diff --git a/Makefile b/Makefile
index 39ade31e..fc7c4c3f 100644
--- a/Makefile
+++ b/Makefile
@@ -421,7 +421,7 @@ MISC_WW_TARGETS = $(MISC_WW_TESTS:%=wwtest/%)
 # test/sep: they run under test-compiler.
 TOOL_WW_TESTS = test/tool/rejects_test.ww test/tool/driver_test.ww \
 	  test/tool/wwdump_test.ww test/tool/ffi_test.ww \
-	  test/tool/attest_test.ww
+	  test/tool/attest_test.ww test/tool/wwirune_test.ww
 TOOL_WW_TARGETS = $(TOOL_WW_TESTS:%=wwtest/%)
 BOOTSTRAP_WRAPPER_SOURCES = test/wcc/950_selfcheck.c \
 	  test/wcc/991_w6a_ww.c \
diff --git a/test/tool/wwirune_test.ww b/test/tool/wwirune_test.ww
new file mode 100644
index 00000000..3f2fc930
--- /dev/null
+++ b/test/tool/wwirune_test.ww
@@ -0,0 +1,104 @@
+package wwirune_test;
+
+// .wwi round-trip observer for \u / \U Unicode escapes in exported
+// rune defs. Port of the retired native carrier
+// test/wcc/110_uniesc_run.c; every assertion preserved, the .wwi
+// inspection added (the audit's optional strengthen).
+//
+// An exported wide-rune `def` re-serialized by the .wwi producer must
+// re-parse into the same codepoint on the importer side (the
+// write-twin of the lexer read-fix — cmd/w6c/wwi.c wwi_rune /
+// selfhost/cmd/wcc/wwi.ww wwirune emit \u/\U above 0xFF). One def per
+// serializer branch: \x (<= 0xFF), \u (<= 0xFFFF), \U. The claim
+// REQUIRES a real multi-file module tree: a single-file two-package
+// fixture builds through __root.unit.ww and never produces a wr.wwi
+// (probed in residual-carrier-audit.json, 110 entry), so no corpus
+// fixture can exercise the serializer.
+//
+// Both driver stages run and their wr.wwi must be byte-identical
+// (rule 10); the serialized escape text itself is pinned per branch.
+// Dropped C machinery, not assertions: the ww_ww-absent skip gate
+// (the Make target declares both drivers) and the unlink/rmdir
+// accounting (testenv.clean asserts the removal).
+
+import os;
+import os.exec;
+import strings;
+import testenv;
+import time;
+
+fn fail(label: str, why: str) void = {
+	let m: str = strings.concat("wwirune 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;
+};
+
+fn runcode(dir: str, name: str, argv: []str) i32 = {
+	let co: testenv.commandout;
+	testenv.runcommand(dir, dir, name, argv, tmo(), &co);
+	if (co.termination != exec.termination.EXIT) { return -1; };
+	return co.code;
+};
+
+@test fn roundtrip() void = {
+	let drvs: []str = ["ww", "ww_ww"];
+	let wwis: []str = ["", ""];
+	let d: i32 = 0;
+	for (d < 2) {
+		let td: str = testenv.fresh();
+		assert(os.mkdir(strings.concat(td, "/wr"), 493) == 0);
+		testenv.writefile(strings.concat(td, "/wr/wr.ww"), strings.concat(
+			"package wr;\n",
+			"export def EACUTE: rune = '\\u00e9';\n",
+			"export def EURO: rune = '\\u20ac';\n",
