diff --git a/infrastructure/spot_train.sh b/infrastructure/spot_train.sh
index cc99a35..39b4959 100755
--- a/infrastructure/spot_train.sh
+++ b/infrastructure/spot_train.sh
@@ -16,6 +16,11 @@
 #   ./infrastructure/spot_train.sh                  # smoke (dry_run) then full
 #   ./infrastructure/spot_train.sh --full-only       # full training only (Saturday SF)
 #   ./infrastructure/spot_train.sh --smoke-only      # smoke only, then terminate
+#   ./infrastructure/spot_train.sh --preflight-only  # boot + import/lib-pin +
+#                                                    # ArcticDB connectivity probe,
+#                                                    # then exit 0 — NO training,
+#                                                    # NO promotion, ZERO S3/config
+#                                                    # writes (Friday shell_run dry path)
 #   ./infrastructure/spot_train.sh --instance-type c5.2xlarge  # override type
 #
 # Prerequisites:
@@ -30,6 +35,9 @@
 #   2. Wait for the SSM agent to register (no SSH)
 #   3. Stage config/predictor.yaml to S3; spot bootstraps + fetches it
 #   4. Run smoke (dry_run=True), then full training (dry_run=False)
+#      — OR, under --preflight-only, run the import/lib-pin + read-only
+#        ArcticDB connectivity probe and exit 0 (no training, no promotion,
+#        no S3/config writes; Friday shell_run dry path)
 #   5. Terminate the spot instance + clean the S3 staging prefix
 #
 # Rollback: `git revert` this commit restores the SSH/SCP script. Port 22
@@ -68,11 +76,12 @@ IAM_PROFILE="alpha-engine-executor-profile"
 REPO_URL="https://github.com/cipher813/alpha-engine-predictor.git"  # public repo, no auth
 
 # Parse flags
-MODE="both"  # both | full-only | smoke-only
+MODE="both"  # both | full-only | smoke-only | preflight-only
 while [ $# -gt 0 ]; do
   case "$1" in
     --full-only) MODE="full-only" ;;
     --smoke-only) MODE="smoke-only" ;;
+    --preflight-only) MODE="preflight-only" ;;
     --instance-type) shift; INSTANCE_TYPE="$1" ;;
   esac
   shift
@@ -278,6 +287,100 @@ $PIP list --format=columns | grep -iE 'numpy|pandas|lightgbm|scikit-learn|scipy|
 DEPS
 )" 900
 
+# ── Preflight-only (Friday shell_run dry path) ────────────────────────────────
+# Boot + lib-pin/import + read-only ArcticDB/universe-freshness probe, then
+# exit 0. This runs the SAME bootstrap+deps steps the real Saturday run uses
+# (so it catches lib-pin drift, sys.path breakage, image gaps, SSM timeouts,
+# stale ArcticDB) but stops HERE — before the smoke step and before the
+# full-training step.
+#
+# Hard invariant under this mode:
+#   • run_meta_training() is NEVER invoked → NO model training, NO walk-forward.
+#   • The `if not dry_run:` upload/promote block in meta_trainer.py is never
+#     reached → NO weights/meta/* write, NO manifest, NO dated archive.
+#   • train_handler.main()'s training_summary / triple-barrier-gate / email /
+#     health-status writes are never reached (they live after run_meta_training).
+#   • The probe imports the training package + runs TrainingPreflight (env +
+#     S3-bucket *reachability* check — no object writes) + a read-only
+#     ArcticDB `list_symbols()` / latest-index probe. No put_object, no
+#     config write, no external API (yfinance/Anthropic) call.
+# The `exit 0` is a clean dispatcher exit; `trap cleanup EXIT` still fires
+# (terminates the spot, clears the S3 staging prefix — staging cleanup only).
+if [ "$MODE" = "preflight-only" ]; then
+  echo ""
+  echo "═══════════════════════════════════════════════════════════════"
+  echo "  PREFLIGHT-ONLY (no training, no promotion, no writes)"
+  echo "═══════════════════════════════════════════════════════════════"
+  run_ssm "preflight-only" "$(cat <<'PREFLIGHT'
+set -eo pipefail
+export HOME=/home/ec2-user XDG_CACHE_HOME=/tmp AWS_REGION=us-east-1 AWS_DEFAULT_REGION=us-east-1
+cd /home/ec2-user/predictor
+command -v python3.12 >/dev/null && PY=python3.12 || PY=python3
+$PY - <<'PYEOF'
+import os, sys
+sys.path.insert(0, '.')
