diff --git a/Makefile b/Makefile
index aa499028..26a7f24a 100644
--- a/Makefile
+++ b/Makefile
@@ -330,6 +330,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_f64cgen_run \
 	  $(BIN)/test_f64crossmod_run \
 	  $(BIN)/test_tuprecv_run \
+	  $(BIN)/test_f64xmm_run \
 	  $(BIN)/test_floats_run \
 	  $(BIN)/test_bufio_run $(BIN)/test_random_run
 
@@ -1082,6 +1083,11 @@ $(BIN)/test_tuprecv_run: test/wcc/954_tuprecv_run.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_f64xmm_run: test/wcc/955_f64xmm_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 sizelint:
 	@sh tools/sizelint
 
diff --git a/test/wcc/955_f64xmm_run.c b/test/wcc/955_f64xmm_run.c
new file mode 100644
index 00000000..0731a627
--- /dev/null
+++ b/test/wcc/955_f64xmm_run.c
@@ -0,0 +1,209 @@
+/*
+ * 955_f64xmm_run — runtime + byte-id regression net for #103: an f64
+ * value failing to materialise in XMM (X0) before an SSE op. Two faces,
+ * same class, both GATE-BLIND (cstage cgen.c and wwstage cgenexpr.ww
+ * emitted byte-identical-but-wrong asm, so the 990-997 byte-id gates
+ * could never catch a reintroduction — only an executed-and-checked
+ * runtime probe can).
+ *
+ *   FACE X — a no-decimal float-typed integer literal (`0f64`, `8f64`)
+ *     is an N_INTLIT carrying float TYPE; the integer-immediate path
+ *     stranded it in AX, so `n == 0f64` compared a stale X0 (true for
+ *     all n) and `(8f64 * 10.0): i32` read garbage. Fixed by routing
+ *     the float-typed N_INTLIT through the float-constant-in-X0 emit
+ *     (cgen.c cgexpr_float / cgenexpr.ww cgfloatbits), plus the wwstage
+ *     exprfloatkind N_INTLIT arm so the downstream f64->i32 cast emits
+ *     CVTTSD2SI not MOVSXD (the #101 structural-vs-stamped asymmetry).
+ *   FACE Z — a tuple positional f64 field read (`r.0`, r:(f64,i64))
+ *     loaded via the integer op into AX, so `r.0 == 0.0` was wrongly
+ *     true. Fixed by a fld_isfloat branch -> MOVSD/MOVSS into X0
+ *     (cgen.c:5910 / cgenexpr.ww tuple arm), mirroring the struct-field
+ *     float load at cgen.c:1462,1838 (the #96 pattern).
+ *
+ * Each row carries BOTH dimensions (like 953_f64crossmod_run):
+ *   (a) cstage `ww build` + run, asserting the exit code.
+ *   (b) w6c vs w6c_ww `.s` cmp — FAILS if the stages diverge. Both
+ *       stages are fixed identically, so this stays byte-identical
+ *       before and after; it catches one stage being fixed without the
+ *       other.
+ */
+#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;
+}
+
+struct row { const char *label; const char *src; int want_exit; };
+
+static const struct row rows[] = {
+	/* FACE X false-case: g(8.0) must be 0. On the bug `n == 0f64`
+	 * compares against a stale X0 (true for all n) -> 1. */
+	{ "x_cmp_false",
+	  "package main;\n"
+	  "fn g(n: f64) i32 = { if (n == 0f64) { return 1; }; return 0; };\n"
+	  "export fn main() i32 = { return g(8.0); };\n", 0 },
+	/* FACE X true-case: g(0.0) must still be 1 — the genuine equal
+	 * case must survive the fix. */
+	{ "x_cmp_true",
+	  "package main;\n"
+	  "fn g(n: f64) i32 = { if (n == 0f64) { return 1; }; return 0; };\n"
+	  "export fn main() i32 = { return g(0.0); };\n", 1 },
+	/* FACE X arith: (8f64 * 10.0): i32 == 80. On the bug 8f64 never
+	 * reaches X0, the MULSD reads stale X0 -> garbage. */
+	{ "x_arith",
+	  "package main;\n"
+	  "export fn main() i32 = { return (8f64 * 10.0): i32; };\n", 80 },
+	/* FACE Z: tuple positional f64 field compare. r.0 == 0.0 with
+	 * r = (8.0, 0) must be false -> 9. On the bug r.0 loads into AX
+	 * (integer op), `== 0.0` reads stale X0 -> wrongly true -> 5. */
+	{ "z_tuple_field",
+	  "package main;\n"
+	  "fn norm(n: f64) (f64, i64) = { return (n, 0); };\n"
