diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index 32d1ea11..ea030102 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -10433,6 +10433,16 @@ type checker = struct {
 				// around its exprtype, nil elsewhere.
 };
 
+// cerr — bare stderr fragment writer for the checker's piecewise
+// diagnostics. Tool-local (NOT a lib wrapper): messages are built from
+// many fragments and we route through os.write to avoid libc stdio.
+// .len replaces the error-prone hand-counted byte literals these sites
+// carried. Lives here (not err.ww) because the selfhost build pulls
+// check.ww but not err.ww (which ports err.c for the C-driven path).
+fn cerr(m: str) void = {
+	os.write(2, m.ptr, m.len: u64);
+};
+
 // seedprimitives — install the built-in type names so `i32`, `str`,
 // etc. can be looked up like ordinary symbols.
 fn seedprimitives(c: *checker) void = {
@@ -10572,12 +10582,12 @@ fn checkmoduleshadow(c: *checker, name: str, kindstr: str) void = {
 	};
 	if (!seen) { return; };
 	if (!srcimports(c.file, c.curmod, name)) { return; };
-	os.write(2, kindstr.ptr, kindstr.len: u64);
-	os.write(2, " '".ptr, 2u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "' shadows imported module '".ptr, 27u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "'\n".ptr, 2u64);
+	cerr(kindstr);
+	cerr(" '");
+	cerr(name);
+	cerr("' shadows imported module '");
+	cerr(name);
+	cerr("'\n");
 	c.errs += 1;
 };
 
@@ -10704,9 +10714,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved id: ".ptr, 17u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved id: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -10743,9 +10753,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved tname: ".ptr, 20u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved tname: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -11674,10 +11684,10 @@ fn foldbinop(op: tkind, a: u64, b: u64, out: *u64) bool = {
 // emitting a missing DATA row silently (rule 7). cstage twin: err()
 // in cmd/wcc/check.c.
 fn deffolderr(c: *checker, n: *node, msg: str) void = {
-	os.write(2, n.file.ptr, n.file.len: u64);
-	os.write(2, ": error: ".ptr, 9u64);
-	os.write(2, msg.ptr, msg.len: u64);
-	os.write(2, "\n".ptr, 1u64);
+	cerr(n.file);
+	cerr(": error: ");
+	cerr(msg);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -12898,9 +12908,9 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							if (e.list.next == nil && e.list.kind == nkind.N_DOT) {
 								let off: i64 = astoffset(c, e.list);
 								if (off < 0i64) {
-									os.write(2, "offset: no field '".ptr, 18u64);
-									os.write(2, e.list.str.ptr, e.list.str.len: u64);
-									os.write(2, "'\n".ptr, 2u64);
+									cerr("offset: no field '");
+									cerr(e.list.str);
+									cerr("'\n");
 									c.errs += 1;
 									off = 0i64;
 								};
@@ -13957,15 +13967,15 @@ fn casecovers(c: *checker, cs: *node, want: *node, unionmod: str) bool = {
 };
 
 fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
-	os.write(2, "match: variant not handled".ptr, 26u64);
+	cerr("match: variant not handled");
 	if (vname != nil) {
 		if (vname.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, vname.str.ptr, vname.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(vname.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14004,15 +14014,15 @@ fn casevariantin(c: *checker, tagged: *node, pat: *node, unionmod: str) bool = {
 };
 
 fn errbadcase(c: *checker, pat: *node) void = {
-	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
+	cerr("case: not a variant of scrutinee");
 	if (pat != nil) {
 		if (pat.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, pat.str.ptr, pat.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(pat.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14105,22 +14115,22 @@ fn checkvariantcovered(c: *checker, n: *node, v: *node, unionmod: str) void = {
 // type inference lives only on the C side.
 
 fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
-	os.write(2, where.ptr, where.len: u64);
-	os.write(2, ": not assignable".ptr, 16u64);
+	cerr(where);
+	cerr(": not assignable");
 	if (src != nil) {
 		if (src.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, src.str.ptr, src.str.len: u64);
-			os.write(2, " → ".ptr, 5u64);
+			cerr(" (");
+			cerr(src.str);
+			cerr(" → ");
 			if (dst != nil) {
 				if (dst.kind == nkind.N_TNAME) {
-					os.write(2, dst.str.ptr, dst.str.len: u64);
+					cerr(dst.str);
 				};
 			};
-			os.write(2, ")".ptr, 1u64);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14165,7 +14175,7 @@ fn checkarrlitfits(c: *checker, arrtn: *node, rhs: *node) void = {
 			if (folded) {
 				if (!defcastfits(et, v)) {
 					let m: str = "array element out of range\n";
-					os.write(2, m.ptr, m.len: u64);
+					cerr(m);
 					c.errs += 1;
 					return;
 				};
@@ -14502,7 +14512,7 @@ fn checkisas(c: *checker, n: *node) void = {
 		};
 	};
 	if (u.kind != nkind.N_TTAGGED) {
-		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		cerr("is/as: operand is not a tagged union\n");
 		c.errs += 1;
 		return;
 	};
@@ -14523,13 +14533,13 @@ fn checkisas(c: *checker, n: *node) void = {
 		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
 	}; };
 	if (casevariantin(c, u, want, taggeddefmod(c, st))) { return; };
-	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	cerr("is/as: not a variant of operand");
 	if (want.kind == nkind.N_TNAME) {
-		os.write(2, " (".ptr, 2u64);
-		os.write(2, want.str.ptr, want.str.len: u64);
-		os.write(2, ")".ptr, 1u64);
+		cerr(" (");
+		cerr(want.str);
+		cerr(")");
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14585,12 +14595,12 @@ fn checktryprop(c: *checker, n: *node) void = {
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
 	if (r == nil) {
-		os.write(2, "?: enclosing fn has no tagged-union return\n".ptr, 43u64);
+		cerr("?: enclosing fn has no tagged-union return\n");
 		c.errs += 1;
 		return;
 	};
 	if (r.kind != nkind.N_TTAGGED) {
-		os.write(2, "?: enclosing fn return is not tagged\n".ptr, 37u64);
+		cerr("?: enclosing fn return is not tagged\n");
 		c.errs += 1;
 		return;
 	};
@@ -14606,7 +14616,7 @@ fn checktryprop(c: *checker, n: *node) void = {
 				} else { rv = rv.next; };
 			};
 			if (!found) {
-				os.write(2, "?: error variant not in enclosing return\n".ptr, 41u64);
+				cerr("?: error variant not in enclosing return\n");
 				c.errs += 1;
 			};
 		};
@@ -14769,22 +14779,22 @@ fn asserttyped(c: *checker, n: *node, indot: bool) void = {
 	};
 	if (isexpr && !skip) {
 		if (n.type_ == nil) {
-			os.write(2, "asserttyped: ".ptr, 13u64);
+			cerr("asserttyped: ");
 			let kn: str = nkname(k);
-			os.write(2, kn.ptr, kn.len: u64);
-			os.write(2, " ".ptr, 1u64);
+			cerr(kn);
+			cerr(" ");
 			if (n.file.len > 0) {
-				os.write(2, n.file.ptr, n.file.len: u64);
-				os.write(2, ":".ptr, 1u64);
+				cerr(n.file);
+				cerr(":");
 				let ls: str = strconv.i32tos(n.line, strconv.base.DEC);
-				os.write(2, ls.ptr, ls.len: u64);
+				cerr(ls);
 			};
 			if (n.str.len > 0) {
-				os.write(2, " '".ptr, 2u64);
-				os.write(2, n.str.ptr, n.str.len: u64);
-				os.write(2, "'".ptr, 1u64);
+				cerr(" '");
+				cerr(n.str);
+				cerr("'");
 			};
-			os.write(2, "\n".ptr, 1u64);
+			cerr("\n");
 			os.exit(1);
 		};
 	};
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index 7c506f3b..43fd6b9d 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -47,6 +47,16 @@ type checker = struct {
 				// around its exprtype, nil elsewhere.
 };
 
