diff --git a/Makefile b/Makefile
index e01ca66d..cba435ce 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_bufio_vstream_run \
+	  $(BIN)/test_log_vstream_run \
 	  $(BIN)/test_use_promote_alias \
 	  $(BIN)/test_field_signed $(BIN)/test_frame_argcount \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
@@ -712,6 +713,14 @@ $(BIN)/test_bufio_vstream_run: test/wcc/778_bufio_vstream_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_log_vstream_run: test/wcc/779_log_vstream_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		lib/log/log.ww lib/log/vstream.ww \
+		lib/fmt/fmt.ww lib/fmt/vstream.ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_arrlit_str_full: test/wcc/711_arrlit_str_full.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 4838e4c7..39e3ffd6 100644
--- a/lib/fmt/vstream.ww
+++ b/lib/fmt/vstream.ww
@@ -170,8 +170,10 @@ fn vformatfield(vs: io.vstream, f: field) (size | io.error) = {
 
 // vfprint — vstream-side fprint. Mirror of fmt.fprint (fmt.ww:177).
 // Per-arg loop with a space separator; returns total bytes written
-// or the first io.error.
-fn vfprint(vs: io.vstream, args: formattable...) (size | io.error) = {
+// or the first io.error. Exported so lib/log's V API (fold-e5) can
+// dispatch through it; eFinal (#50) collapses fprint over the unified
+// surface.
+export fn vfprint(vs: io.vstream, args: formattable...) (size | io.error) = {
 	let total: size = 0;
 	let i: i32 = 0;
 	for (i < args.len) {
@@ -196,8 +198,9 @@ fn vfprint(vs: io.vstream, args: formattable...) (size | io.error) = {
 // {n}-placeholder parser routed through vformatfield + vputbytes
 // instead of fmt.formatfield + fmt.putbytes; the placeholder language
 // (incl. `{N:mods}` modifier subset) is identical — scandigits /
-// scanmods / modsinit reused directly from fmt.ww.
-fn vfprintf(vs: io.vstream, fmt: str, args: field...) (size | io.error) = {
+// scanmods / modsinit reused directly from fmt.ww. Exported (same
+// rationale as vfprint above) so lib/log's V API dispatches through it.
+export fn vfprintf(vs: io.vstream, fmt: str, args: field...) (size | io.error) = {
 	let total: size = 0;
 	let i: i32 = 0;
 	let nextimpl: i32 = 0;
diff --git a/lib/log/vstream.ww b/lib/log/vstream.ww
new file mode 100644
index 00000000..bb9df654
--- /dev/null
+++ b/lib/log/vstream.ww
@@ -0,0 +1,297 @@
+// vstream — Hare-shaped vtable wrappers over the logger surface.
+// Project #94 fold-e5 (Option C, parallel API for lib/log). Last
+// per-caller migration before fold-eFinal (task #50).
+//
+// Adds the ten _v variants of OLD log.ww's surface — new_v /
+// lprintln_v / println_v / lprintfln_v / printfln_v / lfatal_v /
+// fatal_v / lfatalf_v / fatalf_v / setlogger_v — alongside a vlogger
+// vtable + vstdlogger over an io.vstream sink. The OLD surface stays
+// untouched here — fold-eFinal (task #50) atomically flips the
+// package shape: deletes the OLD logger / stdlogger / globals + the
+// pre-vtable stderrsink + ensureinit, renames the `_v` suffix off
+// (drops to bare `new` / `println` / etc. per Hare's
+// ref/hare/log/{logger,funcs,global,silent}.ha), and migrates the
+// few callers (lib/log/logtest.ww plus any downstream).
+//
+// drew acks (3/3):
+//   - module-static stderrsink_ctx (zero-init per #129 A.2/A.3 SSoT)
+//   - vt FIRST field intrusive cast (matches fmt/memio/bufio fold-e
+//     ctxs)
+//   - all 10 OLD fns get _v variants (Option-C consistency)
+//
+// ken mandates honored:
+//   - bootstrap byte-id mechanical pre==post: log is NOT embedded
+//     in any selfhost/cmd/*/main.combined.ww (grep-confirmed
+//     pre-impl), so adding vstream.ww + minimal export bumps in
+//     lib/fmt does not perturb the embedded surfaces. 990-997
+//     gates stay green by virtue of log being test-only.
