diff --git a/lib/strings/strings.ww b/lib/strings/strings.ww
index 3aaeed6e..c8d3cb67 100644
--- a/lib/strings/strings.ww
+++ b/lib/strings/strings.ww
@@ -3,17 +3,18 @@
 //
 // Documented divergences from Hare:
 //
-//   - `concat(a, b)` is 2-arg. Hare ships `concat(strs: str...)`
-//     (ref/hare/strings/concat.ha:5). Blocks on task #16 (cstage
-//     variadic-pack drops .len of multi-field element type). Cite
-//     reverts on fix.
 //   - `trim` / `ltrim` / `rtrim` take a single rune. Hare's are
-//     `(exclude: rune...)` (ref/hare/strings/trim.ha:54). Same
-//     blocker as concat. Hare's no-rune branch (strip whitespace)
-//     is also dropped — depends on a rune set.
+//     `(trim: rune...)` (ref/hare/strings/trim.ha:11,32,54). The
+//     port body uses `iter`/`next` + inner match against the pack +
+//     `prev` step-back; that shape triggers #36 (wwstage scanlocals
+//     misses match-arm `case let` bindings, slot offsets diverge —
+//     `.ai/probe_trim_36extra.{ww,diff}`). Hare's no-rune
+//     strip-whitespace branch additionally needs `lib/bytes`
+//     variadic graduation.
 //   - `contains` is non-variadic. Hare's is
 //     `contains(haystack, needles: (str | rune)...)`
-//     (ref/hare/strings/contains.ha:9). Same blocker.
+//     (ref/hare/strings/contains.ha:9). Gated on `(str|rune)...`
+//     tagged-variadic gather + runtime — task #5.
 //   - `byteindex` / `rbyteindex` rune arms encode via
 //     `utf8.encoderune`; the legacy impls scanned for `r: u8` (an
 //     undocumented ASCII-only restriction that silently dropped
@@ -107,17 +108,30 @@ export fn freeall(s: []str) void = {
 	};
 };
 
-// concat — fresh allocation containing `a` then `b`. Caller releases
-// with `os.free(r.ptr, r.len: u64)`. ref/hare/strings/concat.ha:5
-// (subset: Hare's `(strs: str...)` blocks on task #16).
-export fn concat(a: str, b: str) str = {
-	let total: i32 = a.len + b.len;
-	let buf: *u8 = os.alloc(total: u64): *u8;
+// concat — fresh allocation containing each element of `strs` in
+// order. Caller releases with `os.free(r.ptr, r.len: u64)`.
+// ref/hare/strings/concat.ha:5. Hare's `nomem` return is dropped:
+// `os.alloc` aborts on OOM.
+export fn concat(strs: str...) str = {
+	let total: i32 = 0;
 	let i: i32 = 0;
-	for (i < a.len) { buf[i] = a[i]; i += 1; };
-	let j: i32 = 0;
-	for (j < b.len) { buf[a.len + j] = b[j]; j += 1; };
+	for (i < strs.len) { total += strs[i].len; i += 1; };
 	let r: str;
+	r.ptr = nil;
+	r.len = 0;
+	if (total == 0) { return r; };
+	let buf: *u8 = os.alloc(total: u64): *u8;
+	let off: i32 = 0;
+	i = 0;
+	for (i < strs.len) {
+		let j: i32 = 0;
+		for (j < strs[i].len) {
+			buf[off + j] = strs[i][j];
+			j += 1;
+		};
+		off += strs[i].len;
+		i += 1;
+	};
 	r.ptr = buf;
 	r.len = total;
 	return r;
diff --git a/lib/strings/stringstest.ww b/lib/strings/stringstest.ww
index 7a8a9fa0..5a95db54 100644
--- a/lib/strings/stringstest.ww
+++ b/lib/strings/stringstest.ww
@@ -46,28 +46,57 @@ fn streq(a: str, b: str) bool = {
 };
 
 // ---- concat -----------------------------------------------------------
-// ref/hare/strings/concat.ha:18 (2-arg subset).
+// ref/hare/strings/concat.ha:18. Rows mirror Hare's vectors (0/1/2/3-arg,
+// empty-mid, 2-empty) plus empty-first / empty-last / multibyte. The
+// per-row `signalled` bump narrows a failure exit code to the offending
+// row.
 
 @test fn concat_cases() void = {
-	let a: str = strings.concat("hello ", "world");
-	if (!streq(a, "hello world")) { fail(); };
-	defer os.free(a.ptr: *void, a.len: u64);
+	let pool: [17]str;
+	pool[0]  = "hello";
+	pool[1]  = "hello ";
+	pool[2]  = "world";
+	pool[3]  = "hello";
+	pool[4]  = " ";
+	pool[5]  = "world";
+	pool[6]  = "hello";
+	pool[7]  = "";
+	pool[8]  = "world";
+	pool[9]  = "";
+	pool[10] = "";
+	pool[11] = "";
+	pool[12] = "world";
+	pool[13] = "hello";
+	pool[14] = "";
+	pool[15] = "こん";
+	pool[16] = "にちは";
 
-	let e: str = strings.concat("", "");
-	if (!streq(e, "")) { fail(); };
-	// e.len == 0 — os.free guarded, skip.
+	let argo:   [9]i32;
+	let argn:   [9]i32;
+	let want:   [9]str;
+	let labels: [9]str;
+	argo[0]=0;  argn[0]=0; want[0]="";              labels[0]="0-arg";
+	argo[1]=0;  argn[1]=1; want[1]="hello";         labels[1]="1-arg";
+	argo[2]=1;  argn[2]=2; want[2]="hello world";   labels[2]="2-arg";
+	argo[3]=3;  argn[3]=3; want[3]="hello world";   labels[3]="3-arg";
+	argo[4]=6;  argn[4]=3; want[4]="helloworld";    labels[4]="empty-mid";
+	argo[5]=9;  argn[5]=2; want[5]="";              labels[5]="2-empty";
+	argo[6]=11; argn[6]=2; want[6]="world";         labels[6]="empty-first";
+	argo[7]=13; argn[7]=2; want[7]="hello";         labels[7]="empty-last";
+	argo[8]=15; argn[8]=2; want[8]="こんにちは";    labels[8]="multibyte";
 
-	let l: str = strings.concat("", "world");
-	if (!streq(l, "world")) { fail(); };
-	defer os.free(l.ptr: *void, l.len: u64);
-
-	let r: str = strings.concat("hello", "");
-	if (!streq(r, "hello")) { fail(); };
-	defer os.free(r.ptr: *void, r.len: u64);
-
-	let m: str = strings.concat("こん", "にちは");
-	if (!streq(m, "こんにちは")) { fail(); };
-	defer os.free(m.ptr: *void, m.len: u64);
+	let i: i32 = 0;
+	for (i < 9) {
+		signalled = 200 + i;
+		let argv: []str;
+		argv.ptr = &pool[argo[i]];
+		argv.len = argn[i];
+		argv.cap = argn[i];
+		let got: str = strings.concat(argv...);
+		if (!streq(got, want[i])) { fail(); };
+		if (got.len > 0) { os.free(got.ptr: *void, got.len: u64); };
+		i += 1;
+	};
 };
 
 // ---- hasprefix --------------------------------------------------------