diff --git a/app/metrics/builtin.go b/app/metrics/builtin.go
index 74fe5243..a0c633c3 100644
--- a/app/metrics/builtin.go
+++ b/app/metrics/builtin.go
@@ -141,12 +141,18 @@ func RecordDuration(integration string, d time.Duration) {
 	h.Observe(d.Seconds())
 }
 
+func writePromType(w http.ResponseWriter, name, metricType string) {
+	fmt.Fprintf(w, "# TYPE %s %s\n", name, metricType)
+}
+
 // WriteProm emits all Prometheus metrics to w in text format.
 func WriteProm(w http.ResponseWriter) {
 	w.Header().Set("Content-Type", "text/plain; version=0.0.4")
+	writePromType(w, "authtranslator_requests_total", "counter")
 	requestCounts.Do(func(kv expvar.KeyValue) {
 		fmt.Fprintf(w, "authtranslator_requests_total{integration=%q} %s\n", kv.Key, kv.Value.String())
 	})
+	writePromType(w, "authtranslator_request_duration_seconds", "histogram")
 	durationHistsMu.Lock()
 	type histSnapshot struct {
 		name string
@@ -160,12 +166,15 @@ func WriteProm(w http.ResponseWriter) {
 	for _, hs := range hists {
 		hs.h.writeProm(w, hs.name)
 	}
+	writePromType(w, "authtranslator_rate_limit_events_total", "counter")
 	rateLimitCounts.Do(func(kv expvar.KeyValue) {
 		fmt.Fprintf(w, "authtranslator_rate_limit_events_total{integration=%q} %s\n", kv.Key, kv.Value.String())
 	})
+	writePromType(w, "authtranslator_auth_failures_total", "counter")
 	authFailureCounts.Do(func(kv expvar.KeyValue) {
 		fmt.Fprintf(w, "authtranslator_auth_failures_total{integration=%q} %s\n", kv.Key, kv.Value.String())
 	})
+	writePromType(w, "authtranslator_internal_responses_total", "counter")
 	internalResponseCounts.Do(func(kv expvar.KeyValue) {
 		parts := strings.SplitN(kv.Key, metricKeySeparator, 3)
 		if len(parts) != 3 {
@@ -174,6 +183,7 @@ func WriteProm(w http.ResponseWriter) {
 		integ, code, reason := parts[0], parts[1], parts[2]
 		fmt.Fprintf(w, "authtranslator_internal_responses_total{integration=%q,code=%q,reason=%q} %s\n", integ, code, reason, kv.Value.String())
 	})
+	writePromType(w, "authtranslator_upstream_responses_total", "counter")
 	upstreamStatusCounts.Do(func(kv expvar.KeyValue) {
 		parts := strings.SplitN(kv.Key, metricKeySeparator, 2)
 		if len(parts) != 2 {
diff --git a/app/metrics/metrics_test.go b/app/metrics/metrics_test.go
index 70b9228e..a0f7ea6f 100644
--- a/app/metrics/metrics_test.go
+++ b/app/metrics/metrics_test.go
@@ -45,8 +45,22 @@ func TestMetricsHandlerEmpty(t *testing.T) {
 	if ct := rr.Header().Get("Content-Type"); ct != "text/plain; version=0.0.4" {
 		t.Fatalf("expected content type text/plain; version=0.0.4, got %s", ct)
 	}
-	if body := rr.Body.String(); body != "" {
-		t.Fatalf("expected empty body, got %q", body)
+
+	body := rr.Body.String()
+	for _, line := range []string{
+		"# TYPE authtranslator_requests_total counter",
+		"# TYPE authtranslator_request_duration_seconds histogram",
+		"# TYPE authtranslator_rate_limit_events_total counter",
+		"# TYPE authtranslator_auth_failures_total counter",
+		"# TYPE authtranslator_internal_responses_total counter",
+		"# TYPE authtranslator_upstream_responses_total counter",
+	} {
+		if !strings.Contains(body, line) {
+			t.Fatalf("missing metric type line %q in %q", line, body)
+		}
+	}
+	if strings.Contains(body, `{integration="`) {
+		t.Fatalf("expected no metric samples, got %q", body)
 	}
 }
 
