diff --git a/internal/http3/body.go b/internal/http3/body.go
index cdde482e..868f85c4 100644
--- a/internal/http3/body.go
+++ b/internal/http3/body.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/body_test.go b/internal/http3/body_test.go
index 599e0df8..c2288d3d 100644
--- a/internal/http3/body_test.go
+++ b/internal/http3/body_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -256,7 +256,7 @@ func TestReadData(t *testing.T) {
 
 		}
 
-		runSynctestSubtest(t, test.name+"/client", func(t testing.TB) {
+		synctestSubtest(t, test.name+"/client", func(t *testing.T) {
 			tc := newTestClientConn(t)
 			tc.greet()
 
diff --git a/internal/http3/conn.go b/internal/http3/conn.go
index 5eb80311..6a3c962b 100644
--- a/internal/http3/conn.go
+++ b/internal/http3/conn.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/conn_test.go b/internal/http3/conn_test.go
index a9afb1f9..78f19c46 100644
--- a/internal/http3/conn_test.go
+++ b/internal/http3/conn_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -142,11 +142,11 @@ func TestConnPeerCreatesBadUnidirectionalStream(t *testing.T) {
 
 func runConnTest(t *testing.T, f func(testing.TB, *testQUICConn)) {
 	t.Helper()
-	runSynctestSubtest(t, "client", func(t testing.TB) {
+	synctestSubtest(t, "client", func(t *testing.T) {
 		tc := newTestClientConn(t)
 		f(t, tc.testQUICConn)
 	})
-	runSynctestSubtest(t, "server", func(t testing.TB) {
+	synctestSubtest(t, "server", func(t *testing.T) {
 		ts := newTestServer(t)
 		tc := ts.connect()
 		f(t, tc.testQUICConn)
diff --git a/internal/http3/errors.go b/internal/http3/errors.go
index db46acfc..273ad014 100644
--- a/internal/http3/errors.go
+++ b/internal/http3/errors.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import "fmt"
diff --git a/internal/http3/files_test.go b/internal/http3/files_test.go
index 9c97a6ce..fcb4596d 100644
--- a/internal/http3/files_test.go
+++ b/internal/http3/files_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
@@ -13,13 +13,11 @@ import (
 	"testing"
 )
 
-// TestFiles checks that every file in this package has a build constraint on Go 1.24.
+// TestFiles checks that every test file in this package has a build constraint
+// on Go 1.25. Tests rely on synctest.Test, added in Go 1.25.
 //
-// Package tests rely on testing/synctest, added as an experiment in Go 1.24.
-// When moving internal/http3 to an importable location, we can decide whether
-// to relax the constraint for non-test files.
-//
-// Drop this test when the x/net go.mod depends on 1.24 or newer.
+// TODO(nsh): drop this test and Go 1.25 build contraints once x/net go.mod
+// depends on 1.25 or newer.
 func TestFiles(t *testing.T) {
 	f, err := os.Open(".")
 	if err != nil {
@@ -45,12 +43,10 @@ func TestFiles(t *testing.T) {
 		if name == "doc.go" {
 			continue
 		}
-		if !bytes.Contains(b, []byte("//go:build go1.24")) {
-			t.Errorf("%v: missing constraint on go1.24", name)
-		}
-		if bytes.Contains(b, []byte(`"testing/synctest"`)) &&
-			!bytes.Contains(b, []byte("//go:build go1.24 && goexperiment.synctest")) {
-			t.Errorf("%v: missing constraint on go1.24 && goexperiment.synctest", name)
+		if strings.HasSuffix(name, "_test.go") {
+			if !bytes.Contains(b, []byte("//go:build go1.25")) {
+				t.Errorf("%v: missing constraint on go1.25", name)
+			}
 		}
 	}
 }
diff --git a/internal/http3/http3.go b/internal/http3/http3.go
index 1f606705..edbba0ca 100644
--- a/internal/http3/http3.go
+++ b/internal/http3/http3.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import "fmt"
diff --git a/internal/http3/http3_synctest_test.go b/internal/http3/http3_synctest_test.go
deleted file mode 100644
index a9c0ac29..00000000
--- a/internal/http3/http3_synctest_test.go
+++ /dev/null
@@ -1,64 +0,0 @@
-// Copyright 2024 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.24 && goexperiment.synctest
-
-package http3
-
-import (
-	"context"
-	"slices"
-	"testing"
-	"testing/synctest"
-)
-
-// runSynctest runs f in a synctest.Run bubble.
-// It arranges for t.Cleanup functions to run within the bubble.
-// TODO: Replace with synctest.Test, which handles all this properly.
-func runSynctest(t *testing.T, f func(t testing.TB)) {
-	synctest.Run(func() {
-		// Create a context within the bubble, rather than using t.Context.
-		ctx, cancel := context.WithCancel(context.Background())
-		ct := &cleanupT{
-			T:      t,
-			ctx:    ctx,
-			cancel: cancel,
-		}
-		defer ct.done()
-		f(ct)
-	})
-}
-
-// runSynctestSubtest runs f in a subtest in a synctest.Run bubble.
-func runSynctestSubtest(t *testing.T, name string, f func(t testing.TB)) {
-	t.Run(name, func(t *testing.T) {
-		runSynctest(t, f)
-	})
-}
-
-// cleanupT wraps a testing.T and adds its own Cleanup method.
-// Used to execute cleanup functions within a synctest bubble.
-type cleanupT struct {
-	*testing.T
-	ctx      context.Context
-	cancel   context.CancelFunc
-	cleanups []func()
-}
-
-// Cleanup replaces T.Cleanup.
-func (t *cleanupT) Cleanup(f func()) {
-	t.cleanups = append(t.cleanups, f)
-}
-
-// Context replaces T.Context.
-func (t *cleanupT) Context() context.Context {
-	return t.ctx
-}
-
-func (t *cleanupT) done() {
-	t.cancel()
-	for _, f := range slices.Backward(t.cleanups) {
-		f()
-	}
-}
diff --git a/internal/http3/http3_test.go b/internal/http3/http3_test.go
index f6fb2e9b..ef6bd027 100644
--- a/internal/http3/http3_test.go
+++ b/internal/http3/http3_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
@@ -10,6 +10,8 @@ import (
 	"encoding/hex"
 	"os"
 	"strings"
+	"testing"
+	"testing/synctest"
 )
 
