diff --git a/bin/git-check-assertions b/bin/git-check-assertions
index fec0805..b634c09 100755
--- a/bin/git-check-assertions
+++ b/bin/git-check-assertions
@@ -63,6 +63,10 @@ assert_output() {
 export -f run assert_success assert_failure assert_output
 
 # main flow
+if [ ! -z "$(git status --porcelain)" ]; then
+  echo "Uncommitted changes. Refusing to run." >&2
+  exit 1
+fi
 orig_ref="$(git symbolic-ref --quiet --short HEAD || git rev-parse HEAD)"
 if git show-ref --verify --quiet refs/heads/main; then
   base_branch=main
diff --git a/test/git-check-assertions.bats b/test/git-check-assertions.bats
index ee5097b..6a152bd 100755
--- a/test/git-check-assertions.bats
+++ b/test/git-check-assertions.bats
@@ -46,6 +46,34 @@ commit_with_assertion() {
   assert_file_not_exists ../test
 }
 
+@test "should refuse to run when the index has changes" {
+  echo "staged" >>readme
+  git add readme
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Uncommitted changes. Refusing to run."
+}
+
+@test "should refuse to run when the working tree has changes" {
+  echo "unstaged" >>readme
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Uncommitted changes. Refusing to run."
+}
+
+@test "should refuse to run when there are untracked files" {
+  echo "new" >untracked
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Uncommitted changes. Refusing to run."
+}
+
 @test "should not run any assertion blocks from main when on a feature branch" {
   commit_with_assertion "touch ../test"
   git checkout -b feature