diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f085b541..3175e0db 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -17,6 +17,11 @@ repos:
       - id: check-toml
       - id: check-added-large-files
         args: ["--maxkb=512"]
+        # #1966: the frozen backtest regression-gate fixture is gzipped prod
+        # inputs (~3 MB). It is re-extracted rarely (only when ground-truth
+        # refreshes), so it does not bloat history per estimator change — the
+        # per-change artifact is the 3 KB backtest_baseline.json.
+        exclude: ^tradein-mvp/backend/tests/fixtures/backtest_full_fixture\.json\.gz$
       - id: check-merge-conflict
       - id: detect-private-key
 
diff --git a/tradein-mvp/backend/scripts/backtest_estimator.py b/tradein-mvp/backend/scripts/backtest_estimator.py
index 0508c85a..ffa1e65f 100644
--- a/tradein-mvp/backend/scripts/backtest_estimator.py
+++ b/tradein-mvp/backend/scripts/backtest_estimator.py
@@ -30,7 +30,8 @@ For a held-out sample of ДКП deals we, per deal:
      ``_fetch_house_imv_anchor``) + inject the DB callables
      (``_get_asking_sold_ratio``, ``_lookup_quarter_index(es)``).
   2. Call ``_price_from_inputs`` for a byte-identical headline + expected_sold.
-     Deals the spine cannot price (median<=0 / <3 analogs) are skipped.
+     Deals the spine cannot price (median<=0) are skipped — prod parity (#1966):
+     there is NO analog-count floor, low-analog deals surface at low confidence.
   3. Score ``expected_sold_per_m2`` vs the realised SOLD ppm².
 
 METRICS (full spine)
@@ -87,10 +88,16 @@ USAGE
 from __future__ import annotations
 
 import argparse
+import dataclasses
+import gzip
 import json
 import logging
+import math
 import statistics
+from collections.abc import Callable
 from dataclasses import dataclass
+from datetime import date
+from decimal import Decimal
 from pathlib import Path
 from types import SimpleNamespace
 from typing import Any
@@ -1146,7 +1153,199 @@ def _select_analogs_full(
     return listings, analog_tier, fallback_used, area_widened
 
