diff --git a/test/wcc/993_ww_ww.c b/test/wcc/993_ww_ww.c
index fccb0581..66998769 100644
--- a/test/wcc/993_ww_ww.c
+++ b/test/wcc/993_ww_ww.c
@@ -134,6 +134,28 @@ run_exit_code(const char *bin, const char *driver, int code)
 	return rc;
 }
 
+/* Stage a program that fails to compile, run it via the named driver,
+ * and return the exit code. Pins the build-step caller contract: a
+ * failing build must still report failure (non-zero) — the #16 fix that
+ * lets procrun return the real exit code must not regress this, since the
+ * w6c/w6a/w6l callers only test `!= 0`. */
+static int
+run_build_fail(const char *bin, const char *driver)
+{
+	char src[64];
+	snprintf(src, sizeof src, "/tmp/ww_badbuild_%d.ww", getpid());
+	FILE *f = fopen(src, "w");
+	if (!f) return -1;
+	fputs("export fn main() i32 = {\n\treturn 1\n", f);	/* no ; no } */
+	fclose(f);
+
+	char cmd[256];
+	snprintf(cmd, sizeof cmd, "%s/%s run %s 2>/dev/null", bin, driver, src);
+	int rc = runwait(cmd);
+	unlink(src);
+	return rc;
+}
+
 int
 main(void)
 {
@@ -196,9 +218,10 @@ main(void)
 	/* #16: `ww_ww run` must propagate the child program's real exit code
 	 * (the shared procrun helper was collapsing every non-zero exit to 1).
 	 * Table-driven; the non-zero rows fail under the old collapse-to-1.
-	 * The C ww driver is the reference (do_run -> WEXITSTATUS); assert
-	 * ww_ww matches it row-for-row. */
-	static const int exit_codes[] = { 0, 7, 42 };
+	 * 255 is the single-byte boundary (WEXITSTATUS max). The C ww driver
+	 * is the reference (do_run -> WEXITSTATUS); assert ww_ww matches it
+	 * row-for-row. */
+	static const int exit_codes[] = { 0, 7, 42, 255 };
 	int nrc = (int)(sizeof exit_codes / sizeof exit_codes[0]);
 	int rcfail = 0;
 	for (int i = 0; i < nrc; i++) {
@@ -217,6 +240,16 @@ main(void)
 		}
 	}
 
+	/* Build-step caller contract: a failing build must still report
+	 * failure (non-zero), and both drivers must agree. */
+	int cbad = run_build_fail(bin, "ww");
+	int wbad = run_build_fail(bin, "ww_ww");
+	if (cbad == 0 || wbad == 0 || cbad != wbad) {
+		fprintf(stderr, "ww_ww FAIL: build-fail exit C ww=%d ww_ww=%d "
+		    "(want equal and non-zero)\n", cbad, wbad);
+		rcfail++;
+	}
+
 	if (fail || rcfail) {
 		fprintf(stderr, "ww_ww: %d/%d diff(s) failed, %d exit-code "
 		    "row(s) failed\n", fail, n, rcfail);
@@ -224,6 +257,7 @@ main(void)
 	}
 	printf("ww_ww: byte-identical to C ww on %d corpus builds "
 	    "(single-file + selfhost/wwdump multi-import); "
-	    "run exit-code propagated on %d rows (#16)\n", n, nrc);
+	    "run exit-code propagated on %d rows + build-fail parity (#16)\n",
+	    n, nrc);
 	return 0;
 }