diff --git a/Makefile b/Makefile
index 40717e95..879f418a 100644
--- a/Makefile
+++ b/Makefile
@@ -247,6 +247,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_strslice_elem \
 	  $(BIN)/test_arr_tagged_elem \
 	  $(BIN)/test_dup_main_reject \
+	  $(BIN)/test_size_untyped_int \
 	  $(BIN)/test_tagged_staticinit \
 	  $(BIN)/test_arr_infer_len \
 	  $(BIN)/test_arr_cap_reject \
@@ -1445,6 +1446,12 @@ $(BIN)/test_tagged_staticinit: test/wcc/843_tagged_staticinit.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_size_untyped_int: test/wcc/844_size_untyped_int.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_slice_str_global_zero: test/wcc/686_slice_str_global_zero.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index 8d2eee96..02b6fc5c 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -13551,11 +13551,10 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 					};
 						// Post-fold the node IS an N_INTLIT-shaped
 						// untyped_int constant. Mirrors cstage
-						// cmd/wcc/check.c:926/958 which stamps
-						// ty_untyped_int after the fold. The return
-						// tnode mktname("i32") is the assignability
-						// target for callers, not the constant's
-						// own type.
+						// cmd/wcc/check.c:1570/1602 which stamps
+						// ty_untyped_int after the fold AND returns it
+						// to the caller (the Hare-correct type), so a
+						// `let x: size = size(T)` binding is assignable.
 						let utn: *node = mktname(c, "untyped_int");
 						if (issize) {
 							// #108(b): rule-10 twin of the cstage
@@ -13566,7 +13565,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astsize(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						if (isalign) {
 							if (astunsized(c, e.list)) {
@@ -13575,7 +13574,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astalign(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						// offset(e.f): the arg is a value expression
 						// (N_DOT), parsed via parsearglist — not a
@@ -13592,7 +13591,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 								};
 								foldtointlit(c, e, off);
 								e.type_ = tinfofornode(c, utn): *void;
-								return mktname(c, "i32");
+								return mktname(c, "untyped_int");
 							};
 						};
 				};
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index 40e57452..e422cba9 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -3081,11 +3081,10 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 					};
 						// Post-fold the node IS an N_INTLIT-shaped
 						// untyped_int constant. Mirrors cstage
-						// cmd/wcc/check.c:926/958 which stamps
-						// ty_untyped_int after the fold. The return
-						// tnode mktname("i32") is the assignability
-						// target for callers, not the constant's
-						// own type.
+						// cmd/wcc/check.c:1570/1602 which stamps
+						// ty_untyped_int after the fold AND returns it
+						// to the caller (the Hare-correct type), so a
+						// `let x: size = size(T)` binding is assignable.
 						let utn: *node = mktname(c, "untyped_int");
 						if (issize) {
 							// #108(b): rule-10 twin of the cstage
@@ -3096,7 +3095,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astsize(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						if (isalign) {
 							if (astunsized(c, e.list)) {
@@ -3105,7 +3104,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astalign(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						// offset(e.f): the arg is a value expression
 						// (N_DOT), parsed via parsearglist — not a
@@ -3122,7 +3121,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 								};
 								foldtointlit(c, e, off);
 								e.type_ = tinfofornode(c, utn): *void;
-								return mktname(c, "i32");
+								return mktname(c, "untyped_int");
 							};
 						};
 				};
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 0d6a2e6d..ba68f0a2 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -13551,11 +13551,10 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 					};
 						// Post-fold the node IS an N_INTLIT-shaped
 						// untyped_int constant. Mirrors cstage
-						// cmd/wcc/check.c:926/958 which stamps
-						// ty_untyped_int after the fold. The return
-						// tnode mktname("i32") is the assignability
-						// target for callers, not the constant's
-						// own type.
