diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..9c49aae
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,168 @@
+"""
+Shared test fixtures for the Paper Dynasty database test suite.
+
+Uses in-memory SQLite with foreign_keys pragma enabled. Each test
+gets a fresh set of tables via the setup_test_db fixture (autouse).
+
+All models are bound to the in-memory database before table creation
+so that no connection to the real storage/pd_master.db occurs during
+tests.
+"""
+
+import os
+import pytest
+from peewee import SqliteDatabase
+
+# Set DATABASE_TYPE=postgresql so that the module-level SKIP_TABLE_CREATION
+# flag is True.  This prevents db_engine.py from calling create_tables()
+# against the real storage/pd_master.db during import — those calls would
+# fail if indexes already exist and would also contaminate the dev database.
+# The PooledPostgresqlDatabase object is created but never actually connects
+# because our fixture rebinds all models to an in-memory SQLite db before
+# any query is executed.
+os.environ["DATABASE_TYPE"] = "postgresql"
+
+from app.db_engine import (
+    Rarity,
+    Event,
+    Cardset,
+    MlbPlayer,
+    Player,
+    Team,
+    PackType,
+    Pack,
+    Card,
+    Roster,
+    RosterSlot,
+    StratGame,
+    StratPlay,
+    Decision,
+    PlayerSeasonStats,
+    EvolutionTrack,
+    EvolutionCardState,
+    EvolutionTierBoost,
+    EvolutionCosmetic,
+    ScoutOpportunity,
+    ScoutClaim,
+)
+
+_test_db = SqliteDatabase(":memory:", pragmas={"foreign_keys": 1})
+
+# All models in dependency order (parents before children) so that
+# create_tables and drop_tables work without FK violations.
+_TEST_MODELS = [
+    Rarity,
+    Event,
+    Cardset,
+    MlbPlayer,
+    Player,
+    Team,
+    PackType,
+    Pack,
+    Card,
+    Roster,
+    RosterSlot,
+    StratGame,
+    StratPlay,
+    Decision,
+    ScoutOpportunity,
+    ScoutClaim,
+    PlayerSeasonStats,
+    EvolutionTrack,
+    EvolutionCardState,
+    EvolutionTierBoost,
+    EvolutionCosmetic,
+]
+
+
+@pytest.fixture(autouse=True)
+def setup_test_db():
+    """Bind all models to in-memory SQLite and create tables.
+
+    The fixture is autouse so every test automatically gets a fresh,
+    isolated database schema without needing to request it explicitly.
+    Tables are dropped in reverse dependency order after each test to
+    keep the teardown clean and to catch any accidental FK reference
+    direction bugs early.
+    """
+    _test_db.bind(_TEST_MODELS)
+    _test_db.connect()
+    _test_db.create_tables(_TEST_MODELS)
+    yield _test_db
+    _test_db.drop_tables(list(reversed(_TEST_MODELS)), safe=True)
+    _test_db.close()
+
+
+# ---------------------------------------------------------------------------
+# Minimal shared fixtures — create just enough data for FK dependencies
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture
+def rarity():
+    """A single Common rarity row used as FK seed for Player rows."""
+    return Rarity.create(value=1, name="Common", color="#ffffff")
+
+
+@pytest.fixture
+def player(rarity):
+    """A minimal Player row with all required (non-nullable) columns filled.
+
+    Player.p_name is the real column name (not 'name').  All FK and
+    non-nullable varchar fields are provided so SQLite's NOT NULL
+    constraints are satisfied even with foreign_keys=ON.
+    """
+    cardset = Cardset.create(
+        name="Test Set",
+        description="Test cardset",
+        total_cards=100,
+    )
+    return Player.create(
+        p_name="Test Player",
+        rarity=rarity,
+        cardset=cardset,
+        set_num=1,
+        pos_1="1B",
+        image="https://example.com/image.png",
+        mlbclub="TST",
+        franchise="TST",
+        description="A test player",
+    )
+
+
+@pytest.fixture
+def team():
+    """A minimal Team row.
+
+    Team uses abbrev/lname/sname/gmid/gmname/gsheet/wallet/team_value/
+    collection_value — not the 'name'/'user_id' shorthand described in
+    the spec, which referred to the real underlying columns by
+    simplified names.
+    """
+    return Team.create(
+        abbrev="TST",
+        sname="Test",
+        lname="Test Team",
+        gmid=100000001,
+        gmname="testuser",
+        gsheet="https://docs.google.com/spreadsheets/test",
+        wallet=500,
+        team_value=1000,
+        collection_value=1000,
+        season=11,
+        is_ai=False,
+    )
+
+
+@pytest.fixture
+def track():
+    """A minimal EvolutionTrack for batter cards."""
+    return EvolutionTrack.create(
+        name="Batter Track",
+        card_type="batter",
+        formula="pa + tb * 2",
+        t1_threshold=37,
+        t2_threshold=149,
+        t3_threshold=448,
+        t4_threshold=896,
+    )
diff --git a/tests/test_evolution_models.py b/tests/test_evolution_models.py
new file mode 100644
index 0000000..62f5108
--- /dev/null
+++ b/tests/test_evolution_models.py
@@ -0,0 +1,332 @@
+"""
+Tests for evolution-related models and PlayerSeasonStats.