 func init() {
@@ -42,3 +44,10 @@ type testReader struct {
 }
 
 func (r testReader) Read(p []byte) (n int, err error) { return r.readFunc(p) }
+
+// synctestSubtest runs f in a subtest in a synctest.Run bubble.
+func synctestSubtest(t *testing.T, name string, f func(t *testing.T)) {
+	t.Run(name, func(t *testing.T) {
+		synctest.Test(t, f)
+	})
+}
diff --git a/internal/http3/qpack.go b/internal/http3/qpack.go
index 8fb4860b..64ce99aa 100644
--- a/internal/http3/qpack.go
+++ b/internal/http3/qpack.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/qpack_decode.go b/internal/http3/qpack_decode.go
index 018867af..7348ae76 100644
--- a/internal/http3/qpack_decode.go
+++ b/internal/http3/qpack_decode.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/qpack_decode_test.go b/internal/http3/qpack_decode_test.go
index 3b9a995f..d812d6dd 100644
--- a/internal/http3/qpack_decode_test.go
+++ b/internal/http3/qpack_decode_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -102,7 +102,7 @@ func TestQPACKDecode(t *testing.T) {
 			{mayIndex, "accept-encoding", ""},
 		},
 	}} {
-		runSynctestSubtest(t, test.name, func(t testing.TB) {
+		synctestSubtest(t, test.name, func(t *testing.T) {
 			st1, st2 := newStreamPair(t)
 			st1.Write(test.enc)
 			st1.Flush()
@@ -176,7 +176,7 @@ func TestQPACKDecodeErrors(t *testing.T) {
 		name: "too high static table index",
 		enc:  unhex("0000ff23ff24"),
 	}} {
-		runSynctestSubtest(t, test.name, func(t testing.TB) {
+		synctestSubtest(t, test.name, func(t *testing.T) {
 			st1, st2 := newStreamPair(t)
 			st1.Write(test.enc)
 			st1.Flush()
diff --git a/internal/http3/qpack_encode.go b/internal/http3/qpack_encode.go
index 0f35e0c5..193f7f93 100644
--- a/internal/http3/qpack_encode.go
+++ b/internal/http3/qpack_encode.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 type qpackEncoder struct {
diff --git a/internal/http3/qpack_encode_test.go b/internal/http3/qpack_encode_test.go
index f426d773..f642821d 100644
--- a/internal/http3/qpack_encode_test.go
+++ b/internal/http3/qpack_encode_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
diff --git a/internal/http3/qpack_static.go b/internal/http3/qpack_static.go
index cb0884eb..6c0b51c5 100644
--- a/internal/http3/qpack_static.go
+++ b/internal/http3/qpack_static.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import "sync"
diff --git a/internal/http3/qpack_test.go b/internal/http3/qpack_test.go
index 6e16511f..29473bf2 100644
--- a/internal/http3/qpack_test.go
+++ b/internal/http3/qpack_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
diff --git a/internal/http3/quic.go b/internal/http3/quic.go
index 6d2b1200..4f1cca17 100644
--- a/internal/http3/quic.go
+++ b/internal/http3/quic.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/quic_test.go b/internal/http3/quic_test.go
index bc3b110f..f494811b 100644
--- a/internal/http3/quic_test.go
+++ b/internal/http3/quic_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
diff --git a/internal/http3/roundtrip.go b/internal/http3/roundtrip.go
index bf55a131..d52c8455 100644
--- a/internal/http3/roundtrip.go
+++ b/internal/http3/roundtrip.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/roundtrip_test.go b/internal/http3/roundtrip_test.go
index ba6a234a..230ff82c 100644
--- a/internal/http3/roundtrip_test.go
+++ b/internal/http3/roundtrip_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -18,7 +18,7 @@ import (
 )
 
 func TestRoundTripSimple(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -44,7 +44,7 @@ func TestRoundTripSimple(t *testing.T) {
 }
 