+//   - module-static ctx via #129 A.2/A.3 (cs == ww symmetric)
+//   - ctx shape simple scalars/ptrs (vt is the established intrusive
+//     first-field idiom; the only other field is `fd: i32`. No f32
+//     per #165b; no nested aggregate beyond vt per #18).
+//
+// Hare reference: ref/hare/log/{logger.ha,funcs.ha,global.ha,silent.ha}
+// + ref/hare/log/log.ha (re-export hub).
+//
+// Cast workaround per #206: bare `&fn_name` does not type-check as
+// a `(*<alias> | void)` field-init / let-binding. Explicit
+// `(&fn_name): *io.<role>` cast at each store site is the Hare-
+// faithful minimum-touch route — same workaround memio/vstream.ww
+// + fmt/vstream.ww + bufio/vstream.ww use. 2 cast tokens here at
+// the stderrsink_ctx_g vt wire-up (reader + writer; no closer —
+// the process owns fd 2, same divergence as OLD stderrclose at
+// log.ww:162). Both casts drop out wholesale once #206 closes.
+//
+// Chained-N_DOT-on-module-static field-assign per OLD log.ww:134-140
+// (`_default.logger.println = stdprintln;`) — proven-working spine.
+// stderrsink_ctx_g.vt.X writes pattern through same shape; the
+// tagged-union assign with widening cast composes via #199 α
+// (concrete *fn → NAMED variant of (*reader | void) parent).
+//
+// Newline emission: vfprint / vfprintf are bare (no trailing \n);
+// Hare's ref/hare/fmt/wrappers.ha:60-68 appends the newline at the
+// fprintln site (after fprint). ww mirrors: stdprintln_v / stdprintfln_v
+// call fmt.vfprint / fmt.vfprintf then io.st_write a one-byte slice
+// `[10u8]` for the LF. Same shape OLD log.stdprintln uses via
+// fmt.fprintln (log.ww:169).
+//
+// Sibling tasks parked here (filed, NOT fixed):
+//
+//   - eFinal (#50): atomic flip + delete OLD log surface + drop the
+//     `_v` suffix wholesale (graduates to Hare's bare names).
+//
+//   - #206 (bare &fn → (*alias|void)): 2 cast sites at ensureinit_v;
+//     drop out wholesale on close.
+//
+//   - #173 (TRY-on-tagged-return both-stages broken): stderrwrite_v
+//     constructs nomem and widens to io.error explicitly rather than
+//     using `os.trywrite(...)?`; same shape memio.vstream.ww +
+//     fmt.vstream.ww + bufio.vstream.ww adopt.
+//
+//   - #207 (struct-lit multi-tagged-field copy drops past first):
+//     not bitten here — module-static stderrsink_ctx_g is zero-init
+//     at link time per #129; vt fields wired via post-decl chained
+//     assigns rather than struct-lit, sidestepping #207 wholesale.
+//     The pattern is the module-static analogue of memio.vstream.ww's
+//     alloc-zero-chain.
+//
+//   - #209 (wwstage formattable match-arm bail when fmt imported):
+//     log imports fmt transitively (via the formattable / field
+//     types in vlogger's vtable + fmt.vfprint / vfprintf calls in
+//     the std callbacks). All probe rows in test/wcc/779 run
+//     STAGE_CS-only; eFinal closes the wwstage half when #209 lands.
+
+package log;
+
+import fmt;
+import io;
+import os;
+
+// stderrsink_ctx — module-static state for the default vlogger's
+// stderr sink. vt FIRST field for the intrusive vstream→*stderrsink_ctx
+// cast in stderrwrite_v. Mirrors fmt.fd_ctx (fmt/vstream.ww:66) +
+// memio.fixed_ctx (memio/vstream.ww:48); only scalars/ptrs beyond
+// vt per ken's mandate.
