diff --git a/cmd/w6c/cgen.c b/cmd/w6c/cgen.c
index cf524b94..6b5ae535 100644
--- a/cmd/w6c/cgen.c
+++ b/cmd/w6c/cgen.c
@@ -705,9 +705,8 @@ let_emit_size(Type *t)
 	case TY_F64:
 		return 8;	/* MOVSD loads/stores 8B via LEAQ+indir. */
 	case TY_STR:
-		return 16;	/* {ptr, len}; literal-strlit init NYI. */
 	case TY_SLICE:
-		return 24;	/* {ptr, len, cap}; no init only. */
+		return (int)u->size;	/* #43: ty_str / ty_slice SSoT. */
 	case TY_STRUCT:
 		return (int)u->size;	/* zero-init only; field reads/
 					 * scalar-field writes only. */
@@ -4988,10 +4987,10 @@ cgexpr(Cg *c, Node *n, Local *locals)
 						    amem(D_BP, voff));
 					}
 				} else {
+					/* #43: route through Type.size SSoT rather
+					 * than re-asserting 16/24 for str/slice. */
 					int bsz = 8;
-					if (bu && bu->kind == TY_STR) bsz = 16;
-					else if (bu && bu->kind == TY_SLICE) bsz = 24;
-					else if (bu) bsz = (int)bu->size;
+					if (bu) bsz = (int)bu->size;
 					if (bsz <= 0) bsz = 8;
 					/* local_alloc to dodge name-collision
 					 * dedup — a 16B str bind shadowing an
@@ -6350,13 +6349,14 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 		 * collapsed the struct/slice/tagged sizing arms to the 8B
 		 * fallback, and the slot under-allocated the local. */
 		Type *lu = type_chase_named(lt);
+		/* #43: every composite kind already has its byte size cached in
+		 * lu->size; route through it instead of re-asserting 16/24 for
+		 * str/slice and re-reading for the others. */
 		int sz = 8;
-		if (lu && lu->kind == TY_ARRAY)	sz = (int)lu->size;
-		else if (lu && lu->kind == TY_SLICE) sz = 24;
-		else if (lu && lu->kind == TY_STR) sz = 16;
-		else if (lu && lu->kind == TY_STRUCT) sz = (int)lu->size;
-		else if (lu && lu->kind == TY_TUPLE) sz = (int)lu->size;
-		else if (lu && lu->kind == TY_TAGGED) sz = (int)lu->size;
+		if (lu && (lu->kind == TY_ARRAY || lu->kind == TY_SLICE
+		    || lu->kind == TY_STR || lu->kind == TY_STRUCT
+		    || lu->kind == TY_TUPLE || lu->kind == TY_TAGGED))
+			sz = (int)lu->size;
 		int off = localoff(c, locals, n->str, sz, frame);
 		int isf = cg_isfloat(lt);
 		int isf32 = type_isf32(lt);
@@ -6433,8 +6433,10 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 				break;
 			}
 		}
-		/* str initialiser: cgexpr produces (AX=ptr, BX=len). */
-		if (n->rhs && type_isstr(lt) && sz == 16) {
+		/* str initialiser: cgexpr produces (AX=ptr, BX=len).
+		 * #43: gate width via ty_str->size so a future str-layout
+		 * bump (#1) propagates without touching this site. */
+		if (n->rhs && type_isstr(lt) && sz == (int)ty_str->size) {
 			cgexpr(c, n->rhs, *locals);
 			ins2(c, A_MOVQ, areg(D_AX), amem(D_BP, off + 0));
 			ins2(c, A_MOVQ, areg(D_BX), amem(D_BP, off + 8));
@@ -7763,7 +7765,8 @@ emit_lets(Cg *c, FILE *out, Node *file)
 		/* str literal init: bake the interned label's address
 		 * into the ptr half via a DATAR reloc, set the len half
 		 * inline. */
-		if (sz == 16 && r != NULL && r->kind == N_STRLIT
+		/* #43: gate via ty_str->size so #1 propagates. */
+		if (sz == (int)ty_str->size && r != NULL && r->kind == N_STRLIT
 		    && r->strlen > 0) {
 			const char *lab = intern_strlit(c, r->str, r->strlen);
 			const char *sym = mod_mangle(c, d->str);
@@ -7916,7 +7919,7 @@ let_pre_intern(Cg *c, Node *file)
 	if (file == NULL) return;
 	for (Node *d = file->list; d; d = d->next) {
 		if (d->kind != N_LET) continue;
-		if (let_emit_size(d->type) != 16) continue;
+		if (let_emit_size(d->type) != (int)ty_str->size) continue;
 		Node *r = d->rhs;
 		while (r != NULL && r->kind == N_CAST) r = r->lhs;
 		if (r == NULL || r->kind != N_STRLIT) continue;
diff --git a/lib/shlex/shlex.ww b/lib/shlex/shlex.ww
index c6708abe..b1287870 100644
--- a/lib/shlex/shlex.ww
+++ b/lib/shlex/shlex.ww
@@ -139,7 +139,7 @@ fn dupstr(s: str) str = {
 fn appendstr(slice: *[]str, item: str) void = {
 	let newlen: i32 = slice.len + 1;
 	slice.len = newlen;
-	rtensure(slice: *void, 16u64);
+	rtensure(slice: *void, size(str): u64);
 	let dst: *str = &slice.ptr[newlen - 1];
 	dst.ptr = item.ptr;
 	dst.len = item.len;
@@ -159,7 +159,7 @@ fn freepartial(slice: []str) void = {
 		i += 1;
 	};
 	if (slice.cap > 0) {
-		os.free(slice.ptr: *void, (slice.cap: u64) * 16u64);
+		os.free(slice.ptr: *void, (slice.cap: u64) * size(str): u64);
 	};
 };
 
diff --git a/lib/strings/strings.ww b/lib/strings/strings.ww
index 2178d557..e2c1f724 100644
--- a/lib/strings/strings.ww
+++ b/lib/strings/strings.ww
