diff --git a/backend/services/navi_offroute/hpa_build.py b/backend/services/navi_offroute/hpa_build.py
new file mode 100644
index 0000000..4993a41
--- /dev/null
+++ b/backend/services/navi_offroute/hpa_build.py
@@ -0,0 +1,307 @@
+"""HPA* offline precompute pipeline (spec HPA-SPEC.md §7, Phase H2).
+
+Standalone CLI: for every chunk a region's bbox covers, run small entrance-to-entrance A*
+searches on the chunk's native-resolution cost grid and store the scalar costs into a
+per-region SQLite (spec §6). No router/kernel wiring — H3 consumes these tiles.
+
+    python -m services.navi_offroute.hpa_build \
+        --region idaho --bbox 41.9,-117.3,49.1,-110.9 --output /mnt/nav/hpa/idaho.db
+
+Deliberate v1 scope / deviations (flag for spec amendment):
+- BOTH directions stored, not `from < to`. The anisotropic slope model (signed Tobler /
+  Herzog / linear) makes uphill cost != downhill, so entrance-pair costs are NOT symmetric:
+  20×19 = 380 ordered pairs per mode per chunk, not the 190 spec §4 implies. Recommend
+  amending §4/§6 to say "directed pairs".
+- Costs are PURE TERRAIN (slope + WorldCover friction + trails + wilderness). Boundary-mode
+  rules (PAD-US barriers, MVUM closures) and network_affinity are NOT baked — they are
+  boundary-mode/request dependent and belong at query time / in the fallback (H3). The build
+  passes a zero barrier grid; wilderness impassability (mode-fixed, not boundary-dependent)
+  IS baked.
+- Only the 20 border entrances (spec §4). Transition cells (entrances ≥20, spec §5) are NOT
+  built here: they need the TrailheadIndex/OSMParkingIndex runtime singletons, deferred to
+  H3 / a follow-up. Known v1 limitation.
+- Chunk grid: fixed-degree plate-carrée pinned at lat=0/lon=0, step CHUNK_DEG = 1.5 km / 111
+  (spec §3/§13). No cos(lat) correction — a per-cell cos would make chunk boundaries
+  latitude-dependent and break the deterministic global grid. Chunks are thus ~1.5 km tall
+  and (cos lat)·1.5 km wide; entrance cells scale to each chunk's actual fetched grid shape,
+  so the nominal "50×50" is not assumed. The §3 Z12-tile 5×5 alignment (an I/O optimization)
+  is deferred: v1 reads per chunk.
+"""
+import argparse
+import hashlib
+import math
+import os
+import sqlite3
+import subprocess
+import sys
+import time
+from concurrent.futures import ProcessPoolExecutor
+from datetime import datetime, timezone
+
+import numpy as np
+
+from shared.dem import DEMReader, dem_path
+from .astar import astar_multigoal, inflate_cost_multiplier
+from .barriers import WildernessReader, wilderness_tif_path
+from .cost import MODE_PROFILES, compute_cost_multiplier_grid
+from .friction import FrictionReader, friction_to_multiplier
+from .trails import TrailReader
+
+MODE_ORDER = ["foot", "2w", "4w", "vehicle"]
+_SPEED_FN_ID = {"tobler": 0, "herzog": 1, "linear": 2}
+
+# Deterministic chunk grid (spec §3/§13): plate-carrée, pinned at lat=0/lon=0.
+CHUNK_KM = 1.5
+KM_PER_DEG = 111.0
+CHUNK_DEG = CHUNK_KM / KM_PER_DEG                 # ~0.013514 deg per chunk, both axes
+ENTRANCES_PER_EDGE = 5
+EDGE_FRACTIONS = (1.0 / 6, 2.0 / 6, 3.0 / 6, 4.0 / 6, 5.0 / 6)  # spec §4
+
+_SCHEMA = """
+CREATE TABLE IF NOT EXISTS chunk_costs (
+  chunk_x INTEGER NOT NULL,
+  chunk_y INTEGER NOT NULL,
+  mode_idx INTEGER NOT NULL,
+  from_entrance INTEGER NOT NULL,
+  to_entrance INTEGER NOT NULL,
+  cost_s REAL NOT NULL,
+  PRIMARY KEY (chunk_x, chunk_y, mode_idx, from_entrance, to_entrance)
+);
+CREATE INDEX IF NOT EXISTS idx_chunk_mode ON chunk_costs (chunk_x, chunk_y, mode_idx);
+CREATE TABLE IF NOT EXISTS meta (key TEXT PRIMARY KEY, value TEXT);
+"""
+_INSERT = ("INSERT OR REPLACE INTO chunk_costs "
+           "(chunk_x, chunk_y, mode_idx, from_entrance, to_entrance, cost_s) VALUES (?,?,?,?,?,?)")