+type stderrsink_ctx = struct {
+	vt: io.vtable,
+	fd: i32,
+};
+
+// Zero-init at link time per #129 A.2/A.3 SSoT; ensureinit_v wires
+// vt.reader / vt.writer + fd lazily on first dispatch. Same shape
+// OLD log.ww uses for the pre-vtable stderrsink (log.ww:104).
+let stderrsink_ctx_g: stderrsink_ctx;
+
+// vlogger — V-side counterpart to OLD logger (log.ww:85). Same shape
+// modulo the `_v` field-suffix (collapses on #50). Dispatchers route
+// through io.vstream rather than *io.stream.
+//
+// Mirrors ref/hare/log/logger.ha:9.
+export type vlogger = struct {
+	println_v:  fn(l: *vlogger, args: fmt.formattable...) void,
+	printfln_v: fn(l: *vlogger, format: str, fields: fmt.field...) void,
+};
+
+// vstdlogger — concrete vlogger forwarding to an io.vstream sink.
+// First-field embed: `&sl.logger` yields a `*vlogger` (intrusive
+// outer→inner cast); the vtable callback recovers the outer via
+// `*vstdlogger` cast.
+//
+// Mirrors ref/hare/log/logger.ha:18 (stdlogger).
+export type vstdlogger = struct {
+	logger: vlogger,
+	sink:   io.vstream,
+};
+
+// _silent_v / _default_v — backing storage for the silent_v /
+// default_v *vlogger globals. Zero-init at link time; ensureinit_v
+// fills the println_v / printfln_v slots.
+let _silent_v:  vlogger;
+let _default_v: vstdlogger;
+
+// silent_v / default_v / global_v — Hare-style *vlogger globals.
+// Zero-init at link time; lazily wired by ensureinit_v on the first
+// call to any exported fn. Same divergence as OLD log.ww:116-118
+// (header rationale for the lazy-init route applies wholesale: ww
+// cstage emit_lets only constexprs primitive / str-literal /
+// array-literal lets — struct- and pointer-literal init aren't
+// reachable). #50 / eFinal graduates if static-init lands.
+export let silent_v:  *vlogger;
+export let default_v: *vlogger;
+export let global_v:  *vlogger;
+
+// initdone_v — guards ensureinit_v so the one-shot wiring runs once.
+// Mirrors OLD initdone (log.ww:122) + lib/temp's rnginit pattern.
+let initdone_v: i32 = 0;
+
+fn ensureinit_v() void = {
+	if (initdone_v != 0) { return; };
+	stderrsink_ctx_g.fd = 2;
+	stderrsink_ctx_g.vt.reader = (&stderrread_v):  *io.reader;
+	stderrsink_ctx_g.vt.writer = (&stderrwrite_v): *io.writer;
+
+	_silent_v.println_v  = silentprintln_v;
+	_silent_v.printfln_v = silentprintfln_v;
+
+	_default_v.logger.println_v  = stdprintln_v;
+	_default_v.logger.printfln_v = stdprintfln_v;
+	_default_v.sink = &stderrsink_ctx_g.vt;
+
+	silent_v  = &_silent_v;
+	default_v = &_default_v.logger;
+	global_v  = default_v;
+	initdone_v = 1;
+};
+
+// stderrread_v / stderrwrite_v — vtable callbacks for the private
+// stderr sink. Read is eof-only — lib/log writes; never reads.
+// Mirror of OLD stderrread / stderrwrite (log.ww:148-160) modulo the
+// (size | io.eof | io.error) / (size | io.error) return surface;
+// -errno collapses to a nomem-widened io.error per the
+// fmt.fdsinkwrite_v precedent (fmt/vstream.ww:88-92).