+	  "export fn main() i32 = {\n"
+	  "	const r = norm(8.0);\n"
+	  "	if (r.0 == 0.0) { return 5; };\n"
+	  "	return 9;\n"
+	  "};\n", 9 },
+	/* CONTROL: the let-bound spelling (`const m = r.0; m == 0.0`) was
+	 * already correct (the let-init is float-aware) and must STAY
+	 * correct -> 9. Guards against the FACE-Z fix over- or
+	 * under-reaching. */
+	{ "z_letbound_control",
+	  "package main;\n"
+	  "fn norm(n: f64) (f64, i64) = { return (n, 0); };\n"
+	  "export fn main() i32 = {\n"
+	  "	const r = norm(8.0);\n"
+	  "	const m = r.0;\n"
+	  "	if (m == 0.0) { return 5; };\n"
+	  "	return 9;\n"
+	  "};\n", 9 },
+	{ NULL, NULL, 0 }
+};
+
+static int
+slurp_eq(const char *a, const char *b)
+{
+	FILE *fa = fopen(a, "rb");
+	FILE *fb = fopen(b, "rb");
+	if (!fa || !fb) { if (fa) fclose(fa); if (fb) fclose(fb); return -1; }
+	int rc = 0;
+	for (;;) {
+		int ca = fgetc(fa);
+		int cb = fgetc(fb);
+		if (ca != cb) { rc = -1; break; }
+		if (ca == EOF) break;
+	}
+	fclose(fa); fclose(fb);
+	return rc;
+}
+
+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 w6c[1100], w6c_ww[1100];
+	snprintf(w6c, sizeof w6c, "%s/w6c", bin);
+	snprintf(w6c_ww, sizeof w6c_ww, "%s/w6c_ww", bin);
+	if (access(w6c_ww, X_OK) != 0) {
+		fprintf(stderr, "f64xmm: w6c_ww missing — cannot run the "
+		    "cs==ww byte-id gate (the whole point of this test)\n");
+		return 1;
+	}
+
+	int n = 0, fail = 0;
+	for (int i = 0; rows[i].src; i++, n++) {
+		char src[64];
+		snprintf(src, sizeof src, "/tmp/wwf64m_%d_%d.ww", getpid(), i);
+		FILE *f = fopen(src, "wb");
+		if (f == NULL) { fail++; continue; }
+		fputs(rows[i].src, f);
+		fclose(f);
+
+		/* (a) cstage build + run. */
+		char tmpdir[64];
+		snprintf(tmpdir, sizeof tmpdir, "/tmp/wwf64m_%d_d_%d",
+		    getpid(), i);
+		mkdir(tmpdir, 0755);
+
+		char cmd[2048];
+		snprintf(cmd, sizeof cmd, "cd %s && %s/ww build %s",
+		    tmpdir, bin, src);
+		if (runwait(cmd) != 0) {
+			fprintf(stderr, "row[%s]: cstage build failed\n",
+			    rows[i].label);
+			fail++;
+			unlink(src); rmdir(tmpdir);
+			continue;
+		}
+
+		char outbin[128];
+		const char *base = strrchr(src, '/');
+		base = base ? base + 1 : src;
+		snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+		char *dot = strrchr(outbin, '.');
+		if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+
+		int got = runwait(outbin);
+		if (got != rows[i].want_exit) {
+			fprintf(stderr, "row[%s]: cstage exit %d, want %d\n",
+			    rows[i].label, got, rows[i].want_exit);
+			fail++;
+		}
+		unlink(outbin); rmdir(tmpdir);
+
+		/* (b) cs==ww byte-id gate: emit .s from both stages, cmp. */
+		char cs_s[64], ws_s[64];
+		snprintf(cs_s, sizeof cs_s, "/tmp/wwf64m_%d_%d_cs.s",
+		    getpid(), i);
+		snprintf(ws_s, sizeof ws_s, "/tmp/wwf64m_%d_%d_ww.s",
+		    getpid(), i);
+
+		snprintf(cmd, sizeof cmd, "%s -o %s %s 2>/dev/null",
+		    w6c, cs_s, src);
+		if (runwait(cmd) != 0) {
+			fprintf(stderr, "row[%s]: w6c failed\n", rows[i].label);
+			fail++; unlink(src); continue;
+		}
+		snprintf(cmd, sizeof cmd, "%s -o %s %s 2>/dev/null",
+		    w6c_ww, ws_s, src);
+		if (runwait(cmd) != 0) {
+			fprintf(stderr, "row[%s]: w6c_ww failed\n",
+			    rows[i].label);
+			fail++; unlink(src); unlink(cs_s); continue;
+		}
+		if (slurp_eq(cs_s, ws_s) != 0) {
+			fprintf(stderr,
+			    "row[%s]: cstage/wwstage .s DIFFER (rule-10 "
+			    "byte-id violation)\n", rows[i].label);
+			fail++;
+		}
+		unlink(src); unlink(cs_s); unlink(ws_s);
+	}
+
+	if (fail) {
+		fprintf(stderr, "%d/%d f64 xmm-materialise tests failed\n",
+		    fail, n);
+		return 1;
+	}
+	printf("f64xmm: %d/%d ok (cstage run + cs==ww byte-id)\n", n, n);
+	return 0;
+}