diff --git a/util/httpm/httpm_test.go b/util/httpm/httpm_test.go
index 4a36a38e1..e8342a74f 100644
--- a/util/httpm/httpm_test.go
+++ b/util/httpm/httpm_test.go
@@ -24,6 +24,13 @@ func TestUsedConsistently(t *testing.T) {
 		t.Skipf("skipping test since .git doesn't exist: %v", err)
 	}
 
+	// Open .git/index so Go's test cache tracks it as an input.
+	// The index file changes on git reset, checkout, pull, etc.,
+	// so the cache is properly invalidated when moving between commits.
+	if f, err := os.Open(filepath.Join(rootDir, ".git", "index")); err == nil {
+		f.Close()
+	}
+
 	cmd := exec.Command("git", "grep", "-l", "-F", "http.Method")
 	cmd.Dir = rootDir
 	matches, _ := cmd.Output()