+
+
+# ── deterministic chunk geometry ─────────────────────────────────────────────
+
+def chunk_coords(lat, lon):
+    """(chunk_x, chunk_y) for a lat/lon on the global plate-carrée grid pinned at 0/0."""
+    return int(math.floor(lon / CHUNK_DEG)), int(math.floor(lat / CHUNK_DEG))
+
+
+def chunk_bounds(chunk_x, chunk_y):
+    """(south, west, north, east) degrees for a chunk."""
+    return (chunk_y * CHUNK_DEG, chunk_x * CHUNK_DEG,
+            (chunk_y + 1) * CHUNK_DEG, (chunk_x + 1) * CHUNK_DEG)
+
+
+def chunks_in_bbox(south, west, north, east):
+    """All (chunk_x, chunk_y) whose cells the bbox touches (inclusive)."""
+    cx0, cy0 = chunk_coords(south, west)
+    cx1, cy1 = chunk_coords(north, east)
+    return [(cx, cy)
+            for cx in range(min(cx0, cx1), max(cx0, cx1) + 1)
+            for cy in range(min(cy0, cy1), max(cy0, cy1) + 1)]
+
+
+def _entrance_cells(rows, cols):
+    """The 20 border entrance (row, col) cells, indexed per spec §4:
+    0..4 top L→R, 5..9 right T→B, 10..14 bottom L→R, 15..19 left T→B."""
+    r_last, c_last = rows - 1, cols - 1
+    cells = []
+    cells += [(0, int(round(f * c_last))) for f in EDGE_FRACTIONS]            # top
+    cells += [(int(round(f * r_last)), c_last) for f in EDGE_FRACTIONS]       # right
+    cells += [(r_last, int(round(f * c_last))) for f in EDGE_FRACTIONS]       # bottom
+    cells += [(int(round(f * r_last)), 0) for f in EDGE_FRACTIONS]            # left
+    return cells
+
+
+# ── per-chunk cost computation (pure: testable without readers/processes) ─────
+
+def compute_chunk_rows(chunk_x, chunk_y, elevation, friction_mult, friction_raw,
+                       trails, wilderness, cell_size_m, modes=MODE_ORDER):
+    """Entrance-to-entrance directed costs for one chunk. Returns a list of
+    (chunk_x, chunk_y, mode_idx, from_entrance, to_entrance, cost_s) rows (finite only)."""
+    rows, cols = elevation.shape
+    if rows < 2 or cols < 2:
+        return []
+    entrances = _entrance_cells(rows, cols)
+    elev = np.ascontiguousarray(elevation, dtype=np.float64)
+    barrier_grid = np.zeros((rows, cols), dtype=np.uint8)        # barriers not baked (see docstring)
+    trail_grid = np.ascontiguousarray(
+        trails if trails is not None else np.zeros((rows, cols), np.uint8), dtype=np.uint8)
+    cs = float(cell_size_m)
+    out = []
+    for mi, mode in enumerate(modes):
+        prof = MODE_PROFILES[mode]
+        cm = compute_cost_multiplier_grid(
+            elevation, cell_size_lat_m=cs, cell_size_lon_m=cs,
+            friction=friction_mult, friction_raw=friction_raw, wilderness=wilderness, mode=mode)
+        cm = np.ascontiguousarray(inflate_cost_multiplier(cm), dtype=np.float64)
+        tfl = np.full(256, np.inf, dtype=np.float64)
+        for tv, fric in prof.trail_friction.items():
+            tfl[tv] = np.inf if fric is None else float(fric)
+        sfid = _SPEED_FN_ID.get(prof.speed_function, 0)
+        mg = float(np.tan(np.radians(prof.max_slope_deg)))
+        bspd = float(prof.base_speed_kmh)
+        for fi, (fr, fc) in enumerate(entrances):
+            for ti, (gr, gc) in enumerate(entrances):
+                if ti == fi:
+                    continue
+                _, _, cost = astar_multigoal(
+                    cm, elev, cs, cs, mg, sfid, bspd, trail_grid, tfl, barrier_grid, 1,
+                    int(fr), int(fc),
+                    np.array([gr], dtype=np.int64), np.array([gc], dtype=np.int64))
+                if np.isfinite(cost):
+                    out.append((chunk_x, chunk_y, mi, fi, ti, float(cost)))
+    return out
+
+
+# ── reader-backed chunk fetch + process worker ───────────────────────────────
+
+_READERS = {}
+
+
+def _get_readers():
+    """Lazily build the readers once per (worker) process."""