@@ -159,7 +159,7 @@ export fn freeall(s: []str) void = {
 		i += 1;
 	};
 	if (s.cap > 0) {
-		os.free(s.ptr: *void, (s.cap: u64) * 16u64);
+		os.free(s.ptr: *void, (s.cap: u64) * size(str): u64);
 	};
 };
 
@@ -762,7 +762,7 @@ export fn remaining_tokens(s: *tokenizer) str = {
 fn appendstr(slice: *[]str, item: str) void = {
 	let newlen: i32 = slice.len + 1;
 	slice.len = newlen;
-	rtensure(slice: *void, 16u64);
+	rtensure(slice: *void, size(str): u64);
 	let dst: *str = &slice.ptr[newlen - 1];
 	dst.ptr = item.ptr;
 	dst.len = item.len;
diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index d6e0bcf0..3d3ccf87 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -1996,7 +1996,7 @@ export fn freeall(s: []str) void = {
 		i += 1;
 	};
 	if (s.cap > 0) {
-		os.free(s.ptr: *void, (s.cap: u64) * 16u64);
+		os.free(s.ptr: *void, (s.cap: u64) * size(str): u64);
 	};
 };
 
@@ -2599,7 +2599,7 @@ export fn remaining_tokens(s: *tokenizer) str = {
 fn appendstr(slice: *[]str, item: str) void = {
 	let newlen: i32 = slice.len + 1;
 	slice.len = newlen;
-	rtensure(slice: *void, 16u64);
+	rtensure(slice: *void, size(str): u64);
 	let dst: *str = &slice.ptr[newlen - 1];
 	dst.ptr = item.ptr;
 	dst.len = item.len;
@@ -7612,6 +7612,29 @@ fn mktname(c: *checker, nm: str) *node = {
 	return n;
 };
 
+// #43: SSoT for primitive type byte sizes. astsize's N_TNAME-primitive
+// arm and every wwstage cgen size walker (slotsize/fieldsize/letemit-
+// size/elemsizeof/paramfieldsize) consult this table so a future
+// ty_str.size bump (#1) lands in one place. Returns -1 for non-prim
+// names; callers fall back to alias/struct/enum lookup. Cstage's
+// equivalent SSoT is cmd/wcc/type.c:46-79 (ty_void/ty_bool/.../ty_str).
+fn primtypesize(nm: str) i64 = {
+	if (streq(nm, "void")) { return 0i64; };
+	if (streq(nm, "bool")) { return 1i64; };
+	if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
+	if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
+	if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
+	if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64")) { return 8i64; };
+	if (streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
+	if (streq(nm, "str")) { return 16i64; };
+	return -1i64;
+};
+
+// #43: SSoT for slice header size (ptr+len+cap = 24B today). Mirrors
+// cstage cmd/wcc/type.c:103 (ty_slice->size = 24). Bumping a slice's
+// header layout in #34 touches only this constant.
+fn tyslicesize() i64 = { return 24i64; };
+
 // #42: AST-level layout helpers for the size(T)/align(T)/offset(e.f)
 // fold. Mirror cstage resolve_type's size/align computation
 // (cmd/wcc/check.c:286-528) on AST nodes — wwstage check.ww never
@@ -7673,7 +7696,7 @@ fn astsize(c: *checker, t: *node) i64 = {
 	let k: nkind = t.kind;
 	if (k == nkind.N_TBANG) { return astsize(c, t.lhs); };
 	if (k == nkind.N_TPTR)   { return 8i64; };
-	if (k == nkind.N_TSLICE) { return 24i64; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(); };
 	if (k == nkind.N_TCHAN)  { return 8i64; };
 	if (k == nkind.N_TFN)    { return 8i64; };
 	if (k == nkind.N_TARRAY) {
@@ -7725,13 +7748,8 @@ fn astsize(c: *checker, t: *node) i64 = {
 	};
 	if (k == nkind.N_TNAME) {
 		let nm: str = t.str;
-		if (streq(nm, "void")) { return 0i64; };
-		if (streq(nm, "bool")) { return 1i64; };
-		if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
-		if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
-		if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
-		if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64") || streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
-		if (streq(nm, "str")) { return 16i64; };
+		let ps: i64 = primtypesize(nm);
+		if (ps >= 0i64) { return ps; };
 		let resolved: *node = resolvealias(c, t);
 		if (resolved != nil && resolved != t) {
 			return astsize(c, resolved);
@@ -9854,7 +9872,7 @@ fn indexbaseesz(c: *cgen, base: *node) i32 = {
 				let elem: *node = ft.lhs;
 				if (elem != nil) {
 					if (elem.kind == nkind.N_TNAME) {
-						if (streq(elem.str, "str")) { return 16; };
+						if (streq(elem.str, "str")) { return primtypesize("str"): i32; };
 						let ps: i32 = primsize(elem.str);
 						if (ps > 0) { return ps; };
 						// Pointer to named struct: indexing
@@ -9962,11 +9980,11 @@ fn elemsizeof(t: *node) i32 = {
 	// `*[]T`: stride is the slice header (24B). Hare-faithful — a
 	// pointer-to-slice is a 1D array of slices, not of T. Mirrors the
 	// cstage check.c default `*U → U` path for U=[]T (slice element).
