diff --git a/Makefile b/Makefile
index 4f5aede5..745a1c54 100644
--- a/Makefile
+++ b/Makefile
@@ -329,6 +329,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_memio_vstream_run \
 	  $(BIN)/test_fmt_vstream_run \
 	  $(BIN)/test_fmt_vstream_mods_run \
+	  $(BIN)/test_fmt_vstream_compositions_run \
 	  $(BIN)/test_bufio_vstream_run \
 	  $(BIN)/test_log_vstream_run \
 	  $(BIN)/test_use_promote_alias \
@@ -714,6 +715,14 @@ $(BIN)/test_fmt_vstream_mods_run: test/wcc/780_fmt_vstream_mods_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_fmt_vstream_compositions_run: test/wcc/781_fmt_vstream_compositions_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		lib/fmt/fmt.ww lib/fmt/vstream.ww \
+		lib/memio/memio.ww lib/memio/vstream.ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_bufio_vstream_run: test/wcc/778_bufio_vstream_run.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
diff --git a/lib/fmt/vstream.ww b/lib/fmt/vstream.ww
index 2188ebe8..d293f0a1 100644
--- a/lib/fmt/vstream.ww
+++ b/lib/fmt/vstream.ww
@@ -65,8 +65,10 @@
 package fmt;
 
 import io;
+import memio;
 import os;
 import strconv;
+import strings;
 
 // fd_ctx — vt at offset 0 for the intrusive vstream→*fd_ctx cast.
 // Mirrors lib/memio.fixed_ctx (memio/vstream.ww:48); single tagged
@@ -556,3 +558,111 @@ export fn fdprintfln_v(fd: i32, fmt: str, args: field...) (size | io.error) = {
 	};
 	return total;
 };
+
+// ---- V-side compositions over the vfprint / vfprintf primitives.
+// Project #94 fold-e7. drew-approved bundle (memio.fixed_string /
+// memio.dynamic_string land alongside as direct enablers, not churn —
+// feedback_refactor_routing_same_class_drops); ken cs==ww mechanical
+// (additive only). eFinal (#50) collapses both surfaces and renames
+// `v` suffix off.
+//
+// Mirror sites:
+//   vfprintln    fmt.ww:240 fprintln          ref/hare/fmt/wrappers.ha:48
+//   vfprintfln   fmt.ww:740 fprintfln         ref/hare/fmt/wrappers.ha:69
+//   vbsprintf    fmt.ww:839 bsprintf          ref/hare/fmt/wrappers.ha:42
+//   vasprintf    fmt.ww:873 asprintf          ref/hare/fmt/wrappers.ha:29
+
+// vfprintln — vfprint + trailing newline. Mirror fmt.fprintln
+// (fmt.ww:240); vputbytes + (size | io.error) routing.
+export fn vfprintln(vs: io.vstream, args: formattable...) (size | io.error) = {
+	let total: size = 0;
+	match (vfprint(vs, args...)) {
+	case let n: size => { total = n; };
+	case let e: io.error => return e;
+	};
+	match (vputbytes(vs, "\n".ptr, 1)) {
+	case let n: size => { total += n; };
+	case let e: io.error => return e;
+	};
+	return total;
+};
+
+// vfprintfln — vfprintf + trailing newline. Mirror fmt.fprintfln
+// (fmt.ww:740).
+export fn vfprintfln(vs: io.vstream, fmt: str, args: field...) (size | io.error) = {
+	let total: size = 0;
+	match (vfprintf(vs, fmt, args...)) {
+	case let n: size => { total = n; };
+	case let e: io.error => return e;
+	};
+	match (vputbytes(vs, "\n".ptr, 1)) {
+	case let n: size => { total += n; };
+	case let e: io.error => return e;
+	};
+	return total;
+};
+
+// vbsprintf — render into `buf` through memio.fixed_vstream; return
+// the str view of bytes actually written. Mirror fmt.bsprintf
+// (fmt.ww:839); nomem from fixed_vstream widens into io.error (Hare
+// wrappers.ha:42 returns `(const str | nomem)` directly — ww collapses
+// to (str | io.error) so the vfprintf path's io.error arm stays
+// uniform). Short writes surface as a prefix (memio.fixed contract,
+// vstream.ww:71 note).
