diff --git a/renews_test.go b/renews_test.go
new file mode 100644
index 0000000..042298e
--- /dev/null
+++ b/renews_test.go
@@ -0,0 +1,105 @@
+package agent
+
+import (
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+)
+
+// fakeCore answers certificate requests and remembers what each one said.
+type fakeCore struct {
+	mu     sync.Mutex
+	bodies []map[string]any
+}
+
+func (f *fakeCore) handler(t *testing.T) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/agent/certificates" {
+			http.NotFound(w, r)
+			return
+		}
+		raw, _ := io.ReadAll(r.Body)
+		var body map[string]any
+		_ = json.Unmarshal(raw, &body)
+		f.mu.Lock()
+		f.bodies = append(f.bodies, body)
+		f.mu.Unlock()
+		now := time.Now().UTC()
+		_ = json.NewEncoder(w).Encode(map[string]any{"data": map[string]any{
+			"certificate_id":  "cert-1",
+			"common_name":     "web-01.example.com",
+			"sans":            []string{"web-01.example.com"},
+			"certificate_pem": "-----BEGIN CERTIFICATE-----\nMA==\n-----END CERTIFICATE-----\n",
+			"not_after":       now.Add(90 * 24 * time.Hour).Format(time.RFC3339),
+			"renew_after":     now.Add(60 * 24 * time.Hour).Format(time.RFC3339),
+		}})
+	})
+}
+
+func (f *fakeCore) last() map[string]any {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.bodies[len(f.bodies)-1]
+}
+
+func fakeRunner(t *testing.T, core *fakeCore) *Runner {
+	t.Helper()
+	srv := httptest.NewServer(core.handler(t))
+	t.Cleanup(srv.Close)
+	_, key, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return NewRunner(NewClient(srv.URL, "agent-1", key), State{AgentID: "agent-1", Server: srv.URL}, t.TempDir())
+}
+
+// TestARenewalSaysWhichCertificateItReplaces.
+//
+// The core recorded every request as a new certificate, so the one a renewal
+// replaced stayed ISSUED in the inventory and alerted as it neared expiry about
+// a certificate nothing was serving. This host is the only thing that knows
+// which certificate it is replacing, so it says so.
+func TestARenewalSaysWhichCertificateItReplaces(t *testing.T) {
+	core := &fakeCore{}
+	r := fakeRunner(t, core)
+
+	if _, err := r.Request(context.Background(), RequestOptions{Names: []string{"web-01.example.com"}}); err != nil {
+		t.Fatalf("first request: %v", err)
+	}
+	if _, ok := core.last()["renews"]; ok {
+		t.Fatalf("a first request claimed to renew something: %v", core.last())
+	}
+
+	// Due now.
+	metaPath := filepath.Join(r.stateDir, certsDir, "web-01.example.com", metaFileName)
+	raw, err := os.ReadFile(metaPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var held Held
+	if err := json.Unmarshal(raw, &held); err != nil {
+		t.Fatal(err)
+	}
+	held.RenewAfter = time.Now().Add(-time.Minute)
+	held.IssuedAt = time.Now().Add(-time.Hour)
+	raw, _ = json.Marshal(held)
+	if err := os.WriteFile(metaPath, raw, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	if n := r.RenewDue(context.Background()); n != 1 {
+		t.Fatalf("renewed %d certificates, want 1", n)
+	}
+	if got := core.last()["renews"]; got != "cert-1" {
+		t.Fatalf("the renewal told the core it replaces %v, want cert-1", got)
+	}
+}
diff --git a/request.go b/request.go
index cf35f1a..d7e73ee 100644
--- a/request.go
+++ b/request.go
@@ -61,6 +61,11 @@ type RequestOptions struct {
 	Names   []string
 	KeyType string
 	KeySize int
+	// Renews is the certificate this replaces, when renewing one this host
+	// holds. Without it the core recorded the renewal as a new certificate and
+	// left the old one ISSUED, alerting later about a certificate nothing was
+	// serving. Cores that predate it ignore it.
+	Renews string
 }
 
 // Request obtains a certificate for this host.
@@ -97,10 +102,14 @@ func (r *Runner) Request(ctx context.Context, opts RequestOptions) (*Held, error
 	var envelope struct {
 		Data Issued `json:"data"`
 	}
-	err = r.client.post(ctx, "/api/v1/agent/certificates", map[string]any{
+	payload := map[string]any{
 		"csr_pem":      string(csrPEM),
 		"install_path": dir,
-	}, &envelope)
+	}
+	if opts.Renews != "" {
+		payload["renews"] = opts.Renews
+	}
+	err = r.client.post(ctx, "/api/v1/agent/certificates", payload, &envelope)
 	if err != nil {
 		// The key is discarded rather than kept for a retry. A key that exists
 		// without a certificate is a secret nobody is tracking, and generating
@@ -237,7 +246,7 @@ func (r *Runner) RenewDue(ctx context.Context) int {
 		slog.Info("renewing a certificate this host holds",
 			"names", held.Names, "expires", held.NotAfter.Format(time.RFC3339))
 
-		if _, err := r.Request(ctx, RequestOptions{Names: held.Names}); err != nil {
+		if _, err := r.Request(ctx, RequestOptions{Names: held.Names, Renews: held.CertificateID}); err != nil {
 			// Not fatal, and not retried tightly. The next cycle tries again,
 			// and RenewAfter leaves room for a great many cycles before the
 			// certificate actually expires.