diff --git a/lib/CLAUDE.md b/lib/CLAUDE.md
index a253fcc5..d46dc0e8 100644
--- a/lib/CLAUDE.md
+++ b/lib/CLAUDE.md
@@ -34,8 +34,8 @@ Signatures mirror Hare too, modulo:
   the two uses don't overlap because `T...` only attaches to a
   *param* decl. `lib/fmt` ships both the print family (`print` /
   `println` / `fprint` / `fprintln`) and the {n}-placeholder family
-  (`printf` / `fprintf` / `bsprintf` / `fatalf`); the `%`-parametric
-  modifier form is parsed but its `*mods` arg slot aborts on dispatch.
+  (`printf` / `fprintf` / `bsprintf` / `fatalf`), including Hare's
+  `%`-parametric `*mods` form.
 
 - `(T | U)` sum-typed parameters dispatch via `match` inside the
   callee. `strings.byteindex(haystack: str, needle: (str | rune))`,
diff --git a/lib/fmt/fmt.ww b/lib/fmt/fmt.ww
index d2864255..1bfc3a2f 100644
--- a/lib/fmt/fmt.ww
+++ b/lib/fmt/fmt.ww
@@ -6,60 +6,20 @@ package fmt;
 
 import io;
 import encoding.utf8;
-import math;
 import memio;
 import os;
 import strings;
 import strconv;
 
-// i64dec_buf — scratch buffer for [[i64dec]] below. Module-level
-// because Hare's `strconv::i64tos` is a static-buffer view and we
-// match that shape here. 21 bytes is enough for `-9223372036854775808`
-// (20 digits + sign).
-let i64dec_buf: [21]u8;
-
-// i64dec — render `v` as a base-10 ASCII string into [[i64dec_buf]],
-// returning a borrowed view. Preserved as a base-10 specialisation
-// of strconv.i64tos for the pre-printf-family print path; the
-// printf-family path below dispatches through strconv directly.
-fn i64dec(v: i64) str = {
-	// ref/hare/fmt/print.ha:124-129: magnitude via math::absi64 into u64,
-	// sign tested separately. `n = -n` on an i64 wraps at i64::MIN
-	// (-MIN == MIN), so the old loop never ran and only the '-' printed.
-	let neg: bool = v < 0;
-	let n: u64 = math.absi64(v);
-	let tmp: [20]u8;
-	let i: i32 = 0;
-	if (n == 0u64) { tmp[0] = '0'; i = 1; };
-	for (n > 0u64) {
-		let d: u64 = n % 10u64;
-		tmp[i] = (d + 48u64): u8;
-		n = n / 10u64;
-		i += 1;
-	};
-	let out: i32 = 0;
-	if (neg) { i64dec_buf[out] = '-'; out += 1; };
-	for (i > 0) {
-		i -= 1;
-		i64dec_buf[out] = tmp[i];
-		out += 1;
-	};
-	let r: str;
-	r.ptr = &i64dec_buf[0];
-	r.len = out;
-	return r;
-};
-
 // formattable — tagged union of types fmt can render. Mirrors Hare's
 // `fmt::formattable = (...types::numeric | uintptr | str | rune |
 // bool | nullable *opaque | void)`, narrowed to the set ww actually
 // has codegen for. Slot size is 24B (8 tag + 16 str payload — f64
 // arm is only 8B and rides under the str payload).
 //
-// No `f32` arm: strconv ships no `f32tos` and there is no in-tree
-// caller. Callers with an `f32` cast at the call site (`myf: f64`),
-// mirroring how `i64` covers every int width today. Ship the `f32`
-// arm when the first in-tree caller needs it.
+// No `f32` arm: strconv has the primitive, but there is no in-tree fmt
+// caller and adding a tagged-union arm is a public ABI change. Callers
+// cast to f64 until consumer evidence justifies widening this surface.
 //
 // `int`/`uint` appended LAST: variant tags follow declaration order
 // (cstage cg_tag_for_variant / wwstage flatvariantidxt), so the
@@ -74,16 +34,33 @@ fn i64dec(v: i64) str = {
 export type formattable = (i64 | str | bool | rune | f64 | int | uint);
 
