diff --git a/cache.go b/cache.go
index dca1d23..58d0852 100644
--- a/cache.go
+++ b/cache.go
@@ -14,9 +14,9 @@
 // responseCache is a common interface for cache implementations.
 type responseCache interface {
 	// Set sets the value for a key.
-	Set(key string, v interface{}) error
+	Set(key string, v any) error
 	// Get sets v to the value stored for a key.
-	Get(key string, v interface{}) error
+	Get(key string, v any) error
 }
 
 // gobCache stores and retrieves values using a memcache client using the gob
@@ -30,7 +30,7 @@
 	return &gobCache{memcache.New(addr)}
 }
 
-func (c *gobCache) Set(key string, v interface{}) error {
+func (c *gobCache) Set(key string, v any) error {
 	if c == nil {
 		return nil
 	}
@@ -41,7 +41,7 @@
 	return c.client.Set(&memcache.Item{Key: key, Value: buf.Bytes()})
 }
 
-func (c *gobCache) Get(key string, v interface{}) error {
+func (c *gobCache) Get(key string, v any) error {
 	if c == nil {
 		return memcache.ErrCacheMiss
 	}
diff --git a/examples.go b/examples.go
index d834d1f..bf4bb28 100644
--- a/examples.go
+++ b/examples.go
@@ -66,12 +66,12 @@
 		// Read examples ending in .txt, skipping those ending in .gotip.txt if
 		// gotip is not set.
 		prefix := "" // if non-empty, this is a relevant example file
-		if strings.HasSuffix(name, ".gotip.txt") {
+		if p, ok := strings.CutSuffix(name, ".gotip.txt"); ok {
 			if gotip {
-				prefix = strings.TrimSuffix(name, ".gotip.txt")
+				prefix = p
 			}
-		} else if strings.HasSuffix(name, ".txt") {
-			prefix = strings.TrimSuffix(name, ".txt")
+		} else if p, ok := strings.CutSuffix(name, ".txt"); ok {
+			prefix = p
 		}
 
 		if prefix == "" {
diff --git a/internal/internal_test.go b/internal/internal_test.go
index 337bbbc..25ea9ca 100644
--- a/internal/internal_test.go
+++ b/internal/internal_test.go
@@ -13,7 +13,7 @@
 func TestPeriodicallyDo(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	didWork := make(chan time.Time, 2)
-	done := make(chan interface{})
+	done := make(chan any)
 	go func() {
 		PeriodicallyDo(ctx, 100*time.Millisecond, func(ctx context.Context, t time.Time) {
 			select {
diff --git a/logger.go b/logger.go
index 2d36289..3b0afce 100644
--- a/logger.go
+++ b/logger.go
@@ -10,9 +10,9 @@
 )
 
 type logger interface {
-	Printf(format string, args ...interface{})
-	Errorf(format string, args ...interface{})
-	Fatalf(format string, args ...interface{})
+	Printf(format string, args ...any)
+	Errorf(format string, args ...any)
+	Fatalf(format string, args ...any)
 }
 
 // stdLogger implements the logger interface using the log package.
@@ -30,14 +30,14 @@
 	}
 }
 
-func (l *stdLogger) Printf(format string, args ...interface{}) {
+func (l *stdLogger) Printf(format string, args ...any) {
 	l.stdout.Printf(format, args...)
 }
 
-func (l *stdLogger) Errorf(format string, args ...interface{}) {
+func (l *stdLogger) Errorf(format string, args ...any) {
 	l.stderr.Printf(format, args...)
 }
 
-func (l *stdLogger) Fatalf(format string, args ...interface{}) {
+func (l *stdLogger) Fatalf(format string, args ...any) {
 	l.stderr.Fatalf(format, args...)
 }
diff --git a/play.go b/play.go
index 41e7058..ad83591 100644
--- a/play.go
+++ b/play.go
@@ -85,10 +85,7 @@
 	)
 
 	for _, e := range events {
-		delay := e.time.Sub(now)
-		if delay < 0 {
-			delay = 0
-		}
+		delay := max(e.time.Sub(now), 0)
 		out = append(out, Event{
 			Message: string(sanitize(e.msg)),
 			Kind:    e.kind,
@@ -165,10 +162,7 @@
 
