diff --git a/internal/transport/handshake_log_test.go b/internal/transport/handshake_log_test.go
index aa01092..3461501 100644
--- a/internal/transport/handshake_log_test.go
+++ b/internal/transport/handshake_log_test.go
@@ -2,8 +2,11 @@ package transport
 
 import (
 	"bytes"
+	"crypto/x509"
+	"encoding/pem"
 	"errors"
 	"log/slog"
+	"os"
 	"strings"
 	"testing"
 )
@@ -93,6 +96,34 @@ func TestLogHandshakeFromCert_NilCert(t *testing.T) {
 	}
 }
 
+func TestLogHandshakeFromCert_WithCert(t *testing.T) {
+	var buf bytes.Buffer
+	log := newTestLogger(&buf)
+	dir := t.TempDir()
+	certPath, _, _ := generateTestCerts(t, dir, "localhost")
+	certPEM, err := os.ReadFile(certPath)
+	if err != nil {
+		t.Fatalf("read cert: %v", err)
+	}
+	block, _ := pem.Decode(certPEM)
+	if block == nil {
+		t.Fatal("pem.Decode: no cert block")
+	}
+	leaf, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		t.Fatalf("ParseCertificate: %v", err)
+	}
+	LogHandshakeFromCert(log, "peer-cert", leaf)
+	out := buf.String()
+	if !strings.Contains(out, "result=ok") {
+		t.Errorf("expected result=ok: %s", out)
+	}
+	expectedFP := FingerprintOfCert(leaf)
+	if !strings.Contains(out, "cert_fp="+expectedFP) {
+		t.Errorf("expected cert_fp=%s in: %s", expectedFP, out)
+	}
+}
+
 func TestFingerprintOfCert_Nil(t *testing.T) {
 	if got := FingerprintOfCert(nil); got != "" {
 		t.Errorf("FingerprintOfCert(nil) = %q, want empty", got)
diff --git a/internal/transport/idempotency_test.go b/internal/transport/idempotency_test.go
index bf979a1..135e0f3 100644
--- a/internal/transport/idempotency_test.go
+++ b/internal/transport/idempotency_test.go
@@ -112,6 +112,43 @@ func TestRetryContextCancel(t *testing.T) {
 	}
 }
 