+// cerr — bare stderr fragment writer for the checker's piecewise
+// diagnostics. Tool-local (NOT a lib wrapper): messages are built from
+// many fragments and we route through os.write to avoid libc stdio.
+// .len replaces the error-prone hand-counted byte literals these sites
+// carried. Lives here (not err.ww) because the selfhost build pulls
+// check.ww but not err.ww (which ports err.c for the C-driven path).
+fn cerr(m: str) void = {
+	os.write(2, m.ptr, m.len: u64);
+};
+
 // seedprimitives — install the built-in type names so `i32`, `str`,
 // etc. can be looked up like ordinary symbols.
 fn seedprimitives(c: *checker) void = {
@@ -186,12 +196,12 @@ fn checkmoduleshadow(c: *checker, name: str, kindstr: str) void = {
 	};
 	if (!seen) { return; };
 	if (!srcimports(c.file, c.curmod, name)) { return; };
-	os.write(2, kindstr.ptr, kindstr.len: u64);
-	os.write(2, " '".ptr, 2u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "' shadows imported module '".ptr, 27u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "'\n".ptr, 2u64);
+	cerr(kindstr);
+	cerr(" '");
+	cerr(name);
+	cerr("' shadows imported module '");
+	cerr(name);
+	cerr("'\n");
 	c.errs += 1;
 };
 
@@ -318,9 +328,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved id: ".ptr, 17u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved id: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -357,9 +367,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved tname: ".ptr, 20u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved tname: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -1288,10 +1298,10 @@ fn foldbinop(op: tkind, a: u64, b: u64, out: *u64) bool = {
 // emitting a missing DATA row silently (rule 7). cstage twin: err()
 // in cmd/wcc/check.c.
 fn deffolderr(c: *checker, n: *node, msg: str) void = {
-	os.write(2, n.file.ptr, n.file.len: u64);
-	os.write(2, ": error: ".ptr, 9u64);
-	os.write(2, msg.ptr, msg.len: u64);
-	os.write(2, "\n".ptr, 1u64);
+	cerr(n.file);
+	cerr(": error: ");
+	cerr(msg);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -2512,9 +2522,9 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							if (e.list.next == nil && e.list.kind == nkind.N_DOT) {
 								let off: i64 = astoffset(c, e.list);
 								if (off < 0i64) {
-									os.write(2, "offset: no field '".ptr, 18u64);
-									os.write(2, e.list.str.ptr, e.list.str.len: u64);
-									os.write(2, "'\n".ptr, 2u64);
+									cerr("offset: no field '");
+									cerr(e.list.str);
+									cerr("'\n");
 									c.errs += 1;
 									off = 0i64;
 								};
@@ -3571,15 +3581,15 @@ fn casecovers(c: *checker, cs: *node, want: *node, unionmod: str) bool = {
 };
 
 fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
-	os.write(2, "match: variant not handled".ptr, 26u64);
+	cerr("match: variant not handled");
 	if (vname != nil) {
 		if (vname.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, vname.str.ptr, vname.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(vname.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -3618,15 +3628,15 @@ fn casevariantin(c: *checker, tagged: *node, pat: *node, unionmod: str) bool = {
 };
 
 fn errbadcase(c: *checker, pat: *node) void = {
-	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
+	cerr("case: not a variant of scrutinee");
 	if (pat != nil) {
 		if (pat.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, pat.str.ptr, pat.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(pat.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -3719,22 +3729,22 @@ fn checkvariantcovered(c: *checker, n: *node, v: *node, unionmod: str) void = {
 // type inference lives only on the C side.
 
 fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
-	os.write(2, where.ptr, where.len: u64);
-	os.write(2, ": not assignable".ptr, 16u64);
+	cerr(where);
+	cerr(": not assignable");
 	if (src != nil) {
 		if (src.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, src.str.ptr, src.str.len: u64);
-			os.write(2, " → ".ptr, 5u64);
+			cerr(" (");
+			cerr(src.str);
+			cerr(" → ");
 			if (dst != nil) {
 				if (dst.kind == nkind.N_TNAME) {
-					os.write(2, dst.str.ptr, dst.str.len: u64);
+					cerr(dst.str);
 				};
 			};
-			os.write(2, ")".ptr, 1u64);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -3779,7 +3789,7 @@ fn checkarrlitfits(c: *checker, arrtn: *node, rhs: *node) void = {
 			if (folded) {
 				if (!defcastfits(et, v)) {
 					let m: str = "array element out of range\n";
-					os.write(2, m.ptr, m.len: u64);
+					cerr(m);
 					c.errs += 1;
 					return;
 				};
@@ -4116,7 +4126,7 @@ fn checkisas(c: *checker, n: *node) void = {
 		};
 	};
 	if (u.kind != nkind.N_TTAGGED) {
-		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		cerr("is/as: operand is not a tagged union\n");
 		c.errs += 1;
 		return;
 	};
@@ -4137,13 +4147,13 @@ fn checkisas(c: *checker, n: *node) void = {
 		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
 	}; };
 	if (casevariantin(c, u, want, taggeddefmod(c, st))) { return; };
-	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	cerr("is/as: not a variant of operand");
 	if (want.kind == nkind.N_TNAME) {
-		os.write(2, " (".ptr, 2u64);
-		os.write(2, want.str.ptr, want.str.len: u64);
-		os.write(2, ")".ptr, 1u64);
+		cerr(" (");
+		cerr(want.str);
+		cerr(")");
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -4199,12 +4209,12 @@ fn checktryprop(c: *checker, n: *node) void = {
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
 	if (r == nil) {
-		os.write(2, "?: enclosing fn has no tagged-union return\n".ptr, 43u64);
+		cerr("?: enclosing fn has no tagged-union return\n");
 		c.errs += 1;
 		return;
 	};
 	if (r.kind != nkind.N_TTAGGED) {
-		os.write(2, "?: enclosing fn return is not tagged\n".ptr, 37u64);
+		cerr("?: enclosing fn return is not tagged\n");
 		c.errs += 1;
 		return;
 	};
@@ -4220,7 +4230,7 @@ fn checktryprop(c: *checker, n: *node) void = {
 				} else { rv = rv.next; };
 			};
 			if (!found) {
-				os.write(2, "?: error variant not in enclosing return\n".ptr, 41u64);
+				cerr("?: error variant not in enclosing return\n");
 				c.errs += 1;
 			};
 		};
@@ -4383,22 +4393,22 @@ fn asserttyped(c: *checker, n: *node, indot: bool) void = {
 	};
 	if (isexpr && !skip) {
 		if (n.type_ == nil) {
-			os.write(2, "asserttyped: ".ptr, 13u64);
+			cerr("asserttyped: ");
 			let kn: str = nkname(k);
-			os.write(2, kn.ptr, kn.len: u64);
-			os.write(2, " ".ptr, 1u64);
+			cerr(kn);
+			cerr(" ");
 			if (n.file.len > 0) {
-				os.write(2, n.file.ptr, n.file.len: u64);
-				os.write(2, ":".ptr, 1u64);
+				cerr(n.file);
+				cerr(":");
 				let ls: str = strconv.i32tos(n.line, strconv.base.DEC);
-				os.write(2, ls.ptr, ls.len: u64);
+				cerr(ls);
 			};
 			if (n.str.len > 0) {
-				os.write(2, " '".ptr, 2u64);
-				os.write(2, n.str.ptr, n.str.len: u64);
-				os.write(2, "'".ptr, 1u64);
+				cerr(" '");
+				cerr(n.str);
+				cerr("'");
 			};
-			os.write(2, "\n".ptr, 1u64);
+			cerr("\n");
 			os.exit(1);
 		};
 	};
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index eed95047..44673903 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -10433,6 +10433,16 @@ type checker = struct {
 				// around its exprtype, nil elsewhere.
 };
 