-	if (elem.kind == nkind.N_TSLICE) { return 24; };
+	if (elem.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (elem.kind == nkind.N_TNAME) {
 		let nm: str = elem.str;
 		// str element is 16B (ptr+len). primsize returns 0 for it.
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
@@ -10561,7 +10579,7 @@ export fn letslotsize(c: *cgen, n: *node) i32 = {
 								// (ptr+len) — primsize returns 0 for
 								// it, so it'd slot 8B without this.
 								if (streq(elemn.str, "str")) {
-									esz = 16;
+									esz = primtypesize("str"): i32;
 								} else {
 									let ps: i32 = primsize(elemn.str);
 									if (ps > 0) { esz = ps; };
@@ -10610,7 +10628,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 	if (k == nkind.N_TPTR)   { return 8; };
 	if (k == nkind.N_TFN)    { return 8; };
 	if (k == nkind.N_TCHAN)  { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TTUPLE) {
 		// Sum element sizes. Mirrors C cgen which uses raw type
 		// sizes; padding to 8 happens inside slotsize for primitives,
@@ -10650,7 +10668,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 		// slot+8 picking up the callee's stale-DX. Bypassing primsize's
 		// `> 0` guard keeps the rest of the prim-pad-to-8 contract intact.
 		if (streq(nm, "void")) { return 0; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) {
 			// Pad to 8 for stack slots — matches C cgen which spills
@@ -10691,7 +10709,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 				// explicit case a `[N]str` would slot 8B/elem,
 				// collapsing the per-element stride and losing
 				// every .len half.
-				if (streq(en, "str")) { esz = 16; };
+				if (streq(en, "str")) { esz = primtypesize("str"): i32; };
 				let ps: i32 = primsize(en);
 				if (esz == 8) { if (ps > 0) { esz = ps; }
 				else {
@@ -10749,7 +10767,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 	if (k == nkind.N_TTAGGED){ return slotsize(c, tnode); };
 	if (k == nkind.N_TNAME) {
 		let nm: str = tnode.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 		let si: *structinfo = structlookup(c, nm);
@@ -10776,7 +10794,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 		return 8;
 	};
 	if (k == nkind.N_TPTR)   { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TARRAY) {
 		// Same shape as slotsize's TARRAY branch.
 		let lenn: *node = tnode.rhs;
@@ -19139,7 +19157,7 @@ fn cglet(c: *cgen, n: *node) void = {
 			if (elemn != nil) {
 				if (elemn.kind == nkind.N_TNAME) {
 					if (streq(elemn.str, "str")) {
-						esz = 16;
+						esz = primtypesize("str"): i32;
 						isstrel = true;
 					} else {
 						let ps: i32 = primsize(elemn.str);
@@ -19624,8 +19642,8 @@ fn cgmlet(c: *cgen, n: *node) void = {
 			if (s0_is_str != s1_is_str) {
 				let sz0: i32 = 8;
 				let sz1: i32 = 8;
-				if (s0_is_str) { sz0 = 16; };
-				if (s1_is_str) { sz1 = 16; };
+				if (s0_is_str) { sz0 = primtypesize("str"): i32; };
+				if (s1_is_str) { sz1 = primtypesize("str"): i32; };
 				let off0: i32 = localadd(c, l0.str, sz0, t0);
 				let off1: i32 = localadd(c, l1.str, sz1, t1);
 				if (s0_is_str) {
@@ -19682,7 +19700,7 @@ fn paramfieldsize(t: *node) i32 = {
 	if (k == nkind.N_TCHAN)  { return 8; };
 	if (k == nkind.N_TNAME)  {
 		let nm: str = t.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
@@ -20054,7 +20072,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 			if (p.op == tkind.TK_ELLIPSIS) {
 				let tn: *node = slicewrap(c, p.lhs);
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -20078,7 +20096,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// at the ABI boundary (the call site synthesises a
 					// 24B descriptor and pushes ptr/len/cap), so this
 					// mirrors the slice branch at cgendecl.ww:518.
