diff --git a/Makefile b/Makefile
index cf551678..23b56280 100644
--- a/Makefile
+++ b/Makefile
@@ -335,6 +335,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_tuprecv_f64_run \
 	  $(BIN)/test_floats_run \
 	  $(BIN)/test_size_type_run \
+	  $(BIN)/test_types_sizelim_run \
 	  $(BIN)/test_bufio_run $(BIN)/test_random_run
 
 $(BIN)/test_smoke: test/wcc/000_smoke.c $(LIB)/libwcc.a | $(BIN)
@@ -1086,6 +1087,10 @@ $(BIN)/test_size_type_run: test/wcc/957_size_type_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_types_sizelim_run: test/wcc/958_types_sizelim_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_f64crossmod_run: test/wcc/953_f64crossmod_run.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
diff --git a/lib/types/types.ww b/lib/types/types.ww
index daa16f01..b38caf87 100644
--- a/lib/types/types.ww
+++ b/lib/types/types.ww
@@ -24,4 +24,12 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
diff --git a/selfhost/cmd/w6a/main.combined.ww b/selfhost/cmd/w6a/main.combined.ww
index b899ee9b..e340f5f0 100644
--- a/selfhost/cmd/w6a/main.combined.ww
+++ b/selfhost/cmd/w6a/main.combined.ww
@@ -776,6 +776,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index 5a812483..e82cebad 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -776,6 +776,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/selfhost/cmd/w6l/main.combined.ww b/selfhost/cmd/w6l/main.combined.ww
index 90daa61d..ec497b7d 100644
--- a/selfhost/cmd/w6l/main.combined.ww
+++ b/selfhost/cmd/w6l/main.combined.ww
@@ -885,6 +885,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/selfhost/cmd/ww/main.combined.ww b/selfhost/cmd/ww/main.combined.ww
index a43bb127..259f141e 100644
--- a/selfhost/cmd/ww/main.combined.ww
+++ b/selfhost/cmd/ww/main.combined.ww
@@ -776,6 +776,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 68ab61df..7e78d5f2 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -753,6 +753,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/selfhost/test/smoke.combined.ww b/selfhost/test/smoke.combined.ww
index 2054f01b..731d0df6 100644
--- a/selfhost/test/smoke.combined.ww
+++ b/selfhost/test/smoke.combined.ww
@@ -753,6 +753,14 @@ def U16_MIN: u16 = 0;
 def U32_MIN: u32 = 0;
 def U64_MIN: u64 = 0;
 
+// size is 8B on amd64; no cast needed (size ∈ unsigned class per #113).
+def SIZE_MIN: size = U64_MIN;
+def SIZE_MAX: size = U64_MAX;
+
+// uintptr not in the unsigned class, so the cast is required (Hare's form).
+def UINTPTR_MIN: uintptr = U64_MIN: uintptr;
+def UINTPTR_MAX: uintptr = U64_MAX: uintptr;
+
 def RUNE_MIN: rune = '\0';
 