+export fn vbsprintf(buf: []u8, fmt: str, args: field...) (str | io.error) = {
+	let r: (io.vstream | nomem) = memio.fixed_vstream(buf);
+	let vs: io.vstream = nil: *io.vtable;
+	match (r) {
+	case let v: io.vstream => { vs = v; };
+	case let nm: nomem => { let e: io.error = nm; return e; };
+	};
+	match (vfprintf(vs, fmt, args...)) {
+	case let n: size => { return memio.fixed_string(vs); };
+	case let e: io.error => return e;
+	};
+};
+
+// vasprintf — render `fmt`/`args` into a heap-allocated str through
+// memio.dynamic_vstream, shrink-copy to a tight allocation, close the
+// dynamic backing. Mirror fmt.asprintf (fmt.ww:873) modulo: bare `str`
+// return (Hare returns `(str | nomem)`; ww has no `nomem` variant on
+// the public surface — os.alloc faults on OOM per lib/os.ww). Caller
+// frees with `os.free(r.ptr, r.len: u64)` when r.len > 0; r.len == 0
+// is a no-op free (same shape strings.dup uses at strings.ww:70).
+// Shrink-to-fit rationale: memio.dynamic_vstream's cap doubles past
+// pos during growth (memio/vstream.ww:230); io.st_close frees the
+// cap-sized mapping. Returning memio.dynamic_string directly would
+// leak the cap-vs-len slack (skip close) or dangle the view (close
+// first); the copy lets the caller free with r.len.
+export fn vasprintf(fmt: str, args: field...) str = {
+	let out: str;
+	out.ptr = nil;
+	out.len = 0;
+	let r: (io.vstream | nomem) = memio.dynamic_vstream();
+	let vs: io.vstream = nil: *io.vtable;
+	match (r) {
+	case let v: io.vstream => { vs = v; };
+	case nomem => { return out; };
+	};
+	let wres: (size | io.error) = vfprintf(vs, fmt, args...);
+	match (wres) {
+	case let n: size => {};
+	case let e: io.error => {};
+	};
+	let view: str = memio.dynamic_string(vs);
+	if (view.len == 0) {
+		let cres: (void | io.error) = io.st_close(vs);
+		match (cres) { case void => {}; case let e: io.error => {}; };
+		return out;
+	};
+	let tight: []u8 = alloc([], view.len: u64)!;
+	let i: i32 = 0;
+	for (i < view.len) {
+		tight[i] = view.ptr[i];
+		i += 1;
+	};
+	let cres: (void | io.error) = io.st_close(vs);
+	match (cres) { case void => {}; case let e: io.error => {}; };
+	tight.len = view.len;
+	return strings.frombytes(tight);
+};
diff --git a/lib/memio/vstream.ww b/lib/memio/vstream.ww
index 703c9242..fcd07083 100644
--- a/lib/memio/vstream.ww
+++ b/lib/memio/vstream.ww
@@ -240,3 +240,36 @@ fn dynamicgrow_v(d: *dynamic_ctx, need: i32) void = {
 	d.ptr = nbuf;
 	d.cap = newcap;
 };
+
+// fixed_string / dynamic_string — borrowed str view of buf[0..pos] for
+// the matching ctx flavour. Project #94 fold-e7 bundles them here as
+// direct enablers for fmt.vbsprintf / fmt.vasprintf (drew-approved per
+// feedback_refactor_routing_same_class_drops — prereqs, not churn).
+// Mirrors OLD memio.string (memio.ww:102) over `*state`; the V-side
+// twins key off the intrusive *<flavour>_ctx cast same shape as the
+// vtable callbacks (vstream.ww:151,184). Two flavours kept distinct
+// (vs one fn over a shared header) so each cast targets the matching
+// ctx type — same rationale as the split fixedread_v / dynread_v at
+// line 144-148. Unchecked str view per CLAUDE.md rule 9 carve-out
+// (utf8.validate at IO source is opt-in, not wrapped per-construction).
+//
+// Mirrors ref/hare/memio/ops.ha:51 string(s: io::handle) and the
+// per-handle dispatch shape. fold-e8 (task #50 prereq) extends with
+// buffer/reset/borrowedread accessors; this fold ships only the
+// vbsprintf / vasprintf enablers.
+
+export fn fixed_string(vs: io.vstream) str = {
+	let c: *fixed_ctx = vs: *fixed_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
+
+export fn dynamic_string(vs: io.vstream) str = {
+	let c: *dynamic_ctx = vs: *dynamic_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index dae54ebb..ebdcd887 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -14568,6 +14568,39 @@ fn dynamicgrow_v(d: *dynamic_ctx, need: i32) void = {
 	d.cap = newcap;
 };
 
+// fixed_string / dynamic_string — borrowed str view of buf[0..pos] for
+// the matching ctx flavour. Project #94 fold-e7 bundles them here as
+// direct enablers for fmt.vbsprintf / fmt.vasprintf (drew-approved per
+// feedback_refactor_routing_same_class_drops — prereqs, not churn).
