diff --git a/cmd/w6c/cgen.c b/cmd/w6c/cgen.c
index 4b0d943b..19b8ce5b 100644
--- a/cmd/w6c/cgen.c
+++ b/cmd/w6c/cgen.c
@@ -9395,6 +9395,23 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					memarg[i] =
 					    tagged_memarg_size(args[i]->type);
 		}
+		/* C default argument promotion (#14): an f32 passed in the
+		 * variadic region of a C-variadic call must widen to f64 — the
+		 * callee reads it with va_arg(double) (8B). The region is every
+		 * arg past the fixed params (the C `...` adds no Tparam). The
+		 * push promotes via CVTSS2SD and stores a full 8B double; the
+		 * pop then reloads MOVSD and counts it as exactly one SSE reg.
+		 * Fixed params keep their declared width. Ref SysV §3.5.7;
+		 * C99 §6.5.2.2p6. */
+		int promote_f32[64] = {0};
+		if (cu && cu->kind == TY_FN && cu->variadic) {
+			int nfixed = 0;
+			for (Tparam *p = callee_params; p; p = p->next)
+				nfixed++;
+			for (int i = nfixed; i < argcount; i++)
+				if (args[i] && node_isf32(args[i]))
+					promote_f32[i] = 1;
+		}
 		/* #38b MEMORY-class pre-pass: stage every >48B tagged arg on
 		 * the stack BELOW all register-class words (rightmost-first,
 		 * so the leftmost mem arg lands at the lowest address = the
@@ -9791,8 +9808,16 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				 * float class drives the width per ref/qbe
 				 * amd64/emit.c:524 (slot-copy single→movss). The
 				 * slot is 8B either way; the pop reads the same
-				 * width back. #143. */
-				int fmov = op_for(args[i], A_MOVSD, A_MOVSS);
+				 * width back. #143.
+				 *
+				 * #14: a variadic-region f32 is promoted to f64
+				 * here (CVTSS2SD in X0) and spilled MOVSD, so the
+				 * slot holds a real 8B double for the pop. */
+				int fmov = promote_f32[i] ? A_MOVSD
+				    : op_for(args[i], A_MOVSD, A_MOVSS);
+				if (promote_f32[i])
+					ins2(c, A_CVTSS2SD, areg(D_X0),
+					    areg(D_X0));
 				ins2(c, A_SUBQ, aimm(8), areg(D_SP));
 				ins2(c, fmov, areg(D_X0), amem(D_SP, 0));
 			} else if (node_isstr(args[i])) {
@@ -10017,9 +10042,11 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				if (fi < 8) {
 					/* Reload the spilled f32/f64 at its class
 					 * width — MOVSS for f32, MOVSD for f64 —
-					 * matching the push above (#143). */
-					int fmov = op_for(args[i], A_MOVSD,
-					    A_MOVSS);
+					 * matching the push above (#143). A #14
+					 * variadic-region f32 was promoted to f64
+					 * at push, so its slot reloads MOVSD. */
+					int fmov = promote_f32[i] ? A_MOVSD
+					    : op_for(args[i], A_MOVSD, A_MOVSS);
 					ins2(c, fmov, amem(D_SP, 0),
 					    areg(sysv_fargregs[fi]));
 					ins2(c, A_ADDQ, aimm(8), areg(D_SP));
diff --git a/test/wcc/989_ffivariadic_run.c b/test/wcc/989_ffivariadic_run.c
index af0d6455..8e715936 100644
--- a/test/wcc/989_ffivariadic_run.c
+++ b/test/wcc/989_ffivariadic_run.c
@@ -79,6 +79,52 @@ static const struct row rows[] = {
 	  "	return 1;\n"
 	  "};\n",
 	  0 },
+
+	/* 3 f32 args (#14): C default arg promotion widens each to f64, so
+	 * the callee's va_arg(double) reads 1.5+2.5+3.0 == 7.0. Values are
+	 * exact in f32, so the f64 compare is exact. Unpromoted (MOVSS, 4B)
+	 * the high 4B of each 8B slot is stale stack, so va_arg(double)
+	 * pulls garbage and the sum misses 7.0 — this row FAILS pre-fix. */
+	{ "f32three",
+	  "package main;\n"
+	  "@symbol(\"fixture\") fn fixture(n: i64, ...) f64;\n"
+	  "export fn main() int = {\n"
+	  "	let r: f64 = fixture(3, 1.5: f32, 2.5: f32, 3.0: f32);\n"
+	  "	if (r == 7.0) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n",
+	  0 },
+
+	/* Sharper than f32three: 0.1 is INEXACT in f32, so the f32-rounded
+	 * value differs from the f64 literal 0.1. The expected side `(x: f64)`
+	 * is the SAME f32 var widened at runtime (CVTSS2SD), so equality holds
+	 * iff the variadic arg carried the f32-rounded value promoted to f64 —
+	 * NOT the original f64 literal and NOT stale-high-bit garbage. Both a
+	 * no-promote (MOVSS) and a hypothetical direct-f64 pass would miss it.
+	 * Single f32 arg also exercises the fi==1 boundary. */
+	{ "f32inexact",
+	  "package main;\n"
+	  "@symbol(\"fixture\") fn fixture(n: i64, ...) f64;\n"
+	  "export fn main() int = {\n"
+	  "	let x: f32 = 0.1: f32;\n"
+	  "	let r: f64 = fixture(1, x);\n"
+	  "	if (r == (x: f64)) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n",
+	  0 },
+
+	/* Mixed f32 + f64 in one variadic call: only the f32 args (1.5, 3.5)
+	 * promote; the f64 arg (2.0) passes at its native width and must NOT be
+	 * double-promoted or skewed. 1.5+2.0+3.5 == 7.0, exact in both widths. */
+	{ "f32mixed",
+	  "package main;\n"
+	  "@symbol(\"fixture\") fn fixture(n: i64, ...) f64;\n"
+	  "export fn main() int = {\n"
+	  "	let r: f64 = fixture(3, 1.5: f32, 2.0, 3.5: f32);\n"
+	  "	if (r == 7.0) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n",
+	  0 },
 };
 
 /* run_build — build+run `src` via cstage `driver`, linking libffifix.a from