 fn putbytes(s: io.handle, p: *u8, n: i32) (size | io.error) = {
-	let v: []u8;
-	v.ptr = p;
-	v.len = n;
-	return io.write(s, v);
+	let off: i32 = 0;
+	for (off < n) {
+		let v: []u8;
+		v.ptr = p + (off: u64);
+		v.len = n - off;
+		v.cap = v.len;
+		match (io.write(s, v)) {
+		case let z: size => {
+			assert(z <= (v.len: size),
+				"fmt.putbytes: writer returned an oversized count");
+			if (z == 0) {
+				let nm: nomem;
+				let e: io.error = nm;
+				return e;
+			};
+			off += z: i32;
+		};
+		case let e: io.error => return e;
+		};
+	};
+	return n: size;
 };
 
 fn writeone(s: io.handle, a: formattable) (size | io.error) = {
 	match (a) {
 	case let n: i64 => {
-		let v: str = i64dec(n);
+		let v: str = strconv.i64tos(n, strconv.base.DEC);
 		return putbytes(s, v.ptr, v.len);
 	};
 	case let v: str => return putbytes(s, v.ptr, v.len);
@@ -110,7 +87,7 @@ fn writeone(s: io.handle, a: formattable) (size | io.error) = {
 		return putbytes(s, v2.ptr, v2.len);
 	};
 	case let n: int => {
-		let v: str = i64dec(n: i64);
+		let v: str = strconv.i64tos(n: i64, strconv.base.DEC);
 		return putbytes(s, v.ptr, v.len);
 	};
 	case let n: uint => {
@@ -128,6 +105,7 @@ fn writeone(s: io.handle, a: formattable) (size | io.error) = {
 //   {N}               explicit-positional arg N
 //   {:mods}           inline modifiers on next arg
 //   {N:mods}          inline modifiers on arg N
+//   {%} / {N%M}       modifiers supplied by a *mods argument
 //   {{   }}           literal '{' / '}'
 //
 // Modifier set (subset of iter.ha:129 scan_modifiers):
@@ -141,9 +119,9 @@ fn writeone(s: io.handle, a: formattable) (size | io.error) = {
 //   <digits>   width (first char 1..9)
 //   .<digits>  precision
 //
-// Not v1: '%' parametric form ({0%1}); float modifiers (e/f/g/F…);
-// void / null arms (not in formattable). Invalid format aborts via
-// os.exit — Hare uses `abort()`; same effect.
+// Not v1: float format selectors (e/f/g/F…) and void/null arms (not in
+// formattable). Invalid format aborts via os.exit — Hare uses `abort()`;
+// same effect.
 
 // neg, alignment — mirror iter.ha:19 / iter.ha:26.
 export type neg       = enum i32 { NONE = 0, SPACE = 1, PLUS = 2 };
@@ -162,9 +140,7 @@ export type mods = struct {
 };
 
 // field — variadic arg slot. `(...formattable | *mods)` per iter.ha:11.
-// The *mods arm is the parametric-modifier form ({0%1}); accepted by
-// the type checker today, but the '%' parser branch is deferred —
-// passing a *mods that the parser reaches will abort the program.
+// The *mods arm supplies the parametric-modifier form ({0%1}).
 export type field = (...formattable | *mods);
 
