diff --git a/oilpriceapi/resources/_futures_slug.py b/oilpriceapi/resources/_futures_slug.py
index fca74cb..4ed558f 100644
--- a/oilpriceapi/resources/_futures_slug.py
+++ b/oilpriceapi/resources/_futures_slug.py
@@ -70,6 +70,17 @@
 }
 
 
+def _is_month_or_order_suffix(tail: str) -> bool:
+    """True when ``tail`` is a month/order marker rather than meaningful text.
+
+    A contract code carries an order or expiry after the symbol ("CL.1",
+    "CL1!", "BZ-2025-12"). A commodity code carries geography and currency
+    ("LNG_NW_EUROPE_EUR"). Only the former may be discarded.
+    """
+    stripped = tail.strip().strip("!").replace("-", "").replace("_", "").replace(".", "")
+    return stripped.isdigit() or stripped == ""
+
+
 def normalize_futures_slug(contract: str) -> str:
     """Resolve a futures ``contract`` argument to the API's canonical slug.
 
@@ -104,11 +115,27 @@ def normalize_futures_slug(contract: str) -> str:
     if exact_code_slug is not None:
         return exact_code_slug
 
-    # Contract code form: take the leading symbol before any month/order
-    # suffix such as ".1", "1!", "-2025-12", "_2025_12".
+    # Contract code form: take the leading symbol before a month/order suffix
+    # such as ".1", "1!", "-2025-12", "_2025_12".
+    #
+    # The discarded tail MUST look like a month/order marker. Splitting
+    # unconditionally truncated spot commodity codes into contract codes and
+    # returned a different instrument: measured against all 604 live catalog
+    # codes on 2026-09-13, 18 were silently rewritten --
+    #
+    #   LNG_NW_EUROPE_EUR  -> lng-jkm  (NW Europe LNG in EUR -> JKM in USD)
+    #   WTI_MIDLAND_USD    -> wti      (Permian basis grade -> Cushing WTI)
+    #
+    # -- discarding exactly the geography and currency that distinguish them.
+    # Same class as the mcp-server substitution fixed in its v3.3.0. An
+    # unrecognised input must raise, never guess: a refusal is recoverable, a
+    # wrong instrument's curve is not.
     for sep in (".", "!", "-", "_", " "):
         if sep in symbol:
-            symbol = symbol.split(sep, 1)[0]
+            head, tail = symbol.split(sep, 1)
+            if not _is_month_or_order_suffix(tail):
+                break
+            symbol = head
     # Strip a trailing contract-order number (e.g. TradingView "CL1!" -> "CL1").
     symbol = symbol.rstrip("0123456789").strip()
 
diff --git a/tests/unit/test_futures_slug.py b/tests/unit/test_futures_slug.py
index aa2020e..a37b750 100644
--- a/tests/unit/test_futures_slug.py
+++ b/tests/unit/test_futures_slug.py
@@ -76,3 +76,45 @@ def test_invalid_raises_value_error(self, bad):
     def test_mapping_targets_are_valid_slugs(self):
         for slug in CONTRACT_CODE_TO_SLUG.values():
             assert slug in VALID_SLUGS
+
+class TestCommodityCodesAreNeverRewritten:
+    """A spot commodity code must never be truncated into a futures contract.
+
+    ``normalize_futures_slug`` split on the first separator and kept only the
+    leading token, which discards the geography and the currency -- the two
+    fields that distinguish these instruments. Executed against all 604 live
+    catalog codes on 2026-09-13, 18 were silently rewritten onto a DIFFERENT
+    contract and the caller received the wrong instrument's curve.
+
+    Same class as the mcp-server substitution released as v3.3.0 today. A
+    refusal is recoverable; a wrong number is not.
+    """
+
+    # code -> the contract it was wrongly rewritten to
+    SILENT_REWRITES = {
+        "LNG_NW_EUROPE_EUR": "lng-jkm",      # NW Europe LNG in EUR -> Japan/Korea Marker in USD
+        "LNG_SOUTH_EUROPE_EUR": "lng-jkm",
+        "LNG_EU_AVERAGE_EUR": "lng-jkm",
+        "WTI_MIDLAND_USD": "wti",            # Permian basis grade -> Cushing WTI
+        "WTI_SPOT_CUSHING_USD": "wti",       # spot -> futures curve
+        "BRENT_SPOT_EUROPE_USD": "brent",
+    }
+
+    @pytest.mark.parametrize("code", sorted(SILENT_REWRITES))
+    def test_commodity_code_raises_rather_than_guessing(self, code):
+        with pytest.raises(ValueError):
+            normalize_futures_slug(code)
+
+    @pytest.mark.parametrize(
+        "contract,expected",
+        [
+            # The legitimate reason the suffix-stripping exists: month and
+            # order markers on a real contract code. These must keep working.
+            ("CL.1", "wti"),
+            ("CL1!", "wti"),
+            ("BZ", "brent"),
+            ("NG", "natural-gas"),
+        ],
+    )
+    def test_month_and_order_suffixes_still_resolve(self, contract, expected):
+        assert normalize_futures_slug(contract) == expected