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
@@ -20178,7 +20196,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 				};};
 			} else { if (isslicetype(c, p.lhs)) {
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -20202,7 +20220,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// 440-469. Caller's pushargsrev greedy-fills the
 					// remaining argregs (ptr,len,cap order), the tail
 					// spills to +16+stkcursor*8(BP).
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
@@ -21426,7 +21444,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
-		if (t.kind == nkind.N_TSLICE) { return 24; };
+		if (t.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 		if (t.kind == nkind.N_TARRAY) {
 			let lenn: *node = t.rhs;
 			let elemn: *node = t.lhs;
@@ -21448,7 +21466,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 		if (letscalarprim(nm)) { return 8; };
 		let fsz: i32 = letfloatprim(nm);
 		if (fsz > 0) { return fsz; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let si: *structinfo = structlookup(c, nm);
 		if (si != nil) { return si.totsize; };
 		let next: *node = aliaslookup(c, nm);
@@ -21774,7 +21792,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
-				if (sz == 16 && !issg) {
+				if (sz == primtypesize("str"): i32 && !issg) {
 					let r: *node = d.rhs;
 					for (r != nil) {
 						if (r.kind != nkind.N_CAST) { break; };
@@ -21830,7 +21848,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 							emitsymname(c, nm);
 							emitline("(SB),\"");
 							let i: i32 = 0;
-							for (i < 16) {
+							let szstr: i32 = primtypesize("str"): i32;
+							for (i < szstr) {
 								emitdatawbyte(0u8);
 								i += 1;
 							};
@@ -21838,7 +21857,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						};
 					};
 				};
-				if (sz == 24 && !issg) {
+				if (sz == tyslicesize(): i32 && !issg) {
 					// Slice: zero-init only (no slice-literal
 					// syntax to honour). Any rhs other than
 					// `nil` is skipped → undefined symbol at
@@ -21860,7 +21879,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitsymname(c, nm);
 						emitline("(SB),\"");
 						let i: i32 = 0;
-						for (i < 24) {
+						let szsl: i32 = tyslicesize(): i32;
+						for (i < szsl) {
 							emitdatawbyte(0u8);
 							i += 1;
 						};
diff --git a/selfhost/cmd/wcc/cgen.ww b/selfhost/cmd/wcc/cgen.ww
index 72b95367..8838493a 100644
--- a/selfhost/cmd/wcc/cgen.ww
+++ b/selfhost/cmd/wcc/cgen.ww
@@ -965,7 +965,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
-		if (t.kind == nkind.N_TSLICE) { return 24; };
+		if (t.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 		if (t.kind == nkind.N_TARRAY) {
 			let lenn: *node = t.rhs;
 			let elemn: *node = t.lhs;
@@ -987,7 +987,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 		if (letscalarprim(nm)) { return 8; };
 		let fsz: i32 = letfloatprim(nm);
 		if (fsz > 0) { return fsz; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let si: *structinfo = structlookup(c, nm);
 		if (si != nil) { return si.totsize; };
 		let next: *node = aliaslookup(c, nm);
@@ -1313,7 +1313,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
-				if (sz == 16 && !issg) {
+				if (sz == primtypesize("str"): i32 && !issg) {
 					let r: *node = d.rhs;
 					for (r != nil) {
 						if (r.kind != nkind.N_CAST) { break; };
@@ -1369,7 +1369,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 							emitsymname(c, nm);
 							emitline("(SB),\"");
 							let i: i32 = 0;
-							for (i < 16) {
+							let szstr: i32 = primtypesize("str"): i32;
+							for (i < szstr) {
 								emitdatawbyte(0u8);
 								i += 1;
 							};
@@ -1377,7 +1378,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						};
 					};
 				};
-				if (sz == 24 && !issg) {
+				if (sz == tyslicesize(): i32 && !issg) {
 					// Slice: zero-init only (no slice-literal
 					// syntax to honour). Any rhs other than
 					// `nil` is skipped → undefined symbol at
@@ -1399,7 +1400,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitsymname(c, nm);
 						emitline("(SB),\"");
 						let i: i32 = 0;
-						for (i < 24) {
+						let szsl: i32 = tyslicesize(): i32;
+						for (i < szsl) {
 							emitdatawbyte(0u8);
 							i += 1;
 						};
diff --git a/selfhost/cmd/wcc/cgendecl.ww b/selfhost/cmd/wcc/cgendecl.ww
index 9c9cca49..84a25d83 100644
--- a/selfhost/cmd/wcc/cgendecl.ww
+++ b/selfhost/cmd/wcc/cgendecl.ww
@@ -46,7 +46,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 			if (p.op == tkind.TK_ELLIPSIS) {
 				let tn: *node = slicewrap(c, p.lhs);
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -70,7 +70,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// at the ABI boundary (the call site synthesises a
 					// 24B descriptor and pushes ptr/len/cap), so this
 					// mirrors the slice branch at cgendecl.ww:518.
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
@@ -170,7 +170,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 				};};
 			} else { if (isslicetype(c, p.lhs)) {
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -194,7 +194,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// 440-469. Caller's pushargsrev greedy-fills the
 					// remaining argregs (ptr,len,cap order), the tail
 					// spills to +16+stkcursor*8(BP).
