Go 1 25

Go 1.25

This overview applies when the project's target Go version is at least 1.25.

Language

Do not introduce new syntax on the basis of Go 1.25; it makes no language changes that affect programs.

Tools and modules

• New capability: ignore directives to go.mod for generated, vendored, or otherwise excluded directories that package patterns such as ./... must skip. Remember that ignored directories remain in module zip files.
• Available capability: the work package pattern when a command should cover every package in the current module or workspace modules.
• go version -m -json is available when build information must be consumed programmatically, and go doc -http for local browsable documentation.
• go vet can catch sync.WaitGroup.Add calls made from the wrong place and host/port strings assembled with fmt.Sprintf; replace the latter with net.JoinHostPort for IPv6 correctness.
• Unsupported or discouraged: depend on an automatically added toolchain line when the go command updates the go directive; Go 1.25 no longer adds one.

Runtime and diagnostics

• Runtime behaviour: the runtime choose GOMAXPROCS in containers unless the application has a measured reason to override it. On Linux it accounts for cgroup CPU bandwidth limits and periodically tracks changes. A manual GOMAXPROCS setting disables these defaults; call runtime.SetDefaultGOMAXPROCS to restore them.
• runtime/trace.FlightRecorder can retain a bounded in-memory execution trace and snapshot recent activity when a rare event occurs.
• Available capability: GODEBUG=checkfinalizers=1 while diagnosing stalled finalisers or cleanup queues.
• Experimental option: the Green Tea collector only with GOEXPERIMENT=greenteagc. Treat it as experimental and validate it against representative workloads.

Standard library

• Available capability: testing/synctest.Test and testing/synctest.Wait for deterministic concurrent tests that benefit from virtual time and explicit quiescence. Do not depend on the earlier experimental API.
• Modern alternative: sync.WaitGroup.Go for the common pattern of starting and tracking goroutines when no per-goroutine error result is required.
• net/http.CrossOriginProtection can reject unsafe cross-origin browser requests when Fetch Metadata or origin checks fit the service's CSRF model. Configure only the required origin or pattern bypasses.
• Interface support: io/fs.ReadLinkFS when a filesystem exposes symbolic links. This enables symlink-aware behaviour in archive/tar.Writer.AddFS, os.CopyFS, os.DirFS, os.Root.FS, and testing/fstest.
• Modern alternative: os.Root operations for filesystem work that must remain beneath a designated directory.
• reflect.TypeAssert[T] is the modern alternative to asserting Value.Interface() when converting a reflected value to a known Go type.
• Available capability: the crypto/ecdsa raw key parsing and Bytes APIs instead of assembling encodings through crypto/elliptic or math/big.
• TLS 1.2 SHA-1 signatures are rejected, servers prefer their highest mutually supported protocol version, and malformed peer behaviour is rejected more strictly. GODEBUG=tlssha1=1 provides temporary compatibility.

Experimental JSON

• Experimental or compatibility option: GOEXPERIMENT=jsonv2 only for evaluation. Test existing encoding/json users for compatibility, and use encoding/json/v2 or encoding/json/jsontext only while accepting that their APIs may change.

Testing and compatibility

• Behavioural requirement: an error immediately before using any accompanying result. Go 1.25 corrected delayed nil checks that could previously let invalid result use appear to succeed.
• go build -asan detects leaked C allocations at process exit. ASAN_OPTIONS=detect_leaks=0 disables that check.
• Platform requirement: macOS 12 or later. Treat windows/arm as unavailable for forward-looking work because Go 1.25 is its final release.
• GOAMD64=v3 and higher fuse multiply-add operations, which can alter exact floating-point results. An explicit conversion between multiplication and addition preserves unfused rounding when required.