diff --git a/lib/strconv/decimal.ww b/lib/strconv/decimal.ww
index 38c947ef..78de7c89 100644
--- a/lib/strconv/decimal.ww
+++ b/lib/strconv/decimal.ww
@@ -48,8 +48,6 @@
 
 package strconv;
 
-import os;
-
 // ref/hare/strconv/decimal.ha:5.
 def maxshift: u8 = 60u8;
 
@@ -59,7 +57,7 @@ def decimal_point_range: u16 = 2047u16;
 // ref/hare/strconv/decimal.ha:8-26. Field layout 1:1. The 800-digit
 // bound covers subnormal doubles (min exp -1074, max mantissa 4e16
 // → at most 767 digits; 800 leaves headroom).
-export type decimal = struct {
+type decimal = struct {
 	digits: [800]u8,
 	nd: size,
 	dp: i32,
diff --git a/lib/strconv/ftos.ww b/lib/strconv/ftos.ww
index 26a64404..2146942b 100644
--- a/lib/strconv/ftos.ww
+++ b/lib/strconv/ftos.ww
@@ -54,7 +54,6 @@
 package strconv;
 
 import math;
-import os;
 
 // ref/hare/strconv/ftos_ryu.ha:33. (hi:lo) >> s, low 64 bits. Hare's
 // "TODO: use 128-bit integers" — ww has no u128; pure-u64 decomposition.
diff --git a/lib/strconv/ftos_test.ww b/lib/strconv/ftos_test.ww
index ae85bf8e..b43e68d8 100644
--- a/lib/strconv/ftos_test.ww
+++ b/lib/strconv/ftos_test.ww
@@ -14,7 +14,6 @@
 package strconv_test;
 
 import strconv;
-import os;
 import math;
 
 
@@ -43,6 +42,15 @@ fn rt64(x: f64) bool = {
 	return false;
 };
 
+fn rt32(x: f32) bool = {
+	match (stof32(f32tos(x), base.DEC)) {
+	case let v: f32 => { return math.f32bits(v) == math.f32bits(x); };
+	case let e: invalid => { return false; };
+	case let e: overflow => { return false; };
+	};
+	return false;
+};
+
 // ftos_test.ha:57-64 (G/void/NONE-equivalent) — fixed-point renders.
 @test fn f64tos_fixed() void = {
 	assert(!(!chk(13.37, "13.37")));
@@ -136,6 +144,16 @@ fn chkf32(n: f32, want: str) bool = {
 	assert(!(!chkf32(maxnorm, "3.4028235e38")));
 };
 
+@test fn f32_roundtrip_boundaries() void = {
+	assert(!(!rt32(math.f32frombits(0x00000001u32))));
+	assert(!(!rt32(math.f32frombits(0x007FFFFFu32))));
+	assert(!(!rt32(math.f32frombits(0x00800000u32))));
+	assert(!(!rt32(math.f32frombits(0x00800001u32))));
+	assert(!(!rt32(math.f32frombits(0x7F7FFFFFu32))));
+	assert(!(!rt32(math.f32frombits(0x807FFFFFu32))));
+	assert(!(!rt32(math.f32frombits(0x80800000u32))));
+};
+
 // ftos_test.ha:11/15/26 — zero, ±infinity, nan (f32 bit patterns).
 @test fn f32tos_special() void = {
 	assert(!(!chkf32(0.0f32, "0")));
@@ -162,6 +180,10 @@ fn chkf32(n: f32, want: str) bool = {
 	assert(!(!rt64(90071992547409915.0)));	// 17-digit halfway pair
 	assert(!(!rt64(90071992547409925.0)));
 	assert(!(!rt64(math.f64frombits(0x0000000000000001u64))));	// F64_MIN_SUBNORMAL (5e-324)
+	assert(!(!rt64(math.f64frombits(0x000FFFFFFFFFFFFFu64))));	// F64_MAX_SUBNORMAL
 	assert(!(!rt64(math.f64frombits(0x0010000000000000u64))));	// F64_MIN_NORMAL
+	assert(!(!rt64(math.f64frombits(0x0010000000000001u64))));	// next normal
+	assert(!(!rt64(math.f64frombits(0x800FFFFFFFFFFFFFu64))));	// negative max subnormal
+	assert(!(!rt64(math.f64frombits(0x8010000000000000u64))));	// negative min normal
 	assert(!(!rt64(math.f64frombits(0x7FEFFFFFFFFFFFFFu64))));	// F64_MAX_NORMAL
 };
diff --git a/lib/strconv/int_test.ww b/lib/strconv/int_test.ww
index f0766843..4ef9000c 100644
--- a/lib/strconv/int_test.ww
+++ b/lib/strconv/int_test.ww
@@ -129,6 +129,9 @@ fn ck_uint_ovf(id: i32, s: str, b: base) void = {
 
 	cki_ovf(7, "9223372036854775808",  base.DEC);
 	cki_ovf(8, "-9223372036854775809", base.DEC);
+	// The last multiply wraps to a value larger than its prefix, so a
+	// post-arithmetic `n < old` check misses this overflow.
+	cki_ovf(19, "21000000000000000000", base.DEC);
 
 	cki(9,  "0",  base.DEC, 0);
 	cki(10, "1",  base.DEC, 1);
@@ -168,6 +171,7 @@ fn ck_uint_ovf(id: i32, s: str, b: base) void = {
 	cku_ovf(35, "18446744073709551616",  base.DEC);
 	cku_ovf(36, "184467440737095516150", base.DEC);
 	cku_ovf(37, "-1", base.DEC);
+	cku_ovf(46, "21000000000000000000", base.DEC);
 
 	cku(38, "0", base.DEC, 0u64);
 	cku(39, "1", base.DEC, 1u64);
@@ -207,6 +211,87 @@ fn ck_uint_ovf(id: i32, s: str, b: base) void = {
 	ck_uint(62, "110101", base.BIN, 53u64: uint);        // 0b110101
 };
 