+			"export def SMILEY: rune = '\\U0001F600';\n"));
+		testenv.writefile(strings.concat(td, "/main.ww"), strings.concat(
+			"package main;\n",
+			"import wr;\n",
+			"fn main() i32 = {\n",
+			"  let a: rune = wr.EACUTE;\n",
+			"  let b: rune = wr.EURO;\n",
+			"  let c: rune = wr.SMILEY;\n",
+			"  if ((a: u32) != 233u32) { return 1; };\n",
+			"  if ((b: u32) != 8364u32) { return 2; };\n",
+			"  if ((c: u32) != 128512u32) { return 3; };\n",
+			"  return 42;\n",
+			"};\n"));
+		let av: []str = [testenv.driver(drvs[d]), "build", "main.ww"];
+		if (runcode(td, strings.concat("build_", drvs[d]), av) != 0) {
+			fail("roundtrip", strings.concat("build via ", drvs[d],
+				" failed"));
+		};
+		let rav: []str = [strings.concat(td, "/main")];
+		if (runcode(td, strings.concat("run_", drvs[d]), rav) != 42) {
+			fail("roundtrip", strings.concat(drvs[d], " built program ",
+				"exit != 42 (a wide-rune def re-parsed wrong)"));
+		};
+		wwis[d] = testenv.readfile(strings.concat(td,
+			"/main.sepwork/wr.wwi"));
+		testenv.clean(td);
+		d += 1;
+	};
+
+	// one needle per serializer branch, exact re-serialized text
+	let needles: []str = [
+		"export def EACUTE: rune = '\\xe9';\n",
+		"export def EURO: rune = '\\u20ac';\n",
+		"export def SMILEY: rune = '\\U0001f600';\n"];
+	let i: i32 = 0;
+	for (i < needles.len) {
+		if (!testenv.has(wwis[0], needles[i])) {
+			fail("roundtrip", strings.concat("wr.wwi lacks `", needles[i],
+				"` (serializer branch drifted)"));
+		};
+		i += 1;
+	};
+	if (!testenv.same(wwis[0], wwis[1])) {
+		fail("roundtrip", "cs wr.wwi != ww wr.wwi (rule 10)");
+	};
+};
diff --git a/test/wcc/110_uniesc_run.c b/test/wcc/110_uniesc_run.c
deleted file mode 100644
index 814d803a..00000000
--- a/test/wcc/110_uniesc_run.c
+++ /dev/null
@@ -1,184 +0,0 @@
-/*
- * 110_uniesc_run — .wwi round-trip carrier for \u / \U Unicode escapes.
- *
- * The value-row half (rune/string \u \U \x decode asserts) migrated to the
- * in-language row-table test/lang/uniesc_test.ww (fold-6, task #4); this
- * file retains ONLY the .wwi round-trip case, which is a two-package
- * sep-build observer with no in-language @test home (it asserts the
- * PRODUCER re-serializes a wide-rune `def` byte-exact, observed across a
- * package boundary — not expressible as a single-compile @test).
- *
- * An exported wide-rune `def` re-serialized by the producer must re-parse
- * into the same codepoint (the write-twin of the lexer read-fix —
- * cmd/w6c/wwi.c wwi_rune / selfhost/cmd/wcc/wwi.ww wwirune emit \u/\U above
- * 0xFF). Runs through both the cstage `ww` and wwstage `ww_ww` drivers
- * (task #50); the wwstage arm is skipped when ww_ww is absent (a
- * cstage-only build), mirroring 640_int_cast_signed.
