diff --git a/Makefile b/Makefile
index 406d529..468bf99 100644
--- a/Makefile
+++ b/Makefile
@@ -17,9 +17,18 @@ help: ## Show this help
 	  | awk 'BEGIN{FS=":.*?## "}{printf "  \033[36m%-14s\033[0m %s\n", $$1, $$2}'
 
 .PHONY: setup
-setup: ## Create the venv and install the test extra
+setup: ## Create the venv, install the test extra, and prove it actually works
 	$(PYTHON) -m venv .venv
 	$(VENV)/bin/pip install -e ".[test]"
+	@# Exit 0 from `pip install` is not evidence (D-24) — pip reports success even
+	@# when the result is unusable. Prove the environment works instead of trusting
+	@# the install step: `--collect-only` imports every test module and therefore
+	@# every src module each one pulls in, which is exactly the failure mode this
+	@# target exists to catch (T-47 — this repo had no venv at all on 2026-08-09,
+	@# and the documented test command could not work). It runs zero tests, so it
+	@# stays cheap, and unlike a bare `import src.main` it exercises the tests/
+	@# tree too, not just the package.
+	$(VENV)/bin/python -m pytest tests/ --collect-only -q
 
 .PHONY: test
 test: ## Run the test suite