diff --git a/Makefile b/Makefile
index 87201c6..cc6ecab 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 SHELL := /bin/bash
 
 .PHONY: precommit
-precommit: check-links check-json
+precommit: check-links check-json check-index
 
 .PHONY: release-check
 release-check: precommit check-versions
@@ -26,3 +26,15 @@ check-versions:
 build-index:
 	@python3 scripts/build-index.py > rules/index.json
 	@echo "rules/index.json updated"
+
+.PHONY: check-index
+check-index:
+	@python3 scripts/build-index.py > /tmp/coding-rules-index-check.json
+	@if ! diff -q rules/index.json /tmp/coding-rules-index-check.json > /dev/null 2>&1; then \
+		echo "ERROR: rules/index.json is stale. Run 'make build-index' and commit the result."; \
+		diff -u rules/index.json /tmp/coding-rules-index-check.json | head -40; \
+		rm -f /tmp/coding-rules-index-check.json; \
+		exit 1; \
+	fi
+	@rm -f /tmp/coding-rules-index-check.json
+	@echo "rules/index.json up-to-date"