- */
-#include <stdio.h>
-#include <stdlib.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;
-}
-
-/* run_roundtrip — sep-build a two-package program where the imported
- * package exports wide-rune defs; the producer must serialize them as
- * \u/\U so the importer re-parses the same codepoint. Returns the built
- * program's exit code (42 on a correct round-trip). */
-static int
-run_roundtrip(const char *driver, int id)
-{
-	char dir[80], wrdir[112], wrp[176], mainp[160], cmd[1024];
-	char outbin[160], scratch[176];
-	snprintf(dir, sizeof dir, "/tmp/wwue_rt_%d_%d", getpid(), id);
-	snprintf(wrdir, sizeof wrdir, "%s/wr", dir);
-	snprintf(wrp, sizeof wrp, "%s/wr.ww", wrdir);
-	snprintf(mainp, sizeof mainp, "%s/main.ww", dir);
-	snprintf(outbin, sizeof outbin, "%s/main", dir);
-	snprintf(scratch, sizeof scratch, "%s/main.sepwork", dir);
-
-	if (mkdir(dir, 0755) != 0) {
-		perror(dir);
-		return -1;
-	}
-	int got = -1;
-	int wr_owned = 0;
-	if (mkdir(wrdir, 0755) != 0) {
-		perror(wrdir);
-		goto cleanup;
-	}
-	wr_owned = 1;
-
-	FILE *f = fopen(wrp, "wb");
-	if (!f) {
-		perror(wrp);
-		goto cleanup;
-	}
-	/* one def per serializer branch: \x (<=0xFF), \u (<=0xFFFF), \U. */
-	fputs("package wr;\n"
-	      "export def EACUTE: rune = '\\u00e9';\n"
-	      "export def EURO: rune = '\\u20ac';\n"
-	      "export def SMILEY: rune = '\\U0001F600';\n", f);
-	if (fclose(f) != 0) {
-		perror(wrp);
-		goto cleanup;
-	}
-	f = fopen(mainp, "wb");
-	if (!f) {
-		perror(mainp);
-		goto cleanup;
-	}
-	fputs("package main;\n"
-	      "import wr;\n"
-	      "fn main() i32 = {\n"
-	      "  let a: rune = wr.EACUTE;\n"
-	      "  let b: rune = wr.EURO;\n"
-	      "  let c: rune = wr.SMILEY;\n"
-	      "  if ((a: u32) != 233u32) { return 1; };\n"
-	      "  if ((b: u32) != 8364u32) { return 2; };\n"
-	      "  if ((c: u32) != 128512u32) { return 3; };\n"
-	      "  return 42;\n"
-	      "};\n", f);
-	if (fclose(f) != 0) {
-		perror(mainp);
-		goto cleanup;
-	}
-
-	snprintf(cmd, sizeof cmd, "cd %s && %s build main.ww", dir, driver);
-	if (runwait(cmd) != 0) {
-		fprintf(stderr, "roundtrip: build via %s failed\n", driver);
-		goto cleanup;
-	}
-	got = runwait(outbin);
-
-cleanup:
-	/* `ww build` retains this exact caller-owned scratch tree. */
-	snprintf(cmd, sizeof cmd, "rm -rf %s", scratch);
-	int cleanfail = runwait(cmd) != 0;
-	if (unlink(outbin) != 0 && errno != ENOENT) {
-		perror(outbin);
-		cleanfail = 1;
-	}
-	if (unlink(mainp) != 0 && errno != ENOENT) {
-		perror(mainp);
-		cleanfail = 1;
-	}
-	if (wr_owned) {
-		if (unlink(wrp) != 0 && errno != ENOENT) {
-			perror(wrp);
-			cleanfail = 1;
-		}
-		if (rmdir(wrdir) != 0) {
-			perror(wrdir);
-			cleanfail = 1;
-		}
-	}
-	if (rmdir(dir) != 0) {
-		perror(dir);
-		cleanfail = 1;
-	}
-	if (cleanfail && got == 42)
-		got = -1;
-	return got;
-}
-
-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 cdrv[1024];
-	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
-	char wdrv[1024];
-	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
-
-	struct { const char *name; const char *path; int gated_on_existence; }
-	drivers[] = {
-		{ "cstage", cdrv, 0 },
-		{ "wwstage", wdrv, 1 },
-		{ NULL, NULL, 0 },
-	};
-
-	int total = 0, fail = 0;
-	for (int d = 0; drivers[d].name; d++) {
-		if (drivers[d].gated_on_existence
-		    && access(drivers[d].path, X_OK) != 0) {
-			fprintf(stderr, "uniesc_run: skip %s (no %s)\n",
-			    drivers[d].name, drivers[d].path);
-			continue;
-		}
-		int rt = run_roundtrip(drivers[d].path, d);
-		total++;
-		if (rt != 42) {
-			fprintf(stderr,
-			    "uniesc_run[%s][wwi_roundtrip]: exit=%d want=42\n",
-			    drivers[d].name, rt);
-			fail++;
-		}
-	}
-	if (fail) {
-		fprintf(stderr, "uniesc_run: %d/%d fixtures failed\n",
-		    fail, total);
-		return 1;
-	}
-	printf("uniesc_run: %d/%d ok\n", total, total);
-	return 0;
-}