diff --git a/docs/sandbox/Flex.md b/docs/sandbox/Flex.md
index cafbc7956bf..9a4c12b4a4f 100644
--- a/docs/sandbox/Flex.md
+++ b/docs/sandbox/Flex.md
@@ -11,7 +11,7 @@
 To enable this turn on `FlexRouting` as a feature in `otp-config.json`.
 
 The GTFS feeds should conform to
-the [GTFS-Flex v2.1 draft](https://github.com/MobilityData/gtfs-flex/blob/master/spec/reference.md)
+the [GTFS-Flex v2 draft PR](https://github.com/google/transit/pull/388)
 
 ## Configuration