diff --git a/test/docker-compose.yml b/test/docker-compose.yml
index eead3ec..10fbac2 100644
--- a/test/docker-compose.yml
+++ b/test/docker-compose.yml
@@ -72,8 +72,15 @@ services:
       EULA: "TRUE"
       TYPE: "FORGE"
       VERSION: "1.7.10"
-      # Same build pinned in ../gradle.properties (forge_1_7_10_version).
-      FORGE_VERSION: "10.13.4.1614-1.7.10"
+      # Same build pinned in ../gradle.properties (forge_1_7_10_version=10.13.4.1614-1.7.10), but
+      # itzg's installer rejects that full string ("Invalid value for --forge-version") — its
+      # --forge-version validation only accepts the plain dot-separated build number, not the
+      # "-1.7.10" suffix Forge's own artifact naming appends for this era. Confirmed by actually
+      # booting the container: the suffixed value fails immediately with that validation error;
+      # the trimmed value passes validation and proceeds to actually download the installer
+      # (any further failures at that point are files.minecraftforge.net connectivity, unrelated
+      # to this value).
+      FORGE_VERSION: "10.13.4.1614"
       MEMORY: "${MC_1_7_10_MEMORY:-2G}"
       ONLINE_MODE: "${MC_1_7_10_ONLINE_MODE:-FALSE}"
     volumes: