diff --git a/Makefile b/Makefile
index 18544b3d..9d7da2cc 100644
--- a/Makefile
+++ b/Makefile
@@ -409,7 +409,8 @@ OBJECT_WW_TARGETS = $(OBJECT_WW_TESTS:%=wwtest/%)
 # under test-compiler.
 MISC_WW_TESTS = test/misc/arrglob_test.ww test/misc/packedwwi_test.ww \
 	  test/misc/reject_test.ww test/misc/heldasym_test.ww \
-	  test/misc/stampdiag_test.ww test/misc/qualast_test.ww
+	  test/misc/stampdiag_test.ww test/misc/qualast_test.ww \
+	  test/misc/abortmsg_test.ww
 MISC_WW_TARGETS = $(MISC_WW_TESTS:%=wwtest/%)
 BOOTSTRAP_WRAPPER_SOURCES = test/wcc/950_selfcheck.c \
 	  test/wcc/991_w6a_ww.c \
diff --git a/test/misc/abortmsg_test.ww b/test/misc/abortmsg_test.ww
new file mode 100644
index 00000000..a522a194
--- /dev/null
+++ b/test/misc/abortmsg_test.ww
@@ -0,0 +1,65 @@
+package abortmsg_test;
+
+// Run-STDERR content net for the #58 assert/abort builtins, ported
+// from the retired native carrier test/wcc/957_assert_builtin_run.c.
+// The eight exit-code rows and the checker rejects are owned verbatim
+// by the r957_* fixtures, and the w6c-vs-w6c_ww byte-id by the
+// test-data-byteid blanket; the fixture protocol checks run EXIT
+// only, so the run-stderr dimension — msg content for the msg forms,
+// MANDATORY EMPTINESS for the bare forms (rt_abort(NULL, 0) writes
+// nothing) — lives here. The four abort-shaped fixture sources are
+// built IN PLACE so fixture and probe can never drift; the cstage
+// driver suffices, byte-id extends the lowering to wwstage (the
+// carrier's own argument).
+
+import os;
+import os.exec;
+import strings;
+import testenv;
+import time;
+
+fn fail(label: str, why: str) void = {
+	let m: str = strings.concat("abortmsg FAIL: ", label, " -- ", why,
+		"\n");
+	os.write(2, m.ptr, m.len: u64);
+	assert(false);
+};
+
+fn tmo() time.duration = {
+	return (240i64 * (time.second: i64)): time.duration;
+};
+
+// needle "" = run stderr must be EXACTLY empty.
+fn stderrrow(label: str, fixture: str, needle: str) void = {
+	let src: str = strings.concat(testenv.repo(), "/test/wcc/data/",
+		fixture, "/case.ww");
+	let td: str = testenv.fresh();
+	let co: testenv.commandout;
+	let bav: []str = [testenv.driver("ww"), "build", "-o", "bin", src];
+	testenv.runcommand(td, td, "build", bav, tmo(), &co);
+	if (co.termination != exec.termination.EXIT || co.code != 0) {
+		fail(label, "cstage build failed");
+	};
+	let rav: []str = [strings.concat(td, "/bin")];
+	testenv.runcommand(td, td, "run", rav, tmo(), &co);
+	if (co.termination != exec.termination.EXIT || co.code != 1) {
+		fail(label, "run-exit != 1");
+	};
+	if (needle.len != 0) {
+		if (!testenv.has(co.stderr, needle)) {
+			fail(label, strings.concat("run stderr missing \"",
+				needle, "\""));
+		};
+	} else if (co.stderr.len != 0) {
+		fail(label, strings.concat("run stderr not empty ",
+			"(rt_abort no-msg must be (NULL, 0))"));
+	};
+	testenv.clean(td);
+};
+
+@test fn abortstderr() void = {
+	stderrrow("fail_nomsg", "r957_assert_fail_nomsg", "");
+	stderrrow("fail_msg", "r957_assert_fail_msg", "assert fired");
+	stderrrow("abort_msg", "r957_abort_msg", "boom");
+	stderrrow("abort_bare", "r957_abort_bare", "");
+};
diff --git a/test/wcc/957_assert_builtin_run.c b/test/wcc/957_assert_builtin_run.c
deleted file mode 100644
index c4e8a56e..00000000
--- a/test/wcc/957_assert_builtin_run.c
+++ /dev/null
@@ -1,291 +0,0 @@
-/*
- * 957_assert_builtin_run — stderr + byte-id net for #58: the
- * `assert(cond[, msg])` / `abort([msg])` builtins (the harec
- * EXPR_ASSERT family) in BOTH stages.