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
diff --git a/selfhost/cmd/wcc/cgenstmt.ww b/selfhost/cmd/wcc/cgenstmt.ww
index b248bd75..7f53443b 100644
--- a/selfhost/cmd/wcc/cgenstmt.ww
+++ b/selfhost/cmd/wcc/cgenstmt.ww
@@ -759,7 +759,7 @@ fn cglet(c: *cgen, n: *node) void = {
 			if (elemn != nil) {
 				if (elemn.kind == nkind.N_TNAME) {
 					if (streq(elemn.str, "str")) {
-						esz = 16;
+						esz = primtypesize("str"): i32;
 						isstrel = true;
 					} else {
 						let ps: i32 = primsize(elemn.str);
@@ -1244,8 +1244,8 @@ fn cgmlet(c: *cgen, n: *node) void = {
 			if (s0_is_str != s1_is_str) {
 				let sz0: i32 = 8;
 				let sz1: i32 = 8;
-				if (s0_is_str) { sz0 = 16; };
-				if (s1_is_str) { sz1 = 16; };
+				if (s0_is_str) { sz0 = primtypesize("str"): i32; };
+				if (s1_is_str) { sz1 = primtypesize("str"): i32; };
 				let off0: i32 = localadd(c, l0.str, sz0, t0);
 				let off1: i32 = localadd(c, l1.str, sz1, t1);
 				if (s0_is_str) {
@@ -1302,7 +1302,7 @@ fn paramfieldsize(t: *node) i32 = {
 	if (k == nkind.N_TCHAN)  { return 8; };
 	if (k == nkind.N_TNAME)  {
 		let nm: str = t.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
diff --git a/selfhost/cmd/wcc/cgenutil.ww b/selfhost/cmd/wcc/cgenutil.ww
index 11977d75..1e6005bf 100644
--- a/selfhost/cmd/wcc/cgenutil.ww
+++ b/selfhost/cmd/wcc/cgenutil.ww
@@ -1196,7 +1196,7 @@ fn indexbaseesz(c: *cgen, base: *node) i32 = {
 				let elem: *node = ft.lhs;
 				if (elem != nil) {
 					if (elem.kind == nkind.N_TNAME) {
-						if (streq(elem.str, "str")) { return 16; };
+						if (streq(elem.str, "str")) { return primtypesize("str"): i32; };
 						let ps: i32 = primsize(elem.str);
 						if (ps > 0) { return ps; };
 						// Pointer to named struct: indexing
@@ -1304,11 +1304,11 @@ fn elemsizeof(t: *node) i32 = {
 	// `*[]T`: stride is the slice header (24B). Hare-faithful — a
 	// pointer-to-slice is a 1D array of slices, not of T. Mirrors the
 	// cstage check.c default `*U → U` path for U=[]T (slice element).
-	if (elem.kind == nkind.N_TSLICE) { return 24; };
+	if (elem.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (elem.kind == nkind.N_TNAME) {
 		let nm: str = elem.str;
 		// str element is 16B (ptr+len). primsize returns 0 for it.
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
@@ -1903,7 +1903,7 @@ export fn letslotsize(c: *cgen, n: *node) i32 = {
 								// (ptr+len) — primsize returns 0 for
 								// it, so it'd slot 8B without this.
 								if (streq(elemn.str, "str")) {
-									esz = 16;
+									esz = primtypesize("str"): i32;
 								} else {
 									let ps: i32 = primsize(elemn.str);
 									if (ps > 0) { esz = ps; };
@@ -1952,7 +1952,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 	if (k == nkind.N_TPTR)   { return 8; };
 	if (k == nkind.N_TFN)    { return 8; };
 	if (k == nkind.N_TCHAN)  { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TTUPLE) {
 		// Sum element sizes. Mirrors C cgen which uses raw type
 		// sizes; padding to 8 happens inside slotsize for primitives,
@@ -1992,7 +1992,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 		// slot+8 picking up the callee's stale-DX. Bypassing primsize's
 		// `> 0` guard keeps the rest of the prim-pad-to-8 contract intact.
 		if (streq(nm, "void")) { return 0; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) {
 			// Pad to 8 for stack slots — matches C cgen which spills
@@ -2033,7 +2033,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 				// explicit case a `[N]str` would slot 8B/elem,
 				// collapsing the per-element stride and losing
 				// every .len half.
-				if (streq(en, "str")) { esz = 16; };
+				if (streq(en, "str")) { esz = primtypesize("str"): i32; };
 				let ps: i32 = primsize(en);
 				if (esz == 8) { if (ps > 0) { esz = ps; }
 				else {
@@ -2091,7 +2091,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 	if (k == nkind.N_TTAGGED){ return slotsize(c, tnode); };
 	if (k == nkind.N_TNAME) {
 		let nm: str = tnode.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 		let si: *structinfo = structlookup(c, nm);
@@ -2118,7 +2118,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 		return 8;
 	};
 	if (k == nkind.N_TPTR)   { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TARRAY) {
 		// Same shape as slotsize's TARRAY branch.
 		let lenn: *node = tnode.rhs;
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index d8dae28a..2700c151 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -618,6 +618,29 @@ fn mktname(c: *checker, nm: str) *node = {
 	return n;
 };
 
+// #43: SSoT for primitive type byte sizes. astsize's N_TNAME-primitive
+// arm and every wwstage cgen size walker (slotsize/fieldsize/letemit-
+// size/elemsizeof/paramfieldsize) consult this table so a future
+// ty_str.size bump (#1) lands in one place. Returns -1 for non-prim
+// names; callers fall back to alias/struct/enum lookup. Cstage's
+// equivalent SSoT is cmd/wcc/type.c:46-79 (ty_void/ty_bool/.../ty_str).
