diff --git a/openfut-utas-host/mutation-battery.sh b/openfut-utas-host/mutation-battery.sh
new file mode 100755
index 0000000..172b757
--- /dev/null
+++ b/openfut-utas-host/mutation-battery.sh
@@ -0,0 +1,140 @@
+#!/usr/bin/env bash
+# FIFA 17 squad HOST mutation battery.
+#
+# Each mutation injects a specific wrong behaviour into the committed source
+# (host, or the adapter it composes), runs the ONE host test that defends the
+# invariant, and requires that test to FAIL (mutant killed). Source is reverted
+# via `git checkout` after every mutation. A SURVIVOR fails the battery.
+#
+# Run from the host crate root:  bash mutation-battery.sh
+set -u
+cd "$(dirname "$0")"
+
+H=src/lib.rs
+PROJ=../openfut-adapter-fifa17/src/fut/squad_projection.rs
+EXT=../openfut-adapter-fifa17/src/fut/squad_ext.rs
+ITEM=../openfut-adapter-fifa17/src/fut/item.rs
+
+PASS=0
+FAIL=0
+declare -a SURVIVORS=()
+
+mutate() {
+  OLD="$2" NEW="$3" perl -0777 -pi -e \
+    's/\Q$ENV{OLD}\E/$ENV{NEW}/g or die "PATTERN NOT FOUND in '"$1"'\n"' "$1"
+}
+
+kill_test() {
+  local n="$1" label="$2" filter="$3" file="$4" old="$5" new="$6"
+  git checkout -- "$file"
+  if ! mutate "$file" "$old" "$new"; then
+    echo "  [#$n] SETUP ERROR — $label"; FAIL=$((FAIL+1)); SURVIVORS+=("#$n $label (setup)"); git checkout -- "$file"; return
+  fi
+  if cargo test --quiet --test host_test "$filter" >/dev/null 2>&1; then
+    echo "  [#$n] SURVIVED  — $label"; FAIL=$((FAIL+1)); SURVIVORS+=("#$n $label")
+  else
+    echo "  [#$n] killed    — $label"; PASS=$((PASS+1))
+  fi
+  git checkout -- "$file"
+}
+
+echo "== FIFA17 squad host mutation battery =="
+
+kill_test 1 "resolvable wire id skips authorization" \
+  put_rejects_wire_id_owned_by_another_profile_core_unchanged "$H" \
+  'if !owned_set.contains(&slot.owned_card_id) {' 'if false {'
+
+kill_test 2 "authorization loop never runs (wrong-profile accepted)" \
+  put_rejects_wire_id_owned_by_another_profile_core_unchanged "$H" \
+  'for slot in &build.canonical.slots {' 'for slot in build.canonical.slots.iter().take(0) {'
+
+kill_test 3 "PUT/core failure masked as success (not failed loudly)" \
+  put_core_failure_returns_error_never_python "$H" \
+  '                error_response(502, "core_error"),' '                json_response(&save_ack(put.id)),'
+
+kill_test 4 "PUT forwarded as a partial slot diff" \
+  put_full_replacement_commits_canonical_and_extension_and_acks_id0 "$H" \
+  '.iter()
+            .map(|s| CoreSquadSlot {' '.iter()
+            .take(1)
+            .map(|s| CoreSquadSlot {'
+
+kill_test 5 "extension dropped from the atomic write" \
+  put_full_replacement_commits_canonical_and_extension_and_acks_id0 "$H" \
+  'ext_payload: build.extension.to_payload(),' 'ext_payload: String::new(),'
+
+kill_test 6 "stale extension accepted and projected" \
+  stale_extension_is_not_applied_on_reads "$H" \
+  '        CoreExtState::Stale { .. } => return HostProjection::Stale,' \
+  '        CoreExtState::Stale { schema_version, payload } => match Fifa17SquadExtensionV1::from_payload(*schema_version, payload) { Ok(e) => e, Err(_) => return HostProjection::Missing },'
+
+kill_test 7 "missing extension fabricates defaults" \
+  missing_extension_is_explicit_on_reads "$H" \
+  '        CoreExtState::Missing => return HostProjection::Missing,' \
+  '        CoreExtState::Missing => Fifa17SquadExtensionV1::from_payload(1, "{}").unwrap(),'
+
+kill_test 8 "overlay clobbers userInfo" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$H" \
+  '    root["squad"] = squad_val;' '    root["squad"] = squad_val; root["userInfo"] = serde_json::json!({});'
+
+kill_test 9 "overlay clobbers settings" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$H" \
+  '    root["squad"] = squad_val;' '    root["squad"] = squad_val; root["settings"] = serde_json::json!({});'
+
+kill_test 10 "overlay clobbers pileSizeClientData" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$H" \
+  '    root["squad"] = squad_val;' '    root["squad"] = squad_val; root["pileSizeClientData"] = serde_json::json!({});'
+
+kill_test 11 "/squad/list uses separate shaping logic" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$H" \
+  'json_response(&squad_list(&v)),' 'json_response(&serde_json::json!({ "squad": [{ "formation": "WRONG" }] })),'
+
+kill_test 12 "captain emitted using resourceId (adapter projector)" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$PROJ" \
