diff --git a/docker-compose.yml b/docker-compose.yml
index 605fcdb7..f5745603 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -64,6 +64,9 @@ services:
       GATEWAY_PORT: '14242'
       DATABASE_URL: postgresql://mosaic:mosaic@postgres:5432/mosaic
       VALKEY_URL: valkey://valkey:6379
+      # The compose IS the standalone tier by declaration (mode contract:
+      # mode chosen at install); pinning skips cross-container probe races.
+      MOSAIC_STORAGE_TIER: standalone
       # Standalone-tier secrets: generated at install (see .env.example).
       # Enterprise tier replaces these with Vault/Openbao plumbing.
       BETTER_AUTH_SECRET: '${BETTER_AUTH_SECRET:?set in .env — openssl rand -hex 32}'