 
-def _predict_full_spine(db: Session, deal: DealSample, est: SimpleNamespace) -> Prediction | None:
+# --------------------------------------------------------------------------- #
+# Fixture capture + hermetic replay (#1966 PR 3/3)
+#
+# ``--dump-fixture`` freezes per-deal RESOLVED inputs to ``_price_from_inputs``
+# into a committed JSON file; ``replay_fixture`` replays the spine offline (ZERO
+# DB) so a CI test can assert metrics match a frozen baseline. The 3 DB-backed
+# callables (asking→sold ratio + quarter-index lookups) hide all I/O, so we
+# capture each as an ordered ``[arg -> return]`` call-list and replay it via an
+# exact-match stub. Everything else fed to ``_price_from_inputs`` is pure data.
+# --------------------------------------------------------------------------- #
+
+FIXTURE_SCHEMA_VERSION = 1
+
+
+def _sanitize_json(obj: Any) -> Any:
+    """Recursively coerce a captured value into a finite, JSON-plain structure.
+
+    - non-finite float (inf / nan) → None (``json.dump(allow_nan=False)`` would
+      otherwise raise);
+    - Decimal → float; date / datetime → ISO str; tuple → list; dict / list
+      recursed; bool / int / str / None pass through; any other type → ``str()``.
+
+    Idempotent on already-plain data, so applying it both at capture time (to the
+    recorded call args) and at replay time (to the live args before stub lookup)
+    keeps the lookup keys byte-stable across capture and replay.
+    """
+    if obj is None or isinstance(obj, bool | int | str):
+        return obj
+    if isinstance(obj, float):
+        return obj if math.isfinite(obj) else None
+    if isinstance(obj, Decimal):
+        f = float(obj)
+        return f if math.isfinite(f) else None
+    if isinstance(obj, dict):
+        return {str(k): _sanitize_json(v) for k, v in obj.items()}
+    if isinstance(obj, list | tuple):
+        return [_sanitize_json(x) for x in obj]
+    if isinstance(obj, date):  # date | datetime (datetime is a date subclass)
+        return obj.isoformat()
+    return str(obj)
+
+
+def _coerce_ratio_return(ret: Any) -> tuple[Any, Any]:
+    """Recorded ``[ratio, basis]`` → ``(ratio, basis)`` tuple (unpacked at the call site)."""
+    return (ret[0], ret[1])
+
+
+def _coerce_qi_return(ret: Any) -> tuple[Any, Any] | None:
+    """Recorded ``[qi, n]`` or ``null`` → ``(qi, n)`` tuple or None."""
+    return None if ret is None else (ret[0], ret[1])
+
+
+def _coerce_qis_return(ret: Any) -> dict[str, float]:
+    """Recorded ``{quarter: qi}`` dict → ``dict[str, float]`` (keys forced to str)."""
+    return {str(k): v for k, v in dict(ret).items()}
+
+
+def load_fixture(path: str) -> dict[str, Any]:
+    """Load a backtest fixture JSON — transparently handles gzip (``.gz``) files.
+
+    The committed fixture is gzipped (~3 MB raw); a plain ``.json`` path is also
+    accepted for ad-hoc runs. Exported for the CI regression-gate test.
+    """
+    if str(path).endswith(".gz"):
+        with gzip.open(path, "rt", encoding="utf-8") as fh:
+            return json.loads(fh.read())
+    return json.loads(Path(path).read_text(encoding="utf-8"))
+
+
+def _make_call_stub(
+    calls: list[Any], *, label: str, coerce: Callable[[Any], Any]
+) -> Callable[[Any], Any]:
+    """Build an ORDER-based (FIFO) replay stub from recorded ``[arg, return]`` pairs.
+
+    The stub returns the recorded RETURNS in invocation order and IGNORES the arg
+    value. Rationale: the DB-derived ratio / quarter-index is a FROZEN input — when
+    estimator logic later shifts e.g. ``median_ppm2``, the regression gate must
+    surface that as a clean metric diff vs the baseline, NOT mask it as a lookup
+    miss. Matching on the exact computed float arg is also not bit-stable across
+    platforms (libm last-ulp jitter), so a Linux-captured fixture would crash when
+    replayed off-Linux. The recorded arg is kept (for debuggability) but never
+    matched. Each callable fires ≤1×/deal today (so index 0 in practice); the FIFO
+    stays correct if a call site ever loops. Calling the stub MORE times than
+    recorded raises RuntimeError — control flow diverged from capture.
+
+    ``coerce`` maps each JSON-plain recorded return back to the live callable's
+    return type (tuple / dict) so unpacking at the call site behaves identically.
+    """
+    returns = [coerce(ret) for _arg, ret in calls]
+    idx = 0
+
+    def _stub(_arg: Any) -> Any:
+        nonlocal idx
+        if idx >= len(returns):
+            raise RuntimeError(
+                f"{label}: replay made call #{idx + 1} but fixture recorded only "
+                f"{len(returns)} — control flow diverged from capture"
+            )
+        ret = returns[idx]
+        idx += 1
+        return ret
+
+    return _stub
+
+
+def replay_fixture(fixture: dict[str, Any]) -> dict[str, Any]:
+    """Replay a frozen backtest fixture through the full spine — hermetic, ZERO DB.
+
+    For every captured deal record: rebuild the ``GeocodeResult``, build order-based
+    (FIFO) stubs for the 3 DB callables from the recorded call-lists, call
+    ``_price_from_inputs`` with the frozen kwargs, and rebuild the ``Prediction``
+    EXACTLY as ``_predict_full_spine`` does. Runs the harness's own
+    ``_compute_full_metrics`` over the replayed predictions plus a city-wide
+    headline computed from the stored (priced) deals.
+
+    The returned dict keeps ``expected_sold`` / ``range_coverage`` / ``calibration``
+    / ``sharpness`` / ``confidence_order`` / ``headline`` and DROPS the volatile
+    ``params`` block. Touches NO DB / network and does NOT consult
+    ``settings_at_capture`` — it prices against the live committed
+    ``estimator.settings`` defaults (so a settings change is caught as a metric
+    drift, not silently honoured). Deterministic: same fixture → identical dict.
+    """
+    est = _import_estimator_full()
+    m = est.m
+
+    deals = fixture.get("deals") or []
+    predictions: list[Prediction] = []
+    sold_ppm2_all: list[float] = []
+    pred_ppm2_all: list[float] = []
+
+    for rec in deals:
+        kw = dict(rec["kwargs"])
+        sold_ppm2_all.append(float(rec["sold_ppm2"]))
+        kw["geo"] = est.GeocodeResult(**kw["geo"])
+        kw["ratio_resolver"] = _make_call_stub(
+            rec.get("ratio_calls") or [], label="ratio_resolver", coerce=_coerce_ratio_return
+        )
+        kw["quarter_index_lookup"] = _make_call_stub(
+            rec.get("qi_calls") or [], label="quarter_index_lookup", coerce=_coerce_qi_return
+        )
+        kw["quarter_indexes_lookup"] = _make_call_stub(
+            rec.get("qis_calls") or [], label="quarter_indexes_lookup", coerce=_coerce_qis_return
+        )
+
+        pr = m._price_from_inputs(**kw)
+
+        es_ppm2 = float(pr.expected_sold_per_m2) if pr.expected_sold_per_m2 is not None else None
+        es_price = float(pr.expected_sold_price) if pr.expected_sold_price is not None else None
+        r_low = (
+            float(pr.expected_sold_range_low) if pr.expected_sold_range_low is not None else None
+        )
+        r_high = (
+            float(pr.expected_sold_range_high) if pr.expected_sold_range_high is not None else None
+        )
+        prediction = Prediction(
+            deal_id=int(rec["deal_id"]),
+            rooms=rec["rooms"],
+            area_m2=float(rec["area_m2"]),
+            sold_ppm2=float(rec["sold_ppm2"]),
+            median_ppm2=float(pr.median_ppm2),
+            confidence=pr.confidence,
+            anchor_tier=pr.anchor_tier,
+            expected_sold_ppm2=es_ppm2,
+            expected_sold_price=es_price,
+            range_low=r_low,
+            range_high=r_high,
+        )
+        predictions.append(prediction)
+        pred_ppm2_all.append(prediction.median_ppm2)
+
+    # The fixture stores ONLY priced deals, so n_no_prediction is 0 here.
+    metrics = _compute_full_metrics(predictions, n_no_prediction=0)
+
+    deal_median = statistics.median(sold_ppm2_all) if sold_ppm2_all else None
+    ask_median = statistics.median(pred_ppm2_all) if pred_ppm2_all else None
+    spread_pct: float | None = None
+    if deal_median and ask_median and deal_median > 0:
+        spread_pct = round(100.0 * (ask_median - deal_median) / deal_median, 2)
+    metrics["headline"] = {
+        "deal_median_ppm2": deal_median,
+        "ask_median_ppm2": ask_median,
+        "spread_pct": spread_pct,
+    }
+    return metrics
+
+
+def _predict_full_spine(
+    db: Session,
+    deal: DealSample,
+    est: SimpleNamespace,
+    *,
+    capture: list[dict[str, Any]] | None = None,
+) -> Prediction | None:
     """Predict one deal through the FULL deterministic spine (#1966).
 
     Selects analogs via the replicated tier ladder, pre-fetches the spine inputs
@@ -1156,7 +1355,13 @@ def _predict_full_spine(db: Session, deal: DealSample, est: SimpleNamespace) ->
     the network valuation layers excluded (imv_eval=None, yandex/cian absent).
 