 // fmtabort — invalid format string. Matches Hare's abort() (iter.ha:71)
@@ -220,8 +196,18 @@ fn scanmods(s: str, pos: *i32, m: *mods) void = {
 		else if (c == '=') { m.alignment = alignment.CENTER; }
 		else if (c == '_') {
 			if (*pos >= s.len) { fmtabort(); };
-			m.pad = s[*pos]: rune;
-			*pos += 1;
+			let src: []u8;
+			src.ptr = s.ptr + ((*pos): u64);
+			src.len = s.len - *pos;
+			src.cap = src.len;
+			let d: utf8.decoder = utf8.decode(src);
+			match (utf8.next(&d)) {
+			case let r: rune => m.pad = r;
+			case let dn: utf8.done => fmtabort();
+			case let mr: utf8.more => fmtabort();
+			case let e: utf8.invalid => fmtabort();
+			};
+			*pos += utf8.position(&d);
 		}
 		else if (c == ' ') { m.neg = neg.SPACE; }
 		else if (c == '+') { m.neg = neg.PLUS; }
@@ -302,11 +288,27 @@ fn rawlenu64(v: u64, m: *mods) i32 = {
 	return signlen + inner;
 };
 
-// rawlenstr — bytes the raw render of `s` would emit (after `prec`
-// truncation, per Hare print.ha:86).
+// precstr — borrowed rune-wise precision view. A precision larger than the
+// rune count leaves the string whole even when its byte length is larger.
+fn precstr(s: str, m: *mods) str = {
+	if (m.prec <= 0 || m.prec >= s.len) { return s; };
+	let it: strings.iterator = strings.iter(s);
+	let n: i32 = 0;
+	for (n < m.prec) {
+		match (strings.next(&it)) {
+		case let r: rune => n += 1;
+		case utf8.done => return s;
+		};
+	};
+	let out: str = s;
+	out.len = strings.position(&it);
+	out.cap = out.len;
+	return out;
+};
+
+// rawlenstr — bytes the rune-precision view would emit.
 fn rawlenstr(s: str, m: *mods) i32 = {
-	if (m.prec > 0 && m.prec < s.len) { return m.prec; };
-	return s.len;
+	return precstr(s, m).len;
 };
 
 // rawlenf64 — bytes the raw render of `v` under `m` would emit; the
@@ -315,6 +317,10 @@ fn rawlenstr(s: str, m: *mods) i32 = {
 // safe (strconv emits no leading '-' on nan/inf, so the peel is a no-op
 // on those views). Mirror print.ha.
 fn rawlenf64(v: f64, m: *mods) i32 = {
+	if (m.prec != 0 || (m.base != strconv.base.DEFAULT &&
+	    m.base != strconv.base.DEC)) {
+		fmtabort();
+	};
 	let view: str = strconv.f64tos(v);
 	let body: i32 = view.len;
 	let had_neg: bool = false;
@@ -345,9 +351,8 @@ fn rawlen(arg: formattable, m: *mods) i32 = {
 };
 
 // formatraw — write the bare value (no width padding) to `s`. Mirror
-// print.ha:76 format_raw. `prec` ignored on f64 (strconv.f64tos is
-// shortest-G with no precision knob); base ignored on f64 (Hare too —
-// base is int-only). drew NaN/Inf signoff: nan/infinity strings emitted
+// print.ha:76 format_raw. Unsupported f64 precision/base combinations are
+// rejected rather than silently ignored. NaN/infinity strings are emitted
 // unchanged.
 fn formatraw(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 	match (arg) {
@@ -391,8 +396,8 @@ fn formatraw(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 		return total;
 	};
 	case let v: str => {
-		let n: i32 = rawlenstr(v, m);
-		return putbytes(s, v.ptr, n);
+		let view: str = precstr(v, m);
+		return putbytes(s, view.ptr, view.len);
 	};
 	case let b: bool => {
 		let v: str = "false";
@@ -410,6 +415,10 @@ fn formatraw(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 		return putbytes(s, &buf[0], nn);
 	};
 	case let v: f64 => {
+		if (m.prec != 0 || (m.base != strconv.base.DEFAULT &&
+		    m.base != strconv.base.DEC)) {
+			fmtabort();
+		};
 		// strconv.f64tos prepends '-' for negative values; peel here so
 		// signof folds neg/plus/space mods uniformly with the i64 arm.
 		// drew NaN/Inf signoff: nan/infinity views have no leading '-',
@@ -525,49 +534,57 @@ fn formatraw(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 	let z: size = 0; return z;	// unreachable — match is exhaustive
 };
 