- *
- * Root: wwstage had NO builtin intercept for assert/abort — the checker
- * left the call untyped (asserttyped gate 4 deliberately skipped it)
- * and cgcall fell through to the regular call path, emitting
- * `CALL assert(SB)` for a symbol that exists nowhere → link-fail.
- * cstage lowers both inline via rt_abort (checker tags the callee
- * ty_err at cmd/wcc/check.c:1536-1572, cgen keys on the tag at
- * cmd/w6c/cgen.c:6618-6663). The fix mirrors that tag-then-lower pair
- * into check.ww exprtype N_CALL + cgenexpr.ww cgcall.
- *
- * Shared runtime and reject contracts live in r957_* fixtures. This carrier
- * retains runtime stderr for aborting rows and w6c vs w6c_ww `.s` identity.
- * The latter fails if the stages diverge. On
- *       pre-fix master every assert/abort row diverges (wwstage emits
- *       the bogus CALL).
- *
- * Shadow rows pin the gate's other half: a user fn named assert/abort
- * (same-module or cross-module, the #45 shape) suppresses the builtin
- * and routes the regular call path in BOTH stages. The cross-module
- * row pins the CURRENT flat-scope semantics — if the #45 root (filed
- * as task #14: cross-module bare resolution of a foreign decl) is
- * later ruled a reject, its fixture should change polarity with it.
- *
- * The migrated reject fixtures retain the checker diagnostics and the
- * alias-cond rule-10 down-alignment.
- */
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <errno.h>
-#include <unistd.h>
-#include <sys/stat.h>
-#include <sys/wait.h>
-
-static int
-runwait(const char *cmd)
-{
-	int rc = system(cmd);
-	if (rc == -1) return -1;
-	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
-	return -1;
-}
-
-static long
-slurp(const char *path, char *buf, long cap)
-{
-	FILE *f = fopen(path, "rb");
-	if (!f) return -1;
-	long n = (long)fread(buf, 1, cap - 1, f);
-	fclose(f);
-	if (n < 0) n = 0;
-	buf[n] = '\0';
-	return n;
-}
-
-struct row {
-	const char *label;
-	const char *src;
-	int want_exit;
-	const char *want_msg;	/* run stderr must contain this; NULL = must be empty */
-	int observe_output;
-};
-
-static const struct row rows[] = {
-	/* Passing asserts (bare and with msg) fall through; exit 42. */
-	{ "pass",
-	  "package main;\n"
-	  "export fn main() i32 = {\n"
-	  "\tassert(1 + 1 == 2);\n"
-	  "\tassert(2 > 1, \"never fires\");\n"
-	  "\treturn 42;\n"
-	  "};\n", 42, NULL, 0 },
-	/* Failing assert without msg: rt_abort(NULL, 0), exit 1, silent. */
-	{ "fail_nomsg",
-	  "package main;\n"
-	  "export fn main() i32 = {\n"
-	  "\tassert(1 == 2);\n"
-	  "\treturn 0;\n"
-	  "};\n", 1, NULL, 1 },
-	/* Failing assert with msg: rt_abort(ptr, len) writes it to stderr. */
-	{ "fail_msg",
-	  "package main;\n"
-	  "export fn main() i32 = {\n"
-	  "\tassert(false, \"assert fired\\n\");\n"
-	  "\treturn 0;\n"
-	  "};\n", 1, "assert fired", 1 },
-	/* abort with msg: unconditional rt_abort, exit 1, msg on stderr. */
-	{ "abort_msg",
-	  "package main;\n"
