diff --git a/doc/godebug.md b/doc/godebug.md
index 43375deb5e..3a7a798028 100644
--- a/doc/godebug.md
+++ b/doc/godebug.md
@@ -167,6 +167,11 @@ escaped. Setting `htmlmetacontentescape=0` disables this behavior. To avoid
 content injection attacks, this setting and default was backported to Go 1.25.8
 and Go 1.26.1.
 
+Go 1.27 changes the default for `tracebacklabels` (added in [Go 1.26][#go-126])
+to `1`. This opt-out is expected to be kept indefinitely in case goroutine
+labels acquire sensitive information that shouldn't be made available in
+tracebacks.
+
 ### Go 1.26
 
 Go 1.26 added a new `httpcookiemaxnum` setting that controls the maximum number
diff --git a/doc/next/4-runtime.md b/doc/next/4-runtime.md
index 1f8e445e0b..4880a95af2 100644
--- a/doc/next/4-runtime.md
+++ b/doc/next/4-runtime.md
@@ -1 +1,10 @@
 ## Runtime {#runtime}
+
+<!-- CL 742580 -->
+
+Tracebacks for modules with `go` directives configuring Go 1.27 or later will now
+include [runtime/pprof](https://pkg.go.dev/runtime/pprof) goroutine labels in
+the header line. This behavior can be disabled with `GODEBUG=tracebacklabels=0`
+(added in [Go 1.26](/doc/godebug#go-126)). This opt-out is expected to be
+kept indefinitely in case goroutine labels acquire sensitive information that
+shouldn't be made available in tracebacks.
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index 5e33a0b0f3..7ddfad14ea 100644
--- a/src/internal/godebugs/table.go
+++ b/src/internal/godebugs/table.go
@@ -68,6 +68,10 @@ var All = []Info{
 	{Name: "tlssecpmlkem", Package: "crypto/tls", Changed: 26, Old: "0", Opaque: true},
 	{Name: "tlssha1", Package: "crypto/tls", Changed: 25, Old: "1"},
 	{Name: "tlsunsafeekm", Package: "crypto/tls", Changed: 22, Old: "1"},
+	// Mark tracebacklabels as Opaque so we don't generate a metric that we can't increment.
+	// IncNonDefault uses a sync.Once, which involves sync.Mutex, and is not safe from a signal handler.
+	// (Tracebacks are generated in signal-handlers.)
+	{Name: "tracebacklabels", Package: "runtime", Changed: 27, Old: "0", Opaque: true},
 	{Name: "updatemaxprocs", Package: "runtime", Changed: 25, Old: "0"},
 	{Name: "urlmaxqueryparams", Package: "net/url", Changed: 24, Old: "0"},
 	{Name: "urlstrictcolons", Package: "net/url", Changed: 26, Old: "0"},
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index d5bce17563..10754eee1c 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -398,7 +398,7 @@ var dbgvars = []*dbgVar{
 	{name: "traceallocfree", atomic: &debug.traceallocfree},
 	{name: "tracecheckstackownership", value: &debug.traceCheckStackOwnership},
 	{name: "tracebackancestors", value: &debug.tracebackancestors},
-	{name: "tracebacklabels", atomic: &debug.tracebacklabels, def: 0},
+	{name: "tracebacklabels", atomic: &debug.tracebacklabels, def: 1},
 	{name: "tracefpunwindoff", value: &debug.tracefpunwindoff},
 	{name: "updatemaxprocs", value: &debug.updatemaxprocs, def: 1},
 }