+
+Covers WP-01 acceptance criteria:
+  - EvolutionTrack: CRUD and unique-name constraint
+  - EvolutionCardState: CRUD, defaults, unique-(player,team) constraint,
+    and FK resolution back to EvolutionTrack
+  - EvolutionTierBoost: CRUD and unique-(track, tier, boost_type, boost_target)
+  - EvolutionCosmetic: CRUD and unique-name constraint
+  - PlayerSeasonStats: CRUD with defaults, unique-(player, team, season),
+    and in-place stat accumulation
+
+Each test class is self-contained: fixtures from conftest.py supply the
+minimal parent rows needed to satisfy FK constraints, and every assertion
+targets a single, clearly-named behaviour so failures are easy to trace.
+"""
+
+import pytest
+from peewee import IntegrityError
+from playhouse.shortcuts import model_to_dict
+
+from app.db_engine import (
+    EvolutionCardState,
+    EvolutionCosmetic,
+    EvolutionTierBoost,
+    EvolutionTrack,
+    PlayerSeasonStats,
+)
+
+# ---------------------------------------------------------------------------
+# EvolutionTrack
+# ---------------------------------------------------------------------------
+
+
+class TestEvolutionTrack:
+    """Tests for the EvolutionTrack model.
+
+    EvolutionTrack defines a named progression path (formula +
+    tier thresholds) for a card type.  The name column carries a
+    UNIQUE constraint so that accidental duplicates are caught at
+    the database level.
+    """
+
+    def test_create_track(self, track):
+        """Creating a track persists all fields and they round-trip correctly.
+
+        Reads back via model_to_dict (recurse=False) to verify the raw
+        column values, not Python-object representations, match what was
+        inserted.
+        """
+        data = model_to_dict(track, recurse=False)
+        assert data["name"] == "Batter Track"
+        assert data["card_type"] == "batter"
+        assert data["formula"] == "pa + tb * 2"
+        assert data["t1_threshold"] == 37
+        assert data["t2_threshold"] == 149
+        assert data["t3_threshold"] == 448
+        assert data["t4_threshold"] == 896
+
+    def test_track_unique_name(self, track):
+        """Inserting a second track with the same name raises IntegrityError.
+
+        The UNIQUE constraint on EvolutionTrack.name must prevent two
+        tracks from sharing the same identifier, as the name is used as
+        a human-readable key throughout the evolution system.
+        """
+        with pytest.raises(IntegrityError):
+            EvolutionTrack.create(
+                name="Batter Track",  # duplicate
+                card_type="sp",
+                formula="outs * 3",
+                t1_threshold=10,
+                t2_threshold=40,
+                t3_threshold=120,
+                t4_threshold=240,
+            )
+
+
+# ---------------------------------------------------------------------------
+# EvolutionCardState
+# ---------------------------------------------------------------------------
+
+
+class TestEvolutionCardState:
+    """Tests for EvolutionCardState, which tracks per-player evolution progress.
+
+    Each row represents one card (player) owned by one team, linked to a
+    specific EvolutionTrack.  The model records the current tier (0-4),
+    accumulated progress value, and whether the card is fully evolved.
+    """
+
+    def test_create_card_state(self, player, team, track):
+        """Creating a card state stores all fields and defaults are correct.
+
+        Defaults under test:
+          current_tier  → 0       (fresh card, no tier unlocked yet)
+          current_value → 0.0     (no formula progress accumulated)
+          fully_evolved → False   (evolution is not complete at creation)
+          last_evaluated_at → None (never evaluated yet)
+        """
+        state = EvolutionCardState.create(player=player, team=team, track=track)
+
+        fetched = EvolutionCardState.get_by_id(state.id)
+        assert fetched.player_id == player.player_id
+        assert fetched.team_id == team.id
+        assert fetched.track_id == track.id
+        assert fetched.current_tier == 0
+        assert fetched.current_value == 0.0
+        assert fetched.fully_evolved is False
+        assert fetched.last_evaluated_at is None
+
+    def test_card_state_unique_player_team(self, player, team, track):
+        """A second card state for the same (player, team) pair raises IntegrityError.
+
+        The unique index on (player, team) enforces that each player card
+        has at most one evolution state per team roster slot, preventing
+        duplicate evolution progress rows for the same physical card.
+        """
+        EvolutionCardState.create(player=player, team=team, track=track)
+        with pytest.raises(IntegrityError):
+            EvolutionCardState.create(player=player, team=team, track=track)
+
+    def test_card_state_fk_track(self, player, team, track):
+        """Accessing card_state.track returns the original EvolutionTrack instance.
+
+        This confirms the FK is correctly wired and that Peewee resolves
+        the relationship, returning an object with the same primary key and
+        name as the track used during creation.
