diff --git a/backend/internal/security/url_validator.go b/backend/internal/security/url_validator.go
index fa368925..0d8d3c62 100644
--- a/backend/internal/security/url_validator.go
+++ b/backend/internal/security/url_validator.go
@@ -294,7 +294,14 @@ func ValidateExternalURL(rawURL string, options ...ValidationOption) (string, er
 					continue
 				}
 				if network.IsPrivateIP(ipv4) {
-					return "", fmt.Errorf("connection to private ip addresses is blocked for security (detected IPv4-mapped IPv6: %s)", ip.String())
+					// Normalize to the extracted IPv4 for both the cloud-metadata special-case
+					// and sanitization, so ::ffff:169.254.169.254 produces the same error as
+					// 169.254.169.254 and doesn't leak the raw IPv6 form in messages.
+					sanitizedIPv4 := sanitizeIPForError(ipv4.String())
+					if ipv4.String() == "169.254.169.254" {
+						return "", fmt.Errorf("access to cloud metadata endpoints is blocked for security (detected: %s)", sanitizedIPv4)
+					}
+					return "", fmt.Errorf("connection to private ip addresses is blocked for security (detected IPv4-mapped IPv6: %s)", sanitizedIPv4)
 				}
 			}
 
diff --git a/backend/internal/security/url_validator_test.go b/backend/internal/security/url_validator_test.go
index 240c4c50..6fe99a11 100644
--- a/backend/internal/security/url_validator_test.go
+++ b/backend/internal/security/url_validator_test.go
@@ -1175,7 +1175,8 @@ func TestValidateExternalURL_WithAllowRFC1918_IPv4MappedIPv6Allowed(t *testing.T
 
 func TestValidateExternalURL_WithAllowRFC1918_IPv4MappedMetadataBlocked(t *testing.T) {
 	t.Parallel()
-	// ::ffff:169.254.169.254 maps to the cloud metadata IP; must stay blocked.
+	// ::ffff:169.254.169.254 maps to the cloud metadata IP; must stay blocked and
+	// produce the same cloud-metadata error as the non-mapped address.
 	_, err := ValidateExternalURL(
 		"http://[::ffff:169.254.169.254]",
 		WithAllowHTTP(),
@@ -1185,4 +1186,10 @@ func TestValidateExternalURL_WithAllowRFC1918_IPv4MappedMetadataBlocked(t *testi
 	if err == nil {
 		t.Fatal("expected IPv4-mapped metadata address to be blocked, got nil")
 	}
+	if !strings.Contains(err.Error(), "cloud metadata") {
+		t.Errorf("expected cloud-metadata error for ::ffff:169.254.169.254, got: %v", err)
+	}
+	if strings.Contains(err.Error(), "ffff") {
+		t.Errorf("error message must not leak the raw IPv6 form, got: %v", err)
+	}
 }