+// Mirrors OLD memio.string (memio.ww:102) over `*state`; the V-side
+// twins key off the intrusive *<flavour>_ctx cast same shape as the
+// vtable callbacks (vstream.ww:151,184). Two flavours kept distinct
+// (vs one fn over a shared header) so each cast targets the matching
+// ctx type — same rationale as the split fixedread_v / dynread_v at
+// line 144-148. Unchecked str view per CLAUDE.md rule 9 carve-out
+// (utf8.validate at IO source is opt-in, not wrapped per-construction).
+//
+// Mirrors ref/hare/memio/ops.ha:51 string(s: io::handle) and the
+// per-handle dispatch shape. fold-e8 (task #50 prereq) extends with
+// buffer/reset/borrowedread accessors; this fold ships only the
+// vbsprintf / vasprintf enablers.
+
+export fn fixed_string(vs: io.vstream) str = {
+	let c: *fixed_ctx = vs: *fixed_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
+
+export fn dynamic_string(vs: io.vstream) str = {
+	let c: *dynamic_ctx = vs: *dynamic_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
+
 // selfhost/cmd/wcc/cgenutil.ww — split out of cgen.ww.
 //
 // General helpers used across cgenexpr / cgenstmt / cgendecl:
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 6fd80599..7be553db 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -14568,6 +14568,39 @@ fn dynamicgrow_v(d: *dynamic_ctx, need: i32) void = {
 	d.cap = newcap;
 };
 
+// fixed_string / dynamic_string — borrowed str view of buf[0..pos] for
+// the matching ctx flavour. Project #94 fold-e7 bundles them here as
+// direct enablers for fmt.vbsprintf / fmt.vasprintf (drew-approved per
+// feedback_refactor_routing_same_class_drops — prereqs, not churn).
+// Mirrors OLD memio.string (memio.ww:102) over `*state`; the V-side
+// twins key off the intrusive *<flavour>_ctx cast same shape as the
+// vtable callbacks (vstream.ww:151,184). Two flavours kept distinct
+// (vs one fn over a shared header) so each cast targets the matching
+// ctx type — same rationale as the split fixedread_v / dynread_v at
+// line 144-148. Unchecked str view per CLAUDE.md rule 9 carve-out
+// (utf8.validate at IO source is opt-in, not wrapped per-construction).
+//
+// Mirrors ref/hare/memio/ops.ha:51 string(s: io::handle) and the
+// per-handle dispatch shape. fold-e8 (task #50 prereq) extends with
+// buffer/reset/borrowedread accessors; this fold ships only the
+// vbsprintf / vasprintf enablers.
+
+export fn fixed_string(vs: io.vstream) str = {
+	let c: *fixed_ctx = vs: *fixed_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
+
+export fn dynamic_string(vs: io.vstream) str = {
+	let c: *dynamic_ctx = vs: *dynamic_ctx;
+	let r: str;
+	r.ptr = c.ptr;
+	r.len = c.pos;
+	return r;
+};
+
 // selfhost/cmd/wcc/cgenutil.ww — split out of cgen.ww.
 //
 // General helpers used across cgenexpr / cgenstmt / cgendecl:
diff --git a/test/wcc/781_fmt_vstream_compositions_run.c b/test/wcc/781_fmt_vstream_compositions_run.c
new file mode 100644
index 00000000..72d7382e
--- /dev/null
+++ b/test/wcc/781_fmt_vstream_compositions_run.c
@@ -0,0 +1,324 @@
+/*
+ * 781_fmt_vstream_compositions_run — project #94 fold-e7 sentinel.
+ * Pins the V-side composition wrappers in lib/fmt/vstream.ww —
+ * vfprintln / vfprintfln / vbsprintf / vasprintf — plus the two
+ * memio enablers fold-e7 bundles alongside (memio.fixed_string /
+ * memio.dynamic_string in lib/memio/vstream.ww). Coexists with the
+ * OLD fmt.fprintln / fprintfln / bsprintf / asprintf surface in
+ * fmt.ww:240,740,839,873 (fmt.ww UNCHANGED this fold; eFinal
+ * task #50 collapses both surfaces and renames the `v` suffix off).