+func TestIdempotencyStoreSweep(t *testing.T) {
+	s := NewIdempotencyStore()
+	s.Put("live-1", "job-1")
+	s.entries["expired"] = dedupeEntry{
+		key:       "expired",
+		jobID:     "old-job",
+		expiresAt: time.Now().Add(-1 * time.Minute),
+	}
+	s.Sweep()
+	if _, ok := s.entries["expired"]; ok {
+		t.Error("Sweep did not remove expired entry")
+	}
+	if _, ok := s.entries["live-1"]; !ok {
+		t.Error("Sweep removed live entry")
+	}
+}
+
+func TestIdempotencyStorePutEmpty(t *testing.T) {
+	s := NewIdempotencyStore()
+	s.Put("", "job-1")
+	s.Put("k1", "")
+	if _, ok := s.Get("k1"); ok {
+		t.Error("Put with empty jobID should not store")
+	}
+	if _, ok := s.Get(""); ok {
+		t.Error("Get with empty key should return false")
+	}
+}
+
+func TestWithIdempotencyKeyEmpty(t *testing.T) {
+	ctx := context.Background()
+	got := WithIdempotencyKey(ctx, "")
+	if got != ctx {
+		t.Error("WithIdempotencyKey with empty key should return ctx unchanged")
+	}
+}
+
 func TestIsTransient(t *testing.T) {
 	cases := []struct {
 		err  error
diff --git a/internal/transport/retry_test.go b/internal/transport/retry_test.go
new file mode 100644
index 0000000..e492acb
--- /dev/null
+++ b/internal/transport/retry_test.go
@@ -0,0 +1,203 @@
+package transport
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestDefaultRetryPolicy(t *testing.T) {
+	p := DefaultRetryPolicy()
+	if p.Initial != RetryInitial {
+		t.Errorf("Initial = %v, want %v", p.Initial, RetryInitial)
+	}
+	if p.Max != RetryMax {
+		t.Errorf("Max = %v, want %v", p.Max, RetryMax)
+	}
+	if p.MaxAttempts != RetryMaxAttempts {
+		t.Errorf("MaxAttempts = %d, want %d", p.MaxAttempts, RetryMaxAttempts)
+	}
+}
+
+func TestRetrySucceedsFirstAttempt(t *testing.T) {
+	calls := 0
+	got, err := Do(context.Background(), DefaultRetryPolicy(),
+		func(_ context.Context, attempt int) (string, bool, error) {
+			calls++
+			if attempt != 1 {
+				t.Errorf("attempt = %d, want 1", attempt)
+			}
+			return "ok", true, nil
+		})
+	if err != nil {
+		t.Fatalf("Do: %v", err)
+	}
+	if got != "ok" {
+		t.Errorf("got = %q, want ok", got)
+	}
+	if calls != 1 {
+		t.Errorf("calls = %d, want 1", calls)
+	}
+}
+
+func TestRetryIdempotentVerbRetries(t *testing.T) {
+	calls := 0
+	_, err := Do(context.Background(), DefaultRetryPolicy(),
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", true, errors.New("connection refused")
+		})
+	if err == nil {
+		t.Fatal("expected error after exhausting attempts")
+	}
+	if calls != RetryMaxAttempts {
+		t.Errorf("calls = %d, want %d", calls, RetryMaxAttempts)
+	}
+}
+
+func TestRetryWithIdempotencyKeyRetries(t *testing.T) {
+	calls := 0
+	ctx := WithIdempotencyKey(context.Background(), "key-1")
+	_, err := Do(ctx, DefaultRetryPolicy(),
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", false, errors.New("i/o timeout")
+		})
+	if err == nil {
+		t.Fatal("expected error after exhausting attempts")
+	}
+	if calls != RetryMaxAttempts {
+		t.Errorf("calls = %d, want %d (idempotency key enables retry)", calls, RetryMaxAttempts)
+	}
+}
+
+func TestRetryMaxAttemptsReached(t *testing.T) {
+	p := RetryPolicy{Initial: time.Millisecond, Max: 5 * time.Millisecond, MaxAttempts: 3}
+	calls := 0
+	_, err := Do(context.Background(), p,
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", true, errors.New("EOF")
+		})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !IsTransient(err) {
+		t.Errorf("expected transient error, got %v", err)
+	}
+	if calls != 3 {
+		t.Errorf("calls = %d, want 3", calls)
+	}
+}
+
+func TestRetryZeroMaxAttemptsDefaults(t *testing.T) {
+	calls := 0
+	p := RetryPolicy{}
+	_, err := Do(context.Background(), p,
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "ok", true, nil
+		})
+	if err != nil {
+		t.Fatalf("Do: %v", err)
+	}
+	if calls != 1 {
+		t.Errorf("calls = %d, want 1", calls)
+	}
+}
+
+func TestRetryNonTransientIdempotentRetries(t *testing.T) {
+	calls := 0
+	_, err := Do(context.Background(), DefaultRetryPolicy(),
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", true, errors.New("invalid spec")
+		})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if calls != RetryMaxAttempts {
+		t.Errorf("calls = %d, want %d (non-transient idempotent still retries)", calls, RetryMaxAttempts)
+	}
+}
+
+func TestRetryTransientNonIdempotentNoKeyBails(t *testing.T) {
+	calls := 0
+	_, err := Do(context.Background(), DefaultRetryPolicy(),
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", false, errors.New("connection refused")
+		})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if calls != 1 {
+		t.Errorf("calls = %d, want 1 (transient+non-idempotent+no key = bail)", calls)
+	}
+}
+
+func TestRetryContextCancelledMidBackoff(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	p := RetryPolicy{Initial: 100 * time.Millisecond, Max: time.Second, MaxAttempts: 5}
+	calls := 0
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		cancel()
+	}()
+	_, err := Do(ctx, p,
+		func(_ context.Context, _ int) (string, bool, error) {
+			calls++
+			return "", true, errors.New("connection refused")
+		})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("expected context.Canceled, got %v", err)
+	}
+}
+
+func TestBackoffGrowsExponentially(t *testing.T) {
+	initial := 10 * time.Millisecond
+	max := 1 * time.Second
+	d1 := backoff(initial, max, 1)
+	d2 := backoff(initial, max, 2)
+	d3 := backoff(initial, max, 3)
+	if d1 < 0 {
+		t.Errorf("backoff(1) = %v, want >= 0", d1)
+	}
+	if d2 < d1 {
+		t.Errorf("backoff(2)=%v < backoff(1)=%v (should grow)", d2, d1)
+	}
+	if d3 < d2 {
+		t.Errorf("backoff(3)=%v < backoff(2)=%v (should grow)", d3, d2)
+	}
+}
+
+func TestBackoffCapsAtMax(t *testing.T) {
+	initial := 100 * time.Millisecond
+	max := 200 * time.Millisecond
+	d := backoff(initial, max, 10)
+	if d > max+max/2 {
+		t.Errorf("backoff(10) = %v, want <= ~max=%v", d, max)
+	}
+}
+
+func TestContains(t *testing.T) {
+	cases := []struct {
+		s, sub string
+		want   bool
+	}{
+		{"hello world", "world", true},
+		{"hello", "xyz", false},
+		{"hello", "", true},
+		{"", "", true},
+		{"abc", "abcd", false},
+	}
+	for _, c := range cases {
+		if got := contains(c.s, c.sub); got != c.want {
+			t.Errorf("contains(%q, %q) = %v, want %v", c.s, c.sub, got, c.want)
+		}
+	}
+}