@@ -73,9 +87,17 @@ func TestMetricsHandlerOutput(t *testing.T) {
 	}
 
 	body := rr.Body.String()
-	lines := strings.Split(strings.TrimSpace(body), "\n")
-	if len(lines) < 27 {
-		t.Fatalf("expected at least 27 metrics lines, got %d", len(lines))
+	for _, line := range []string{
+		"# TYPE authtranslator_requests_total counter",
+		"# TYPE authtranslator_request_duration_seconds histogram",
+		"# TYPE authtranslator_rate_limit_events_total counter",
+		"# TYPE authtranslator_auth_failures_total counter",
+		"# TYPE authtranslator_internal_responses_total counter",
+		"# TYPE authtranslator_upstream_responses_total counter",
+	} {
+		if !strings.Contains(body, line) {
+			t.Fatalf("missing metric type line %q in %q", line, body)
+		}
 	}
 	if !strings.Contains(body, `authtranslator_requests_total{integration="foo"} 2`) {
 		t.Fatal("missing foo request metric")
diff --git a/app/metrics/plugins/example/plugin.go b/app/metrics/plugins/example/plugin.go
index 0ec37bcf..c35562d3 100644
--- a/app/metrics/plugins/example/plugin.go
+++ b/app/metrics/plugins/example/plugin.go
@@ -3,10 +3,8 @@
 package example
 
 import (
-	"bytes"
 	"encoding/json"
 	"fmt"
-	"io"
 	"net/http"
 	"sync"
 
@@ -47,6 +45,10 @@ func (t *tokenCounter) OnResponse(integ, caller string, r *http.Request, resp *h
 func (t *tokenCounter) WriteProm(w http.ResponseWriter) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
+	if len(t.totals) == 0 {
+		return
+	}
+	fmt.Fprintln(w, "# TYPE authtranslator_tokens_total counter")
 	for caller, total := range t.totals {
 		fmt.Fprintf(w, "authtranslator_tokens_total{caller=%q} %d\n", caller, total)
 	}
diff --git a/app/metrics/plugins/example/plugin_test.go b/app/metrics/plugins/example/plugin_test.go
index d879116e..aa48fd2e 100644
--- a/app/metrics/plugins/example/plugin_test.go
+++ b/app/metrics/plugins/example/plugin_test.go
@@ -32,6 +32,9 @@ func TestTokenCounter(t *testing.T) {
 
 	rr := httptest.NewRecorder()
 	metrics.Handler(rr, httptest.NewRequest(http.MethodGet, "/metrics", nil), "", "")
+	if !strings.Contains(rr.Body.String(), "# TYPE authtranslator_tokens_total counter") {
+		t.Fatalf("token metric type missing: %s", rr.Body.String())
+	}
 	if !strings.Contains(rr.Body.String(), `authtranslator_tokens_total{caller="caller"} 42`) {
 		t.Fatalf("token metric missing: %s", rr.Body.String())
 	}
diff --git a/docs/metrics-plugins.md b/docs/metrics-plugins.md
index 57961710..c197294d 100644
--- a/docs/metrics-plugins.md
+++ b/docs/metrics-plugins.md
@@ -22,7 +22,9 @@ type Plugin interface {
 
 `OnRequest` fires just before the proxy forwards a request upstream and
 `OnResponse` runs once the upstream reply is received. `WriteProm` lets a plugin
-append custom Prometheus metrics. The integration name is passed so you can
+append custom Prometheus metrics. For custom families, emit the matching
+Prometheus metadata lines such as `# TYPE ... counter` so functions like
+`rate()` are recognized correctly. The integration name is passed so you can
 apply per-service logic.
 
 ---
@@ -97,6 +99,10 @@ func (t *tokenCounter) OnResponse(integ, caller string, r *http.Request, resp *h
 func (t *tokenCounter) WriteProm(w http.ResponseWriter) {
     t.mu.Lock()
     defer t.mu.Unlock()
+    if len(t.totals) == 0 {
+        return
+    }
+    fmt.Fprintln(w, "# TYPE authtranslator_tokens_total counter")
     for caller, total := range t.totals {
         fmt.Fprintf(w, "authtranslator_tokens_total{caller=%q} %d\n", caller, total)
     }