-// formatone — render `arg` to `s` with `m`'s width / alignment / pad
-// applied. Mirror print.ha:45 format, modulo the tail-pad loop: it
-// drives on a `need` counter rather than Hare's `total < m.width` form.
-// memio.fixedwrite now returns nomem on a full sink, so the width form
-// would terminate too; restoring it is a deferred follow-up — kept
-// counter-driven here to stay in F-R scope.
+// formatone — render `arg` to `s` with `m`'s minimum byte width,
+// alignment, and complete UTF-8 pad runes applied. Mirrors print.ha:45.
 fn formatone(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
+	if (m.width < 0 || m.prec < 0) { fmtabort(); };
 	let start: i32 = 0;
-	if (m.width > 0 && m.alignment != alignment.LEFT) {
+	let needpad: bool = false;
+	if (m.width > 0) {
 		let raw: i32 = rawlen(arg, m);
 		let pad: i32 = 0;
-		if (raw < m.width) { pad = m.width - raw; };
-		if (m.alignment == alignment.CENTER) { start = (pad + 1) / 2; }
-		else { start = pad; };
+		if (raw < m.width) {
+			pad = m.width - raw;
+			needpad = true;
+		};
+		if (m.alignment != alignment.LEFT) {
+			if (m.alignment == alignment.CENTER) {
+				start = (pad + 1) / 2;
+			} else {
+				start = pad;
+			};
+		};
 	};
 	let total: size = 0;
-	let i: i32 = 0;
-	let padb: [1]u8;
-	padb[0] = m.pad: u8;
-	for (i < start) {
-		let r: (size | io.error) = putbytes(s, &padb[0], 1);
+	let padb: [4]u8;
+	let pads: []u8;
+	let padn: i32 = 0;
+	if (needpad) {
+		pads.ptr = &padb[0];
+		pads.len = 4;
+		pads.cap = 4;
+		padn = utf8.encoderune(pads, m.pad);
+	};
+	let lead: size = 0;
+	for (lead < (start: size)) {
+		let r: (size | io.error) = putbytes(s, &padb[0], padn);
 		match (r) {
-		case let n: size => { total += n; };
+		case let n: size => { total += n; lead += n; };
 		case let e: io.error => return e;
 		};
-		i += 1;
 	};
 	let r1: (size | io.error) = formatraw(s, arg, m);
 	match (r1) {
 	case let n: size => { total += n; };
 	case let e: io.error => return e;
 	};
-	let need: i32 = 0;
 	let twidth: size = m.width: size;
-	if (twidth > total) { need = (twidth - total): i32; };
-	let j: i32 = 0;
-	for (j < need) {
-		let r: (size | io.error) = putbytes(s, &padb[0], 1);
+	for (total < twidth) {
+		let r: (size | io.error) = putbytes(s, &padb[0], padn);
 		match (r) {
 		case let n: size => { total += n; };
 		case let e: io.error => return e;
 		};
-		j += 1;
 	};
 	return total;
 };
@@ -575,8 +592,8 @@ fn formatone(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 // formatfield — field→formattable inline-per-arm dispatch through
 // formatone. A field→formattable widen helper called from fprintf's
 // for-loop would trip #18 (silent miscompile of 24B return-by-value in
-// for-loop context); inline-per-arm sidesteps it. `*mods` arm aborts
-// (parametric '%' form not implemented). Mirror print.ha:648.
+// for-loop context); inline-per-arm sidesteps it. A *mods is metadata,
+// never a value to render. Mirror print.ha:648.
 fn formatfield(s: io.handle, f: field, m: *mods) (size | io.error) = {
 	match (f) {
 	case let v: i64 => {
@@ -611,14 +628,33 @@ fn formatfield(s: io.handle, f: field, m: *mods) (size | io.error) = {
 	};
 };
 