+@test fn test_narrow_parse_boundaries() void = {
+	match (stoi16("32767", base.DEC)) {
+	case let v: i16 => assert(v == 32767i16);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stoi16("-32768", base.DEC)) {
+	case let v: i16 => assert(v == -32768i16);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stoi16("32768", base.DEC)) {
+	case let v: i16 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+	match (stoi16("-32769", base.DEC)) {
+	case let v: i16 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+
+	match (stoi8("127", base.DEC)) {
+	case let v: i8 => assert(v == 127i8);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stoi8("-128", base.DEC)) {
+	case let v: i8 => assert(v == -128i8);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stoi8("128", base.DEC)) {
+	case let v: i8 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+	match (stoi8("-129", base.DEC)) {
+	case let v: i8 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+
+	match (stou16("65535", base.DEC)) {
+	case let v: u16 => assert(v == 65535u16);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stou16("65536", base.DEC)) {
+	case let v: u16 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+	match (stou8("255", base.DEC)) {
+	case let v: u8 => assert(v == 255u8);
+	case let e: invalid => abort();
+	case let e: overflow => abort();
+	};
+	match (stou8("256", base.DEC)) {
+	case let v: u8 => abort();
+	case let e: invalid => abort();
+	case let e: overflow => { };
+	};
+};
+
+@test fn test_strerror_static() void = {
+	let inv: invalid = 0: invalid;
+	let ie: error = inv;
+	let ia: str = strerror(ie);
+	let ib: str = strerror(ie);
+	assert(ia.ptr == ib.ptr);
+	assert(streq(ia, "input is not a valid number"));
+
+	let ov: overflow;
+	let oe: error = ov;
+	let oa: str = strerror(oe);
+	let ob: str = strerror(oe);
+	assert(oa.ptr == ob.ptr);
+	assert(streq(oa, "input number doesn't fit target type"));
+};
+
 // Verbatim ports of ref/hare/strconv/utos.ha:74-103 (utos/utos_bases) and
 // itos.ha:54-87 (itos/itos_bases) — Hare's format tests are flat assert
 // sequences too. Radix-literal inputs are written in DECIMAL (ww value
diff --git a/lib/strconv/stof.ww b/lib/strconv/stof.ww
index c8c2d50b..51c8b99f 100644
--- a/lib/strconv/stof.ww
+++ b/lib/strconv/stof.ww
@@ -53,7 +53,6 @@ package strconv;
 
 import ascii;
 import math;
-import os;
 import strings;
 
 // ref/hare/strconv/ftos_ryu.ha:12. 64×64→128 result halves.
diff --git a/lib/strconv/strconv.ww b/lib/strconv/strconv.ww
index b981b147..ae69774c 100644
--- a/lib/strconv/strconv.ww
+++ b/lib/strconv/strconv.ww
@@ -8,7 +8,6 @@ package strconv;
 
 import ascii;
 import bytes;
-import os;
 import strings;
 
 // invalid — input wasn't a valid number in the requested format.
@@ -224,14 +223,12 @@ fn parseint(s: str, b: base) ((bool, u64) | invalid | overflow) = {
 			return i: invalid;
 		};
 
-		let old: u64 = n;
-
-		n = n * nb;
-		n = n + digit;
-
-		if (n < old) {
+		// Check before multiplying: a wrapped value is not necessarily
+		// smaller than the preceding prefix.
+		if (n > (18446744073709551615u64 - digit) / nb) {
 			return overflow{};
 		};
+		n = n * nb + digit;
 
 		i += 1;
 	};
@@ -403,13 +400,12 @@ export fn stoz(s: str, b: base) (size | invalid | overflow) = {
 // both"); ftos.ww's f64tos is the live one. f32tos follows in fold-5b
 // (task #67, gated on the #143 f32-arg-push cgen fix).
 
-// strerror — convert an strconv error to a user-readable string.
-// Returns owned str; release via os.free. Mirrors Hare's
-// strconv::strerror.
+// strerror — convert a strconv error to a user-readable string.
+// The returned string has static storage and must not be freed.
 export fn strerror(e: error) str = {
 	match (e) {
-	case let v: invalid  => return strings.dup("input is not a valid number");
-	case let v: overflow => return strings.dup("input number doesn't fit target type");
+	case let v: invalid  => return "input is not a valid number";
+	case let v: overflow => return "input number doesn't fit target type";
 	};
-	return strings.dup("");
+	return "";
 };