+						// cmd/wcc/check.c:1570/1602 which stamps
+						// ty_untyped_int after the fold AND returns it
+						// to the caller (the Hare-correct type), so a
+						// `let x: size = size(T)` binding is assignable.
 						let utn: *node = mktname(c, "untyped_int");
 						if (issize) {
 							// #108(b): rule-10 twin of the cstage
@@ -13566,7 +13565,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astsize(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						if (isalign) {
 							if (astunsized(c, e.list)) {
@@ -13575,7 +13574,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							let v: i64 = astalign(c, e.list);
 							foldtointlit(c, e, v);
 							e.type_ = tinfofornode(c, utn): *void;
-							return mktname(c, "i32");
+							return mktname(c, "untyped_int");
 						};
 						// offset(e.f): the arg is a value expression
 						// (N_DOT), parsed via parsearglist — not a
@@ -13592,7 +13591,7 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 								};
 								foldtointlit(c, e, off);
 								e.type_ = tinfofornode(c, utn): *void;
-								return mktname(c, "i32");
+								return mktname(c, "untyped_int");
 							};
 						};
 				};
diff --git a/test/wcc/844_size_untyped_int.c b/test/wcc/844_size_untyped_int.c
new file mode 100644
index 00000000..ebb5012c
--- /dev/null
+++ b/test/wcc/844_size_untyped_int.c
@@ -0,0 +1,244 @@
+/*
+ * 844_size_untyped_int — the size/align/offset builtins fold to an
+ * untyped_int constant and RETURN untyped_int to the caller (the
+ * Hare-correct type), so `let x: size = size(T)` is assignable
+ * (catB-9; 2026-06-14).
+ *
+ * The fold stamps the node `e.type_ = untyped_int` in BOTH stages
+ * (correct), but pre-fix wwstage's check.ww RETURNED `i32` to the
+ * caller while cstage returned `untyped_int` (check.c:1570/1602).
+ * untyped_int is assignable to any integer type (size, u64, ...);
+ * i32 is NOT assignable to `size`. So `let lenbytes: size =
+ * size(u64);` — the canonical Hare idiom (sha256.ww:189) — was
+ * FALSE-REJECTED by wwstage (`let: not assignable (i32 -> size)`)
+ * while cstage accepted. The fix aligns wwstage UP: check.ww:3099/
+ * 3108/3125 now return mktname("untyped_int"). cstage is unchanged.
+ *
+ * DISCRIMINATION (verified against the pre-fix master out/bin/ww_ww):
+ * p1 built rc=1 with `let: not assignable (i32 -> size)` on wwstage
+ * and rc=0 on cstage; post-fix both accept. (A live discrimination
+ * row can't run here — the post-fix binary accepts p1 — so the
+ * pre-fix reject is pinned in this comment, not as a runtime row.)
+ *
+ *   row     | shape                                  | want
+ *   --------+----------------------------------------+-----
+ *   p1      | let lenbytes: size = size(u64);         | 8
+ *   p2      | let x: u64 = size(int);                 | 8
+ *   p3      | let pad: size = size(u64) - n; (n=1)    | 7
+ *   p7_ctrl | let x = size(u64);  (UNANNOTATED)       | 8
+ *
+ * p7_ctrl is drew's de-risk control: an unannotated `let` defaults
+ * the untyped_int to int and behaves IDENTICALLY in both stages,
+ * pre AND post fix (a both-ACCEPT, run 8). It proves the return-type
+ * flip did not perturb the default-int path.
+ *
+ * BYTE-ID (rule 10): the bid program mixes a `size`-typed local with
+ * `size(u64)` in a comparison — the context where the fold's new
+ * untyped_int could converge a type and shift CMPL->CMPQ width. Post-
+ * fix cstage's w6c and wwstage's w6c_ww emit byte-identical asm (both
+ * CMPQ); the row diffs the two .s files.
+ *
+ * Table-driven, BOTH stages (cstage `ww`, wwstage `ww_ww` gated on
+ * existence), mirrors 682_arr_enum_elem / 842_dup_main_reject.
+ * Intermediates land in tmpdirs, never the source tree (rule 14).