+fn copymods(f: field, m: *mods) void = {
+	match (f) {
+	case let p: *mods => {
+		m.alignment = p.alignment;
+		m.pad = p.pad;
+		m.neg = p.neg;
+		m.width = p.width;
+		m.prec = p.prec;
+		m.base = p.base;
+		return;
+	};
+	case let v: i64 => fmtabort();
+	case let v: str => fmtabort();
+	case let v: bool => fmtabort();
+	case let v: rune => fmtabort();
+	case let v: f64 => fmtabort();
+	case let v: int => fmtabort();
+	case let v: uint => fmtabort();
+	};
+};
+
 // fprint — write the formatted form of each `args` element to `s`,
 // separated by spaces. Returns total bytes written or the first
 // io.error. Mirrors ref/hare/fmt/print.ha (fprint) + wrappers.ha.
 //
-// A short write (sink accepts fewer bytes than asked) is reported by
-// the returned count, not as an error — matches [[io.write]]'s contract
-// per [[memio.fixed]]. Callers that need write-all semantics layer it
-// on top, the same way they do over raw [[io.write]].
+// Each value is written completely. A writer that stops making progress
+// returns nomem through io.error rather than a successful truncated prefix.
 export fn fprint(s: io.handle, args: formattable...) (size | io.error) = {
 	let total: size = 0;
 	let i: i32 = 0;
@@ -674,6 +710,19 @@ export fn fprintf(s: io.handle, fmt: str, args: field...) (size | io.error) = {
 				if (i < fmt.len && fmt[i] == ':') {
 					i += 1;
 					scanmods(fmt, &i, &m);
+				} else if (i < fmt.len && fmt[i] == '%') {
+					i += 1;
+					if (i >= fmt.len) { fmtabort(); };
+					let midx: i32 = 0;
+					if (fmt[i] >= '0' && fmt[i] <= '9') {
+						checkunused = false;
+						midx = scandigits(fmt, &i);
+					} else {
+						midx = nextimpl;
+						nextimpl += 1;
+					};
+					if (midx >= args.len) { fmtabort(); };
+					copymods(args[midx], &m);
 				};
 				if (i >= fmt.len || fmt[i] != '}') { fmtabort(); };
 				i += 1;
@@ -694,12 +743,16 @@ export fn fprintf(s: io.handle, fmt: str, args: field...) (size | io.error) = {
 			};
 			i += 1;
 		} else {
-			let r: (size | io.error) = putbytes(s, fmt.ptr + i: u64, 1);
+			let start: i32 = i;
+			for (i < fmt.len && fmt[i] != '{' && fmt[i] != '}') {
+				i += 1;
+			};
+			let r: (size | io.error) = putbytes(s,
+				fmt.ptr + (start: u64), i - start);
 			match (r) {
 			case let n: size => { total += n; };
 			case let e: io.error => return e;
 			};
-			i += 1;
 		};
 	};
 	if (checkunused && nextimpl != args.len) { fmtabort(); };
diff --git a/lib/fmt/fmt_test.ww b/lib/fmt/fmt_test.ww
index e4621333..c145e09e 100644
--- a/lib/fmt/fmt_test.ww
+++ b/lib/fmt/fmt_test.ww
@@ -10,6 +10,8 @@ import fmt;
 import io;
 import memio;
 import os;
+import strconv;
+import test;
 
 fn streq(a: str, b: str) bool = {
 	if (a.len != b.len) { return false; };
@@ -35,6 +37,38 @@ fn errsource() io.stream = {
 	return &errvt;
 };
 