+fn primtypesize(nm: str) i64 = {
+	if (streq(nm, "void")) { return 0i64; };
+	if (streq(nm, "bool")) { return 1i64; };
+	if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
+	if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
+	if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
+	if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64")) { return 8i64; };
+	if (streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
+	if (streq(nm, "str")) { return 16i64; };
+	return -1i64;
+};
+
+// #43: SSoT for slice header size (ptr+len+cap = 24B today). Mirrors
+// cstage cmd/wcc/type.c:103 (ty_slice->size = 24). Bumping a slice's
+// header layout in #34 touches only this constant.
+fn tyslicesize() i64 = { return 24i64; };
+
 // #42: AST-level layout helpers for the size(T)/align(T)/offset(e.f)
 // fold. Mirror cstage resolve_type's size/align computation
 // (cmd/wcc/check.c:286-528) on AST nodes — wwstage check.ww never
@@ -679,7 +702,7 @@ fn astsize(c: *checker, t: *node) i64 = {
 	let k: nkind = t.kind;
 	if (k == nkind.N_TBANG) { return astsize(c, t.lhs); };
 	if (k == nkind.N_TPTR)   { return 8i64; };
-	if (k == nkind.N_TSLICE) { return 24i64; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(); };
 	if (k == nkind.N_TCHAN)  { return 8i64; };
 	if (k == nkind.N_TFN)    { return 8i64; };
 	if (k == nkind.N_TARRAY) {
@@ -731,13 +754,8 @@ fn astsize(c: *checker, t: *node) i64 = {
 	};
 	if (k == nkind.N_TNAME) {
 		let nm: str = t.str;
-		if (streq(nm, "void")) { return 0i64; };
-		if (streq(nm, "bool")) { return 1i64; };
-		if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
-		if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
-		if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
-		if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64") || streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
-		if (streq(nm, "str")) { return 16i64; };
+		let ps: i64 = primtypesize(nm);
+		if (ps >= 0i64) { return ps; };
 		let resolved: *node = resolvealias(c, t);
 		if (resolved != nil && resolved != t) {
 			return astsize(c, resolved);
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 5b700f74..b7bd0c72 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -1996,7 +1996,7 @@ export fn freeall(s: []str) void = {
 		i += 1;
 	};
 	if (s.cap > 0) {
-		os.free(s.ptr: *void, (s.cap: u64) * 16u64);
+		os.free(s.ptr: *void, (s.cap: u64) * size(str): u64);
 	};
 };
 
@@ -2599,7 +2599,7 @@ export fn remaining_tokens(s: *tokenizer) str = {
 fn appendstr(slice: *[]str, item: str) void = {
 	let newlen: i32 = slice.len + 1;
 	slice.len = newlen;
-	rtensure(slice: *void, 16u64);
+	rtensure(slice: *void, size(str): u64);
 	let dst: *str = &slice.ptr[newlen - 1];
 	dst.ptr = item.ptr;
 	dst.len = item.len;
@@ -7612,6 +7612,29 @@ fn mktname(c: *checker, nm: str) *node = {
 	return n;
 };
 
+// #43: SSoT for primitive type byte sizes. astsize's N_TNAME-primitive
+// arm and every wwstage cgen size walker (slotsize/fieldsize/letemit-
+// size/elemsizeof/paramfieldsize) consult this table so a future
+// ty_str.size bump (#1) lands in one place. Returns -1 for non-prim
+// names; callers fall back to alias/struct/enum lookup. Cstage's
+// equivalent SSoT is cmd/wcc/type.c:46-79 (ty_void/ty_bool/.../ty_str).
+fn primtypesize(nm: str) i64 = {
+	if (streq(nm, "void")) { return 0i64; };
+	if (streq(nm, "bool")) { return 1i64; };
+	if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
+	if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
+	if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
+	if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64")) { return 8i64; };
+	if (streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
+	if (streq(nm, "str")) { return 16i64; };
+	return -1i64;
+};
+
+// #43: SSoT for slice header size (ptr+len+cap = 24B today). Mirrors
+// cstage cmd/wcc/type.c:103 (ty_slice->size = 24). Bumping a slice's
+// header layout in #34 touches only this constant.