-	  "export fn main() i32 = {\n"
-	  "\tabort(\"boom\\n\");\n"
-	  "\treturn 0;\n"
-	  "};\n", 1, "boom", 1 },
-	/* bare abort(): rt_abort(NULL, 0), exit 1, silent. */
-	{ "abort_bare",
-	  "package main;\n"
-	  "export fn main() i32 = {\n"
-	  "\tabort();\n"
-	  "\treturn 0;\n"
-	  "};\n", 1, NULL, 1 },
-	/* assert inside an IMPORTED module's fn (single-file multi-package
-	 * form, like 953) — the regex fold-5 consumer shape. */
-	{ "imported_mod",
-	  "package m;\n"
-	  "export fn f() i32 = {\n"
-	  "\tassert(3 > 2, \"m.f invariant\");\n"
-	  "\treturn 42;\n"
-	  "};\n"
-	  "package main;\n"
-	  "import m;\n"
-	  "export fn main() i32 = { return m.f(); };\n", 42, NULL, 0 },
-	/* SHADOW control, same-module: a user fn `assert` suppresses the
-	 * builtin; the call routes the regular path (no-op fn), so the
-	 * false cond does NOT abort. Exit 7 proves the user fn ran. */
-	{ "shadow_samemod",
-	  "package main;\n"
-	  "let g: i32 = 0;\n"
-	  "fn assert(b: bool) void = { g = 7; };\n"
-	  "export fn main() i32 = {\n"
-	  "\tassert(false);\n"
-	  "\treturn g;\n"
-	  "};\n", 7, NULL, 0 },
-	/* SHADOW control, cross-module (#45 shape, task #14): a foreign
-	 * exported fn `assert` in the flat scope also suppresses the
-	 * builtin; bare `assert(false, ...)` binds it and does NOT abort. */
-	{ "shadow_xmod",
-	  "package m;\n"
-	  "export fn assert(b: bool, msg: str) void = { };\n"
-	  "package main;\n"
-	  "import m;\n"
-	  "export fn main() i32 = {\n"
-	  "\tassert(false, \"routed to m.assert\");\n"
-	  "\treturn 42;\n"
-	  "};\n", 42, NULL, 0 },
-	{ NULL, NULL, 0, NULL, 0 }
-};
-
-static int
-slurp_eq(const char *a, const char *b)
-{
-	FILE *fa = fopen(a, "rb");
-	FILE *fb = fopen(b, "rb");
-	if (!fa || !fb) { if (fa) fclose(fa); if (fb) fclose(fb); return -1; }
-	int rc = 0;
-	for (;;) {
-		int ca = fgetc(fa);
-		int cb = fgetc(fb);
-		if (ca != cb) { rc = -1; break; }
-		if (ca == EOF) break;
-	}
-	fclose(fa); fclose(fb);
-	return rc;
-}
-
-int
-main(void)
-{
-	const char *bin = getenv("BIN");
-	if (!bin) bin = "out/bin";
-	char absbin[1024];
-	if (bin[0] != '/') {
-		char cwd[1024];
-		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
-		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
-		bin = absbin;
-	}
-
-	char w6c[1100], w6c_ww[1100];
-	snprintf(w6c, sizeof w6c, "%s/w6c", bin);
-	snprintf(w6c_ww, sizeof w6c_ww, "%s/w6c_ww", bin);
-	if (access(w6c_ww, X_OK) != 0) {
-		fprintf(stderr, "assert_builtin: w6c_ww missing — cannot run "
-		    "the cs==ww byte-id gate (the whole point of #58)\n");
-		return 1;
-	}
-
-	char errbuf[4096];
-	int n = 0, fail = 0;
-	for (int i = 0; rows[i].src; i++, n++) {
-		char tmpdir[64] = "/tmp/wwasrt_XXXXXX";
-		if (mkdtemp(tmpdir) == NULL) {
-			perror("assert_builtin: mkdtemp");
-			fail++;
-			continue;
-		}
-		char src[128], errf[128], outbin[128], sepwork[160];
-		char cs_s[128], ws_s[128], rmcmd[192];
-		snprintf(src, sizeof src, "%s/wwasrt_%d_%d.ww",
-		    tmpdir, getpid(), i);