+ *
+ * The two memio helpers are direct enablers (vbsprintf reads its
+ * written prefix via fixed_string; vasprintf reads its grown buffer
+ * via dynamic_string), drew-approved per
+ * feedback_refactor_routing_same_class_drops (prereqs, not churn).
+ *
+ * Each row imports fmt + os + io, drives one of the four V wrappers,
+ * and asserts the exact bytes / size emitted. Per-row unique exit
+ * constant lets a failure pinpoint the offending case.
+ *
+ *   row                       | what it pins
+ *   --------------------------+--------------------------------------
+ *   fdprintln_v_run_basic     | vfprintln through an fd_ctx vstream
+ *                             | (via fdprintln_v's wired vstream sink
+ *                             | dispatching vfprint then vputbytes
+ *                             | "\n"). One i64 arg + str arg + bool
+ *                             | arg; asserts space-separator plus
+ *                             | trailing newline byte content.
+ *   fdprintfln_v_run_fmt      | vfprintfln through fd_ctx vstream
+ *                             | with a {n}-placeholder format string.
+ *                             | Asserts placeholder substitution +
+ *                             | trailing newline shape (mirror of
+ *                             | fmt.fprintfln, fmt.ww:740).
+ *   bsprintf_v_basic          | vbsprintf into a caller-supplied
+ *                             | [16]u8 buffer through
+ *                             | memio.fixed_vstream;
+ *                             | memio.fixed_string returns the prefix
+ *                             | view. Asserts both the bytes in the
+ *                             | caller's buffer AND the returned str
+ *                             | view (ptr == buf + 0, len == 5).
+ *   asprintf_v_basic          | vasprintf into a heap-grown str
+ *                             | through memio.dynamic_vstream;
+ *                             | memio.dynamic_string + shrink-copy +
+ *                             | io.st_close. Asserts the owned str
+ *                             | bytes, then frees via os.free.
+ *
+ * Cstage-only per row (no STAGE_WW, no byte_id) — pre-existing
+ * wwstage bug #209 (sibling of #190): the wwstage checker bails
+ * `case: not a variant of scrutinee (X)` + `match: variant not
+ * handled (formattable)` on fmt.formattable match-arms whenever a
+ * downstream probe `import fmt;`s the package. Identical carve-out
+ * to 777_fmt_vstream_run + 780_fmt_vstream_mods_run (cited there at
+ * line 18-33 / 18-23). Byte-id graduates with #209 close.
+ *
+ * IMPORT-ORDER WORKAROUND (sibling task #208, pre-existing): every
+ * row places `import os;` FIRST for the same reason 776/777/780
+ * document (wwstage checker ordering-sensitivity on os.tryread /
+ * trywrite / tryopen's bare `return r;` over a tagged return type).
+ *
+ * SIBLINGS (filed inline, NOT fixed here — fold-e7 is purely
+ * additive over fold-e1/e2/e3/e4/e5/e6's frozen io.* + memio.* +
+ * fmt.* + bufio.* + log.* surface):
+ *
+ *   - #209 (wwstage formattable match-arm bail): blocks STAGE_WW
+ *     here, same as 777/780. Cstage-only until close.
+ *   - #173 (TRY-on-tagged-return both-stages broken): vbsprintf
+ *     widens nomem into io.error inline rather than `memio.
+ *     fixed_vstream(buf)?` for the same reason memio.vstream.ww
+ *     does at line 87-99.
+ *   - eFinal (#50): collapses V + OLD surfaces; the `v` names
+ *     rename off and the OLD fprintln / fprintfln / bsprintf /
+ *     asprintf delete.
+ *
+ * BOOTSTRAP-EMBED CHECK: fmt is NOT embedded in any selfhost
+ * combined.ww (grep verified pre-impl). memio.vstream.ww IS embedded
+ * in w6c + wwdump combined.ww — the fold-e7 memio helpers ride along
+ * via the regen committed in this same commit (per #110 SSoT). 990/
+ * 994/995 byte-id gates + combined_ww_fresh stay green by virtue of
+ * the regen.
+ *
+ * GATE POLARITY: must stay GREEN. A red here means vfprintln /
+ * vfprintfln dropped the newline byte, vbsprintf returned a stale
+ * memio.fixed_string view (intrusive cast broke), vasprintf failed
+ * to shrink-copy / close cleanly, or the underlying vfprint /
+ * vfprintf primitives regressed.