 // bytes — slice operations over []u8. Mirrors Hare's bytes module
diff --git a/test/wcc/958_types_sizelim_run.c b/test/wcc/958_types_sizelim_run.c
new file mode 100644
index 00000000..7b1593c2
--- /dev/null
+++ b/test/wcc/958_types_sizelim_run.c
@@ -0,0 +1,146 @@
+/*
+ * 958_types_sizelim_run — runtime proof of the lib/types SIZE_* and
+ * UINTPTR_* limit constants (faithful port of ref/hare/types/arch+
+ * x86_64.ha:16-26). Asserts both the values (MIN==0, MAX==U64_MAX) and
+ * that the consts are usable as their declared type in arithmetic and an
+ * i32 cast.
+ *
+ * SIZE_MAX is the no-cast def `def SIZE_MAX: size = U64_MAX;` — size is in
+ * the unsigned class and 8B-wide on amd64, so the u64→size init coerces
+ * without a cast (#113). UINTPTR_MAX keeps Hare's explicit cast
+ * (`U64_MAX: uintptr`) because uintptr is outside the unsigned class. A
+ * green row IS the proof both forms compiled and carry the right value.
+ *
+ * INT_MIN/MAX + UINT_MIN/MAX are deferred to #114 (ww int=8B vs Hare's 4B
+ * on amd64 makes the value question open); RUNE_MAX is deferred to #112
+ * (no \U rune-escape lexer yet). Only the 4 SIZE/UINTPTR consts land here.
+ *
+ * Table-driven like 957_size_type_run / 952_floats_run: each row is a
+ * self-contained ww program; the cstage `ww build -I lib` compiles it, we
+ * run the binary and assert the exit code. cstage-only by design (mirrors
+ * 951/952/957): per-program wwstage byte-id is the 990-997 gates' job.
+ */
+#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 *src; int want_exit; };
+
+static const struct row rows[] = {
+	/* size limits: SIZE_MIN is zero; SIZE_MAX equals U64_MAX both by
+	 * direct compare and by the all-ones wrap (MAX + 1 == 0 on 8B). */
+	{ "package main;\n"
+	  "import types;\n"
+	  "export fn main() i32 = {\n"
+	  "	let lo: size = types.SIZE_MIN;\n"
+	  "	if (lo != 0) { return 1; };\n"
+	  "	let hi: size = types.SIZE_MAX;\n"
+	  "	if (hi != (types.U64_MAX): size) { return 2; };\n"
+	  "	if (hi + 1 != 0) { return 3; };\n"
+	  "	return 0;\n"
+	  "};\n", 0 },
+	/* SIZE_MAX usable in size arithmetic, propagated through an i32
+	 * cast: (2^64-1) + 43 wraps to 42. */
+	{ "package main;\n"
+	  "import types;\n"
+	  "export fn main() i32 = {\n"
+	  "	let hi: size = types.SIZE_MAX;\n"
+	  "	let v: size = hi + 43;\n"
+	  "	return v: i32;\n"
+	  "};\n", 42 },
+	/* uintptr limits: UINTPTR_MIN is zero; UINTPTR_MAX equals U64_MAX
+	 * (Hare's explicit-cast def) both by compare and by the wrap. */
+	{ "package main;\n"
+	  "import types;\n"
+	  "export fn main() i32 = {\n"
+	  "	let lo: uintptr = types.UINTPTR_MIN;\n"
+	  "	if (lo != 0) { return 1; };\n"
+	  "	let hi: uintptr = types.UINTPTR_MAX;\n"
+	  "	if (hi != (types.U64_MAX): uintptr) { return 2; };\n"
+	  "	if (hi + 1 != 0) { return 3; };\n"
+	  "	return 0;\n"
+	  "};\n", 0 },
+	/* UINTPTR_MAX usable in uintptr arithmetic, propagated through an
+	 * i32 cast: (2^64-1) + 43 wraps to 42. */
+	{ "package main;\n"
+	  "import types;\n"
+	  "export fn main() i32 = {\n"
+	  "	let hi: uintptr = types.UINTPTR_MAX;\n"
+	  "	let v: uintptr = hi + 43;\n"
+	  "	return v: i32;\n"
+	  "};\n", 42 },
+	{ NULL, 0 }
+};
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+	char absbin[1024];
+	if (bin[0] != '/') {
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	int n = 0, fail = 0;
+	for (int i = 0; rows[i].src; i++, n++) {
+		char src[64];
+		snprintf(src, sizeof src, "/tmp/wwslim_%d_%d.ww", getpid(), i);
+
+		FILE *f = fopen(src, "wb");
+		if (f == NULL) { fail++; continue; }
+		fputs(rows[i].src, f);
+		fclose(f);
+
+		char tmpdir[64];
+		snprintf(tmpdir, sizeof tmpdir, "/tmp/wwslim_%d_d_%d", getpid(), i);
+		mkdir(tmpdir, 0755);
+
+		char cmd[2048];
+		snprintf(cmd, sizeof cmd, "cd %s && %s/ww build -I %s/lib %s",
+		    tmpdir, bin, cwd, src);
+		if (runwait(cmd) != 0) {
+			fprintf(stderr, "row %d: build failed\n  src: %s\n",
+			    i, rows[i].src);
+			fail++;
+			unlink(src); rmdir(tmpdir);
+			continue;
+		}
+
+		char outbin[128];
+		const char *base = strrchr(src, '/');
+		base = base ? base + 1 : src;
+		snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+		char *dot = strrchr(outbin, '.');
+		if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+
+		int got = runwait(outbin);
+		if (got != rows[i].want_exit) {
+			fprintf(stderr, "row %d: exit %d, want %d\n  src: %s\n",
+			    i, got, rows[i].want_exit, rows[i].src);
+			fail++;
+		}
+		unlink(src); unlink(outbin); rmdir(tmpdir);
+	}
+	if (fail) {
+		fprintf(stderr, "%d/%d types_sizelim tests failed\n", fail, n);
+		return 1;
+	}
+	printf("types_sizelim: %d/%d ok\n", n, n);
+	return 0;
+}