diff --git a/lang/runtime/el_runtime.c b/lang/runtime/el_runtime.c
index e0db23e..73c04e0 100644
--- a/lang/runtime/el_runtime.c
+++ b/lang/runtime/el_runtime.c
@@ -712,6 +712,20 @@ el_val_t el_map_set(el_val_t mapv, el_val_t keyv, el_val_t value) {
  * happen to look like aligned heap pointers are exceedingly unlikely to land
  * on a page whose first 4 bytes match either magic.                          */
 
+/* el_tagged — THE gate for "is this slot a heap object carrying this tag".
+ *
+ * el_val_t carries both integers and tagged heap pointers, so deciding which
+ * requires checking the value BEFORE dereferencing it. That check was a
+ * convention every author had to know rather than a gate they had to pass
+ * through, and the result is measurable: geom_of and mfld_of call
+ * looks_like_heap_obj and are correct; el_bin_lookup checked only a 4096 floor
+ * -- no alignment, no small-int, no negative -- and reads EIGHT BYTES BACKWARD
+ * from the pointer. sha256_hex(50000) therefore compiled clean and segfaulted.
+ *
+ * Exported, so the engram siblings stop re-deriving it. Anything that
+ * dereferences a slot without passing through here is the defect. */
+int el_tagged(el_val_t v, uint32_t magic);
+
 static int looks_like_heap_obj(el_val_t v) {
     if (v == 0) return 0;
     int64_t s = (int64_t)v;
@@ -722,6 +736,12 @@ static int looks_like_heap_obj(el_val_t v) {
     return 1;
 }
 
+int el_tagged(el_val_t v, uint32_t magic) {
+    if (!looks_like_heap_obj(v)) return 0;
+    return *(const uint32_t*)(uintptr_t)v == magic;
+}
+
+
 void el_retain(el_val_t v) {
     if (!looks_like_heap_obj(v)) return;
     ElHeader* h = (ElHeader*)(uintptr_t)v;
@@ -18272,8 +18292,11 @@ static int el_bin_lookup(const void* p, size_t* out_len) {
     /* Avoid reading off the front of a page on tiny pointers (e.g. NULs
      * passed in as int-cast values). 4096 is a safe lower bound on any
      * platform we target.                                              */
-    if ((uintptr_t)p < 4096) return 0;
-    const el_bin_hdr_t* hdr = (const el_bin_hdr_t*)((const char*)p - sizeof(el_bin_hdr_t));
+    /* Reads BACKWARD, so the HEADER address is what must be validated -- and a
+     * 4096 floor alone let sha256_hex(50000) through to a SIGSEGV. */
+    const char* hp = (const char*)p - sizeof(el_bin_hdr_t);
+    if (!looks_like_heap_obj((el_val_t)(uintptr_t)hp)) return 0;
+    const el_bin_hdr_t* hdr = (const el_bin_hdr_t*)hp;
     if (hdr->magic != EL_MAGIC_BIN) return 0;
     *out_len = hdr->length;
     return 1;
@@ -18283,7 +18306,12 @@ static int el_bin_lookup(const void* p, size_t* out_len) {
 static size_t el_input_len(const char* s) {
     size_t n;
     if (el_bin_lookup(s, &n)) return n;
-    return s ? strlen(s) : 0;
+    /* The FALLBACK is the hazard, not the tagged lookup. A NULL check does not
+     * establish that a slot is a pointer: el_val_t carries integers too, so
+     * strlen() on `sha256_hex(50000)` walks address 50000. Guarding the tagged
+     * path alone left this untouched and the SIGSEGV unchanged -- measured. */
+    if (!looks_like_heap_obj((el_val_t)(uintptr_t)s)) return 0;
+    return strlen(s);
 }
 