+        """
+        state = EvolutionCardState.create(player=player, team=team, track=track)
+        fetched = EvolutionCardState.get_by_id(state.id)
+        resolved_track = fetched.track
+        assert resolved_track.id == track.id
+        assert resolved_track.name == "Batter Track"
+
+
+# ---------------------------------------------------------------------------
+# EvolutionTierBoost
+# ---------------------------------------------------------------------------
+
+
+class TestEvolutionTierBoost:
+    """Tests for EvolutionTierBoost, the per-tier stat/rating bonus table.
+
+    Each row maps a (track, tier) combination to a single boost — the
+    specific stat or rating column to buff and by how much.  The four-
+    column unique constraint prevents double-booking the same boost slot.
+    """
+
+    def test_create_tier_boost(self, track):
+        """Creating a boost row persists all fields accurately.
+
+        Verifies boost_type, boost_target, and boost_value are stored
+        and retrieved without modification.
+        """
+        boost = EvolutionTierBoost.create(
+            track=track,
+            tier=1,
+            boost_type="rating",
+            boost_target="contact_vl",
+            boost_value=1.5,
+        )
+        fetched = EvolutionTierBoost.get_by_id(boost.id)
+        assert fetched.track_id == track.id
+        assert fetched.tier == 1
+        assert fetched.boost_type == "rating"
+        assert fetched.boost_target == "contact_vl"
+        assert fetched.boost_value == 1.5
+
+    def test_tier_boost_unique_constraint(self, track):
+        """Duplicate (track, tier, boost_type, boost_target) raises IntegrityError.
+
+        The four-column unique index ensures that a single boost slot
+        (e.g. Tier-1 contact_vl rating) cannot be defined twice for the
+        same track, which would create ambiguity during evolution evaluation.
+        """
+        EvolutionTierBoost.create(
+            track=track,
+            tier=2,
+            boost_type="rating",
+            boost_target="power_vr",
+            boost_value=2.0,
+        )
+        with pytest.raises(IntegrityError):
+            EvolutionTierBoost.create(
+                track=track,
+                tier=2,
+                boost_type="rating",
+                boost_target="power_vr",
+                boost_value=3.0,  # different value, same identity columns
+            )
+
+
+# ---------------------------------------------------------------------------
+# EvolutionCosmetic
+# ---------------------------------------------------------------------------
+
+
+class TestEvolutionCosmetic:
+    """Tests for EvolutionCosmetic, decorative unlocks tied to evolution tiers.
+
+    Cosmetics are purely visual rewards (frames, badges, themes) that a
+    card unlocks when it reaches a required tier.  The name column is
+    the stable identifier and carries a UNIQUE constraint.
+    """
+
+    def test_create_cosmetic(self):
+        """Creating a cosmetic persists all fields correctly.
+
+        Verifies all columns including optional ones (css_class, asset_url)
+        are stored and retrieved.
+        """
+        cosmetic = EvolutionCosmetic.create(
+            name="Gold Frame",
+            tier_required=2,
+            cosmetic_type="frame",
+            css_class="evo-frame-gold",
+            asset_url="https://cdn.example.com/frames/gold.png",
+        )
+        fetched = EvolutionCosmetic.get_by_id(cosmetic.id)
+        assert fetched.name == "Gold Frame"
+        assert fetched.tier_required == 2
+        assert fetched.cosmetic_type == "frame"
+        assert fetched.css_class == "evo-frame-gold"
+        assert fetched.asset_url == "https://cdn.example.com/frames/gold.png"
+
+    def test_cosmetic_unique_name(self):
+        """Inserting a second cosmetic with the same name raises IntegrityError.
+
+        The UNIQUE constraint on EvolutionCosmetic.name prevents duplicate
+        cosmetic definitions that could cause ambiguous tier unlock lookups.
+        """
+        EvolutionCosmetic.create(
+            name="Silver Badge",
+            tier_required=1,
+            cosmetic_type="badge",
+        )
+        with pytest.raises(IntegrityError):
+            EvolutionCosmetic.create(
+                name="Silver Badge",  # duplicate
+                tier_required=3,
+                cosmetic_type="badge",
+            )
+
+
+# ---------------------------------------------------------------------------
+# PlayerSeasonStats
+# ---------------------------------------------------------------------------
+
+
+class TestPlayerSeasonStats:
+    """Tests for PlayerSeasonStats, the per-season accumulation table.
+
+    Each row aggregates game-by-game batting and pitching stats for one
+    player on one team in one season.  The three-column unique constraint
+    prevents double-counting and ensures a single authoritative row for
+    each (player, team, season) combination.
+    """
+
+    def test_create_season_stats(self, player, team):
+        """Creating a stats row with explicit values stores everything correctly.
+
+        Also verifies the integer stat defaults (all 0) for columns that
+        are not provided, which is the initial state before any games are
+        processed.
