diff --git a/app/Rules/SafeWebhookUrl.php b/app/Rules/SafeWebhookUrl.php
index 0316da0bc..415f94df7 100644
--- a/app/Rules/SafeWebhookUrl.php
+++ b/app/Rules/SafeWebhookUrl.php
@@ -134,10 +134,17 @@ class SafeWebhookUrl implements ValidationRule
             return [];
         }
 
-        return array_map(
-            fn (string $ip): string => sprintf('%s:%d=%s', $target['host'], $target['port'], filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6) ? '['.$ip.']' : $ip),
+        // The MinIO client keeps one --resolve IP per host and parses it with Go's
+        // netip.ParseAddr, which rejects bracketed IPv6. Emit a single entry, preferring
+        // IPv4 for reachability, without brackets.
+        $preferredIps = array_values(array_filter(
             $target['ips'],
-        );
+            fn (string $ip): bool => filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) !== false,
+        ));
+
+        $ip = $preferredIps[0] ?? $target['ips'][0];
+
+        return [sprintf('%s:%d=%s', $target['host'], $target['port'], $ip)];
     }
 
     public static function redactedUrlForLog(string $url): string
diff --git a/tests/Unit/SafeWebhookUrlTest.php b/tests/Unit/SafeWebhookUrlTest.php
index 84d1b33ff..d02b0057d 100644
--- a/tests/Unit/SafeWebhookUrlTest.php
+++ b/tests/Unit/SafeWebhookUrlTest.php
@@ -239,6 +239,19 @@ it('builds MinIO client resolve options for S3 backup uploads', function () {
     expect($options)->toContain('localhost:9000=127.0.0.1');
 });
 
+it('collapses dual-stack S3 hosts to a single bracket-free IPv4 resolve entry', function () {
+    InstanceSettings::unguarded(fn () => InstanceSettings::query()->updateOrCreate(['id' => 0], [
+        'webhook_allowed_internal_hosts' => ['localhost'],
+        'webhook_allow_localhost' => true,
+    ]));
+
+    // localhost resolves to both 127.0.0.1 and ::1. The MinIO client keeps one IP per
+    // host and rejects bracketed IPv6, so exactly one bracket-free IPv4 entry is produced.
+    $options = SafeWebhookUrl::minioClientResolveOptions('http://localhost:9000');
+
+    expect($options)->toBe(['localhost:9000=127.0.0.1']);
+});
+
 it('rejects trailing-dot hostnames to avoid DNS pinning mismatch', function () {
     $rule = new SafeWebhookUrl(fn (string $host): array => ['93.184.216.34']);