     Returns a Prediction, or None when the spine cannot price the deal
-    (median<=0 or <MIN_CANDIDATES analogs) — mirroring the asking-core skip.
+    (median<=0). Prod parity (#1966): there is NO analog-count skip — a positive
+    median surfaces an estimate (at low confidence for thin samples), matching
+    estimate_quality which never hard-drops a priced result.
+
+    When ``capture`` is a list, every deal that yields a prediction also appends a
+    fully JSON-plain replay record (frozen kwargs + the 3 callables' recorded
+    ``[arg -> return]`` call-lists) — see ``--dump-fixture`` / ``replay_fixture``.
     """
     m = est.m
     settings = est.settings
@@ -1165,15 +1370,21 @@ def _predict_full_spine(db: Session, deal: DealSample, est: SimpleNamespace) ->
 
     # ── Pre-fetch the spine inputs (same calls estimate_quality hoists) ───────
     dkp_raw = m._fetch_dkp_corridor(db, address=deal.address, rooms=deal.rooms, area=deal.area_m2)
-    anchor_comps, anchor_tier = m._fetch_anchor_comps(
-        db,
-        address=deal.address,
-        target_house_id=None,
-        lat=deal.lat,
-        lon=deal.lon,
-        rooms=deal.rooms,
-        area=deal.area_m2,
-    )
+    # #1966 prod parity: same-building anchor pre-fetch is GATED exactly like
+    # estimate_quality (estimator.py L2862-2881) — disabled / no-area / no-address
+    # → ([], None) instead of an unconditional fetch.
+    if settings.estimate_same_building_anchor_enabled and deal.area_m2 and deal.address:
+        anchor_comps, anchor_tier = m._fetch_anchor_comps(
+            db,
+            address=deal.address,
+            target_house_id=None,
+            lat=deal.lat,
+            lon=deal.lon,
+            rooms=deal.rooms,
+            area=deal.area_m2,
+        )
+    else:
+        anchor_comps, anchor_tier = [], None
     imv_anchor = m._fetch_house_imv_anchor(
         db, target_house_id=None, rooms=deal.rooms, area=deal.area_m2
     )
@@ -1191,22 +1402,38 @@ def _predict_full_spine(db: Session, deal: DealSample, est: SimpleNamespace) ->
         confidence="exact",
     )
 
+    # In capture mode each closure RECORDS its (single-arg -> return) call so the
+    # offline replay can rebuild a DB-free stub. The real value is still returned.
+    ratio_calls: list[Any] = []
+    qi_calls: list[Any] = []
+    qis_calls: list[Any] = []
+
     def _ratio_resolver(appm2: float | None) -> tuple[float | None, str | None]:
-        return m._get_asking_sold_ratio(db, deal.rooms, anchor_ppm2=appm2)
+        res = m._get_asking_sold_ratio(db, deal.rooms, anchor_ppm2=appm2)
+        if capture is not None:
+            ratio_calls.append([_sanitize_json(appm2), _sanitize_json(list(res))])
+        return res
 
     def _qi_lookup(q: str) -> tuple[float, int] | None:
-        return m._lookup_quarter_index(
+        res = m._lookup_quarter_index(
             db,
             quarter_cad_number=q,
             min_n_deals=settings.estimate_quarter_index_min_n_deals,
         )
+        if capture is not None:
+            ret = _sanitize_json(list(res) if res is not None else None)
+            qi_calls.append([_sanitize_json(q), ret])
+        return res
 
     def _qis_lookup(qs: list[str]) -> dict[str, float]:
-        return m._lookup_quarter_indexes(
+        res = m._lookup_quarter_indexes(
             db,
             quarter_cad_numbers=qs,
             min_n_deals=settings.estimate_quarter_index_min_n_deals,
         )
+        if capture is not None:
+            qis_calls.append([_sanitize_json(list(qs)), _sanitize_json(dict(res))])
+        return res
 
     pr = m._price_from_inputs(
         listings=listings,
@@ -1235,14 +1462,54 @@ def _predict_full_spine(db: Session, deal: DealSample, est: SimpleNamespace) ->
         dadata_qc_geo=None,
     )
 
-    # Skip when the spine couldn't price it — mirror the asking-core skip.
-    if pr.median_price <= 0 or pr.n_analogs < MIN_CANDIDATES:
+    # #1966 prod parity: skip ONLY when the spine could not price the deal
+    # (median<=0). NO analog-count floor — estimate_quality surfaces any positive
+    # estimate at low confidence. MIN_CANDIDATES still gates the asking-core path.
+    if pr.median_price <= 0:
         return None
 
     es_ppm2 = float(pr.expected_sold_per_m2) if pr.expected_sold_per_m2 is not None else None
     es_price = float(pr.expected_sold_price) if pr.expected_sold_price is not None else None
     r_low = float(pr.expected_sold_range_low) if pr.expected_sold_range_low is not None else None
     r_high = float(pr.expected_sold_range_high) if pr.expected_sold_range_high is not None else None
+
+    if capture is not None:
+        capture.append(
+            {
+                "deal_id": deal.id,
+                "sold_ppm2": float(deal.sold_ppm2),
+                "area_m2": float(deal.area_m2),
+                "rooms": deal.rooms,
+                "deal_date": str(deal.deal_date),
+                "kwargs": {
+                    "listings": _sanitize_json(listings),
+                    "area_m2": float(deal.area_m2),
+                    "rooms": deal.rooms,
+                    "repair_state": None,
+                    "floor": deal.floor,
+                    "total_floors": deal.total_floors,
+                    "target_year": deal.year_built,
+                    "analog_tier": analog_tier,
+                    "fallback_used": fallback_used,
+                    "area_widened": area_widened,
+                    "anchor_comps": _sanitize_json(anchor_comps),
+                    "anchor_tier_fetched": anchor_tier,
+                    "dkp_raw": _sanitize_json(dkp_raw),
+                    "imv_anchor": _sanitize_json(imv_anchor),
+                    "imv_eval": None,
+                    "yandex_val_present": False,
+                    "cian_val_present": False,
+                    "target_house_cadnum": None,
+                    "dadata_coarse": False,
+                    "geo": _sanitize_json(dataclasses.asdict(geo)),
+                    "dadata_qc_geo": None,
+                },
+                "ratio_calls": ratio_calls,
+                "qi_calls": qi_calls,
+                "qis_calls": qis_calls,
+            }
+        )
+
     return Prediction(
         deal_id=deal.id,
         rooms=deal.rooms,
@@ -1390,19 +1657,26 @@ def run_backtest(
     return metrics
 
 
-def run_backtest_full(db: Session, *, sample: int, since: str) -> dict[str, Any]:
+def run_backtest_full(
+    db: Session, *, sample: int, since: str, dump_fixture: str | None = None
+) -> dict[str, Any]:
     """Drive the FULL-spine read-only backtest and return a metrics dict (#1966).
 