+fn tyslicesize() i64 = { return 24i64; };
+
 // #42: AST-level layout helpers for the size(T)/align(T)/offset(e.f)
 // fold. Mirror cstage resolve_type's size/align computation
 // (cmd/wcc/check.c:286-528) on AST nodes — wwstage check.ww never
@@ -7673,7 +7696,7 @@ fn astsize(c: *checker, t: *node) i64 = {
 	let k: nkind = t.kind;
 	if (k == nkind.N_TBANG) { return astsize(c, t.lhs); };
 	if (k == nkind.N_TPTR)   { return 8i64; };
-	if (k == nkind.N_TSLICE) { return 24i64; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(); };
 	if (k == nkind.N_TCHAN)  { return 8i64; };
 	if (k == nkind.N_TFN)    { return 8i64; };
 	if (k == nkind.N_TARRAY) {
@@ -7725,13 +7748,8 @@ fn astsize(c: *checker, t: *node) i64 = {
 	};
 	if (k == nkind.N_TNAME) {
 		let nm: str = t.str;
-		if (streq(nm, "void")) { return 0i64; };
-		if (streq(nm, "bool")) { return 1i64; };
-		if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
-		if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
-		if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
-		if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64") || streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
-		if (streq(nm, "str")) { return 16i64; };
+		let ps: i64 = primtypesize(nm);
+		if (ps >= 0i64) { return ps; };
 		let resolved: *node = resolvealias(c, t);
 		if (resolved != nil && resolved != t) {
 			return astsize(c, resolved);
@@ -9854,7 +9872,7 @@ fn indexbaseesz(c: *cgen, base: *node) i32 = {
 				let elem: *node = ft.lhs;
 				if (elem != nil) {
 					if (elem.kind == nkind.N_TNAME) {
-						if (streq(elem.str, "str")) { return 16; };
+						if (streq(elem.str, "str")) { return primtypesize("str"): i32; };
 						let ps: i32 = primsize(elem.str);
 						if (ps > 0) { return ps; };
 						// Pointer to named struct: indexing
@@ -9962,11 +9980,11 @@ fn elemsizeof(t: *node) i32 = {
 	// `*[]T`: stride is the slice header (24B). Hare-faithful — a
 	// pointer-to-slice is a 1D array of slices, not of T. Mirrors the
 	// cstage check.c default `*U → U` path for U=[]T (slice element).
-	if (elem.kind == nkind.N_TSLICE) { return 24; };
+	if (elem.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (elem.kind == nkind.N_TNAME) {
 		let nm: str = elem.str;
 		// str element is 16B (ptr+len). primsize returns 0 for it.
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
@@ -10561,7 +10579,7 @@ export fn letslotsize(c: *cgen, n: *node) i32 = {
 								// (ptr+len) — primsize returns 0 for
 								// it, so it'd slot 8B without this.
 								if (streq(elemn.str, "str")) {
-									esz = 16;
+									esz = primtypesize("str"): i32;
 								} else {
 									let ps: i32 = primsize(elemn.str);
 									if (ps > 0) { esz = ps; };
@@ -10610,7 +10628,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 	if (k == nkind.N_TPTR)   { return 8; };
 	if (k == nkind.N_TFN)    { return 8; };
 	if (k == nkind.N_TCHAN)  { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TTUPLE) {
 		// Sum element sizes. Mirrors C cgen which uses raw type
 		// sizes; padding to 8 happens inside slotsize for primitives,
@@ -10650,7 +10668,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 		// slot+8 picking up the callee's stale-DX. Bypassing primsize's
 		// `> 0` guard keeps the rest of the prim-pad-to-8 contract intact.
 		if (streq(nm, "void")) { return 0; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) {
 			// Pad to 8 for stack slots — matches C cgen which spills
@@ -10691,7 +10709,7 @@ fn slotsize(c: *cgen, typn: *node) i32 = {
 				// explicit case a `[N]str` would slot 8B/elem,
 				// collapsing the per-element stride and losing
 				// every .len half.
-				if (streq(en, "str")) { esz = 16; };
+				if (streq(en, "str")) { esz = primtypesize("str"): i32; };
 				let ps: i32 = primsize(en);
 				if (esz == 8) { if (ps > 0) { esz = ps; }
 				else {
@@ -10749,7 +10767,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 	if (k == nkind.N_TTAGGED){ return slotsize(c, tnode); };
 	if (k == nkind.N_TNAME) {
 		let nm: str = tnode.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 		let si: *structinfo = structlookup(c, nm);
@@ -10776,7 +10794,7 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 		return 8;
 	};
 	if (k == nkind.N_TPTR)   { return 8; };
-	if (k == nkind.N_TSLICE) { return 24; };
+	if (k == nkind.N_TSLICE) { return tyslicesize(): i32; };
 	if (k == nkind.N_TARRAY) {
 		// Same shape as slotsize's TARRAY branch.
