diff --git a/internal/logs/sanitizer.go b/internal/logs/sanitizer.go
index 24dacba8d..863411395 100644
--- a/internal/logs/sanitizer.go
+++ b/internal/logs/sanitizer.go
@@ -66,10 +66,10 @@ func (s *SecretSanitizer) registerDefaultPatterns() {
 		},
 	})
 
-	// Anthropic API keys (sk-ant-...)
+	// Anthropic API keys (sk-ant-...); the body is base64url, so it carries '_'.
 	s.patterns = append(s.patterns, &secretPattern{
 		name:  "anthropic_key",
-		regex: regexp.MustCompile(`\b(sk-ant-[A-Za-z0-9\-]{30,})\b`),
+		regex: regexp.MustCompile(`\b(sk-ant-[A-Za-z0-9_\-]{30,})\b`),
 		maskFunc: func(key string) string {
 			if len(key) <= 10 {
 				return "****"
diff --git a/internal/logs/sanitizer_anthropic_key_test.go b/internal/logs/sanitizer_anthropic_key_test.go
new file mode 100644
index 000000000..c5cec7c09
--- /dev/null
+++ b/internal/logs/sanitizer_anthropic_key_test.go
@@ -0,0 +1,23 @@
+package logs
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSanitizer_AnthropicKeyWithUnderscore(t *testing.T) {
+	s := newTestSanitizer()
+	for _, key := range []string{
+		"sk-ant-api03-abcdefghij_klmnopqrstuvwxyz0123456789ABCDEFGHIJ-abcdefghijklmnopqrstuvwx_yzAA",
+		"sk-ant-api03-abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJ_abcdefghijklmnopqrstuvwx-yzAA",
+		"sk-ant-admin01-Ab_Cd-EfGhIjKlMnOpQrStUvWxYz0123456789AA",
+	} {
+		out := s.sanitizeString("using key " + key + " for upstream")
+		if strings.Contains(out, key[16:]) {
+			t.Errorf("anthropic key leaked: %q", out)
+		}
+		if !strings.Contains(out, key[:10]+"***") {
+			t.Errorf("anthropic key not masked by anthropic_key: %q", out)
+		}
+	}
+}