+    if not _READERS:
+        _READERS["dem"] = DEMReader(dem_path())
+        _READERS["friction"] = FrictionReader()
+        _READERS["trail"] = TrailReader()
+        _READERS["wild"] = WildernessReader() if wilderness_tif_path().exists() else None
+    return _READERS
+
+
+def _fetch_chunk_rasters(chunk_x, chunk_y, readers):
+    """Fetch (elevation, friction_mult, friction_raw, trails, wilderness, cell_size_m) for a
+    chunk via the existing readers — same pattern as router._fetch_auto_rasters."""
+    s, w, n, e = chunk_bounds(chunk_x, chunk_y)
+    elevation, meta = readers["dem"].get_elevation_grid(south=s, north=n, west=w, east=e)
+    shape = elevation.shape
+    friction_raw = readers["friction"].get_friction_grid(
+        south=s, north=n, west=w, east=e, target_shape=shape)
+    friction_mult = friction_to_multiplier(friction_raw)
+    trails = readers["trail"].get_trails_grid(south=s, north=n, west=w, east=e, target_shape=shape)
+    wilderness = None
+    if readers["wild"] is not None:
+        wilderness = readers["wild"].get_wilderness_grid(
+            south=s, north=n, west=w, east=e, target_shape=shape)
+    return elevation, friction_mult, friction_raw, trails, wilderness, float(meta["cell_size_m"])
+
+
+def _chunk_worker(chunk):
+    """ProcessPool worker: fetch + compute one chunk's rows. Errors -> empty (logged by caller)."""
+    cx, cy = chunk
+    try:
+        readers = _get_readers()
+        elev, fmult, fraw, trails, wild, cs = _fetch_chunk_rasters(cx, cy, readers)
+        return cx, cy, compute_chunk_rows(cx, cy, elev, fmult, fraw, trails, wild, cs), None
+    except Exception as exc:  # pragma: no cover - exercised in H4 ops, not unit tests
+        return cx, cy, [], repr(exc)
+
+
+# ── SQLite assembly ──────────────────────────────────────────────────────────
+
+def _init_schema(conn):
+    conn.executescript(_SCHEMA)
+
+
+def _mode_profile_hash():
+    return hashlib.sha256(repr(MODE_PROFILES).encode()).hexdigest()
+
+
+def _dem_version():
+    try:
+        return f"mtime:{int(os.path.getmtime(dem_path()))}"
+    except Exception:
+        return "unknown"
+
+
+def _git_commit():
+    try:
+        return subprocess.check_output(
+            ["git", "rev-parse", "HEAD"], stderr=subprocess.DEVNULL).decode().strip()
+    except Exception:
+        return "unknown"
+
+
+def _write_meta(conn, region, bbox):
+    meta = {
+        "region_name": region,
+        "region_bbox": ",".join(str(x) for x in bbox),
+        "chunk_size_cells": "50",
+        "chunk_size_km": str(CHUNK_KM),
+        "chunk_deg": repr(CHUNK_DEG),
+        "entrances_per_edge": str(ENTRANCES_PER_EDGE),
+        "mode_profile_hash": _mode_profile_hash(),
+        "dem_version": _dem_version(),
+        "build_timestamp_utc": datetime.now(timezone.utc).isoformat(),
+        "build_git_commit": _git_commit(),
+    }
+    conn.executemany("INSERT OR REPLACE INTO meta (key, value) VALUES (?, ?)", list(meta.items()))
+
+
+def build_region(region, bbox, output, max_workers=None, chunks_only=None):
+    """Build the cost-tile SQLite for `bbox`=(south, west, north, east). Returns a summary dict."""
