diff --git a/tests/test_invariant_fastcommon.c b/tests/test_invariant_fastcommon.c
new file mode 100644
index 0000000..b6fe74e
--- /dev/null
+++ b/tests/test_invariant_fastcommon.c
@@ -0,0 +1,69 @@
+#include <check.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+
+/* Include the actual production header */
+#include "fastcommon.h"
+
+START_TEST(test_sprintf_buffer_bounds)
+{
+    /* Invariant: sprintf must never write beyond the bounds of mc_info buffer */
+    const char *payloads[] = {
+        "fastcommon v%d.%d.%d supported",  /* Original format string */
+        "fastcommon v%999d.%999d.%999d supported",  /* Boundary overflow attempt */
+        "fastcommon v%2147483647d.%2147483647d.%2147483647d supported",  /* Large width overflow */
+        "fastcommon v%hd.%hd.%hd supported",  /* Different format specifier */
+        "fastcommon v%ld.%ld.%ld supported"   /* Another format specifier */
+    };
+    int num_payloads = sizeof(payloads) / sizeof(payloads[0]);
+
+    for (int i = 0; i < num_payloads; i++) {
+        char mc_info[64];
+        int result;
+        
+        /* Direct call to the vulnerable pattern from production code */
+        result = snprintf(mc_info, sizeof(mc_info), payloads[i],
+                         FC_MAJOR_VERSION, FC_MINOR_VERSION, FC_PATCH_VERSION);
+        
+        /* Security property: result must be less than buffer size */
+        ck_assert_msg(result < (int)sizeof(mc_info), 
+                     "Format string '%s' produced %d bytes (buffer size: %zu)", 
+                     payloads[i], result, sizeof(mc_info));
+        
+        /* Additional check: no buffer overflow occurred */
+        ck_assert_msg(result >= 0, 
+                     "Format string '%s' caused encoding error", payloads[i]);
+    }
+}
+END_TEST
+
+Suite *security_suite(void)
+{
+    Suite *s;
+    TCase *tc_core;
+
+    s = suite_create("Security");
+    tc_core = tcase_create("Core");
+
+    tcase_add_test(tc_core, test_sprintf_buffer_bounds);
+    suite_add_tcase(s, tc_core);
+
+    return s;
+}
+
+int main(void)
+{
+    int number_failed;
+    Suite *s;
+    SRunner *sr;
+
+    s = security_suite();
+    sr = srunner_create(s);
+
+    srunner_run_all(sr, CK_NORMAL);
+    number_failed = srunner_ntests_failed(sr);
+    srunner_free(sr);
+
+    return (number_failed == 0) ? EXIT_SUCCESS : EXIT_FAILURE;
+}
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