+fn stderrread_v(s: io.vstream, buf: []u8) (size | io.eof | io.error) = {
+	let e: io.eof;
+	return e;
+};
+
+fn stderrwrite_v(s: io.vstream, buf: []u8) (size | io.error) = {
+	let c: *stderrsink_ctx = s: *stderrsink_ctx;
+	let r: i64 = os.write(c.fd, buf.ptr, buf.len: u64);
+	if (r < 0) {
+		let nm: nomem;
+		let e: io.error = nm;
+		return e;
+	};
+	return r: size;
+};
+
+// stdprintln_v / stdprintfln_v — vtable callbacks for vstdlogger.
+// Forwards to fmt.vfprint / vfprintf on the sink + appends the
+// trailing newline via io.st_write. Hare's fmt.fprintln emits the
+// newline inside one wrapper (ref/hare/fmt/wrappers.ha:60-68); we
+// inline that split at the log site because fmt.vfprint is the bare
+// (no-newline) primitive. The (size | io.error) results are dropped
+// — lib/log's surface is `void` (matches Hare + OLD stdprintln at
+// log.ww:167).
+fn stdprintln_v(l: *vlogger, args: fmt.formattable...) void = {
+	let sl: *vstdlogger = l: *vstdlogger;
+	fmt.vfprint(sl.sink, args...);
+	let nl: [1]u8;
+	nl[0] = 10u8;
+	let v: []u8 = nl[0:1];
+	io.st_write(sl.sink, v);
+};
+
+fn stdprintfln_v(l: *vlogger, format: str, fields: fmt.field...) void = {
+	let sl: *vstdlogger = l: *vstdlogger;
+	fmt.vfprintf(sl.sink, format, fields...);
+	let nl: [1]u8;
+	nl[0] = 10u8;
+	let v: []u8 = nl[0:1];
+	io.st_write(sl.sink, v);
+};
+
+// silentprintln_v / silentprintfln_v — vtable callbacks for the
+// silent vlogger. Discard every record without dispatching through
+// fmt; keeps silent truly silent if fmt ever gets stateful. Mirror
+// of OLD silentprintln / silentprintfln (log.ww:183-187).
+fn silentprintln_v(l: *vlogger, args: fmt.formattable...) void = { };
+fn silentprintfln_v(l: *vlogger, format: str, fields: fmt.field...) void = { };
+
+// ---- public API (10 _v variants) -----------------------------------------
+
+// new_v — wire `sl` as a vstdlogger over `sink`. Hare returns by
+// value (ref/hare/log/logger.ha:20); ww cgen can't return wide
+// structs, so we take an out-parameter pointer (same shape as OLD
+// new at log.ww:193 + memio.fixed + bufio.init).
+export fn new_v(sl: *vstdlogger, sink: io.vstream) void = {
+	ensureinit_v();
+	sl.logger.println_v  = stdprintln_v;
+	sl.logger.printfln_v = stdprintfln_v;
+	sl.sink = sink;
+};
+
+// lprintln_v — dispatch `args` through `log`. Mirrors OLD lprintln
+// (log.ww:202) + ref/hare/log/funcs.ha:8.
+export fn lprintln_v(log: *vlogger, args: fmt.formattable...) void = {
+	ensureinit_v();
+	log.println_v(log, args...);
+};
+
+// println_v — dispatch through the [[global_v]] vlogger. Mirrors
+// OLD println (log.ww:208) + ref/hare/log/funcs.ha:18.
+export fn println_v(args: fmt.formattable...) void = {
+	ensureinit_v();
+	lprintln_v(global_v, args...);
+};
+
+// lfatal_v — lprintln_v to `log` then exit(255). `never` return
+// marks the bottom type so flow-control checks treat callers as
+// terminated. Mirrors OLD lfatal (log.ww:216) +
+// ref/hare/log/funcs.ha:28.
+export fn lfatal_v(log: *vlogger, args: fmt.formattable...) never = {
+	lprintln_v(log, args...);
+	os.exit(255);
+};
+
+// fatal_v — lprintln_v to [[global_v]] then exit(255). Mirrors OLD
+// fatal (log.ww:223) + ref/hare/log/funcs.ha:45.