+    south, west, north, east = bbox
+    chunks = chunks_in_bbox(south, west, north, east)
+    if chunks_only is not None:
+        x0, y0, x1, y1 = chunks_only
+        chunks = [(cx, cy) for (cx, cy) in chunks
+                  if min(x0, x1) <= cx <= max(x0, x1) and min(y0, y1) <= cy <= max(y0, y1)]
+
+    conn = sqlite3.connect(output)
+    conn.execute("PRAGMA journal_mode=WAL")
+    conn.execute("PRAGMA synchronous=NORMAL")
+    _init_schema(conn)
+    _write_meta(conn, region, bbox)
+    conn.commit()
+
+    t0 = time.perf_counter()
+    rows_total, failed = 0, []
+    workers = max_workers or os.cpu_count()
+    with ProcessPoolExecutor(max_workers=workers) as ex:
+        for cx, cy, chunk_rows, err in ex.map(_chunk_worker, chunks):
+            if err is not None:
+                failed.append((cx, cy, err))
+                continue
+            if chunk_rows:
+                conn.executemany(_INSERT, chunk_rows)
+                rows_total += len(chunk_rows)
+    conn.commit()
+    elapsed = time.perf_counter() - t0
+    # Stored rows are finite only; the rest of the 380×modes×chunks possibilities were pruned.
+    possible = len(chunks) * len(MODE_ORDER) * 20 * 19
+    summary = {"region": region, "chunks": len(chunks), "rows": rows_total,
+               "inf_pruned": possible - rows_total, "failed": len(failed),
+               "workers": workers, "elapsed_s": elapsed}
+    conn.execute("VACUUM")
+    conn.close()
+    print(f"HPA build [{region}]: {summary['chunks']} chunks, {rows_total} rows, "
+          f"{summary['inf_pruned']} inf-pruned, {len(failed)} failed, "
+          f"{workers} workers, {elapsed:.1f}s -> {output}")
+    for cx, cy, err in failed[:10]:
+        print(f"  chunk ({cx},{cy}) failed: {err}", file=sys.stderr)
+    return summary
+
+
+def _parse_csv_floats(s, n):
+    parts = [float(x) for x in s.split(",")]
+    if len(parts) != n:
+        raise argparse.ArgumentTypeError(f"expected {n} comma-separated values, got {len(parts)}")
+    return parts
+
+
+def main(argv=None):
+    p = argparse.ArgumentParser(
+        prog="python -m services.navi_offroute.hpa_build",
+        description="Precompute HPA* chunk-cost tiles for a region (HPA-SPEC.md §7).")
+    p.add_argument("--region", required=True, help="region name, recorded in meta.region_name")
+    p.add_argument("--bbox", required=True, type=lambda s: _parse_csv_floats(s, 4),
+                   metavar="S,W,N,E", help="region bbox: south,west,north,east (decimal degrees)")
+    p.add_argument("--output", required=True, help="path to the SQLite tile DB to create")
+    p.add_argument("--max-workers", type=int, default=os.cpu_count(),
+                   help="ProcessPoolExecutor workers (default: os.cpu_count())")
+    p.add_argument("--chunks-only", type=lambda s: [int(x) for x in s.split(",")],
+                   default=None, metavar="X1,Y1,X2,Y2",
+                   help="debug: constrain the build to a chunk-coord bbox")
+    args = p.parse_args(argv)
+    south, west, north, east = args.bbox
+    build_region(args.region, (south, west, north, east), args.output,
+                 max_workers=args.max_workers, chunks_only=args.chunks_only)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/backend/services/navi_offroute/tests/test_hpa_build.py b/backend/services/navi_offroute/tests/test_hpa_build.py
new file mode 100644
index 0000000..b3eeddc
--- /dev/null
+++ b/backend/services/navi_offroute/tests/test_hpa_build.py
@@ -0,0 +1,82 @@
+"""HPA* precompute pipeline tests (Phase H2). Structural correctness on synthetic chunks;
+no real DEM/readers, no ProcessPool — compute_chunk_rows is exercised directly in-process."""
+import math
+import sqlite3
+
+import numpy as np
+
+from services.navi_offroute import hpa_build as hb
+
+
+def test_chunk_coords_deterministic():
+    # Pure function of (lat, lon): identical on repeat, and matches the floor formula.