 		// Slurp output.
 		// Truncated output is OK (probably caused by sandbox limits).
-		end := i + n
-		if end > len(output) {
-			end = len(output)
-		}
+		end := min(i+n, len(output))
 		add(t, output[i:end])
 		i += n
 	}
diff --git a/sandbox/sandbox.go b/sandbox/sandbox.go
index f266caa..3466158 100644
--- a/sandbox/sandbox.go
+++ b/sandbox/sandbox.go
@@ -545,7 +545,7 @@
 	t0 := time.Now()
 	tlast := t0
 	var logmu sync.Mutex
-	logf := func(format string, args ...interface{}) {
+	logf := func(format string, args ...any) {
 		if !*dev {
 			return
 		}
@@ -746,9 +746,9 @@
 // cleanStderr removes spam stderr lines from the beginning of x
 // and returns a slice of x.
 func cleanStderr(x []byte) []byte {
-	i := bytes.Index(x, containedStderrHeader)
-	if i == -1 {
+	_, stderr, ok := bytes.Cut(x, containedStderrHeader)
+	if !ok {
 		return x
 	}
-	return x[i+len(containedStderrHeader):]
+	return stderr
 }
diff --git a/sandbox_test.go b/sandbox_test.go
index 99f1f81..8a83b17 100644
--- a/sandbox_test.go
+++ b/sandbox_test.go
@@ -51,8 +51,8 @@
 	t.Logf("%s:\n%s", strings.Join(cmd.Args, " "), out)
 
 	isTestFunction := map[string]bool{}
-	lines := strings.Split(string(out), "\n")
-	for _, line := range lines {
+	lines := strings.SplitSeq(string(out), "\n")
+	for line := range lines {
 		// We want Test/Benchmark/Example/Fuzz functions.
 		// Reject extraneous output such as "ok ...".
 		if line == "" || !strings.Contains("TBEF", line[:1]) {
diff --git a/server.go b/server.go
index 1ecb134..b80362d 100644
--- a/server.go
+++ b/server.go
@@ -97,7 +97,7 @@
 
 // writeJSONResponse JSON-encodes resp and writes to w with the given HTTP
 // status.
-func (s *server) writeJSONResponse(w http.ResponseWriter, resp interface{}, status int) {
+func (s *server) writeJSONResponse(w http.ResponseWriter, resp any, status int) {
 	w.Header().Set("Content-Type", "application/json")
 	var buf bytes.Buffer
 	if err := json.NewEncoder(&buf).Encode(resp); err != nil {
diff --git a/server_test.go b/server_test.go
index 6a5362c..3d71058 100644
--- a/server_test.go
+++ b/server_test.go
@@ -26,13 +26,13 @@
 	t *testing.T
 }
 
-func (l testLogger) Printf(format string, args ...interface{}) {
+func (l testLogger) Printf(format string, args ...any) {
 	l.t.Logf(format, args...)
 }
-func (l testLogger) Errorf(format string, args ...interface{}) {
+func (l testLogger) Errorf(format string, args ...any) {
 	l.t.Errorf(format, args...)
 }
-func (l testLogger) Fatalf(format string, args ...interface{}) {
+func (l testLogger) Fatalf(format string, args ...any) {
 	l.t.Fatalf(format, args...)
 }
 
@@ -361,7 +361,7 @@
 
 // Set implements the responseCache interface.
 // Set stores a *response in the cache. It panics for other types to ensure test failure.
-func (i *inMemCache) Set(key string, v interface{}) error {
+func (i *inMemCache) Set(key string, v any) error {
 	i.l.Lock()
 	defer i.l.Unlock()
 	if i.m == nil {
@@ -374,7 +374,7 @@
 // Get implements the responseCache interface.
 // Get fetches a *response from the cache, or returns a memcache.ErrCacheMiss.
 // It panics for other types to ensure test failure.
-func (i *inMemCache) Get(key string, v interface{}) error {
+func (i *inMemCache) Get(key string, v any) error {
 	i.l.Lock()
 	defer i.l.Unlock()
 	target := v.(*response)
diff --git a/tests.go b/tests.go
index b0911a1..18cf53b 100644
--- a/tests.go
+++ b/tests.go
@@ -226,7 +226,7 @@
 				return nil
 			}
 			have := map[string]bool{}
-			for _, f := range strings.Split(got, "\n") {
+			for f := range strings.SplitSeq(got, "\n") {
 				have[f] = true
 			}
 			for _, expect := range []string{