-		snprintf(errf, sizeof errf, "%s/wwasrt_%d_%d.err",
-		    tmpdir, getpid(), i);
-		snprintf(outbin, sizeof outbin, "%s/wwasrt_%d_%d",
-		    tmpdir, getpid(), i);
-		snprintf(sepwork, sizeof sepwork, "%s.sepwork", outbin);
-		snprintf(cs_s, sizeof cs_s, "%s/wwasrt_%d_%d_cs.s",
-		    tmpdir, getpid(), i);
-		snprintf(ws_s, sizeof ws_s, "%s/wwasrt_%d_%d_ww.s",
-		    tmpdir, getpid(), i);
-		snprintf(rmcmd, sizeof rmcmd, "rm -rf %s", sepwork);
-		FILE *f = fopen(src, "wb");
-		if (f == NULL) { fail++; goto cleanup; }
-		fputs(rows[i].src, f);
-		fclose(f);
-
-		char cmd[2048];
-		if (rows[i].observe_output) {
-			snprintf(cmd, sizeof cmd, "%s/ww build -o %s %s "
-			    ">/dev/null 2>&1", bin, outbin, src);
-			if (runwait(cmd) != 0) {
-				fprintf(stderr, "row[%s]: cstage build failed\n",
-				    rows[i].label);
-				fail++;
-				goto cleanup;
-			}
-
-			snprintf(cmd, sizeof cmd, "%s >/dev/null 2>%s", outbin, errf);
-			int got = runwait(cmd);
-			if (got != rows[i].want_exit) {
-				fprintf(stderr, "row[%s]: cstage exit %d, want %d\n",
-				    rows[i].label, got, rows[i].want_exit);
-				fail++;
-			}
-			long elen = slurp(errf, errbuf, sizeof errbuf);
-			if (rows[i].want_msg) {
-				if (elen < 0 || strstr(errbuf, rows[i].want_msg) == NULL) {
-					fprintf(stderr, "row[%s]: run stderr missing "
-					    "\"%s\"\n", rows[i].label,
-					    rows[i].want_msg);
-					fail++;
-				}
-			} else if (elen != 0) {
-				fprintf(stderr, "row[%s]: run stderr not empty "
-				    "(rt_abort no-msg must be (NULL, 0))\n",
-				    rows[i].label);
-				fail++;
-			}
-		}
-
-		/* cs==ww byte-id gate. */
-		snprintf(cmd, sizeof cmd, "%s -o %s %s 2>/dev/null",
-		    w6c, cs_s, src);
-		if (runwait(cmd) != 0) {
-			fprintf(stderr, "row[%s]: w6c failed\n", rows[i].label);
-			fail++;
-			goto cleanup;
-		}
-		snprintf(cmd, sizeof cmd, "%s -o %s %s 2>/dev/null",
-		    w6c_ww, ws_s, src);
-		if (runwait(cmd) != 0) {
-			fprintf(stderr, "row[%s]: w6c_ww failed\n",
-			    rows[i].label);
-			fail++;
-			goto cleanup;
-		}
-		if (slurp_eq(cs_s, ws_s) != 0) {
-			fprintf(stderr,
-			    "row[%s]: cstage/wwstage .s DIFFER (rule-10 "
-			    "byte-id violation)\n", rows[i].label);
-			fail++;
-		}
-
-cleanup: {
-		int cleanbad = 0;
-		if (unlink(src) != 0 && errno != ENOENT) cleanbad = 1;
-		if (unlink(errf) != 0 && errno != ENOENT) cleanbad = 1;
-		if (unlink(outbin) != 0 && errno != ENOENT) cleanbad = 1;
-		if (unlink(cs_s) != 0 && errno != ENOENT) cleanbad = 1;
-		if (unlink(ws_s) != 0 && errno != ENOENT) cleanbad = 1;
-		if (runwait(rmcmd) != 0) cleanbad = 1;
-		if (rmdir(tmpdir) != 0) cleanbad = 1;
-		if (cleanbad) {
-			fprintf(stderr, "row[%s]: temporary cleanup failed\n",
-			    rows[i].label);
-			fail++;
-		}
-	}
-	}
-
-	if (fail) {
-		fprintf(stderr, "%d/%d assert-builtin tests failed\n",
-		    fail, n);
-		return 1;
-	}
-	printf("assert_builtin: %d/%d ok (stderr + cs==ww byte-id)\n",
-	    n, n);
-	return 0;
-}