diff --git a/Makefile b/Makefile
index e89b182e..c69f8f5c 100644
--- a/Makefile
+++ b/Makefile
@@ -249,6 +249,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_dup_main_reject \
 	  $(BIN)/test_tuple_trailing_comma_reject \
 	  $(BIN)/test_voidless_return_reject \
+	  $(BIN)/test_const_reassign_reject \
 	  $(BIN)/test_size_untyped_int \
 	  $(BIN)/test_tagged_staticinit \
 	  $(BIN)/test_arr_infer_len \
@@ -1463,6 +1464,12 @@ $(BIN)/test_voidless_return_reject: test/wcc/846_voidless_return_reject.c $(BIN)
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_const_reassign_reject: test/wcc/847_const_reassign_reject.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_tagged_staticinit: test/wcc/843_tagged_staticinit.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 58ede432..3c8e21bb 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -11247,7 +11247,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -15947,6 +15954,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index 51634b61..a0075869 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -773,7 +773,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -5473,6 +5480,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 7bf81d1c..6b5bad14 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -11247,7 +11247,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -15947,6 +15954,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store
diff --git a/test/wcc/847_const_reassign_reject.c b/test/wcc/847_const_reassign_reject.c
new file mode 100644
index 00000000..bba612e8
--- /dev/null
+++ b/test/wcc/847_const_reassign_reject.c
@@ -0,0 +1,173 @@
+/*
+ * 847_const_reassign_reject (catB-22) — cstage and wwstage LOUD-REJECT
+ * a reassignment of a `const`-declared binding.
+ *
+ * `const x = 5; x = 6;` mutates an immutable binding — a programmer
+ * error. The sym carries an is_const flag (lib/ww/sym.ww), but wwstage
+ * never SET it at the let-install nor CONSUMED it at assignment, so the
+ * reassignment slipped through silently. The fix mirrors cstage's two
+ * sites: set is_const when n.op == TK_CONST at the local let-install
+ * (cmd/wcc/check.c:2408) and reject an N_ASSIGN whose lhs ident resolves
+ * to an is_const sym (cmd/wcc/check.c:1889-1896). cstage already
+ * rejected; this aligns wwstage UP.
+ *
+ * Both rows annotate `: i32` so the inferred-int default (#34 int/i32
+ * cs/ww divergence) does not contaminate the const signal.
+ *
+ *   row        | shape                          | expect
+ *   -----------+--------------------------------+--------
+ *   const_set  | const x: i32 = 5; x = 6;       | REJECT
+ *   let_set    | let y: i32 = 5; y = 6;         | BUILD, run 6
+ */
+#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;
+}
+
+static void
+outbin(char *dst, size_t n, const char *tmpdir, const char *src)
+{
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	snprintf(dst, n, "%s/%s", tmpdir, base);
+	char *dot = strrchr(dst, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+}
+
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128];
+	snprintf(s, sizeof s, "/tmp/creas_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/creas_%d_d_%d", getpid(), i);
+	FILE *f = fopen(s, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, s);
+	int rc = runwait(cmd);
+	outbin(bin, sizeof bin, tmpdir, s);
+	unlink(s);
+	unlink(bin);
+	rmdir(tmpdir);
+	if (rc == 0)
+		fprintf(stderr, "creas[%s][%s]: built ok, expected a reject\n",
+		    driver, label);
+	return rc == 0 ? -1 : 0;
+}
+
+static int
+run_build(const char *driver, const char *label, const char *src, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128];
+	snprintf(s, sizeof s, "/tmp/creas_ok_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/creas_ok_%d_d_%d", getpid(), i);
+	FILE *f = fopen(s, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, s);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "creas[%s][%s]: build failed (a mutable let "
+		    "must compile)\n", driver, label);
+		unlink(s); rmdir(tmpdir);
+		return -1;
+	}
+	outbin(bin, sizeof bin, tmpdir, s);
+	int got = runwait(bin);
+	unlink(s); unlink(bin); rmdir(tmpdir);
+	return got;
+}
+
+struct rejrow { const char *label; const char *src; };
+
+static const struct rejrow reject_rows[] = {
+	{ "const_set",
+	  "package main;\n"
+	  "fn main() i32 = { const x: i32 = 5; x = 6; return x; };\n" },
+};
+
+struct okrow { const char *label; const char *src; int want; };
+
+static const struct okrow ok_rows[] = {
+	{ "let_set",
+	  "package main;\n"
+	  "fn main() i32 = { let y: i32 = 5; y = 6; return y; };\n", 6 },
+};
+
+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 nrej = (int)(sizeof reject_rows / sizeof reject_rows[0]);
+	int nok = (int)(sizeof ok_rows / sizeof ok_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, "creas: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < nrej; i++) {
+			total++;
+			if (build_should_fail(drivers[d].path,
+			    reject_rows[i].label, reject_rows[i].src,
+			    d * 100 + i) != 0)
+				fail++;
+		}
+		for (int i = 0; i < nok; i++) {
+			total++;
+			int got = run_build(drivers[d].path, ok_rows[i].label,
+			    ok_rows[i].src, d * 100 + i);
+			if (got != ok_rows[i].want) {
+				fprintf(stderr, "creas[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, ok_rows[i].label,
+				    got, ok_rows[i].want);
+				fail++;
+			}
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr, "creas: %d/%d fixtures failed\n", fail, total);
+		return 1;
+	}
+	printf("creas: %d/%d ok\n", total, total);
+	return 0;
+}