+        """
+        stats = PlayerSeasonStats.create(
+            player=player,
+            team=team,
+            season=11,
+            games_batting=5,
+            pa=20,
+            ab=18,
+            hits=6,
+            doubles=1,
+            triples=0,
+            hr=2,
+            bb=2,
+            hbp=0,
+            so=4,
+            rbi=5,
+            runs=3,
+            sb=1,
+            cs=0,
+        )
+        fetched = PlayerSeasonStats.get_by_id(stats.id)
+        assert fetched.player_id == player.player_id
+        assert fetched.team_id == team.id
+        assert fetched.season == 11
+        assert fetched.games_batting == 5
+        assert fetched.pa == 20
+        assert fetched.hits == 6
+        assert fetched.hr == 2
+        # Pitching fields were not set — confirm default zero values
+        assert fetched.games_pitching == 0
+        assert fetched.outs == 0
+        assert fetched.wins == 0
+        assert fetched.saves == 0
+        # Nullable meta fields
+        assert fetched.last_game is None
+        assert fetched.last_updated_at is None
+
+    def test_season_stats_unique_constraint(self, player, team):
+        """A second row for the same (player, team, season) raises IntegrityError.
+
+        The unique index on these three columns guarantees that each
+        player-team-season combination has exactly one accumulation row,
+        preventing duplicate stat aggregation that would inflate totals.
+        """
+        PlayerSeasonStats.create(player=player, team=team, season=11)
+        with pytest.raises(IntegrityError):
+            PlayerSeasonStats.create(player=player, team=team, season=11)
+
+    def test_season_stats_increment(self, player, team):
+        """Manually incrementing hits on an existing row persists the change.
+
+        Simulates the common pattern used by the stats accumulator:
+        fetch the row, add the game delta, save.  Verifies that save()
+        writes back to the database and that subsequent reads reflect the
+        updated value.
+        """
+        stats = PlayerSeasonStats.create(
+            player=player,
+            team=team,
+            season=11,
+            hits=10,
+        )
+        stats.hits += 3
+        stats.save()
+
+        refreshed = PlayerSeasonStats.get_by_id(stats.id)
+        assert refreshed.hits == 13
diff --git a/tests/test_evolution_seed.py b/tests/test_evolution_seed.py
new file mode 100644
index 0000000..a3d1842
--- /dev/null
+++ b/tests/test_evolution_seed.py
@@ -0,0 +1,159 @@
+"""
+Tests for app/seed/evolution_tracks.py — seed_evolution_tracks().
+
+What: Verify that the JSON-driven seed function correctly creates, counts,
+and idempotently updates EvolutionTrack rows in the database.
+
+Why: The seed is the single source of truth for track configuration. A
+regression here (duplicates, wrong thresholds, missing formula) would
+silently corrupt evolution scoring for every card in the system.
+
+Each test operates on a fresh in-memory SQLite database provided by the
+autouse `setup_test_db` fixture in conftest.py. The seed reads its data
+from `app/seed/evolution_tracks.json` on disk, so the tests also serve as
+a light integration check between the JSON file and the Peewee model.
+"""
+
+import json
+from pathlib import Path
+
+import pytest
+
+from app.db_engine import EvolutionTrack
+from app.seed.evolution_tracks import seed_evolution_tracks
+
+# Path to the JSON fixture that the seed reads from at runtime
+_JSON_PATH = Path(__file__).parent.parent / "app" / "seed" / "evolution_tracks.json"
+
+
+@pytest.fixture
+def json_tracks():
+    """Load the raw JSON definitions so tests can assert against them.
+
+    This avoids hardcoding expected values — if the JSON changes, tests
+    automatically follow without needing manual updates.
+    """
+    return json.loads(_JSON_PATH.read_text(encoding="utf-8"))
+
+
+def test_seed_creates_three_tracks(json_tracks):
+    """After one seed call, exactly 3 EvolutionTrack rows must exist.
+
+    Why: The JSON currently defines three card-type tracks (batter, sp, rp).
+    If the count is wrong the system would either be missing tracks
+    (evolution disabled for a card type) or have phantom extras.
+    """
+    seed_evolution_tracks()
+    assert EvolutionTrack.select().count() == 3
+
+
+def test_seed_correct_card_types(json_tracks):
+    """The set of card_type values persisted must match the JSON exactly.
+
+    Why: card_type is used as a discriminator throughout the evolution engine.
+    An unexpected value (e.g. 'pitcher' instead of 'sp') would cause
+    track-lookup misses and silently skip evolution scoring for that role.
+    """
+    seed_evolution_tracks()
+    expected_types = {d["card_type"] for d in json_tracks}
+    actual_types = {t.card_type for t in EvolutionTrack.select()}
+    assert actual_types == expected_types
+
+
+def test_seed_thresholds_ascending():
+    """For every track, t1 < t2 < t3 < t4.
+
+    Why: The evolution engine uses these thresholds to determine tier
+    boundaries. If they are not strictly ascending, tier comparisons
+    would produce incorrect or undefined results (e.g. a player could
+    simultaneously satisfy tier 3 and not satisfy tier 2).