+// cerr — bare stderr fragment writer for the checker's piecewise
+// diagnostics. Tool-local (NOT a lib wrapper): messages are built from
+// many fragments and we route through os.write to avoid libc stdio.
+// .len replaces the error-prone hand-counted byte literals these sites
+// carried. Lives here (not err.ww) because the selfhost build pulls
+// check.ww but not err.ww (which ports err.c for the C-driven path).
+fn cerr(m: str) void = {
+	os.write(2, m.ptr, m.len: u64);
+};
+
 // seedprimitives — install the built-in type names so `i32`, `str`,
 // etc. can be looked up like ordinary symbols.
 fn seedprimitives(c: *checker) void = {
@@ -10572,12 +10582,12 @@ fn checkmoduleshadow(c: *checker, name: str, kindstr: str) void = {
 	};
 	if (!seen) { return; };
 	if (!srcimports(c.file, c.curmod, name)) { return; };
-	os.write(2, kindstr.ptr, kindstr.len: u64);
-	os.write(2, " '".ptr, 2u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "' shadows imported module '".ptr, 27u64);
-	os.write(2, name.ptr, name.len: u64);
-	os.write(2, "'\n".ptr, 2u64);
+	cerr(kindstr);
+	cerr(" '");
+	cerr(name);
+	cerr("' shadows imported module '");
+	cerr(name);
+	cerr("'\n");
 	c.errs += 1;
 };
 
@@ -10704,9 +10714,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved id: ".ptr, 17u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved id: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -10743,9 +10753,9 @@ fn resolvewalk(c: *checker, n: *node) void = {
 			if (s == nil) {
 				c.nunresolved += 1;
 				if (c.verbose != 0) {
-					os.write(2, "  unresolved tname: ".ptr, 20u64);
-					os.write(2, nm.ptr, nm.len: u64);
-					os.write(2, "\n".ptr, 1u64);
+					cerr("  unresolved tname: ");
+					cerr(nm);
+					cerr("\n");
 				};
 			} else { c.nresolved += 1; };
 		};
@@ -11674,10 +11684,10 @@ fn foldbinop(op: tkind, a: u64, b: u64, out: *u64) bool = {
 // emitting a missing DATA row silently (rule 7). cstage twin: err()
 // in cmd/wcc/check.c.
 fn deffolderr(c: *checker, n: *node, msg: str) void = {
-	os.write(2, n.file.ptr, n.file.len: u64);
-	os.write(2, ": error: ".ptr, 9u64);
-	os.write(2, msg.ptr, msg.len: u64);
-	os.write(2, "\n".ptr, 1u64);
+	cerr(n.file);
+	cerr(": error: ");
+	cerr(msg);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -12898,9 +12908,9 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 							if (e.list.next == nil && e.list.kind == nkind.N_DOT) {
 								let off: i64 = astoffset(c, e.list);
 								if (off < 0i64) {
-									os.write(2, "offset: no field '".ptr, 18u64);
-									os.write(2, e.list.str.ptr, e.list.str.len: u64);
-									os.write(2, "'\n".ptr, 2u64);
+									cerr("offset: no field '");
+									cerr(e.list.str);
+									cerr("'\n");
 									c.errs += 1;
 									off = 0i64;
 								};
@@ -13957,15 +13967,15 @@ fn casecovers(c: *checker, cs: *node, want: *node, unionmod: str) bool = {
 };
 
 fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
-	os.write(2, "match: variant not handled".ptr, 26u64);
+	cerr("match: variant not handled");
 	if (vname != nil) {
 		if (vname.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, vname.str.ptr, vname.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(vname.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14004,15 +14014,15 @@ fn casevariantin(c: *checker, tagged: *node, pat: *node, unionmod: str) bool = {
 };
 
 fn errbadcase(c: *checker, pat: *node) void = {
-	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
+	cerr("case: not a variant of scrutinee");
 	if (pat != nil) {
 		if (pat.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, pat.str.ptr, pat.str.len: u64);
-			os.write(2, ")".ptr, 1u64);
+			cerr(" (");
+			cerr(pat.str);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14105,22 +14115,22 @@ fn checkvariantcovered(c: *checker, n: *node, v: *node, unionmod: str) void = {
 // type inference lives only on the C side.
 
 fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
-	os.write(2, where.ptr, where.len: u64);
-	os.write(2, ": not assignable".ptr, 16u64);
+	cerr(where);
+	cerr(": not assignable");
 	if (src != nil) {
 		if (src.kind == nkind.N_TNAME) {
-			os.write(2, " (".ptr, 2u64);
-			os.write(2, src.str.ptr, src.str.len: u64);
-			os.write(2, " → ".ptr, 5u64);
+			cerr(" (");
+			cerr(src.str);
+			cerr(" → ");
 			if (dst != nil) {
 				if (dst.kind == nkind.N_TNAME) {
-					os.write(2, dst.str.ptr, dst.str.len: u64);
+					cerr(dst.str);
 				};
 			};
-			os.write(2, ")".ptr, 1u64);
+			cerr(")");
 		};
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14165,7 +14175,7 @@ fn checkarrlitfits(c: *checker, arrtn: *node, rhs: *node) void = {
 			if (folded) {
 				if (!defcastfits(et, v)) {
 					let m: str = "array element out of range\n";
-					os.write(2, m.ptr, m.len: u64);
+					cerr(m);
 					c.errs += 1;
 					return;
 				};
@@ -14502,7 +14512,7 @@ fn checkisas(c: *checker, n: *node) void = {
 		};
 	};
 	if (u.kind != nkind.N_TTAGGED) {
-		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		cerr("is/as: operand is not a tagged union\n");
 		c.errs += 1;
 		return;
 	};
@@ -14523,13 +14533,13 @@ fn checkisas(c: *checker, n: *node) void = {
 		if (flatvariantidxt(utinfo, wanttinfo) >= 0) { return; };
 	}; };
 	if (casevariantin(c, u, want, taggeddefmod(c, st))) { return; };
-	os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+	cerr("is/as: not a variant of operand");
 	if (want.kind == nkind.N_TNAME) {
-		os.write(2, " (".ptr, 2u64);
-		os.write(2, want.str.ptr, want.str.len: u64);
-		os.write(2, ")".ptr, 1u64);
+		cerr(" (");
+		cerr(want.str);
+		cerr(")");
 	};
-	os.write(2, "\n".ptr, 1u64);
+	cerr("\n");
 	c.errs += 1;
 };
 
@@ -14585,12 +14595,12 @@ fn checktryprop(c: *checker, n: *node) void = {
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
 	if (r == nil) {
-		os.write(2, "?: enclosing fn has no tagged-union return\n".ptr, 43u64);
+		cerr("?: enclosing fn has no tagged-union return\n");
 		c.errs += 1;
 		return;
 	};
 	if (r.kind != nkind.N_TTAGGED) {
-		os.write(2, "?: enclosing fn return is not tagged\n".ptr, 37u64);
+		cerr("?: enclosing fn return is not tagged\n");
 		c.errs += 1;
 		return;
 	};
@@ -14606,7 +14616,7 @@ fn checktryprop(c: *checker, n: *node) void = {
 				} else { rv = rv.next; };
 			};
 			if (!found) {
-				os.write(2, "?: error variant not in enclosing return\n".ptr, 41u64);
+				cerr("?: error variant not in enclosing return\n");
 				c.errs += 1;
 			};
 		};
@@ -14769,22 +14779,22 @@ fn asserttyped(c: *checker, n: *node, indot: bool) void = {
 	};
 	if (isexpr && !skip) {
 		if (n.type_ == nil) {
-			os.write(2, "asserttyped: ".ptr, 13u64);
+			cerr("asserttyped: ");
 			let kn: str = nkname(k);
-			os.write(2, kn.ptr, kn.len: u64);
-			os.write(2, " ".ptr, 1u64);
+			cerr(kn);
+			cerr(" ");
 			if (n.file.len > 0) {
-				os.write(2, n.file.ptr, n.file.len: u64);
-				os.write(2, ":".ptr, 1u64);
+				cerr(n.file);
+				cerr(":");
 				let ls: str = strconv.i32tos(n.line, strconv.base.DEC);
-				os.write(2, ls.ptr, ls.len: u64);
+				cerr(ls);
 			};
 			if (n.str.len > 0) {
-				os.write(2, " '".ptr, 2u64);
-				os.write(2, n.str.ptr, n.str.len: u64);
-				os.write(2, "'".ptr, 1u64);
+				cerr(" '");
+				cerr(n.str);
+				cerr("'");
 			};
-			os.write(2, "\n".ptr, 1u64);
+			cerr("\n");
 			os.exit(1);
 		};
 	};