     Per deal: load sample → ``_predict_full_spine`` (replicate the analog tier
     ladder + pre-fetch spine inputs → ``_price_from_inputs``) → collect Prediction
     records + no-prediction counts → ``_compute_full_metrics`` (expected_sold
     error overall/per-rooms/per-segment + range-coverage + calibration +
-    sharpness) + a city-wide asking-vs-deal headline spread. No writes.
+    sharpness) + a city-wide asking-vs-deal headline spread. No writes to the DB.
 
     ``--radius`` / ``--rooms-tolerance`` / ``--holdout-split`` do NOT apply here —
     the tier ladder uses the estimator's OWN constants (DEFAULT_RADIUS_M /
     FALLBACK_RADIUS_M) and there is no per-rooms correction block (the spine
     already emits expected_sold via the asking→sold ratio).
+
+    When ``dump_fixture`` is a path, every priced deal's RESOLVED spine inputs +
+    the 3 DB callables' recorded call-lists are frozen into a committed JSON file
+    (schema_version=1) so ``replay_fixture`` can re-score offline against a frozen
+    baseline (the CI regression gate). This is the only path that writes a file.
     """
     est = _import_estimator_full()
     deals = _load_sample(db, sample=sample, since=since)
@@ -1414,10 +1688,11 @@ def run_backtest_full(db: Session, *, sample: int, since: str) -> dict[str, Any]
 
     sold_ppm2_all: list[float] = [d.sold_ppm2 for d in deals]
     pred_ppm2_all: list[float] = []  # asking headline median_ppm2 (priced deals)
+    capture: list[dict[str, Any]] | None = [] if dump_fixture else None
 
     for i, deal in enumerate(deals, start=1):
         try:
-            pr = _predict_full_spine(db, deal, est)
+            pr = _predict_full_spine(db, deal, est, capture=capture)
         except Exception as exc:
             # Read-only: a failed SELECT can poison the tx → rollback so the next
             # deal's queries run clean. Skip this deal (counts as no-prediction).
@@ -1467,9 +1742,47 @@ def run_backtest_full(db: Session, *, sample: int, since: str) -> dict[str, Any]
         "n_no_prediction": n_no_prediction,
         "price_segments_ppm2": [list(seg) for seg in PRICE_SEGMENTS_PPM2],
     }
+
+    if dump_fixture is not None and capture is not None:
+        _write_fixture(dump_fixture, capture=capture, since=since, est=est)
+
     return metrics
 
 
+def _write_fixture(
+    path: str, *, capture: list[dict[str, Any]], since: str, est: SimpleNamespace
+) -> None:
+    """Freeze captured per-deal replay records into a committed JSON fixture.
+
+    ``settings_at_capture`` records every ``estimate_*`` Settings field as an
+    informational snapshot (NOT consulted by ``replay_fixture``). A final
+    recursive ``_sanitize_json`` pass guarantees the whole document is finite +
+    JSON-plain before ``json.dump(allow_nan=False)``.
+    """
+    settings_at_capture = {
+        name: _sanitize_json(getattr(est.settings, name))
+        for name in sorted(type(est.settings).model_fields)
+        if name.startswith("estimate_")
+    }
+    fixture = {
+        "schema_version": FIXTURE_SCHEMA_VERSION,
+        "engine": "full",
+        "since": since,
+        "n_deals": len(capture),
+        "settings_at_capture": settings_at_capture,
+        "deals": capture,
+    }
+    fixture = _sanitize_json(fixture)
+    out_path = Path(path)
+    if out_path.parent != Path():
+        out_path.parent.mkdir(parents=True, exist_ok=True)
+    with out_path.open("w", encoding="utf-8") as fh:
+        json.dump(fixture, fh, ensure_ascii=False, indent=2, allow_nan=False)
+        fh.write("\n")
+    logger.info("dumped fixture: %s (%d deals)", out_path, len(capture))
+    print(f"dumped fixture: {out_path} ({len(capture)} deals)")
+
+
 # --------------------------------------------------------------------------- #
 # Entry point
 # --------------------------------------------------------------------------- #
@@ -1531,27 +1844,76 @@ def _parse_args(argv: list[str] | None = None) -> argparse.Namespace:
         action="store_true",
         help="Emit machine-readable JSON instead of the text table.",
     )
+    # #1966 PR 3/3 — fixture capture + hermetic replay. --dump-fixture (DB run,
+    # full engine) and --from-fixture (NO DB) are mutually exclusive modes.
+    fixture_mode = p.add_mutually_exclusive_group()
+    fixture_mode.add_argument(
+        "--dump-fixture",
+        metavar="PATH",
+        default=None,
+        help="FULL engine only: freeze each priced deal's resolved _price_from_"
+        "inputs inputs + the 3 DB callables' recorded calls into a committed JSON "
+        "fixture at PATH, so replay_fixture can re-score offline (CI gate).",
+    )
+    fixture_mode.add_argument(
+        "--from-fixture",
+        metavar="PATH",
+        default=None,
+        help="Hermetic replay: load the frozen fixture at PATH (gzip-transparent — "
+        "a .gz path is decompressed on the fly), re-run the spine offline (ZERO DB) "
+        "via replay_fixture, and print the metrics JSON. No DB connection is opened.",
+    )
+    p.add_argument(
+        "--update-baseline",
+        metavar="OUT",
+        default=None,
+        help="With --from-fixture: also write the replayed metrics to OUT "
+        "(sorted-keys JSON + trailing newline) — regenerates the frozen baseline.",
+    )
     return p.parse_args(argv)
 
