diff --git a/data/submissions/drop.py b/data/submissions/drop.py
index 62f6f7f..74d148a 100755
--- a/data/submissions/drop.py
+++ b/data/submissions/drop.py
@@ -29,6 +29,7 @@
     SEASONAL_WORLD_TYPE,
     SeasonalDrop,
 )
+from utils.valuation_guard import sanitize_quantity, exceeds_value_sanity_limit
 
 
 redis_client = RedisClient()
@@ -223,7 +224,9 @@ def log_checkpoint(label: str):
         value = drop_data["value"]
         item_id = drop_data.get("item_id", drop_data.get("id", None))
         item_name = drop_data.get("item_name", drop_data.get("item", None))
-        quantity = drop_data["quantity"]
+        quantity = sanitize_quantity(drop_data.get("quantity"))
+        if quantity is None:
+            return SubmissionResponse(success=False, message="Invalid drop quantity")
         auth_key = drop_data.get("auth_key", None)
         player_name = drop_data.get("player_name", drop_data.get("player", None))
         account_hash = drop_data["acc_hash"]
@@ -346,6 +349,17 @@ def log_checkpoint(label: str):
             f"Drop value calculated - Raw: {raw_drop_value}, Total: {drop_value} ({quantity}x)"
         )
 
+        # Reject implausible totals (spoofed quantity on a valuable item).
+        if exceeds_value_sanity_limit(raw_drop_value, quantity):
+            print(
+                f"[DropGuard] Rejected implausible drop total: {item_name} "
+                f"unit={raw_drop_value} x qty={quantity} from {player_name}"
+            )
+            return SubmissionResponse(
+                success=False,
+                message="Drop value exceeds the plausible maximum and was rejected",
+            )
+
         if drop_value > 1000000:
             debug_print(f"High value drop detected, verifying item/NPC combination...")
             # Offload the wiki lookup to a worker thread so a slow/hanging
diff --git a/tests/unit/test_valuation_guard.py b/tests/unit/test_valuation_guard.py
new file mode 100644
index 0000000..34204c7
--- /dev/null
+++ b/tests/unit/test_valuation_guard.py
@@ -0,0 +1,44 @@
+"""Regression tests for the quantity / total-value sanity guards."""
+from utils.valuation_guard import (
+    MAX_SINGLE_DROP_TOTAL,
+    sanitize_quantity,
+    exceeds_value_sanity_limit,
+)
+
+
+def test_sanitize_quantity_accepts_positive_ints():
+    assert sanitize_quantity(1) == 1
+    assert sanitize_quantity("1000") == 1000
+    assert sanitize_quantity(65_535) == 65_535
+
+
+def test_sanitize_quantity_rejects_non_positive_and_junk():
+    assert sanitize_quantity(0) is None
+    assert sanitize_quantity(-5) is None
+    assert sanitize_quantity("-5") is None
+    assert sanitize_quantity("abc") is None
+    assert sanitize_quantity(None) is None
+
+
+def test_large_but_realistic_stack_is_within_limit():
+    assert exceeds_value_sanity_limit(unit_value=1, quantity=1_000_000) is False
+    assert exceeds_value_sanity_limit(unit_value=1_200_000_000, quantity=1) is False
+
+
+def test_top_tier_single_item_within_limit():
+    # 3rd age-tier item (GE integer ceiling) at quantity 1.
+    assert exceeds_value_sanity_limit(unit_value=2_147_483_647, quantity=1) is False
+
+
+def test_total_above_top_tier_single_item_is_flagged():
+    assert exceeds_value_sanity_limit(unit_value=5_000_000_000, quantity=1) is True
+
+
+def test_absurd_total_is_flagged():
+    # 1.2B x 1000 = the demonstrated exploit.
+    assert exceeds_value_sanity_limit(unit_value=1_200_000_000, quantity=1000) is True
+
+
+def test_limit_boundary():
+    assert exceeds_value_sanity_limit(MAX_SINGLE_DROP_TOTAL, 1) is False
+    assert exceeds_value_sanity_limit(MAX_SINGLE_DROP_TOTAL + 1, 1) is True
diff --git a/tests/unit/test_value_authoritative.py b/tests/unit/test_value_authoritative.py
new file mode 100644
index 0000000..bf04037
--- /dev/null
+++ b/tests/unit/test_value_authoritative.py
@@ -0,0 +1,62 @@
+"""Regression tests: get_true_item_value must not trust a spoofable client value.
+
+Loads the real utils.ge_value in isolation (conftest otherwise stubs it),
+stubbing its infra imports so the valuation branch runs without Redis / GE / aiohttp.
+"""
+import importlib.util
+import os
+import sys
+import types
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+_REPO_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+def _load_real_ge_value():
+    """Import utils/ge_value.py fresh, bypassing the conftest stub."""
+    sys.modules["aiohttp"] = MagicMock()
+
+    redis_stub = types.ModuleType("utils.redis")
+    redis_stub.RedisClient = MagicMock
+    sys.modules["utils.redis"] = redis_stub
+
+    vo_stub = types.ModuleType("utils.value_overrides")
+    vo_stub.match = lambda item_id, item_name: None
+    sys.modules["utils.value_overrides"] = vo_stub
+
+    path = os.path.join(_REPO_ROOT, "utils", "ge_value.py")
+    spec = importlib.util.spec_from_file_location("_real_ge_value_under_test", path)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+@pytest.fixture()
+def ge():
+    return _load_real_ge_value()
+
+
+@pytest.mark.asyncio
+async def test_nonzero_client_value_is_overridden_by_server_price(ge):
+    """A huge client value for a priced item is replaced by the real GE price."""
+    ge._lookup_and_cache_ge_price = AsyncMock(return_value=1_200_000_000)
+    result = await ge.get_true_item_value("Twisted bow", 2_000_000_000, item_id=20997)
+    assert result == 1_200_000_000  # server price wins, not the client's 2,000,000,000
+
+
+@pytest.mark.asyncio
+async def test_client_value_used_when_item_has_no_server_price(ge):
+    """Untradeables (no GE price) still fall back to the client-reported value."""
+    ge._lookup_and_cache_ge_price = AsyncMock(return_value=0)
+    result = await ge.get_true_item_value("Some untradeable", 5000, item_id=999999)
+    assert result == 5000
+
+
+@pytest.mark.asyncio
+async def test_zero_client_value_still_uses_server_price(ge):
+    """Existing behaviour preserved: value=0 recovers the real GE price."""
+    ge._lookup_and_cache_ge_price = AsyncMock(return_value=1_200_000_000)
+    result = await ge.get_true_item_value("Twisted bow", 0, item_id=20997)
+    assert result == 1_200_000_000
diff --git a/utils/ge_value.py b/utils/ge_value.py
index 59780c1..46eb1b9 100755
--- a/utils/ge_value.py
+++ b/utils/ge_value.py
@@ -102,10 +102,11 @@ async def get_true_item_value(item_name, provided_value: int = 0, item_id=None):
         fallback = override.get("fallback_value") or 0
         return fallback if fallback else provided_value
 
