Go 1.25 Recap: Container-Aware by Default

Go recap series — one short post per release. Released August 2025.

TL;DR

• GOMAXPROCS follows your container CPU limit on Linux.
• wg.Go(func() { ... }) replaces Add(1) + defer Done().
• testing/synctest tests timeouts instantly with a fake clock.
• Experiments to watch: Green Tea GC and JSON v2.

1. Container-aware GOMAXPROCS

The old problem: a Go app in a container limited to 2 CPUs, on a 64-core machine, used GOMAXPROCS=64. It tried to do 64 things at once with only 2 CPUs of quota — and got throttled.

What changed: on Linux, the default now respects the cgroup CPU limit (not the request). It also updates if the limit changes.

Why it matters: better latency in Kubernetes for free. You can usually remove automaxprocs.

2. sync.WaitGroup.Go

var wg sync.WaitGroup
for _, url := range urls {
    wg.Go(func() {
        fetch(url)
    })
}
wg.Wait()

Why it matters: no forgotten Add or Done. go vet also warns about misplaced wg.Add now.

3. testing/synctest: a fake clock for tests

What changed: synctest.Test runs your test in a bubble with fake time. Time jumps forward when every goroutine is waiting.

func TestTimeout(t *testing.T) {
    synctest.Test(t, func(t *testing.T) {
        ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
        defer cancel()

        time.Sleep(5 * time.Second) // returns instantly
        synctest.Wait()

        if ctx.Err() == nil {
            t.Fatal("expected timeout")
        }
    })
}

Why it matters: a 5-second timeout test now takes milliseconds — and never flakes.

4. Worth knowing

• Green Tea GC (experiment): a new garbage collector. It becomes the default in 1.26.
• encoding/json/v2 (experiment): faster decoding. It backs encoding/json in 1.27.
• trace.FlightRecorder: keeps the last few seconds of trace in memory, so you can save it right after a problem.

Previous: ← Go 1.24 Recap · Next: Go 1.26 Recap →

Source: Go 1.25 release notes