+    """
+    seed_evolution_tracks()
+    for track in EvolutionTrack.select():
+        assert (
+            track.t1_threshold < track.t2_threshold
+        ), f"{track.name}: t1 ({track.t1_threshold}) >= t2 ({track.t2_threshold})"
+        assert (
+            track.t2_threshold < track.t3_threshold
+        ), f"{track.name}: t2 ({track.t2_threshold}) >= t3 ({track.t3_threshold})"
+        assert (
+            track.t3_threshold < track.t4_threshold
+        ), f"{track.name}: t3 ({track.t3_threshold}) >= t4 ({track.t4_threshold})"
+
+
+def test_seed_thresholds_positive():
+    """All tier threshold values must be strictly greater than zero.
+
+    Why: A zero or negative threshold would mean a card starts the game
+    already evolved (tier >= 1 at 0 accumulated stat points), which would
+    bypass the entire progression system.
+    """
+    seed_evolution_tracks()
+    for track in EvolutionTrack.select():
+        assert track.t1_threshold > 0, f"{track.name}: t1_threshold is not positive"
+        assert track.t2_threshold > 0, f"{track.name}: t2_threshold is not positive"
+        assert track.t3_threshold > 0, f"{track.name}: t3_threshold is not positive"
+        assert track.t4_threshold > 0, f"{track.name}: t4_threshold is not positive"
+
+
+def test_seed_formula_present():
+    """Every persisted track must have a non-empty formula string.
+
+    Why: The formula is evaluated at runtime to compute a player's evolution
+    score. An empty formula would cause either a Python eval error or
+    silently produce 0 for every player, halting all evolution progress.
+    """
+    seed_evolution_tracks()
+    for track in EvolutionTrack.select():
+        assert (
+            track.formula and track.formula.strip()
+        ), f"{track.name}: formula is empty or whitespace-only"
+
+
+def test_seed_idempotent():
+    """Calling seed_evolution_tracks() twice must still yield exactly 3 rows.
+
+    Why: The seed is designed to be safe to re-run (e.g. as part of a
+    migration or CI bootstrap). If it inserts duplicates on a second call,
+    the unique constraint on EvolutionTrack.name would raise an IntegrityError
+    in PostgreSQL, and in SQLite it would silently create phantom rows that
+    corrupt tier-lookup joins.
+    """
+    seed_evolution_tracks()
+    seed_evolution_tracks()
+    assert EvolutionTrack.select().count() == 3
+
+
+def test_seed_updates_on_rerun(json_tracks):
+    """A second seed call must restore any manually changed threshold to the JSON value.
+
+    What: Seed once, manually mutate a threshold in the DB, then seed again.
+    Assert that the threshold is now back to the JSON-defined value.
+
+    Why: The seed must act as the authoritative source of truth. If
+    re-seeding does not overwrite local changes, configuration drift can
+    build up silently and the production database would diverge from the
+    checked-in JSON without any visible error.
+    """
+    seed_evolution_tracks()
+
+    # Pick the first track and corrupt its t1_threshold
+    first_def = json_tracks[0]
+    track = EvolutionTrack.get(EvolutionTrack.name == first_def["name"])
+    original_t1 = track.t1_threshold
+    corrupted_value = original_t1 + 9999
+    track.t1_threshold = corrupted_value
+    track.save()
+
+    # Confirm the corruption took effect before re-seeding
+    track_check = EvolutionTrack.get(EvolutionTrack.name == first_def["name"])
+    assert track_check.t1_threshold == corrupted_value
+
+    # Re-seed — should restore the JSON value
+    seed_evolution_tracks()
+
+    restored = EvolutionTrack.get(EvolutionTrack.name == first_def["name"])
+    assert restored.t1_threshold == first_def["t1_threshold"], (
+        f"Expected t1_threshold={first_def['t1_threshold']} after re-seed, "
+        f"got {restored.t1_threshold}"
+    )
diff --git a/tests/test_season_stats_update.py b/tests/test_season_stats_update.py
new file mode 100644
index 0000000..94f40e5
--- /dev/null
+++ b/tests/test_season_stats_update.py
@@ -0,0 +1,597 @@
+"""
+Tests for app/services/season_stats.py — update_season_stats().
+
+What: Verify that the incremental stat accumulation function correctly
+aggregates StratPlay and Decision rows into PlayerSeasonStats, handles
+duplicate calls idempotently, and accumulates stats across multiple games.
+
+Why: This is the core bookkeeping engine for card evolution scoring. A
+double-count bug, a missed Decision merge, or a team-isolation failure
+would silently produce wrong stats that would then corrupt every
+evolution tier calculation downstream.
+
+Test data is created using real Peewee models (no mocking) against the
+in-memory SQLite database provided by the autouse setup_test_db fixture
+in conftest.py.  All Player and Team creation uses the actual required
+column set discovered from the model definition in db_engine.py.
+"""
+
+import app.services.season_stats as _season_stats_module
+import pytest
+
+from app.db_engine import (
+    Cardset,
+    Decision,
+    Player,
+    PlayerSeasonStats,
+    Rarity,
+    StratGame,
+    StratPlay,
+    Team,
+)
+from app.services.season_stats import update_season_stats
+from tests.conftest import _test_db
+
+# ---------------------------------------------------------------------------
+# Module-level patch: redirect season_stats.db to the test database
+# ---------------------------------------------------------------------------
+# season_stats.py holds a module-level reference to the `db` object imported
+# from db_engine.  When test models are rebound to _test_db via bind(), the
+# `db` object inside season_stats still points at the original production db
+# (SQLite file or PostgreSQL).  We replace it here so that db.atomic() in
+# update_season_stats() operates on the same in-memory connection that the
+# test fixtures write to.
