From 8f5fe8e20b9a242a6aef7a8b46e0ceae93b498a9 Mon Sep 17 00:00:00 2001
From: Dom Parfitt <dom.parfitt@circleci.com>
Date: Thu, 21 Aug 2025 10:14:32 +0100
Subject: [PATCH] Disable automaxprocs on Go 1.25+

---
 rundef/maxprocs.go       |  2 ++
 rundef/maxprocs_go125.go | 21 +++++++++++++++++++++
 rundef/maxprocs_test.go  |  2 ++
 3 files changed, 25 insertions(+)
 create mode 100644 rundef/maxprocs_go125.go

diff --git a/rundef/maxprocs.go b/rundef/maxprocs.go
index 4226084d4..083fd22ec 100644
--- a/rundef/maxprocs.go
+++ b/rundef/maxprocs.go
@@ -1,3 +1,5 @@
+//go:build !go1.25
+
 package rundef
 
 import (
diff --git a/rundef/maxprocs_go125.go b/rundef/maxprocs_go125.go
new file mode 100644
index 000000000..e1be35202
--- /dev/null
+++ b/rundef/maxprocs_go125.go
@@ -0,0 +1,21 @@
+//go:build go1.25
+
+package rundef
+
+import (
+	"context"
+	"runtime"
+
+	"github.com/circleci/ex/o11y"
+)
+
+// MaxProcs is a no-op on Go 1.25+
+func MaxProcs(ctx context.Context) (err error) {
+	ctx, span := o11y.StartSpan(ctx, "rundef: max procs")
+	defer o11y.End(span, &err)
+
+	limit := runtime.GOMAXPROCS(0) // Get the limit the go runtime has determined
+
+	span.AddField("limit", limit)
+	return err
+}
diff --git a/rundef/maxprocs_test.go b/rundef/maxprocs_test.go
index 4c51f7009..1c4d445b2 100644
--- a/rundef/maxprocs_test.go
+++ b/rundef/maxprocs_test.go
@@ -1,3 +1,5 @@
+//go:build !go1.25
+
 package rundef
 
 import (