+os.environ.setdefault('S3_BUCKET', os.environ.get('S3_BUCKET', 'alpha-engine-research'))
+bucket = os.environ.get('S3_BUCKET', 'alpha-engine-research')
+
+import logging
+logging.basicConfig(level=logging.INFO, format='%(asctime)s  %(levelname)-8s  %(message)s')
+log = logging.getLogger('preflight-only')
+
+# 1. Import the training package (catches sys.path / lib-pin / image gaps).
+#    Importing train_handler transitively imports the lib + training stack
+#    WITHOUT invoking main(), so no training runs.
+log.info('[1/3] Importing training package...')
+import alpha_engine_lib  # lib-pin presence (version asserted by requirements.txt pin)
+from training import train_handler  # noqa: F401  (import-only; main() NOT called)
+from training.preflight import TrainingPreflight
+log.info('       OK — alpha_engine_lib + training.train_handler import clean')
+
+# 2. Reuse the EXISTING training preflight (env vars + S3 bucket
+#    *reachability*; check_s3_bucket is a read/head, no object write).
+log.info('[2/3] Running TrainingPreflight (env + S3 connectivity)...')
+TrainingPreflight(bucket=bucket).run()
+log.info('       OK — env vars present, S3 bucket reachable')
+
+# 3. Read-only ArcticDB connectivity + universe-freshness probe.
+#    list_symbols() + a single read().tail(1) — NO download_from_arctic(),
+#    NO parquet writes, NO training array build. Mirrors the connectivity
+#    the real run depends on without doing any work.
+log.info('[3/3] ArcticDB connectivity + universe-freshness probe...')
+from store.arctic_reader import _get_arctic
+arctic = _get_arctic(bucket)
+universe = arctic.get_library('universe')
+symbols = universe.list_symbols()
+n = len(symbols)
+if n == 0:
+    raise RuntimeError(
+        'ArcticDB universe library is empty/unreachable — '
+        'Saturday DataPhase1 + weekly backfill have not run cleanly.'
+    )
+probe = sorted(symbols)[0]
+df_tail = universe.read(probe).data.tail(1)
+latest = df_tail.index.max() if not df_tail.empty else 'n/a'
+log.info('       OK — universe has %d symbols; %s latest index=%s', n, probe, latest)
+
+print()
+print('=' * 60)
+print('  PREFLIGHT-ONLY RESULT: PASS')
+print('=' * 60)
+print(f'  Imports:        alpha_engine_lib + training stack clean')
+print(f'  TrainingPreflight: PASS (env + S3 reachable)')
+print(f'  ArcticDB:       {n} universe symbols (probe {probe} latest={latest})')
+print(f'  Training:       SKIPPED (no run_meta_training call)')
+print(f'  Promotion:      SKIPPED (no weights/meta write)')
+print(f'  S3/config writes: NONE')
+print('=' * 60)
+PYEOF
+PREFLIGHT
+)" 600
+  echo ""
+  echo "==> Preflight-only mode — PASS. No training, no promotion, no writes."
+  echo "    Exiting 0 BEFORE smoke + full-training steps."
+  exit 0
+fi
+
 # ── Smoke test (dry_run=True) ─────────────────────────────────────────────────
 if [ "$MODE" != "full-only" ]; then
   echo ""
diff --git a/tests/test_spot_train_preflight_only.py b/tests/test_spot_train_preflight_only.py
new file mode 100644
index 0000000..6961e5d
--- /dev/null
+++ b/tests/test_spot_train_preflight_only.py
@@ -0,0 +1,126 @@
+"""Pins spot_train.sh `--preflight-only` to the Friday shell_run dry-path
+hard invariant: boot + import/lib-pin + read-only ArcticDB connectivity
+probe, then `exit 0` BEFORE the smoke step and BEFORE the full-training
+step — with NO model training, NO weight promotion, and ZERO S3/config
+writes.
