diff --git a/tools/ai-review/tests/empty_mistral_key.rs b/tools/ai-review/tests/empty_mistral_key.rs
new file mode 100644
index 0000000..de14bde
--- /dev/null
+++ b/tools/ai-review/tests/empty_mistral_key.rs
@@ -0,0 +1,19 @@
+use std::process::Command;
+
+#[test]
+fn empty_mistral_key_stops_the_executable_before_external_calls() {
+    let output = Command::new(env!("CARGO_BIN_EXE_ai-review"))
+        .env_clear()
+        .env("GITHUB_REPOSITORY", "owner/repository")
+        .env("GITHUB_TOKEN", "test-token")
+        .env("MISTRAL_API_KEY", "   ")
+        .env("PR_NUMBER", "1")
+        .output()
+        .expect("ai-review executable should start");
+
+    assert!(!output.status.success());
+
+    let standard_error =
+        String::from_utf8(output.stderr).expect("ai-review should write UTF-8 errors to stderr");
+    assert!(standard_error.contains("MISTRAL_API_KEY is empty"));
+}