Parallel()
+
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Fatalf("os.Hostname() error = %v", err)
+	}
+
+	firstInContext, first := sendRequestID(t, "")
+	_, second := sendRequestID(t, "")
+	t.Logf("host %q, made up %q and %q", hostname, first, second)
+
+	if first == "" || first == second {
+		t.Errorf("made up %q and %q, want two different IDs", first, second)
+	}
+
+	if firstInContext != first {
+		t.Errorf("context has %q, want the response's %q", firstInContext, first)
+	}
+
+	if strings.Contains(first, hostname) {
+		t.Errorf("made-up ID %q holds the host name %q", first, hostname)
+	}
+}