+type shortstream = struct {
+	vt: io.vtable,
+	buf: [32]u8,
+	pos: i32,
+	calls: i32,
+};
+
+fn shortwrite(s: io.stream, buf: []u8) (size | io.error) = {
+	let out: *shortstream = s: *shortstream;
+	let n: i32 = buf.len;
+	if (n > 2) { n = 2; };
+	let i: i32 = 0;
+	for (i < n) {
+		out.buf[out.pos + i] = buf[i];
+		i += 1;
+	};
+	out.pos += n;
+	out.calls += 1;
+	return n: size;
+};
+
+type countstream = struct {
+	vt: io.vtable,
+	calls: i32,
+};
+
+fn countwrite(s: io.stream, buf: []u8) (size | io.error) = {
+	let out: *countstream = s: *countstream;
+	out.calls += 1;
+	return buf.len: size;
+};
+
 @test fn fprintbarestr() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;
@@ -65,10 +99,8 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
-// #67: i64dec used `n = -n` within i64, which wraps at i64::MIN (-MIN == MIN),
-// so the digit loop never ran and only the bare '-' was emitted. The fix
-// takes the magnitude through math.absi64 into u64. These rows pin the
-// boundary value plus a normal negative, zero, and int MIN.
+// These rows preserve the I64_MIN boundary while fprint delegates integer
+// conversion to strconv.i64tos instead of keeping a second decimal engine.
 
 @test fn fprinti64min() void = {
 	let mem: memio.stream = memio.dynamic();
@@ -162,10 +194,8 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
-// memio.fixedwrite caps each call at the remaining buffer space and
-// never closes the stream, so fprint sees a short i32 result, not
-// io.error. Verifies the inner-loop arithmetic adds the actual byte
-// count rather than the requested length.
+// A partial fixed-buffer write cannot be reported as a successful prefix.
+// putbytes retries the unwritten suffix; the full sink then returns nomem.
 
 @test fn fprintfixedshort() void = {
 	let buf: [3]u8;
@@ -174,8 +204,8 @@ fn errsource() io.stream = {
 
 	let r: (size | io.error) = fmt.fprint(s, "hello");
 	match (r) {
-	case let n: size => { assert(!(n: i32 != 3)); };
-	case let eioe: io.error => abort();
+	case let n: size => abort();
+	case let eioe: io.error => { assert(eioe is nomem); };
 	};
 	assert(!(!streq(memio.string(&mem), "hel")));
 