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.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;
+}
+
+struct row { const char *label; const char *src; int want; };
+
+static const struct row rows[] = {
+	{ "p1_let_size",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let lenbytes: size = size(u64);\n"
+	  "	return lenbytes: i32;\n"
+	  "};\n", 8 },
+
+	{ "p2_let_u64",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let x: u64 = size(int);\n"
+	  "	return x: i32;\n"
+	  "};\n", 8 },
+
+	{ "p3_size_arith",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let n: size = 1;\n"
+	  "	let pad: size = size(u64) - n;\n"
+	  "	return pad: i32;\n"
+	  "};\n", 7 },
+
+	{ "p7_ctrl_unannotated",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let x = size(u64);\n"
+	  "	return x: i32;\n"
+	  "};\n", 8 },
+};
+
+/* the byte-id program: a size()-mixing comparison (the CMPL->CMPQ
+ * width context). Both stages must emit identical asm. */
+static const char *bid_src =
+	"package main;\n"
+	"export fn main() i32 = {\n"
+	"	let m: size = 16;\n"
+	"	if (m >= size(u64)) { return 1; };\n"
+	"	return 0;\n"
+	"};\n";
+
+static int
+run_driver(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/szui_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/szui_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: build via %s failed\n",
+		    r->label, driver);
+		unlink(src); rmdir(tmpdir);
+		return -1;
+	}
+
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	char outbin[128];
+	snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+	char *dot = strrchr(outbin, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+	int got = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return got;
+}
+
+static int
+asm_byte_identical(const char *bin, int i)
+{
+	char src[64], cs[64], ws[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/szui_asm_%d_%d.ww", getpid(), i);
+	snprintf(cs,  sizeof cs,  "/tmp/szui_asm_%d_%d_c.s", getpid(), i);
+	snprintf(ws,  sizeof ws,  "/tmp/szui_asm_%d_%d_w.s", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(bid_src, f);
+	fclose(f);
+
+	snprintf(cmd, sizeof cmd, "%s/w6c -o %s %s 2>/dev/null", bin, cs, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "bid: w6c errored\n");
+		unlink(src);
+		return -1;
+	}
+	snprintf(cmd, sizeof cmd, "%s/w6c_ww -o %s %s 2>/dev/null",
+	    bin, ws, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "bid: w6c_ww errored\n");
+		unlink(src); unlink(cs);
+		return -1;
+	}
+
+	FILE *fc = fopen(cs, "rb");
+	FILE *fw = fopen(ws, "rb");
+	int rc = 0;
+	if (!fc || !fw) {
+		rc = -1;
+	} else {
+		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);
+	if (rc != 0)
+		fprintf(stderr, "bid: cstage vs wwstage asm differs\n");
+	unlink(src); unlink(cs); unlink(ws);
+	return rc;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[1024], wdrv[1024];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *path; int gated; } drivers[] = {
+		{ "cstage",  cdrv, 0 },
+		{ "wwstage", wdrv, 1 },
+		{ NULL, NULL, 0 },
+	};
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int total = 0, fail = 0;
+
+	for (int d = 0; drivers[d].name; d++) {
+		if (drivers[d].gated && access(drivers[d].path, X_OK) != 0) {
+			fprintf(stderr, "size_untyped_int: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < n; i++) {
+			total++;
+			int got = run_driver(drivers[d].path, &rows[i], i);
+			if (got != rows[i].want) {
+				fprintf(stderr,
+				    "size_untyped_int[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, rows[i].label,
+				    got, rows[i].want);
+				fail++;
+			}
+		}
+	}
+
+	if (access(wdrv, X_OK) == 0) {
+		total++;
+		if (asm_byte_identical(bin, 0) != 0)
+			fail++;
+	}
+
+	if (fail) {
+		fprintf(stderr,
+		    "size_untyped_int: %d/%d fixtures failed\n", fail, total);
+		return 1;
+	}
+	printf("size_untyped_int: %d/%d ok\n", total, total);
+	return 0;
+}