diff --git a/dev/docker-compose/compose.yaml b/dev/docker-compose/compose.yaml
index cf9d12fe..67a24e08 100644
--- a/dev/docker-compose/compose.yaml
+++ b/dev/docker-compose/compose.yaml
@@ -312,7 +312,7 @@ services:
       dockerfile: ./orchestrator/Dockerfile
     command: ["buttercup-ui"]
     ports:
-      - "127.0.0.1:31323:1323"
+      - "127.0.0.1:31323:31323"
     env_file:
       - env.dev.compose
       - .env
diff --git a/dev/docker-compose/env.dev.compose b/dev/docker-compose/env.dev.compose
index 41934d7d..1bbbba33 100644
--- a/dev/docker-compose/env.dev.compose
+++ b/dev/docker-compose/env.dev.compose
@@ -90,6 +90,10 @@ TRACED_VULNERABILITIES_TASK_TIMEOUT_MS=120000
 
 BUTTERCUP_UI_HOST=buttercup-ui
 BUTTERCUP_UI_EXTERNAL_HOST=buttercup-ui
+# Serve on 31323 internally too, so container-to-container URLs
+# (BUTTERCUP_*_COMPETITION_API_URL=http://buttercup-ui:31323) and the
+# host-published port both use one consistent port.
+BUTTERCUP_UI_PORT=31323
 BUTTERCUP_UI_CRS_BASE_URL="http://task-server:8000"
 BUTTERCUP_UI_STORAGE_DIR="/tmp/buttercup-ui"
 BUTTERCUP_UI_LOG_LEVEL="info"