+    lat, lon = 43.7500, -114.9000
+    assert hb.chunk_coords(lat, lon) == hb.chunk_coords(lat, lon)
+    assert hb.chunk_coords(lat, lon) == (
+        int(math.floor(lon / hb.CHUNK_DEG)), int(math.floor(lat / hb.CHUNK_DEG)))
+    # Adjacent points one chunk-step apart land in adjacent chunks; bounds enclose the point.
+    cx, cy = hb.chunk_coords(lat, lon)
+    s, w, n, e = hb.chunk_bounds(cx, cy)
+    assert s <= lat < n and w <= lon < e
+    assert hb.chunk_coords(lat + hb.CHUNK_DEG, lon) == (cx, cy + 1)
+
+
+def test_entrance_cells_layout():
+    cells = hb._entrance_cells(50, 50)
+    assert len(cells) == 20
+    assert all(0 <= r <= 49 and 0 <= c <= 49 for r, c in cells)
+    # Index convention (spec §4): 0..4 top, 5..9 right, 10..14 bottom, 15..19 left.
+    assert all(r == 0 for r, c in cells[0:5])           # top edge -> row 0
+    assert all(c == 49 for r, c in cells[5:10])         # right edge -> col 49
+    assert all(r == 49 for r, c in cells[10:15])        # bottom edge -> row 49
+    assert all(c == 0 for r, c in cells[15:20])         # left edge -> col 0
+    # Even spacing L→R / T→B along each edge (strictly increasing free coordinate).
+    assert [c for _, c in cells[0:5]] == sorted(c for _, c in cells[0:5])
+    assert [r for r, _ in cells[5:10]] == sorted(r for r, _ in cells[5:10])
+    # Fractions 1/6..5/6 of the 49-cell span.
+    assert [c for _, c in cells[0:5]] == [int(round(f * 49)) for f in hb.EDGE_FRACTIONS]
+
+
+def _flat_chunk(n=50, fill_elev=1000.0):
+    elevation = np.full((n, n), fill_elev, dtype=np.float64)
+    friction_mult = np.ones((n, n), dtype=np.float64)
+    friction_raw = np.full((n, n), 30, dtype=np.uint8)   # grass
+    trails = np.zeros((n, n), dtype=np.uint8)
+    wilderness = np.zeros((n, n), dtype=np.uint8)
+    return elevation, friction_mult, friction_raw, trails, wilderness
+
+
+def _cost(conn, mode_idx, fr, to):
+    row = conn.execute(
+        "SELECT cost_s FROM chunk_costs WHERE chunk_x=0 AND chunk_y=0 AND mode_idx=? "
+        "AND from_entrance=? AND to_entrance=?", (mode_idx, fr, to)).fetchone()
+    return row[0] if row else None
+
+
+def test_build_one_chunk_fixture():
+    elev, fmult, fraw, trails, wild = _flat_chunk()
+    rows = hb.compute_chunk_rows(0, 0, elev, fmult, fraw, trails, wild,
+                                 cell_size_m=30.0, modes=("foot",))
+    conn = sqlite3.connect(":memory:")
+    hb._init_schema(conn)
+    conn.executemany(hb._INSERT, rows)
+
+    # Flat passable grass: every foot entrance pair routes (380 finite directed pairs).
+    assert len(rows) == 20 * 19
+    # Distance monotonicity: along the top edge, 0→1 (adjacent) is cheaper than 0→4 (far).
+    assert _cost(conn, 0, 0, 1) < _cost(conn, 0, 0, 4)
+    # top-middle (2) -> bottom-middle (12): a straight vertical crossing.
+    flat_mid = _cost(conn, 0, 2, 12)
+    assert flat_mid is not None
+
+    # A steep hill on the straight 2→12 column forces a detour / slope penalty -> costlier.
+    elev_hill = elev.copy()
+    elev_hill[20:30, 20:30] = 2600.0                     # ~1600 m bump over ~300 m: cliff-steep
+    rows_h = hb.compute_chunk_rows(0, 0, elev_hill, fmult, fraw, trails, wild,
+                                   cell_size_m=30.0, modes=("foot",))
+    conn_h = sqlite3.connect(":memory:")
+    hb._init_schema(conn_h)
+    conn_h.executemany(hb._INSERT, rows_h)
+    hill_mid = _cost(conn_h, 0, 2, 12)
+    assert hill_mid is not None
+    assert hill_mid > flat_mid                            # the hill is not free to cross