+
+Owed-item #2 of ROADMAP "Friday shell-run — per-module dry-path
+activation" (P1). Static-analysis test (mirrors
+test_spot_train_aws_region.py) — the spot_train.sh path is SSM/EC2 and
+cannot be exercised in CI; these assertions guard the structural
+invariant against a future edit that would let preflight-only fall
+through into training.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+_SCRIPT = Path(__file__).resolve().parent.parent / "infrastructure" / "spot_train.sh"
+
+
+def _text() -> str:
+    return _SCRIPT.read_text()
+
+
+def test_spot_train_exists():
+    assert _SCRIPT.is_file()
+
+
+def test_preflight_only_flag_parses():
+    text = _text()
+    assert "--preflight-only) MODE=\"preflight-only\" ;;" in text, (
+        "--preflight-only flag not wired into the flag parser"
+    )
+
+
+def test_preflight_only_branch_exists_and_exits_zero():
+    text = _text()
+    assert 'if [ "$MODE" = "preflight-only" ]; then' in text, (
+        "no dedicated preflight-only branch found"
+    )
+    # The branch must terminate with `exit 0` (clean dispatcher exit;
+    # trap cleanup still terminates the spot + clears S3 staging).
+    branch = text.split('if [ "$MODE" = "preflight-only" ]; then', 1)[1]
+    branch = branch.split("# ── Smoke test", 1)[0]
+    assert "exit 0" in branch, "preflight-only branch must exit 0"
+
+
+def test_preflight_only_runs_before_smoke_and_full_training():
+    text = _text()
+    i_branch = text.index('if [ "$MODE" = "preflight-only" ]; then')
+    i_smoke = text.index("# ── Smoke test (dry_run=True)")
+    i_full = text.index("# ── Full training (dry_run=False)")
+    assert i_branch < i_smoke < i_full, (
+        "preflight-only branch must precede the smoke and full-training "
+        "sections so its exit 0 short-circuits before any training"
+    )
+
+
+def test_preflight_only_does_not_invoke_training_or_promotion():
+    """The preflight-only SSM step body must not call run_meta_training /
+    train_handler.main / dry_run training — those are the train+promote
+    entrypoints. It may only import the package + run TrainingPreflight +
+    the read-only ArcticDB probe."""
+    text = _text()
+    start = text.index('run_ssm "preflight-only"')
+    # End of the preflight-only run_ssm heredoc payload.
+    end = text.index("PREFLIGHT\n)\"", start)
+    payload = text[start:end]
+
+    # Forbidden as *executable references* (call/import forms), not as
+    # mentions inside the human-readable proof print() strings. We strip
+    # the print(...) / log.info(...) diagnostic lines first so the proof
+    # text ("no run_meta_training call") doesn't false-positive.
+    code_lines = [
+        ln
+        for ln in payload.splitlines()
+        if not ln.lstrip().startswith(("print(", "log.info(", "#"))
+    ]
+    code = "\n".join(code_lines)
+    forbidden = [
+        "run_meta_training(",
+        "import run_meta_training",
+        "train_main(",
+        "from training.train_handler import main",
+        "download_from_arctic",  # that is the training data DOWNLOAD, not a probe
+        "put_object",
+        "upload_file",
+    ]
+    for token in forbidden:
+        assert token not in code, (
+            f"preflight-only step must NOT reference {token!r} — "
+            f"it would break the no-train/no-promote/no-write invariant"
+        )
+
+    # Positive: it must do the reuse-not-rebuild things.
+    assert "TrainingPreflight" in payload, (
+        "preflight-only must reuse the existing training/preflight.py "
+        "TrainingPreflight (do not rebuild a parallel preflight)"
+    )
+    assert "import alpha_engine_lib" in payload, (
+        "preflight-only must import alpha_engine_lib to catch lib-pin drift"
+    )
+    assert "list_symbols()" in payload, (
+        "preflight-only must do a read-only ArcticDB universe probe"
+    )
+
+
+def test_preflight_only_step_keeps_aws_region_export():
+    """Same #247 regression guard as test_spot_train_aws_region.py — the
+    new step's env line must still export AWS_REGION/AWS_DEFAULT_REGION
+    (TrainingPreflight.check_env_vars('AWS_REGION') hard-requires it)."""
+    text = _text()
+    start = text.index('run_ssm "preflight-only"')
+    end = text.index("PREFLIGHT\n)\"", start)
+    payload = text[start:end]
+    export_lines = [
+        ln
+        for ln in payload.splitlines()
+        if ln.startswith("export HOME=/home/ec2-user XDG_CACHE_HOME=/tmp")
+    ]
+    assert export_lines, "preflight-only step missing its per-step export line"
+    for ln in export_lines:
+        assert "AWS_REGION=" in ln and "AWS_DEFAULT_REGION=" in ln, (
+            f"preflight-only export line missing AWS region vars: {ln!r}"
+        )