 		let lenn: *node = tnode.rhs;
@@ -19139,7 +19157,7 @@ fn cglet(c: *cgen, n: *node) void = {
 			if (elemn != nil) {
 				if (elemn.kind == nkind.N_TNAME) {
 					if (streq(elemn.str, "str")) {
-						esz = 16;
+						esz = primtypesize("str"): i32;
 						isstrel = true;
 					} else {
 						let ps: i32 = primsize(elemn.str);
@@ -19624,8 +19642,8 @@ fn cgmlet(c: *cgen, n: *node) void = {
 			if (s0_is_str != s1_is_str) {
 				let sz0: i32 = 8;
 				let sz1: i32 = 8;
-				if (s0_is_str) { sz0 = 16; };
-				if (s1_is_str) { sz1 = 16; };
+				if (s0_is_str) { sz0 = primtypesize("str"): i32; };
+				if (s1_is_str) { sz1 = primtypesize("str"): i32; };
 				let off0: i32 = localadd(c, l0.str, sz0, t0);
 				let off1: i32 = localadd(c, l1.str, sz1, t1);
 				if (s0_is_str) {
@@ -19682,7 +19700,7 @@ fn paramfieldsize(t: *node) i32 = {
 	if (k == nkind.N_TCHAN)  { return 8; };
 	if (k == nkind.N_TNAME)  {
 		let nm: str = t.str;
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let ps: i32 = primsize(nm);
 		if (ps > 0) { return ps; };
 	};
@@ -20054,7 +20072,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 			if (p.op == tkind.TK_ELLIPSIS) {
 				let tn: *node = slicewrap(c, p.lhs);
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -20078,7 +20096,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// at the ABI boundary (the call site synthesises a
 					// 24B descriptor and pushes ptr/len/cap), so this
 					// mirrors the slice branch at cgendecl.ww:518.
-					let off: i32 = localadd(c, nm, 24, tn);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, tn);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
@@ -20178,7 +20196,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 				};};
 			} else { if (isslicetype(c, p.lhs)) {
 				if (idx + 3 <= 6) {
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					emitline("\tMOVQ\t");
 					emitline(argregname(idx));
 					emitline(", ");
@@ -20202,7 +20220,7 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					// 440-469. Caller's pushargsrev greedy-fills the
 					// remaining argregs (ptr,len,cap order), the tail
 					// spills to +16+stkcursor*8(BP).
-					let off: i32 = localadd(c, nm, 24, p.lhs);
+					let off: i32 = localadd(c, nm, tyslicesize(): i32, p.lhs);
 					let regs_left: i32 = 6 - idx;
 					let w: i32 = 0;
 					for (w < regs_left) {
@@ -21426,7 +21444,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
-		if (t.kind == nkind.N_TSLICE) { return 24; };
+		if (t.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 		if (t.kind == nkind.N_TARRAY) {
 			let lenn: *node = t.rhs;
 			let elemn: *node = t.lhs;
@@ -21448,7 +21466,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 		if (letscalarprim(nm)) { return 8; };
 		let fsz: i32 = letfloatprim(nm);
 		if (fsz > 0) { return fsz; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let si: *structinfo = structlookup(c, nm);
 		if (si != nil) { return si.totsize; };
 		let next: *node = aliaslookup(c, nm);
@@ -21774,7 +21792,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
-				if (sz == 16 && !issg) {
+				if (sz == primtypesize("str"): i32 && !issg) {
 					let r: *node = d.rhs;
 					for (r != nil) {
 						if (r.kind != nkind.N_CAST) { break; };
@@ -21830,7 +21848,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 							emitsymname(c, nm);
 							emitline("(SB),\"");
 							let i: i32 = 0;
-							for (i < 16) {
+							let szstr: i32 = primtypesize("str"): i32;
+							for (i < szstr) {
 								emitdatawbyte(0u8);
 								i += 1;
 							};
@@ -21838,7 +21857,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						};
 					};
 				};
-				if (sz == 24 && !issg) {
+				if (sz == tyslicesize(): i32 && !issg) {
 					// Slice: zero-init only (no slice-literal
 					// syntax to honour). Any rhs other than
 					// `nil` is skipped → undefined symbol at
@@ -21860,7 +21879,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitsymname(c, nm);
 						emitline("(SB),\"");
 						let i: i32 = 0;
-						for (i < 24) {
+						let szsl: i32 = tyslicesize(): i32;
+						for (i < szsl) {
 							emitdatawbyte(0u8);
 							i += 1;
 						};
diff --git a/selfhost/test/smoke.combined.ww b/selfhost/test/smoke.combined.ww
index 40191000..404336e2 100644
--- a/selfhost/test/smoke.combined.ww
+++ b/selfhost/test/smoke.combined.ww
@@ -1887,7 +1887,7 @@ export fn freeall(s: []str) void = {
 		i += 1;
 	};
 	if (s.cap > 0) {
-		os.free(s.ptr: *void, (s.cap: u64) * 16u64);
+		os.free(s.ptr: *void, (s.cap: u64) * size(str): u64);
 	};
 };
 
@@ -2490,7 +2490,7 @@ export fn remaining_tokens(s: *tokenizer) str = {
 fn appendstr(slice: *[]str, item: str) void = {
 	let newlen: i32 = slice.len + 1;
 	slice.len = newlen;
-	rtensure(slice: *void, 16u64);
+	rtensure(slice: *void, size(str): u64);
 	let dst: *str = &slice.ptr[newlen - 1];
 	dst.ptr = item.ptr;
 	dst.len = item.len;