+_season_stats_module.db = _test_db
+
+
+# ---------------------------------------------------------------------------
+# Helper factories
+# ---------------------------------------------------------------------------
+
+
+def _make_cardset():
+    """Return a reusable Cardset row (or fetch the existing one by name)."""
+    cs, _ = Cardset.get_or_create(
+        name="Test Set",
+        defaults={"description": "Test cardset", "total_cards": 100},
+    )
+    return cs
+
+
+def _make_rarity():
+    """Return the Common rarity singleton."""
+    r, _ = Rarity.get_or_create(value=1, name="Common", defaults={"color": "#ffffff"})
+    return r
+
+
+def _make_player(name: str, pos: str = "1B") -> Player:
+    """Create a Player row with all required (non-nullable) columns satisfied.
+
+    Why we need this helper: Player has many non-nullable varchar columns
+    (image, mlbclub, franchise, description) and a required FK to Cardset.
+    A single helper keeps test fixtures concise and consistent.
+    """
+    return Player.create(
+        p_name=name,
+        rarity=_make_rarity(),
+        cardset=_make_cardset(),
+        set_num=1,
+        pos_1=pos,
+        image="https://example.com/image.png",
+        mlbclub="TST",
+        franchise="TST",
+        description=f"Test player: {name}",
+    )
+
+
+def _make_team(abbrev: str, gmid: int, season: int = 11) -> Team:
+    """Create a Team row with all required (non-nullable) columns satisfied."""
+    return Team.create(
+        abbrev=abbrev,
+        sname=abbrev,
+        lname=f"Team {abbrev}",
+        gmid=gmid,
+        gmname=f"gm_{abbrev.lower()}",
+        gsheet="https://docs.google.com/spreadsheets/test",
+        wallet=500,
+        team_value=1000,
+        collection_value=1000,
+        season=season,
+        is_ai=False,
+    )
+
+
+def make_play(game, play_num, batter, batter_team, pitcher, pitcher_team, **stats):
+    """Create a StratPlay row with sensible defaults for all required fields.
+
+    Why we provide defaults for every stat column: StratPlay has many
+    IntegerField columns with default=0 at the model level, but supplying
+    them explicitly makes it clear what the baseline state of each play is
+    and keeps the helper signature stable if defaults change.
+    """
+    defaults = dict(
+        on_base_code="000",
+        inning_half="top",
+        inning_num=1,
+        batting_order=1,
+        starting_outs=0,
+        away_score=0,
+        home_score=0,
+        pa=0,
+        ab=0,
+        hit=0,
+        run=0,
+        hr=0,
+        double=0,
+        triple=0,
+        homerun=0,
+        bb=0,
+        so=0,
+        hbp=0,
+        rbi=0,
+        sb=0,
+        cs=0,
+        outs=0,
+        sac=0,
+        ibb=0,
+        gidp=0,
+        bphr=0,
+        bpfo=0,
+        bp1b=0,
+        bplo=0,
+    )
+    defaults.update(stats)
+    return StratPlay.create(
+        game=game,
+        play_num=play_num,
+        batter=batter,
+        batter_team=batter_team,
+        pitcher=pitcher,
+        pitcher_team=pitcher_team,
+        **defaults,
+    )
+
+
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture
+def rarity():
+    return Rarity.create(value=1, name="Common", color="#ffffff")
+
+
+@pytest.fixture
+def team_a():
+    return _make_team("TMA", gmid=1001)
+
+
+@pytest.fixture
+def team_b():
+    return _make_team("TMB", gmid=1002)
+
+
+@pytest.fixture
+def player_batter(rarity):
+    """A batter-type player for team A."""
+    return _make_player("Batter One", pos="CF")
+
+
+@pytest.fixture
+def player_pitcher(rarity):
+    """A pitcher-type player for team B."""
+    return _make_player("Pitcher One", pos="SP")
+
+
+@pytest.fixture
+def game(team_a, team_b):
+    return StratGame.create(
+        season=11,
+        game_type="ranked",
+        away_team=team_a,
+        home_team=team_b,
+    )
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+
+def test_single_game_batting_stats(team_a, team_b, player_batter, player_pitcher, game):
+    """Batting stat totals from StratPlay rows are correctly accumulated.
+
+    What: Create three plate appearances (2 hits, 1 strikeout, a walk, and a
+    home run) for one batter. After update_season_stats(), the
+    PlayerSeasonStats row should reflect the exact sum of all play fields.
+
+    Why: The core of the batting aggregation pipeline. If any field mapping
+    is wrong (e.g. 'hit' mapped to 'doubles' instead of 'hits'), evolution
+    scoring and leaderboards would silently report incorrect stats.