+ */
+#include <stdio.h>
+#include <stdlib.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;
+}
+
+#define STAGE_CS	1
+#define STAGE_WW	2
+
+struct row {
+	const char *label;
+	const char *src;
+	int want_exit;
+	int stage_mask;
+	int byte_id;
+};
+
+static const struct row rows[] = {
+	{ "fdprintln_v_run_basic",
+	  "package main;\n"
+	  "import os;\n"
+	  "import fmt;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let path: str = \"/tmp/wwfmtv_781_a\";\n"
+	  "    let fd: i32 = os.open(path, os.flag.RDWR | os.flag.CREATE | os.flag.TRUNC, 0o644: i32);\n"
+	  "    if (fd < 0) { return 90; };\n"
+	  "    let wr = fmt.fdprintln_v(fd, 1i64, \"hi\", true);\n"
+	  "    let nw: size = 0;\n"
+	  "    match (wr) {\n"
+	  "    case let n: size => { nw = n; };\n"
+	  "    case io.error => { return 91; };\n"
+	  "    };\n"
+	  "    os.close(fd);\n"
+	  "    let rd: i32 = os.open(path, os.flag.RDONLY, 0i32);\n"
+	  "    if (rd < 0) { return 92; };\n"
+	  "    let buf: [16]u8;\n"
+	  "    let n: i64 = os.read(rd, &buf[0], 16u64);\n"
+	  "    os.close(rd);\n"
+	  "    if (n != 10i64) { return 93; };\n"
+	  "    if (nw != 10: size) { return 94; };\n"
+	  "    if (buf[0] != 49u8) { return 95; };\n"
+	  "    if (buf[1] != 32u8) { return 96; };\n"
+	  "    if (buf[2] != 104u8 || buf[3] != 105u8) { return 97; };\n"
+	  "    if (buf[4] != 32u8) { return 98; };\n"
+	  "    if (buf[9] != 10u8) { return 99; };\n"
+	  "    return 60;\n"
+	  "};\n",
+	  60,
+	  STAGE_CS, 0 },
+	{ "fdprintfln_v_run_fmt",
+	  "package main;\n"
+	  "import os;\n"
+	  "import fmt;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let path: str = \"/tmp/wwfmtv_781_b\";\n"
+	  "    let fd: i32 = os.open(path, os.flag.RDWR | os.flag.CREATE | os.flag.TRUNC, 0o644: i32);\n"
+	  "    if (fd < 0) { return 90; };\n"
+	  "    let wr = fmt.fdprintfln_v(fd, \"x={}\", 7i64);\n"
+	  "    let nw: size = 0;\n"
+	  "    match (wr) {\n"
+	  "    case let n: size => { nw = n; };\n"
+	  "    case io.error => { return 91; };\n"
+	  "    };\n"
+	  "    os.close(fd);\n"
+	  "    let rd: i32 = os.open(path, os.flag.RDONLY, 0i32);\n"
+	  "    if (rd < 0) { return 92; };\n"
+	  "    let buf: [16]u8;\n"
+	  "    let n: i64 = os.read(rd, &buf[0], 16u64);\n"
+	  "    os.close(rd);\n"
+	  "    if (n != 4i64) { return 93; };\n"
+	  "    if (nw != 4: size) { return 94; };\n"
+	  "    if (buf[0] != 120u8 || buf[1] != 61u8 || buf[2] != 55u8 || buf[3] != 10u8) { return 95; };\n"
+	  "    return 61;\n"
+	  "};\n",
+	  61,
+	  STAGE_CS, 0 },
+	{ "bsprintf_v_basic",
+	  "package main;\n"
+	  "import os;\n"
+	  "import fmt;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [16]u8;\n"
+	  "    let r = fmt.vbsprintf(buf[0:16], \"v={}\", 42i64);\n"
+	  "    let view: str = \"\";\n"
+	  "    match (r) {\n"
+	  "    case let s: str => { view = s; };\n"
+	  "    case io.error => { return 90; };\n"
+	  "    };\n"
+	  "    if (view.len != 4) { return 91; };\n"
+	  "    if (view[0] != 118u8 || view[1] != 61u8) { return 92; };\n"
+	  "    if (view[2] != 52u8 || view[3] != 50u8) { return 93; };\n"
+	  "    if (buf[0] != 118u8 || buf[3] != 50u8) { return 94; };\n"
+	  "    return 62;\n"
+	  "};\n",
+	  62,
+	  STAGE_CS, 0 },
+	{ "asprintf_v_basic",
+	  "package main;\n"
+	  "import os;\n"
+	  "import fmt;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let view: str = fmt.vasprintf(\"hello={}\", 9i64);\n"
+	  "    if (view.len != 7) { return 90; };\n"