 
 def main(argv: list[str] | None = None) -> int:
-    """CLI entry point. Returns the count of matched (predicted) deals."""
+    """CLI entry point. Returns the count of matched (predicted) / replayed deals."""
     args = _parse_args(argv)
+
+    # ── Hermetic replay path (#1966 PR 3/3) — ZERO DB, no SessionLocal opened. ──
+    if args.from_fixture:
+        fixture = load_fixture(args.from_fixture)
+        metrics = replay_fixture(fixture)
+        print(json.dumps(metrics, ensure_ascii=False, indent=2, sort_keys=True))
+        if args.update_baseline:
+            out = Path(args.update_baseline)
+            out.write_text(
+                json.dumps(metrics, ensure_ascii=False, indent=2, sort_keys=True) + "\n",
+                encoding="utf-8",
+            )
+            logger.info("wrote baseline: %s", out)
+        return len(fixture.get("deals") or [])
+
+    if args.update_baseline:
+        raise SystemExit("--update-baseline requires --from-fixture")
+    if args.dump_fixture and args.engine != "full":
+        raise SystemExit("--dump-fixture is only supported with --engine full")
+
     logger.info(
         "backtest start: engine=%s sample=%d since=%s radius=%dm "
-        "rooms_tolerance=%d holdout_split=%s",
+        "rooms_tolerance=%d holdout_split=%s dump_fixture=%s",
         args.engine,
         args.sample,
         args.since,
         args.radius,
         args.rooms_tolerance,
         args.holdout_split,
+        args.dump_fixture,
     )
 