+export fn fatal_v(args: fmt.formattable...) never = {
+	println_v(args...);
+	os.exit(255);
+};
+
+// setlogger_v — install `log` as the [[global_v]] vlogger. Mirrors
+// OLD setlogger (log.ww:230) + ref/hare/log/global.ha:20.
+export fn setlogger_v(log: *vlogger) void = {
+	ensureinit_v();
+	global_v = log;
+};
+
+// lprintfln_v — dispatch a format-string record through `log`.
+// Mirrors OLD lprintfln (log.ww:237) + ref/hare/log/funcs.ha:13.
+export fn lprintfln_v(log: *vlogger, format: str, fields: fmt.field...) void = {
+	ensureinit_v();
+	log.printfln_v(log, format, fields...);
+};
+
+// printfln_v — dispatch through the [[global_v]] vlogger. Mirrors
+// OLD printfln (log.ww:244) + ref/hare/log/funcs.ha:23.
+export fn printfln_v(format: str, fields: fmt.field...) void = {
+	ensureinit_v();
+	lprintfln_v(global_v, format, fields...);
+};
+
+// lfatalf_v — lprintfln_v to `log` then exit(255). Mirrors OLD
+// lfatalf (log.ww:251) + ref/hare/log/funcs.ha:35.
+export fn lfatalf_v(log: *vlogger, format: str, fields: fmt.field...) never = {
+	lprintfln_v(log, format, fields...);
+	os.exit(255);
+};
+
+// fatalf_v — lprintfln_v to [[global_v]] then exit(255). Mirrors
+// OLD fatalf (log.ww:258) + ref/hare/log/funcs.ha:52.
+export fn fatalf_v(format: str, fields: fmt.field...) never = {
+	printfln_v(format, fields...);
+	os.exit(255);
+};
diff --git a/test/wcc/779_log_vstream_run.c b/test/wcc/779_log_vstream_run.c
new file mode 100644
index 00000000..bd37d0c1
--- /dev/null
+++ b/test/wcc/779_log_vstream_run.c
@@ -0,0 +1,385 @@
+/*
+ * 779_log_vstream_run — project #94 fold-e5 sentinel. Pins the
+ * additive lib/log/vstream.ww Option C parallel API: module-static
+ * stderrsink_ctx_g with `vt: io.vtable` as the first field for the
+ * intrusive vstream cast, the 10 _v variants of OLD log.ww's surface
+ * (new_v / lprintln_v / println_v / lprintfln_v / printfln_v / lfatal_v
+ * / fatal_v / lfatalf_v / fatalf_v / setlogger_v), a vlogger vtable +
+ * vstdlogger over io.vstream sink, and lazy ensureinit_v wiring the
+ * stderr default + silent_v / default_v / global_v *vlogger globals.
+ * Coexists with the pre-vtable log.logger / log.stdlogger / log.new /
+ * log.println / etc. surface in log.ww (fold-eFinal, task #50, retires
+ * the latter and drops the `_v` suffix wholesale to match Hare's bare
+ * names).
+ *
+ * Each row imports os + log + memio + io. log itself imports fmt,
+ * so the formattable / field types flow through transitively.
+ *
+ * Cstage-only per row (no STAGE_WW, no byte_id) — pre-existing
+ * wwstage bug #209 (sibling of #190): the wwstage checker bails on
+ * match-arm-over-formattable when fmt is imported transitively, which
+ * lib/log does for the vlogger vtable signatures + the fmt.vfprint /
+ * vfprintf dispatch in stdprintln_v / stdprintfln_v. The bug bites the
+ * OLD log surface identically — `log.println("hi")` from a probe
+ * trips the same trace. Existing 970 logtest runs the @test fixture
+ * under cstage `ww run` only, so the OLD surface is fine via cstage.
+ * 995 self-rebuild dodges it because no selfhost cmd imports log
+ * transitively. Fix-of-the-bug = one-class checker repair, out-of-
+ * scope for the additive fold-e5; closes the wwstage half here when
+ * it lands. Byte-id graduates with #209 close.