+  'captain_wire = id.item_id as i64;' 'captain_wire = id.asset_id as i64;'
+
+kill_test 13 "kitNumber keyed by slot index (adapter extension)" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$EXT" \
+  '(s.owned_card_id.clone(), s.kit_number)' '(s.index.to_string(), s.kit_number)'
+
+kill_test 14 "client-reported evaluation dropped/mishandled" \
+  put_full_replacement_commits_canonical_and_extension_and_acks_id0 "$H" \
+  'chemistry: build.extension.client_reported.chemistry,' 'chemistry: None,'
+
+kill_test 15 "host trusts/acts on the Core fingerprint" \
+  put_full_replacement_commits_canonical_and_extension_and_acks_id0 "$H" \
+  '        Ok(_) => (
+            json_response(&save_ack(put.id)),' \
+  '        Ok(r) => (
+            if r.canonical_fingerprint.is_empty() { json_response(&save_ack(put.id)) } else { error_response(409, "host_trusts_fp") },'
+
+kill_test 16 "per-slot Core lookup introduced (N+1)" \
+  read_path_is_bounded_no_per_slot_lookup "$H" \
+  '    let owned = match deps.core.all_owned() {' \
+  '    for _ in 0..2 { let _ = deps.core.all_owned(); }
+    let owned = match deps.core.all_owned() {'
+
+kill_test 17 "/squad/active classified to Rust" \
+  classify_squad_and_usermassinfo_routes "$H" \
+  '        Some(id) => !id.is_empty() && id.bytes().all(|b| b.is_ascii_digit()),' \
+  '        Some(id) => !id.is_empty(),'
+
+kill_test 18 "duplicate definition collapses owned instances (adapter shaper)" \
+  duplicate_definition_instances_stay_distinct_through_host "$ITEM" \
+  '"id": id.item_id,' '"id": id.asset_id,'
+
+kill_test 19 "Content-Length left stale after overlay" \
+  coupled_read_after_write_list_and_usermassinfo_agree "$H" \
+  'resp.headers.push(("Content-Length".to_string(), body.len().to_string()));' \
+  'resp.headers.push(("Content-Length".to_string(), "0".to_string()));'
+
+kill_test 20 "Python squad silently used after Rust projection failure" \
+  usermassinfo_never_serves_python_squad_on_integrity_failure "$H" \
+  'empty_squad_overlay(persona)' 'root["squad"].clone()'
+
+echo "== mutants killed: $PASS / $((PASS+FAIL)) =="
+if [ "$FAIL" -ne 0 ]; then
+  printf 'SURVIVORS:\n'; printf '  - %s\n' "${SURVIVORS[@]}"
+  exit 1
+fi
+echo "all mutants killed"
diff --git a/openfut-utas-host/tests/host_test.rs b/openfut-utas-host/tests/host_test.rs
index 714d6b2..8ce1488 100644
--- a/openfut-utas-host/tests/host_test.rs
+++ b/openfut-utas-host/tests/host_test.rs
@@ -1025,4 +1025,42 @@ fn unrelated_route_still_reaches_python() {
     assert_eq!(resp.status, 200);
     assert!(resp.headers.iter().any(|(k, _)| k.eq_ignore_ascii_case("x-from-python")));
     assert_eq!(rec.lock().len(), 1, "reached Python");
+}
+
+#[test]
+fn duplicate_definition_instances_stay_distinct_through_host() {
+    // Two owned copies of ONE definition (card_a): distinct owned ids and wire
+    // ids, one shared resourceId — must not collapse anywhere in the host.
+    let a = item("oc-a", "card_a", 87, "GK", "Argentina", "Premier League", "Chelsea");
+    let c = item("oc-c", "card_a", 87, "ST", "Argentina", "Premier League", "Chelsea");
+    let items = vec![a, c];
+    let (resolver, w) = resolver_with_wires(&items, &[("card_a", 20801)]);
+    assert_ne!(w["oc-a"], w["oc-c"], "two copies get distinct wire ids");
+    let core = FakeCore::new(items.clone(), 2);
+    let ent = entities();
+    let deps = SquadDeps { core: &core, resolver: &resolver, entities: &ent };
+    let body = put_body("f442", w["oc-a"], &[(0, w["oc-a"], 1), (1, w["oc-c"], 7)], "[]");
+    let (put, _) = handle_put_squad(&body, &deps);
+    assert_eq!(put.status, 200);
+
+    let py = json!({"userInfo": {"personaId": 1}, "squad": {"players": []}});
+    let (url, _r) = spawn_mock_python_json(py);
+    let pass = PassClient::new(&url);
+    let (umi, _) =
+        handle_user_mass_info("GET", "/ut/game/fifa17/userMassInfo", &[], b"", &deps, &pass);
+    let v: Value = serde_json::from_slice(&umi.body).unwrap();
+    let occ: Vec<&Value> = v["squad"]["players"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .filter(|p| p["itemData"]["id"].as_i64().unwrap() != 0)
+        .collect();
+    assert_eq!(occ.len(), 2);
+    assert_ne!(
+        occ[0]["itemData"]["id"], occ[1]["itemData"]["id"],
+        "two copies of one definition keep distinct wire ids"
+    );
+    for p in &occ {
+        assert_eq!(p["itemData"]["resourceId"], 20801, "shared asset id");
+    }
 }
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