     db = _session()
     try:
         if args.engine == "full":
-            metrics = run_backtest_full(db, sample=args.sample, since=args.since)
+            metrics = run_backtest_full(
+                db, sample=args.sample, since=args.since, dump_fixture=args.dump_fixture
+            )
         else:
             metrics = run_backtest(
                 db,
diff --git a/tradein-mvp/backend/tests/fixtures/.gitattributes b/tradein-mvp/backend/tests/fixtures/.gitattributes
new file mode 100644
index 00000000..5047b712
--- /dev/null
+++ b/tradein-mvp/backend/tests/fixtures/.gitattributes
@@ -0,0 +1,6 @@
+# #1966 PR 3/3 — frozen backtest regression-gate artifacts.
+# The fixture is gzipped binary: never apply CRLF/text filters (would corrupt it).
+backtest_full_fixture.json.gz binary
+# The baseline is the diff-visible artifact; keep it LF so `--update-baseline`
+# regen (which writes LF) never shows spurious line-ending churn.
+backtest_baseline.json text eol=lf
diff --git a/tradein-mvp/backend/tests/fixtures/backtest_baseline.json b/tradein-mvp/backend/tests/fixtures/backtest_baseline.json
new file mode 100644
index 00000000..56b96a3e
--- /dev/null
+++ b/tradein-mvp/backend/tests/fixtures/backtest_baseline.json
@@ -0,0 +1,154 @@
+{
+  "calibration": {
+    "high": {
+      "coverage_pct": null,
+      "mape_pct": null,
+      "n": 0,
+      "n_covered": 0
+    },
+    "low": {
+      "coverage_pct": 55.27,
+      "mape_pct": 18.63,
+      "n": 275,
+      "n_covered": 152
+    },
+    "medium": {
+      "coverage_pct": 50.0,
+      "mape_pct": 19.18,
+      "n": 2,
+      "n_covered": 1
+    }
+  },
+  "confidence_order": [
+    "high",
+    "medium",
+    "low"
+  ],
+  "expected_sold": {
+    "overall": {
+      "mape_pct": 18.63,
+      "median_bias_pct": -1.2,
+      "n": 277,
+      "n_no_analogs": 0,
+      "p25_pct": -16.25,
+      "p75_pct": 20.26
+    },
+    "per_rooms": {
+      "0": {
+        "label": "студия",
+        "mape_pct": 27.97,
+        "median_bias_pct": 27.56,
+        "n": 37,
+        "n_no_analogs": 0,
+        "p25_pct": -8.97,
+        "p75_pct": 38.45
+      },
+      "1": {
+        "label": "1к",
+        "mape_pct": 19.55,
+        "median_bias_pct": -8.08,
+        "n": 93,
+        "n_no_analogs": 0,
+        "p25_pct": -21.27,
+        "p75_pct": 13.3
+      },
+      "2": {
+        "label": "2к",
+        "mape_pct": 16.22,
+        "median_bias_pct": -8.82,
+        "n": 74,
+        "n_no_analogs": 0,
+        "p25_pct": -21.01,
+        "p75_pct": 6.13
+      },
+      "3": {
+        "label": "3к",
+        "mape_pct": 10.52,
+        "median_bias_pct": 4.08,
+        "n": 43,
+        "n_no_analogs": 0,
+        "p25_pct": -6.54,
+        "p75_pct": 11.96
+      },
+      "4": {
+        "label": "4+",
+        "mape_pct": 23.53,
+        "median_bias_pct": 14.35,
+        "n": 30,
+        "n_no_analogs": 0,
+        "p25_pct": -0.72,
+        "p75_pct": 35.27
+      }
+    },
+    "per_segment": {
+      "бизнес": {
+        "mape_pct": 22.14,
+        "median_bias_pct": -21.49,
+        "n": 46,
+        "p25_pct": -28.22,
+        "p75_pct": -9.82
+      },
+      "комфорт": {
+        "mape_pct": 16.74,
+        "median_bias_pct": -8.05,
+        "n": 104,
+        "p25_pct": -20.63,
+        "p75_pct": 7.55
+      },
+      "премиум": {
+        "mape_pct": 59.37,
+        "median_bias_pct": -59.37,
+        "n": 1,
+        "p25_pct": -59.37,
+        "p75_pct": -59.37
+      },
+      "эконом": {
+        "mape_pct": 18.01,
+        "median_bias_pct": 17.17,
+        "n": 120,
+        "p25_pct": 1.73,
+        "p75_pct": 46.96
+      },
+      "элит": {
+        "mape_pct": 38.62,
+        "median_bias_pct": -38.62,
+        "n": 6,
+        "p25_pct": -47.98,
+        "p75_pct": -33.18
+      }
+    }
+  },
+  "headline": {
+    "ask_median_ppm2": 147545.8502510892,
+    "deal_median_ppm2": 125063.0,
+    "spread_pct": 17.98
+  },
+  "range_coverage": {
+    "overall": {
+      "coverage_pct": 55.23,
+      "n": 277,
+      "n_covered": 153
+    },
+    "per_confidence": {
+      "high": {
+        "coverage_pct": null,
+        "n": 0,
+        "n_covered": 0
+      },
+      "low": {
+        "coverage_pct": 55.27,
+        "n": 275,
+        "n_covered": 152
+      },
+      "medium": {
+        "coverage_pct": 50.0,
+        "n": 2,
+        "n_covered": 1
+      }
+    }
+  },
+  "sharpness": {
+    "median_rel_width": 0.4638,
+    "n": 277
+  }
+}
diff --git a/tradein-mvp/backend/tests/fixtures/backtest_full_fixture.json.gz b/tradein-mvp/backend/tests/fixtures/backtest_full_fixture.json.gz
new file mode 100644
index 00000000..dff67155
Binary files /dev/null and b/tradein-mvp/backend/tests/fixtures/backtest_full_fixture.json.gz differ
diff --git a/tradein-mvp/backend/tests/test_backtest_fixture_roundtrip.py b/tradein-mvp/backend/tests/test_backtest_fixture_roundtrip.py
new file mode 100644
index 00000000..10adf67d
--- /dev/null
+++ b/tradein-mvp/backend/tests/test_backtest_fixture_roundtrip.py
@@ -0,0 +1,280 @@
+"""Synthetic round-trip test for the hermetic fixture replay (#1966 PR 3/3).
+
+Proves the ``--dump-fixture`` / ``replay_fixture`` machinery end-to-end WITHOUT a
+DB: a hand-crafted in-memory fixture (the exact JSON shape ``--dump-fixture``
+writes) is replayed through the full pricing spine via ``bt.replay_fixture`` and
+the resulting metrics dict is asserted for structure + determinism.
+
+The deals are crafted so every call ``_price_from_inputs`` makes to the 3 injected
+callables is recorded up-front, and so each headline ``median_ppm2`` is an exact,
+predictable value (3 listings → the middle ₽/m²; no anchor / quarter-index / ДКП
+mutation), which is what the recorded ``ratio_calls`` key must match.
+
+NOTE: importing scripts.backtest_estimator → app.services.estimator →
+app.core.config.Settings REQUIRES DATABASE_URL. Set a dummy value BEFORE importing
+app modules (same pattern as tests/test_backtest_estimator.py:19-21). The dummy URL
+is never connected to — replay_fixture opens NO session.
+"""
+
+import os
+
+os.environ.setdefault("DATABASE_URL", "postgresql+psycopg://test:test@localhost:5432/test")
+
+import gzip
+import json
+import tempfile
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from scripts import backtest_estimator as bt
+
+# --------------------------------------------------------------------------- #
+# Fixture builders — hand-crafted, fully predictable per-deal records.
+# --------------------------------------------------------------------------- #
+
+
+def _geo(address: str) -> dict[str, Any]:
+    """A synthetic GeocodeResult dict (the asdict shape replay rebuilds)."""
+    return {
+        "lat": 56.84,
+        "lon": 60.60,
+        "full_address": address,
+        "provider": "cache",
+        "confidence": "exact",