+ *
+ *   row                       | what it pins
+ *   --------------------------+--------------------------------------
+ *   println_v_default_stderr  | log.println_v through the [[default_v]]
+ *                             | global, routed to fd 2 via the
+ *                             | module-static stderrsink_ctx_g. Asserts
+ *                             | ensureinit_v wires + dispatches without
+ *                             | crashing; stderr capture is left to the
+ *                             | shell runner. Pins the full chain:
+ *                             | println_v → lprintln_v(global_v) →
+ *                             | (*vlogger).println_v → stdprintln_v →
+ *                             | fmt.vfprint + io.st_write("\n").
+ *   printfln_v_default_stderr | Same chain as row 1 but through the
+ *                             | format-string slot (printfln_v →
+ *                             | lprintfln_v(global_v) → stdprintfln_v
+ *                             | → fmt.vfprintf + io.st_write("\n")).
+ *                             | Pins the {n}-placeholder dispatch
+ *                             | through vformatfield reaches the
+ *                             | default sink.
+ *   lprintln_v_custom_sink    | memio.fixed_vstream-backed vstdlogger;
+ *                             | log.new_v wires the vtable + sink,
+ *                             | log.lprintln_v dispatches "hi" + 7i64
+ *                             | through stdprintln_v. Asserts the
+ *                             | exact bytes ("hi 7\n") land in the
+ *                             | caller's buffer (fixed_vstream borrows
+ *                             | the slice, so the bytes are visible
+ *                             | back through buf[]).
+ *   branched_lprintln_v       | branched callee per #105: two distinct
+ *                             | vstdlogger sinks runtime-selected via
+ *                             | *vlogger pointer. lprintln_v dispatches
+ *                             | through the picked sink only; the
+ *                             | other stays empty. Catches a constant-
+ *                             | fold mistake in the fn-ptr dispatch
+ *                             | (mirror of 778's branched_bufio_vstream).
+ *
+ * SIBLINGS (filed inline, NOT fixed here — fold-e5 is purely
+ * additive over fold-e1/e2/e3/e4's frozen io.* + memio.* + fmt.* +
+ * bufio.* surface plus the minimal `export` bump on fmt.vfprint /
+ * fmt.vfprintf):
+ *
+ *   - eFinal (#50): atomic flip + delete OLD log surface + drop the
+ *     `_v` suffix wholesale.
+ *
+ *   - #206 (bare &fn → (*alias|void)): 2 cast sites at ensureinit_v
+ *     in lib/log/vstream.ww; drop out wholesale on close.
+ *
+ *   - #173 (TRY-on-tagged-return both-stages broken): stderrwrite_v
+ *     constructs nomem and widens to io.error explicitly rather than
+ *     using `os.trywrite(...)?`; same shape memio.vstream.ww +
+ *     fmt.vstream.ww + bufio.vstream.ww adopt.
+ *
+ *   - #209 (wwstage formattable match-arm bail): every row is
+ *     STAGE_CS-only; byte-id deferred until #209 lands.
+ *
+ * BOOTSTRAP-EMBED CHECK: log is NOT embedded in any selfhost
+ * combined.ww (grep `package log\|import log` returned empty pre-
+ * impl). Adding lib/log/vstream.ww + the two `export` bumps on
+ * lib/fmt/vstream.ww does NOT require a Makefile regen (#110); only
+ * lib/log/ + lib/fmt/ test paths see the new module surface. 990-997
+ * byte-id gates stay green by virtue of log being test-only and the
+ * fmt vstream.ww changes being non-embedded.
+ *
+ * GATE POLARITY: must stay GREEN. A red here means vstream.ww
+ * regressed, the module-static stderrsink_ctx_g zero-init or chained
+ * vt-field assigns regressed, the intrusive vstream→*stderrsink_ctx
+ * cast miscomputed offsets, ensureinit_v's one-shot guard broke, or
+ * the fn-ptr dispatch through vlogger.println_v / printfln_v stopped
+ * resolving.
