diff --git a/tools/selfhost/selfhost_build.sh b/tools/selfhost/selfhost_build.sh
index 0ab3e359..e60037f2 100644
--- a/tools/selfhost/selfhost_build.sh
+++ b/tools/selfhost/selfhost_build.sh
@@ -8,9 +8,20 @@
 #
 # Usage:
 #   tools/selfhost/selfhost_build.sh --in source.hako [--json out.json] [--run]
+#   Options:
+#     --in FILE     Input .hako source file (required)
+#     --json FILE   Output Program(JSON v0) to FILE
+#     --run         Run via Core-Direct after compilation
+#     --mir FILE    Also emit MIR(JSON) to FILE
+#     --exe FILE    Build native EXE via ny-llvmc
+#     --keep-tmp    Keep temporary files
+#     --core        Phase 80: Enable JoinIR Core ON (mainline path)
+#     --strict      Phase 81: Enable Strict mode (fail-fast, no fallback)
 #   Env:
 #     NYASH_BIN: path to hakorune/nyash binary (auto-detected if omitted)
 #     NYASH_ROOT: repo root (auto-detected)
+#     NYASH_JOINIR_CORE: Set to 1 for Core ON mode
+#     NYASH_JOINIR_STRICT: Set to 1 for Strict mode
 #
 set -euo pipefail
 
@@ -47,6 +58,14 @@ apply_selfhost_env() {
   # Ensure core plugins (Console/Array/Map/String/Integer) are discoverable
   export NYASH_PLUGIN_PATH="${NYASH_PLUGIN_PATH:-$ROOT/target/release}"
   export NYASH_PLUGIN_PATHS="${NYASH_PLUGIN_PATHS:-$NYASH_PLUGIN_PATH}"
+  # Phase 80/81: JoinIR Core/Strict mode propagation
+  # Pass through NYASH_JOINIR_CORE and NYASH_JOINIR_STRICT if set
+  if [ -n "${NYASH_JOINIR_CORE:-}" ]; then
+    export NYASH_JOINIR_CORE
+  fi
+  if [ -n "${NYASH_JOINIR_STRICT:-}" ]; then
+    export NYASH_JOINIR_STRICT
+  fi
 }
 
 while [ $# -gt 0 ]; do
@@ -57,6 +76,8 @@ while [ $# -gt 0 ]; do
     --mir) MIR_OUT="$2"; shift 2;;
     --keep-tmp) KEEP_TMP=1; shift;;
     --exe) EXE_OUT="$2"; shift 2;;
+    --strict) export NYASH_JOINIR_STRICT=1; shift;; # Phase 81: Fail-Fast mode
+    --core) export NYASH_JOINIR_CORE=1; shift;;     # Phase 80: Core ON mode
     *) echo "[selfhost] unknown arg: $1" >&2; exit 2;;
   esac
 done