+    }
+
+
+def _deal_record(
+    *,
+    deal_id: int,
+    sold_ppm2: float,
+    area_m2: float,
+    rooms: int,
+    listings: list[dict[str, Any]],
+    anchor_comps: list[dict[str, Any]],
+    anchor_tier_fetched: str | None,
+    ratio_calls: list[Any],
+    qi_calls: list[Any],
+    qis_calls: list[Any],
+    address: str,
+) -> dict[str, Any]:
+    """Assemble one fixture deal record in the exact ``--dump-fixture`` schema."""
+    return {
+        "deal_id": deal_id,
+        "sold_ppm2": sold_ppm2,
+        "area_m2": area_m2,
+        "rooms": rooms,
+        "deal_date": "2025-06-15",
+        "kwargs": {
+            "listings": listings,
+            "area_m2": area_m2,
+            "rooms": rooms,
+            "repair_state": None,
+            "floor": 3,
+            "total_floors": 9,
+            "target_year": 2010,
+            "analog_tier": "W",
+            "fallback_used": False,
+            "area_widened": False,
+            "anchor_comps": anchor_comps,
+            "anchor_tier_fetched": anchor_tier_fetched,
+            "dkp_raw": None,
+            "imv_anchor": None,
+            "imv_eval": None,
+            "yandex_val_present": False,
+            "cian_val_present": False,
+            "target_house_cadnum": None,
+            "dadata_coarse": False,
+            "geo": _geo(address),
+            "dadata_qc_geo": None,
+        },
+        "ratio_calls": ratio_calls,
+        "qi_calls": qi_calls,
+        "qis_calls": qis_calls,
+    }
+
+
+def _build_fixture() -> dict[str, Any]:
+    """3 hand-crafted deals spanning эконом / бизнес / элит SOLD segments.
+
+    deal 1: plain — no anchor, no quarter index → single ratio call.
+    deal 2: a listing carries a cadastral number → one recorded quarter-index
+            call (returns null, leaves the median untouched) + a ratio call.
+    deal 3: carries non-empty anchor_comps (2 comps < min_comps=4 → anchor never
+            fires, so the median stays the radius median) + a ratio call.
+    """
+    # ── deal 1 — median of [90k, 100k, 110k] = 100k → ratio_resolver(100000.0). ──
+    deal1 = _deal_record(
+        deal_id=1,
+        sold_ppm2=100_000.0,  # SOLD эконом (< 120k)
+        area_m2=50.0,
+        rooms=1,
+        listings=[
+            {"price_per_m2": 90_000.0, "source": "avito"},
+            {"price_per_m2": 100_000.0, "source": "avito"},
+            {"price_per_m2": 110_000.0, "source": "avito"},
+        ],
+        anchor_comps=[],
+        anchor_tier_fetched=None,
+        ratio_calls=[[100_000.0, [0.95, "per_rooms_all"]]],
+        qi_calls=[],
+        qis_calls=[],
+        address="ул. Тестовая, 1",
+    )
+
+    # ── deal 2 — median 150k; first lot's cadnum → quarter "66:41:0204016". ──
+    deal2 = _deal_record(
+        deal_id=2,
+        sold_ppm2=200_000.0,  # SOLD бизнес (160k..220k)
+        area_m2=60.0,
+        rooms=2,
+        listings=[
+            {
+                "price_per_m2": 140_000.0,
+                "source": "cian",
+                "building_cadastral_number": "66:41:0204016:350",
+            },
+            {"price_per_m2": 150_000.0, "source": "cian"},
+            {"price_per_m2": 160_000.0, "source": "cian"},
+        ],
+        anchor_comps=[],
+        anchor_tier_fetched=None,
+        # quarter-index lookup returns null → spine leaves the median untouched.
+        qi_calls=[["66:41:0204016", None]],
+        qis_calls=[],
+        ratio_calls=[[150_000.0, [0.90, "per_rooms_all"]]],
+        address="ул. Тестовая, 2",
+    )
+
+    # ── deal 3 — median 310k; anchor_comps present but below min_comps → no fire. ──
+    deal3 = _deal_record(
+        deal_id=3,
+        sold_ppm2=290_000.0,  # SOLD элит (220k..300k)
+        area_m2=80.0,
+        rooms=3,
+        listings=[
+            {"price_per_m2": 300_000.0, "source": "yandex"},
+            {"price_per_m2": 310_000.0, "source": "yandex"},
+            {"price_per_m2": 320_000.0, "source": "yandex"},
+        ],
+        anchor_comps=[
+            {"price_per_m2": 305_000.0, "area_m2": 80.0, "rooms": 3, "floor": 5, "total_floors": 9},
+            {"price_per_m2": 308_000.0, "area_m2": 82.0, "rooms": 3, "floor": 6, "total_floors": 9},
+        ],
+        anchor_tier_fetched=None,
+        ratio_calls=[[310_000.0, [0.92, "per_rooms_tier:high"]]],
+        qi_calls=[],
+        qis_calls=[],
+        address="ул. Тестовая, 3",
+    )
+
+    deals = [deal1, deal2, deal3]
+    return {
+        "schema_version": bt.FIXTURE_SCHEMA_VERSION,
+        "engine": "full",
+        "since": "2025-06-01",
+        "n_deals": len(deals),
+        "settings_at_capture": {},
+        "deals": deals,
+    }
+
+
+# --------------------------------------------------------------------------- #
+# Tests
+# --------------------------------------------------------------------------- #
+
+
+def test_replay_fixture_structure_and_keys() -> None:
+    fixture = _build_fixture()
+    metrics = bt.replay_fixture(fixture)
+
+    # Keeps the non-volatile metric blocks ...
+    for key in (
+        "expected_sold",
+        "range_coverage",
+        "calibration",
+        "sharpness",
+        "confidence_order",
+        "headline",
+    ):
+        assert key in metrics, f"missing metric block: {key}"
+    # ... and DROPS the volatile params block.
+    assert "params" not in metrics
+
+    # Every crafted deal resolves a ratio → an expected_sold row, so overall n
+    # equals the number of deals in the fixture.
+    assert metrics["expected_sold"]["overall"]["n"] == len(fixture["deals"])
+
+
+def test_replay_fixture_is_deterministic() -> None:
+    fixture = _build_fixture()
+    first = bt.replay_fixture(fixture)
+    second = bt.replay_fixture(fixture)
+    # Byte-identical across two independent replays (no DB, no RNG, no caches).
+    assert json.dumps(first, ensure_ascii=False, sort_keys=True) == json.dumps(
+        second, ensure_ascii=False, sort_keys=True
+    )
+
+
+def test_replay_fixture_segments_span_multiple_bands() -> None:
+    # The 3 deals sit in distinct SOLD price-segments (эконом / бизнес / элит),
+    # so the per-segment expected_sold breakdown must show ≥ 3 non-empty bands.
+    metrics = bt.replay_fixture(_build_fixture())
+    per_segment = metrics["expected_sold"]["per_segment"]
+    non_empty = [label for label, m in per_segment.items() if m["n"] > 0]
+    assert len(non_empty) >= 3
+
+
+def test_replay_is_arg_insensitive_order_based() -> None:
+    # Order-based (FIFO) replay returns the recorded ratio REGARDLESS of the arg
+    # value the spine actually computes — so a recorded arg that can never equal
+    # the live median (999_999.0) still replays cleanly. This is the cross-platform
+    # robustness contract: a Linux-captured fixture must replay off-Linux even when
+    # libm last-ulp jitter shifts the computed median_ppm2 by an ulp.