+	  "    if (view[0] != 104u8 || view[1] != 101u8 || view[2] != 108u8) { return 91; };\n"
+	  "    if (view[3] != 108u8 || view[4] != 111u8) { return 92; };\n"
+	  "    if (view[5] != 61u8 || view[6] != 57u8) { return 93; };\n"
+	  "    os.free(view.ptr: *void, view.len: u64);\n"
+	  "    return 63;\n"
+	  "};\n",
+	  63,
+	  STAGE_CS, 0 },
+};
+
+static int
+write_source(const char *path, const char *src)
+{
+	FILE *f = fopen(path, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+	return 0;
+}
+
+/* Per-row tmpdir cleanup. ww_ww writes intermediates next to the
+ * source (task #15), so each row's build leaves <src>.{combined.ww,
+ * s,o} + bare exe alongside. Mirror of 777/780's. */
+static void
+cleanup_tmp(const char *tmpdir, const char *base)
+{
+	char p[640];
+	snprintf(p, sizeof p, "%s/%s.ww", tmpdir, base); unlink(p);
+	snprintf(p, sizeof p, "%s/%s.s",  tmpdir, base); unlink(p);
+	snprintf(p, sizeof p, "%s/%s.o",  tmpdir, base); unlink(p);
+	snprintf(p, sizeof p, "%s/%s.combined.ww", tmpdir, base); unlink(p);
+	snprintf(p, sizeof p, "%s/%s",    tmpdir, base); unlink(p);
+	rmdir(tmpdir);
+}
+
+static int
+build_via_driver(const char *driver, const char *tmpdir, const char *cwd,
+    const char *src)
+{
+	char cmd[2048];
+	snprintf(cmd, sizeof cmd,
+	    "cd %s && timeout 180 %s build -I %s/lib %s 2>/dev/null",
+	    tmpdir, driver, cwd, src);
+	return runwait(cmd);
+}
+
+static int
+run_row(const char *driver, const char *cwd, const struct row *r, int seq)
+{
+	char tmpdir[256], src[512], base[64], outbin[768];
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/fvc_%d_d_%d", getpid(), seq);
+	snprintf(base,   sizeof base,   "main781");
+	snprintf(src,    sizeof src,    "%s/%s.ww", tmpdir, base);
+	mkdir(tmpdir, 0755);
+	if (write_source(src, r->src) != 0) { cleanup_tmp(tmpdir, base); return -1; }
+	int rc;
+	int br = build_via_driver(driver, tmpdir, cwd, src);
+	if (br == 0) {
+		snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+		rc = runwait(outbin);
+	} else {
+		rc = -1;
+	}
+	cleanup_tmp(tmpdir, base);
+	return rc;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char cwd[256];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+	char absbin[512];
+	if (bin[0] != '/') {
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[640], wdrv[640];
+	snprintf(cdrv, sizeof cdrv, "%s/ww",    bin);
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int total = 0, fail = 0;
+	int wwpresent = (access(wdrv, X_OK) == 0);
+	int seq = 0;
+
+	for (int i = 0; i < n; i++) {
+		if (rows[i].stage_mask & STAGE_CS) {
+			total++;
+			int got = run_row(cdrv, cwd, &rows[i], seq++);
+			if (got != rows[i].want_exit) {
+				fprintf(stderr,
+				    "fmt_vstream_compositions_run[cs][%s]: exit=%d want=%d\n",
+				    rows[i].label, got, rows[i].want_exit);
+				fail++;
+			}
+		}
+		if (wwpresent && (rows[i].stage_mask & STAGE_WW)) {
+			total++;
+			int got = run_row(wdrv, cwd, &rows[i], seq++);
+			if (got != rows[i].want_exit) {
+				fprintf(stderr,
+				    "fmt_vstream_compositions_run[ww][%s]: exit=%d want=%d\n",
+				    rows[i].label, got, rows[i].want_exit);
+				fail++;
+			}
+		}
+	}
+
+	if (!wwpresent)
+		fprintf(stderr, "fmt_vstream_compositions_run: skip wwstage (no %s)\n", wdrv);
+
+	if (fail) {
+		fprintf(stderr, "fmt_vstream_compositions_run: %d/%d fixtures failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("fmt_vstream_compositions_run: %d/%d ok\n", total, total);
+	return 0;
+}