 /* ─── SHA-256 (Brad Conte / public domain) ──────────────────────────────── */
diff --git a/lang/runtime/el_runtime.h b/lang/runtime/el_runtime.h
index 0bf2d43..2492c68 100644
--- a/lang/runtime/el_runtime.h
+++ b/lang/runtime/el_runtime.h
@@ -888,6 +888,7 @@ el_val_t  engram_age_field_catchup(void);
 el_val_t  engram_chrono_persist_tick(void);
 el_val_t  engram_chrono_tick(void);
 el_val_t  engram_boundary_beat(el_val_t op_name, el_val_t construct);
+int       el_tagged(el_val_t v, uint32_t magic);  /* the gate: validate a slot BEFORE dereferencing it */
 el_val_t  el_seam_run(el_val_t fn_name, el_val_t phase, el_val_t result);  /* runtime construct seam */
 el_val_t  el_seam_wrap(el_val_t fn_name, el_val_t (*body)(void*), void* env);  /* runtime invocation control */  /* API-reshape decorator-seam auto-emit; construct = the decorator that caused the beat */
 el_val_t  engram_self_anchor_capture(void);
diff --git a/lang/tests/integration/tagged_gate.sh b/lang/tests/integration/tagged_gate.sh
new file mode 100755
index 0000000..5445dcc
--- /dev/null
+++ b/lang/tests/integration/tagged_gate.sh
@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# tagged_gate.sh — a slot must be validated before it is dereferenced.
+#
+# el_val_t carries both integers and tagged heap pointers, so "is this a
+# pointer" is undecidable without checking first. That check was a CONVENTION
+# every author had to know rather than a GATE they had to pass through:
+#   geom_of, mfld_of   call looks_like_heap_obj    correct
+#   el_bin_lookup      checked only a 4096 floor   read 8 bytes backward
+#   el_input_len       checked only for NULL       strlen'd an integer
+# sha256_hex(50000) therefore compiled clean and segfaulted (exit 139).
+set -uo pipefail
+ELC="${1:?usage: tagged_gate.sh <elc>}"
+LANG_DIR="${2:-$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)}"
+W=$(mktemp -d); trap 'rm -rf "$W"' EXIT; F=0
+chk(){ [ "$2" = "$3" ] && printf '  ok    %s\n' "$1" || { printf '  FAIL  %s\n    expected %s got %s\n' "$1" "$2" "$3"; F=$((F+1)); }; }
+cd "$LANG_DIR"
+SRCS=$(../scripts/el-runtime-sources.sh runtime)
+CF="-std=c11 -O2 -I runtime"; for d in /opt/homebrew/opt/openssl@3 /usr/local/opt/openssl@3; do [ -d "$d" ] && CF="$CF -I $d/include" && LF="-L $d/lib"; done
+LF="${LF:-} -lcurl -lssl -lcrypto -lpthread -lm"
+
+build(){ "$ELC" "$1" > "$W/t.c" 2>/dev/null && cc $CF -o "$W/t" "$W/t.c" $SRCS $LF 2>/dev/null; }
+
+printf 'fn main() { let h: String = sha256_hex(50000) println("got " + h) }\n' > "$W/a.el"
+build "$W/a.el"; "$W/t" >"$W/o" 2>&1; chk "an integer where a string is expected does not crash" "0" "$?"
+chk "and yields the empty-string hash, not memory" "1" "$(grep -c e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 "$W/o")"
+
+printf 'fn main() { let h: String = sha256_hex(-5) println("got " + h) }\n' > "$W/b.el"
+build "$W/b.el"; "$W/t" >/dev/null 2>&1; chk "a NEGATIVE integer does not crash" "0" "$?"
+
+printf 'fn main() { println(sha256_hex("abc")) }\n' > "$W/c.el"
+build "$W/c.el"; out=$("$W/t" 2>&1)
+chk "a legitimate string still hashes correctly" "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad" "$out"
+
+chk "the gate is exported, so siblings stop re-deriving it" "1" "$(grep -c 'int  *el_tagged(el_val_t' runtime/el_runtime.h)"
+echo; echo "  5 assertions, $((5-F)) passed, $F failed"; exit $F