 func TestRoundTripWithBadHeaders(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -56,7 +56,7 @@ func TestRoundTripWithBadHeaders(t *testing.T) {
 }
 
 func TestRoundTripWithUnknownFrame(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -82,7 +82,7 @@ func TestRoundTripWithInvalidPushPromise(t *testing.T) {
 	// "A client MUST treat receipt of a PUSH_PROMISE frame that contains
 	// a larger push ID than the client has advertised as a connection error of H3_ID_ERROR."
 	// https://www.rfc-editor.org/rfc/rfc9114.html#section-7.2.5-5
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -155,7 +155,7 @@ func TestRoundTripResponseContentLength(t *testing.T) {
 		},
 		wantContentLength: -1,
 	}} {
-		runSynctestSubtest(t, test.name, func(t testing.TB) {
+		synctestSubtest(t, test.name, func(t *testing.T) {
 			tc := newTestClientConn(t)
 			tc.greet()
 
@@ -199,7 +199,7 @@ func TestRoundTripMalformedResponses(t *testing.T) {
 		name:       "no :status",
 		respHeader: http.Header{},
 	}} {
-		runSynctestSubtest(t, test.name, func(t testing.TB) {
+		synctestSubtest(t, test.name, func(t *testing.T) {
 			tc := newTestClientConn(t)
 			tc.greet()
 
@@ -218,7 +218,7 @@ func TestRoundTripCrumbledCookiesInResponse(t *testing.T) {
 	// these MUST be concatenated into a single byte string [...]"
 	// using the two-byte delimiter of "; "''
 	// https://www.rfc-editor.org/rfc/rfc9114.html#section-4.2.1-2
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -238,7 +238,7 @@ func TestRoundTripCrumbledCookiesInResponse(t *testing.T) {
 }
 
 func TestRoundTripRequestBodySent(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
@@ -290,7 +290,7 @@ func TestRoundTripRequestBodyErrors(t *testing.T) {
 			},
 		),
 	}} {
-		runSynctestSubtest(t, test.name, func(t testing.TB) {
+		synctestSubtest(t, test.name, func(t *testing.T) {
 			tc := newTestClientConn(t)
 			tc.greet()
 
@@ -323,7 +323,7 @@ func TestRoundTripRequestBodyErrors(t *testing.T) {
 }
 
 func TestRoundTripRequestBodyErrorAfterHeaders(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 
diff --git a/internal/http3/server.go b/internal/http3/server.go
index ca93c529..bdb3479b 100644
--- a/internal/http3/server.go
+++ b/internal/http3/server.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/server_test.go b/internal/http3/server_test.go
index 8e727d25..1e0cba95 100644
--- a/internal/http3/server_test.go
+++ b/internal/http3/server_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -19,7 +19,7 @@ func TestServerReceivePushStream(t *testing.T) {
 	// "[...] if a server receives a client-initiated push stream,
 	// this MUST be treated as a connection error of type H3_STREAM_CREATION_ERROR."
 	// https://www.rfc-editor.org/rfc/rfc9114.html#section-6.2.2-3
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		ts := newTestServer(t)
 		tc := ts.connect()
 		tc.newStream(streamTypePush)
@@ -28,7 +28,7 @@ func TestServerReceivePushStream(t *testing.T) {
 }
 
 func TestServerCancelPushForUnsentPromise(t *testing.T) {
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		ts := newTestServer(t)
 		tc := ts.connect()
 		tc.greet()
diff --git a/internal/http3/settings.go b/internal/http3/settings.go
index b5e562ec..b3cb2a64 100644
--- a/internal/http3/settings.go
+++ b/internal/http3/settings.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/stream.go b/internal/http3/stream.go
index 345e2f50..93294d43 100644
--- a/internal/http3/stream.go
+++ b/internal/http3/stream.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/stream_test.go b/internal/http3/stream_test.go
index a034cc76..036b2356 100644
--- a/internal/http3/stream_test.go
+++ b/internal/http3/stream_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
+//go:build go1.25
 
 package http3
 
diff --git a/internal/http3/transport.go b/internal/http3/transport.go
index b26524cb..48aaadd8 100644
--- a/internal/http3/transport.go
+++ b/internal/http3/transport.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24
-
 package http3
 
 import (
diff --git a/internal/http3/transport_test.go b/internal/http3/transport_test.go
index b3008663..0b7134ad 100644
--- a/internal/http3/transport_test.go
+++ b/internal/http3/transport_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build go1.25
 
 package http3
 
@@ -27,7 +27,7 @@ func TestTransportServerCreatesBidirectionalStream(t *testing.T) {
 	// "Clients MUST treat receipt of a server-initiated bidirectional
 	// stream as a connection error of type H3_STREAM_CREATION_ERROR [...]"
 	// https://www.rfc-editor.org/rfc/rfc9114.html#section-6.1-3
-	runSynctest(t, func(t testing.TB) {
+	synctest.Test(t, func(t *testing.T) {
 		tc := newTestClientConn(t)
 		tc.greet()
 		st := tc.newStream(streamTypeRequest)