+ */
+#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[] = {
+	{ "println_v_default_stderr",
+	  "package main;\n"
+	  "import os;\n"
+	  "import log;\n"
+	  "export fn main() i32 = {\n"
+	  "    log.println_v(\"hello\");\n"
+	  "    return 42;\n"
+	  "};\n",
+	  42,
+	  STAGE_CS, 0 },
+	{ "printfln_v_default_stderr",
+	  "package main;\n"
+	  "import os;\n"
+	  "import log;\n"
+	  "export fn main() i32 = {\n"
+	  "    log.printfln_v(\"v={}\", 7i64);\n"
+	  "    return 43;\n"
+	  "};\n",
+	  43,
+	  STAGE_CS, 0 },
+	{ "lprintln_v_custom_sink",
+	  "package main;\n"
+	  "import os;\n"
+	  "import log;\n"
+	  "import memio;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [16]u8;\n"
+	  "    let r = memio.fixed_vstream(buf[0:16]);\n"
+	  "    let vs: io.vstream = nil: *io.vtable;\n"
+	  "    match (r) {\n"
+	  "    case let v: io.vstream => { vs = v; };\n"
+	  "    case nomem => { return 50; };\n"
+	  "    };\n"
+	  "    let sl: log.vstdlogger;\n"
+	  "    log.new_v(&sl, vs);\n"
+	  "    log.lprintln_v(&sl.logger, \"hi\", 7i64);\n"
+	  "    if (buf[0] != 104u8) { return 91; };\n"
+	  "    if (buf[1] != 105u8) { return 92; };\n"
+	  "    if (buf[2] != 32u8)  { return 93; };\n"
+	  "    if (buf[3] != 55u8)  { return 94; };\n"
+	  "    if (buf[4] != 10u8)  { return 95; };\n"
+	  "    return 44;\n"
+	  "};\n",
+	  44,
+	  STAGE_CS, 0 },
+	{ "branched_lprintln_v",
+	  "package main;\n"
+	  "import os;\n"
+	  "import log;\n"
+	  "import memio;\n"
+	  "import io;\n"
+	  "export fn main() i32 = {\n"
+	  "    let bufA: [16]u8;\n"
+	  "    let bufB: [16]u8;\n"
+	  "    let rA = memio.fixed_vstream(bufA[0:16]);\n"
+	  "    let vsA: io.vstream = nil: *io.vtable;\n"
+	  "    match (rA) {\n"
+	  "    case let v: io.vstream => { vsA = v; };\n"
+	  "    case nomem => { return 50; };\n"
+	  "    };\n"
+	  "    let rB = memio.fixed_vstream(bufB[0:16]);\n"
+	  "    let vsB: io.vstream = nil: *io.vtable;\n"
+	  "    match (rB) {\n"
+	  "    case let v: io.vstream => { vsB = v; };\n"
+	  "    case nomem => { return 51; };\n"
+	  "    };\n"
+	  "    let slA: log.vstdlogger;\n"
+	  "    let slB: log.vstdlogger;\n"
+	  "    log.new_v(&slA, vsA);\n"
+	  "    log.new_v(&slB, vsB);\n"
+	  "    let sel: i32 = 1;\n"
+	  "    let chosen: *log.vlogger = &slA.logger;\n"
+	  "    if (sel == 0) { chosen = &slB.logger; };\n"
+	  "    log.lprintln_v(chosen, \"pick\");\n"
+	  "    if (bufA[0] != 112u8) { return 91; };\n"
+	  "    if (bufA[1] != 105u8) { return 92; };\n"
+	  "    if (bufA[2] != 99u8)  { return 93; };\n"
+	  "    if (bufA[3] != 107u8) { return 94; };\n"
+	  "    if (bufA[4] != 10u8)  { return 95; };\n"
+	  "    if (bufB[0] != 0u8)   { return 96; };\n"
+	  "    return 45;\n"
+	  "};\n",
+	  45,
+	  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 (filed task #15), so each row's build leaves
+ * <src>.{combined.ww,s,o} + bare exe alongside. Sweep all then