+    """
+    # PA 1: single (hit=1, ab=1, pa=1)
+    make_play(
+        game,
+        1,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        hit=1,
+        outs=0,
+    )
+    # PA 2: home run (hit=1, homerun=1, ab=1, pa=1, rbi=1, run=1)
+    make_play(
+        game,
+        2,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        hit=1,
+        homerun=1,
+        rbi=1,
+        run=1,
+        outs=0,
+    )
+    # PA 3: strikeout (ab=1, pa=1, so=1, outs=1)
+    make_play(
+        game,
+        3,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        so=1,
+        outs=1,
+    )
+    # PA 4: walk (pa=1, bb=1)
+    make_play(
+        game,
+        4,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        bb=1,
+        outs=0,
+    )
+
+    result = update_season_stats(game.id)
+
+    assert result["batters_updated"] >= 1
+    stats = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == player_batter,
+        PlayerSeasonStats.team == team_a,
+        PlayerSeasonStats.season == 11,
+    )
+    assert stats.pa == 4
+    assert stats.ab == 3
+    assert stats.hits == 2
+    assert stats.hr == 1
+    assert stats.so == 1
+    assert stats.bb == 1
+    assert stats.rbi == 1
+    assert stats.runs == 1
+    assert stats.games_batting == 1
+
+
+def test_single_game_pitching_stats(
+    team_a, team_b, player_batter, player_pitcher, game
+):
+    """Pitching stat totals (outs, k, hits_allowed, bb_allowed) are correct.
+
+    What: The same plays that create batting stats for the batter are also
+    the source for the pitcher's opposing stats. This test checks that
+    _build_pitching_groups() correctly inverts batter-perspective fields.
+
+    Why: The batter's 'so' becomes the pitcher's 'k', the batter's 'hit'
+    becomes 'hits_allowed', etc. Any transposition in this mapping would
+    corrupt pitcher stats silently.
+    """
+    # Play 1: strikeout — batter so=1, outs=1
+    make_play(
+        game,
+        1,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        so=1,
+        outs=1,
+    )
+    # Play 2: single — batter hit=1
+    make_play(
+        game,
+        2,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        hit=1,
+        outs=0,
+    )
+    # Play 3: walk — batter bb=1
+    make_play(
+        game,
+        3,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        bb=1,
+        outs=0,
+    )
+
+    update_season_stats(game.id)
+
+    stats = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == player_pitcher,
+        PlayerSeasonStats.team == team_b,
+        PlayerSeasonStats.season == 11,
+    )
+    assert stats.outs == 1  # one strikeout = one out recorded
+    assert stats.k == 1  # batter's so → pitcher's k
+    assert stats.hits_allowed == 1  # batter's hit → pitcher hits_allowed
+    assert stats.bb_allowed == 1  # batter's bb → pitcher bb_allowed
+    assert stats.games_pitching == 1
+
+
+def test_decision_integration(team_a, team_b, player_batter, player_pitcher, game):
+    """Decision.win=1 for a pitcher results in wins=1 in PlayerSeasonStats.
+
+    What: Add a single StratPlay to establish the pitcher in pitching_groups,
+    then create a Decision row recording a win. Call update_season_stats()
+    and verify the wins column is 1.
+
+    Why: Decisions are stored in a separate table from StratPlay. If
+    _apply_decisions() fails to merge them (wrong FK lookup, key mismatch),
+    pitchers would always show 0 wins/losses/saves regardless of actual game
+    outcomes, breaking standings and evolution criteria.
+    """
+    make_play(
+        game,
+        1,
+        player_batter,
+        team_a,
+        player_pitcher,
+        team_b,
+        pa=1,
+        ab=1,
+        outs=1,
+    )
+    Decision.create(
+        season=11,
+        game=game,
+        pitcher=player_pitcher,
+        pitcher_team=team_b,
+        win=1,
+        loss=0,
+        is_save=0,
+        hold=0,
+        b_save=0,
+        is_start=True,
+    )
+
+    update_season_stats(game.id)
+
+    stats = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == player_pitcher,
+        PlayerSeasonStats.team == team_b,
+        PlayerSeasonStats.season == 11,
+    )
+    assert stats.wins == 1
+    assert stats.losses == 0
+
+
+def test_double_count_prevention(team_a, team_b, player_batter, player_pitcher, game):
+    """Calling update_season_stats() twice for the same game must not double the stats.
+
+    What: Process a game once (pa=3), then call the function again. The
+    second call should detect the already-processed state via the
+    PlayerSeasonStats.last_game FK check and return early with 'skipped'=True.
+    The resulting pa should still be 3, not 6.
+
+    Why: The bot infrastructure may deliver game-complete events more than
+    once (network retries, message replays). Without idempotency, stats
+    would accumulate incorrectly and could not be corrected without a full
+    reset.