-    # No override: when the client reports 0 gp, try to recover the real GE price
-    # so that notification thresholds and point awards aren't silently skipped.
-    if provided_value == 0:
-        return await _lookup_and_cache_ge_price(item_name, item_id)
+    # The client-supplied value is spoofable, so prefer the server GE price for
+    # any priceable item; fall back to the client value only when unpriceable.
+    server_price = await _lookup_and_cache_ge_price(item_name, item_id)
+    if server_price:
+        return server_price
     return provided_value
 
 async def get_mapping():
diff --git a/utils/valuation_guard.py b/utils/valuation_guard.py
new file mode 100644
index 0000000..af0621b
--- /dev/null
+++ b/utils/valuation_guard.py
@@ -0,0 +1,25 @@
+"""Sanity guards for client-supplied drop quantities and totals (stdlib only)."""
+from __future__ import annotations
+
+from typing import Optional
+
+# OSRS Grand Exchange integer cap: no legitimate single drop's total value
+# exceeds this. Valuable uniques drop at quantity 1; large stacks are low-value.
+MAX_SINGLE_DROP_TOTAL = 2_147_483_647
+
+
+def sanitize_quantity(raw) -> Optional[int]:
+    """Coerce a submitted quantity to a positive int, or None if invalid."""
+    try:
+        quantity = int(raw)
+    except (TypeError, ValueError):
+        return None
+    return quantity if quantity > 0 else None
+
+
+def exceeds_value_sanity_limit(unit_value, quantity) -> bool:
+    """True when unit_value * quantity exceeds the limit (fail closed on junk)."""
+    try:
+        return int(unit_value) * int(quantity) > MAX_SINGLE_DROP_TOTAL
+    except (TypeError, ValueError):
+        return True