@@ -183,6 +213,24 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+@test fn fprintcompleteshortwrites() void = {
+	let out: shortstream;
+	out.vt.writer = (&shortwrite): *io.writer;
+	out.pos = 0;
+	out.calls = 0;
+	let r: (size | io.error) = fmt.fprint(&out.vt, "hello");
+	match (r) {
+	case let n: size => assert(n == 5: size);
+	case let e: io.error => abort();
+	};
+	let got: str;
+	got.ptr = &out.buf[0];
+	got.len = out.pos;
+	got.cap = got.len;
+	assert(streq(got, "hello"));
+	assert(out.calls == 3);
+};
+
 // Exercises the early-return arm in fprint's inner match — distinct from
 // the short-write path above, which keeps returning i32 from a partial
 // accept. Single arm is enough: every formattable case routes errors
@@ -213,6 +261,20 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+@test fn fprintf_coalesces_literals() void = {
+	let out: countstream;
+	out.vt.writer = (&countwrite): *io.writer;
+	out.calls = 0;
+	let r: (size | io.error) = fmt.fprintf(&out.vt,
+		"hello {} world", 42i64);
+	match (r) {
+	case let n: size => assert(n == 14: size);
+	case let e: io.error => abort();
+	};
+	// One call for each literal run and one for the formatted integer.
+	assert(out.calls == 3);
+};
+
 @test fn fprintf_indexed() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;
@@ -226,6 +288,27 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+@test fn fprintf_parametric_mods() void = {
+	let m: fmt.mods;
+	m.alignment = fmt.alignment.RIGHT;
+	m.pad = '0': rune;
+	m.neg = fmt.neg.NONE;
+	m.width = 5;
+	m.prec = 0;
+	m.base = strconv.base.DEC;
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+	let r: (size | io.error) = fmt.fprintf(s,
+		"{%} {2%3}", 42i64, &m, 7i64, &m);
+	match (r) {
+	case let n: size => assert(n: i32 == 11);
+	case let e: io.error => abort();
+	};
+	assert(streq(memio.string(&mem), "00042 00007"));
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let e: io.error => abort(); };
+};
+
 @test fn fprintf_literal_braces() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;
@@ -330,6 +413,19 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+@test fn fprintf_pad_multibyte() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+	let r: (size | io.error) = fmt.fprintf(s, "{:_€5}", "hi");
+	match (r) {
+	case let n: size => assert(n: i32 == 5);
+	case let e: io.error => abort();
+	};
+	assert(streq(memio.string(&mem), "€hi"));
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let e: io.error => abort(); };
+};
+
 @test fn fprintf_base_hex() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;
@@ -382,6 +478,33 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+@test fn fprintf_prec_str_runes() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+	let r: (size | io.error) = fmt.fprintf(s,
+		"{:.1}|{:.2}|{:.2}", "éx", "éx", "😀");
+	match (r) {
+	case let n: size => assert(n: i32 == 11);
+	case let e: io.error => abort();
+	};
+	assert(streq(memio.string(&mem), "é|éx|😀"));
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let e: io.error => abort(); };
+};
+
+@test fn fprintf_prec_str_rune_width() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+	let r: (size | io.error) = fmt.fprintf(s, "{:5.1}", "éx");
+	match (r) {
+	case let n: size => assert(n: i32 == 5);
+	case let e: io.error => abort();
+	};
+	assert(streq(memio.string(&mem), "   é"));
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let e: io.error => abort(); };
+};
+
 @test fn fprintf_sign_neg() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;
@@ -439,6 +562,15 @@ fn errsource() io.stream = {
 	};
 };
 
+@test fn bsprintf_final_short() void = {
+	let buf: [3]u8;
+	let r: (str | io.error) = fmt.bsprintf(buf[0:3], "hello");
+	match (r) {
+	case let s: str => abort();
+	case let e: io.error => assert(e is nomem);
+	};
+};
+
 // Pins the formatfield bool / rune arms inside fprintf's for-loop —
 // the codegen-smell repro shape (task #18). Pre-task-#18, the str arm
 // is also exercised by fprintf_implicit; this adds the two remaining
@@ -532,14 +664,23 @@ fn errsource() io.stream = {
 	os.free(r.ptr: *void, r.len: u64);
 };
 
-// Pins the f64 formattable arm landed under task #17 (unblocked by #30
-// — the variant-widen-from-X0 fix). strconv.f64tos drives the render;
-// see lib/strconv/strconv.ww for the documented subset (fixed-point,
-// 6 fractional digits, trailing-zero trim, magnitudes ≥ 9e18 → "huge",
-// no NaN/±Inf detection).
+// strconv.f64tos drives shortest round-trippable decimal output, including
+// signed zero, infinities, NaNs, subnormals, and scientific notation.
 //
-// Mods scope under v1: width / alignment / pad / sign mods honored.
-// `prec` and `base` ignored — graduate when strconv grows ffmt/fflags.
+// Width / alignment / pad / sign mods are honored. Precision and
+// non-decimal bases are rejected until strconv grows ffmt/fflags.
+
+@test fn fprintf_f64_precision_aborts() void = {
+	test.expectabort();
+	let buf: [16]u8;
+	let r: (str | io.error) = fmt.bsprintf(buf[0:16], "{:.1}", 1.5);
+};
+
+@test fn fprintf_f64_base_aborts() void = {
+	test.expectabort();
+	let buf: [16]u8;
+	let r: (str | io.error) = fmt.bsprintf(buf[0:16], "{:x}", 1.5);
+};
 
 @test fn fprintf_f64_basic() void = {
 	let mem: memio.stream = memio.dynamic();