+    """
+    for i in range(3):
+        make_play(
+            game,
+            i + 1,
+            player_batter,
+            team_a,
+            player_pitcher,
+            team_b,
+            pa=1,
+            ab=1,
+            outs=1,
+        )
+
+    first_result = update_season_stats(game.id)
+    assert "skipped" not in first_result
+
+    second_result = update_season_stats(game.id)
+    assert second_result.get("skipped") is True
+    assert second_result["batters_updated"] == 0
+    assert second_result["pitchers_updated"] == 0
+
+    stats = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == player_batter,
+        PlayerSeasonStats.team == team_a,
+        PlayerSeasonStats.season == 11,
+    )
+    # Must still be 3, not 6
+    assert stats.pa == 3
+
+
+def test_two_games_accumulate(team_a, team_b, player_batter, player_pitcher):
+    """Stats from two separate games are summed in a single PlayerSeasonStats row.
+
+    What: Process game 1 (pa=2) then game 2 (pa=3) for the same batter/team.
+    After both updates the stats row should show pa=5.
+
+    Why: PlayerSeasonStats is a season-long accumulator, not a per-game
+    snapshot. If the upsert logic overwrites instead of increments, a player's
+    stats would always reflect only their most recent game.
+    """
+    game1 = StratGame.create(
+        season=11, game_type="ranked", away_team=team_a, home_team=team_b
+    )
+    game2 = StratGame.create(
+        season=11, game_type="ranked", away_team=team_a, home_team=team_b
+    )
+
+    # Game 1: 2 plate appearances
+    for i in range(2):
+        make_play(
+            game1,
+            i + 1,
+            player_batter,
+            team_a,
+            player_pitcher,
+            team_b,
+            pa=1,
+            ab=1,
+            outs=1,
+        )
+
+    # Game 2: 3 plate appearances
+    for i in range(3):
+        make_play(
+            game2,
+            i + 1,
+            player_batter,
+            team_a,
+            player_pitcher,
+            team_b,
+            pa=1,
+            ab=1,
+            outs=1,
+        )
+
+    update_season_stats(game1.id)
+    update_season_stats(game2.id)
+
+    stats = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == player_batter,
+        PlayerSeasonStats.team == team_a,
+        PlayerSeasonStats.season == 11,
+    )
+    assert stats.pa == 5
+    assert stats.games_batting == 2
+
+
+def test_two_team_game(team_a, team_b):
+    """Players from both teams in a game each get their own stats row.
+
+    What: Create a batter+pitcher pair for team A and another pair for team B.
+    In the same game, team A bats against team B's pitcher and vice versa.
+    After update_season_stats(), both batters and both pitchers must have
+    correct, isolated stats rows.
+
+    Why: A key correctness guarantee is that stats are attributed to the
+    correct (player, team) combination. If team attribution is wrong,
+    a player's stats could appear under the wrong franchise or be merged
+    with an opponent's row.
+    """
+    batter_a = _make_player("Batter A", pos="CF")
+    pitcher_a = _make_player("Pitcher A", pos="SP")
+    batter_b = _make_player("Batter B", pos="CF")
+    pitcher_b = _make_player("Pitcher B", pos="SP")
+
+    game = StratGame.create(
+        season=11, game_type="ranked", away_team=team_a, home_team=team_b
+    )
+
+    # Team A bats against team B's pitcher (away half)
+    make_play(
+        game,
+        1,
+        batter_a,
+        team_a,
+        pitcher_b,
+        team_b,
+        pa=1,
+        ab=1,
+        hit=1,
+        outs=0,
+        inning_half="top",
+    )
+    make_play(
+        game,
+        2,
+        batter_a,
+        team_a,
+        pitcher_b,
+        team_b,
+        pa=1,
+        ab=1,
+        so=1,
+        outs=1,
+        inning_half="top",
+    )
+
+    # Team B bats against team A's pitcher (home half)
+    make_play(
+        game,
+        3,
+        batter_b,
+        team_b,
+        pitcher_a,
+        team_a,
+        pa=1,
+        ab=1,
+        bb=1,
+        outs=0,
+        inning_half="bottom",
+    )
+
+    update_season_stats(game.id)
+
+    # Team A's batter: 2 PA, 1 hit, 1 SO
+    stats_ba = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == batter_a,
+        PlayerSeasonStats.team == team_a,
+    )
+    assert stats_ba.pa == 2
+    assert stats_ba.hits == 1
+    assert stats_ba.so == 1
+
+    # Team B's batter: 1 PA, 1 BB
+    stats_bb = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == batter_b,
+        PlayerSeasonStats.team == team_b,
+    )
+    assert stats_bb.pa == 1
+    assert stats_bb.bb == 1
+
+    # Team B's pitcher (faced team A's batter): 1 hit allowed, 1 K
+    stats_pb = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == pitcher_b,
+        PlayerSeasonStats.team == team_b,
+    )
+    assert stats_pb.hits_allowed == 1
+    assert stats_pb.k == 1
+
+    # Team A's pitcher (faced team B's batter): 1 BB allowed
+    stats_pa = PlayerSeasonStats.get(
+        PlayerSeasonStats.player == pitcher_a,
+        PlayerSeasonStats.team == team_a,
+    )
+    assert stats_pa.bb_allowed == 1