+    fixture = _build_fixture()
+    fixture["deals"][0]["ratio_calls"] = [[999_999.0, [0.95, "per_rooms_all"]]]
+    metrics = bt.replay_fixture(fixture)
+    # Replay succeeded (no KeyError) and still priced every deal.
+    assert metrics["expected_sold"]["overall"]["n"] == len(fixture["deals"])
+    # The recorded ratio (0.95) was applied to deal 1 (median 100k → expected_sold
+    # ppm² 95k), so its signed error vs SOLD 100k is -5% — proving the recorded
+    # RETURN drove the result, not the (mismatched) arg.
+    econom = metrics["expected_sold"]["per_segment"]["эконом"]
+    assert econom["n"] == 1
+    assert econom["median_bias_pct"] == pytest.approx(-5.0)
+
+
+def test_replay_exhaustion_raises_runtime_error() -> None:
+    # If a callable's recorded list is SHORTER than the spine's call count (here:
+    # the spine calls ratio_resolver once but nothing is recorded), the exhaustion
+    # guard must raise a clear RuntimeError — control flow diverged from capture,
+    # NOT a silent wrong answer.
+    fixture = _build_fixture()
+    fixture["deals"][0]["ratio_calls"] = []  # spine will still request the ratio
+    with pytest.raises(RuntimeError) as exc:
+        bt.replay_fixture(fixture)
+    assert "ratio_resolver" in str(exc.value)
+    assert "diverged from capture" in str(exc.value)
+
+
+def test_load_fixture_plain_and_gzip_roundtrip() -> None:
+    # load_fixture is gzip-transparent: a .gz path is decompressed, a plain .json
+    # path is read directly. Both must yield a replay-able fixture dict.
+    fixture = _build_fixture()
+    payload = json.dumps(fixture, ensure_ascii=False)
+    with tempfile.TemporaryDirectory() as d:
+        plain = Path(d) / "fx.json"
+        plain.write_text(payload, encoding="utf-8")
+        gz = Path(d) / "fx.json.gz"
+        with gzip.open(gz, "wt", encoding="utf-8") as fh:
+            fh.write(payload)
+
+        loaded_plain = bt.load_fixture(str(plain))
+        loaded_gz = bt.load_fixture(str(gz))
+        assert loaded_plain == loaded_gz == fixture
+        # Both load paths produce identical replay metrics.
+        m_plain = bt.replay_fixture(loaded_plain)
+        m_gz = bt.replay_fixture(loaded_gz)
+        assert json.dumps(m_plain, sort_keys=True) == json.dumps(m_gz, sort_keys=True)
diff --git a/tradein-mvp/backend/tests/test_backtest_regression_gate.py b/tradein-mvp/backend/tests/test_backtest_regression_gate.py
new file mode 100644
index 00000000..15c19299
--- /dev/null
+++ b/tradein-mvp/backend/tests/test_backtest_regression_gate.py
@@ -0,0 +1,84 @@
+"""Hermetic estimator regression gate (#1966 PR 3/3).
+
+Replays the committed frozen backtest fixture through the full pricing spine
+(``app.services.estimator._price_from_inputs``) with ZERO DB / network, recomputes
+the backtest metrics, and asserts they match the committed baseline. Any change to
+the spine, the metric code, or a config default that moves a metric beyond float
+jitter fails this test → regenerate the baseline deliberately:
+
+    cd tradein-mvp/backend
+    uv run python -m scripts.backtest_estimator \
+        --from-fixture tests/fixtures/backtest_full_fixture.json.gz \
+        --update-baseline tests/fixtures/backtest_baseline.json
+
+and justify the per-segment MAPE / coverage deltas in the PR. This is a RELATIVE
+regression gate, not an absolute SLA (live coverage ~55% is data-blocked, see #1966).
+
+The fixture is gzipped frozen prod inputs (opaque, rarely changes); the baseline is
+the small diff-visible artifact that surfaces accuracy movement right in the PR diff.
+"""
+
+from __future__ import annotations
+
+import json
+import math
+import os
+from pathlib import Path
+
+os.environ.setdefault("DATABASE_URL", "postgresql+psycopg://test:test@localhost:5432/test")
+
+from scripts.backtest_estimator import load_fixture, replay_fixture
+
+_FIXTURES = Path(__file__).parent / "fixtures"
+_FIXTURE_PATH = _FIXTURES / "backtest_full_fixture.json.gz"
+_BASELINE_PATH = _FIXTURES / "backtest_baseline.json"
+
+# Floats: a small relative+absolute tolerance absorbs cross-platform / Python
+# libm last-ulp jitter (the replay is otherwise deterministic). A real regression
+# moves a metric by orders of magnitude more than this, so it is still caught.
+_REL_TOL = 1e-6
+_ABS_TOL = 1e-6
+
+
+def _assert_match(path: str, expected: object, actual: object) -> None:
+    if isinstance(expected, dict):
+        assert isinstance(actual, dict), f"{path}: expected dict, got {type(actual).__name__}"
+        assert (
+            expected.keys() == actual.keys()
+        ), f"{path}: key set differs\n  expected={sorted(expected)}\n  actual=  {sorted(actual)}"
+        for k in expected:
+            _assert_match(f"{path}.{k}", expected[k], actual[k])
+    elif isinstance(expected, list):
+        assert isinstance(actual, list), f"{path}: expected list, got {type(actual).__name__}"
+        assert len(expected) == len(actual), f"{path}: list length {len(actual)} != {len(expected)}"
+        for i, (e, a) in enumerate(zip(expected, actual, strict=True)):
+            _assert_match(f"{path}[{i}]", e, a)
+    elif expected is None or isinstance(expected, bool):
+        assert actual is expected or actual == expected, f"{path}: {actual!r} != {expected!r}"
+    elif isinstance(expected, int):  # bool already handled above
+        assert actual == expected, f"{path}: int {actual!r} != {expected!r}"
+    elif isinstance(expected, float):
+        assert isinstance(actual, int | float), f"{path}: {type(actual).__name__} not numeric"
+        assert math.isclose(actual, expected, rel_tol=_REL_TOL, abs_tol=_ABS_TOL), (
+            f"{path}: {actual!r} != baseline {expected!r} (Δ={actual - expected:.3e}). "
+            f"The estimator/metrics changed — if intentional, regenerate the baseline "
+            f"(--from-fixture --update-baseline) and justify the deltas in the PR."
+        )
+    else:
+        assert actual == expected, f"{path}: {actual!r} != {expected!r}"
+
+
+def test_fixture_and_baseline_committed() -> None:
+    assert _FIXTURE_PATH.exists(), f"frozen fixture missing: {_FIXTURE_PATH}"
+    assert _BASELINE_PATH.exists(), f"frozen baseline missing: {_BASELINE_PATH}"
+
+
+def test_backtest_regression_gate() -> None:
+    fixture = load_fixture(_FIXTURE_PATH)
+    baseline = json.loads(_BASELINE_PATH.read_text(encoding="utf-8"))
+    # Round-trip the replay output through JSON before comparing: the committed
+    # baseline is JSON (string object keys), while replay_fixture returns native
+    # dicts whose per_rooms buckets are int keys (0..4). Round-tripping normalises
+    # key types to match — the same transform `--update-baseline` applies on write.
+    metrics = json.loads(json.dumps(replay_fixture(fixture), ensure_ascii=False))
+    _assert_match("metrics", baseline, metrics)