+ * rmdir. Mirror of 778's cleanup_tmp. */
+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/lvs_%d_d_%d", getpid(), seq);
+	snprintf(base,   sizeof base,   "main779");
+	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);
+		/* Suppress stderr for rows 1/2 — the default-sink probes
+		 * write through fd 2; don't pollute the test runner's
+		 * stderr. */
+		char runcmd[1024];
+		snprintf(runcmd, sizeof runcmd, "%s 2>/dev/null", outbin);
+		rc = runwait(runcmd);
+	} else {
+		rc = -1;
+	}
+	cleanup_tmp(tmpdir, base);
+	return rc;
+}
+
+/* asm_byte_identical — diff cstage vs wwstage .s. Parallel trees so
+ * ww_ww writing intermediates next to the source doesn't clobber the
+ * cstage .s (CLAUDE.md rule 14 phase split). Mirror of 778's. Unused
+ * for fold-e5 — every row is STAGE_CS-only per #209 — but kept
+ * scaffolded for when #209 closes and byte-id graduates. */
+static int
+asm_byte_identical(const char *cdrv, const char *wdrv, const char *cwd,
+    const struct row *r, int seq)
+{
+	char src[512], tdc[256], tdw[256], base[64], cs[512], ws[512];
+	snprintf(tdc, sizeof tdc, "/tmp/lvs_%d_c_%d", getpid(), seq);
+	snprintf(tdw, sizeof tdw, "/tmp/lvs_%d_w_%d", getpid(), seq);
+	snprintf(base, sizeof base, "main779");
+	mkdir(tdc, 0755);
+	mkdir(tdw, 0755);
+	snprintf(src, sizeof src, "%s/%s.ww", tdc, base);
+	if (write_source(src, r->src) != 0) { cleanup_tmp(tdc, base); cleanup_tmp(tdw, base); return -1; }
+	int rc = -1;
+	if (build_via_driver(cdrv, tdc, cwd, src) != 0) goto out;
+	snprintf(cs, sizeof cs, "%s/%s.s", tdc, base);
+
+	snprintf(src, sizeof src, "%s/%s.ww", tdw, base);
+	if (write_source(src, r->src) != 0) goto out;
+	if (build_via_driver(wdrv, tdw, cwd, src) != 0) goto out;
+	snprintf(ws, sizeof ws, "%s/%s.s", tdw, base);
+
+	FILE *fc = fopen(cs, "rb");
+	FILE *fw = fopen(ws, "rb");
+	if (fc && fw) {
+		rc = 0;
+		for (;;) {
+			int a = fgetc(fc);
+			int b = fgetc(fw);
+			if (a != b) { rc = -1; break; }
+			if (a == EOF) break;
+		}
+	}
+	if (fc) fclose(fc);
+	if (fw) fclose(fw);
+out:
+	cleanup_tmp(tdc, base);
+	cleanup_tmp(tdw, 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,
+				    "log_vstream_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,
+				    "log_vstream_run[ww][%s]: exit=%d want=%d\n",
+				    rows[i].label, got, rows[i].want_exit);
+				fail++;
+			}
+			if (rows[i].byte_id) {
+				total++;
+				if (asm_byte_identical(cdrv, wdrv, cwd, &rows[i], seq++) != 0) {
+					fprintf(stderr,
+					    "log_vstream_run[byte-id][%s]: cstage vs wwstage asm differs\n",
+					    rows[i].label);
+					fail++;
+				}
+			}
+		}
+	}
+
+	if (!wwpresent)
+		fprintf(stderr, "log_vstream_run: skip wwstage (no %s)\n", wdrv);
+
+	if (fail) {
+		fprintf(stderr, "log_vstream_run: %d/%d fixtures failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("log_vstream_run: %d/%d ok\n", total, total);
+	return 0;
+}