diff --git a/kalshi/models/events.py b/kalshi/models/events.py
index d77f487..bd06845 100644
--- a/kalshi/models/events.py
+++ b/kalshi/models/events.py
@@ -13,6 +13,15 @@
 """Event status filter for GET /events. Spec: GetEvents.status query enum."""
 
 
+class SettlementSource(BaseModel):
+    """A settlement source for an event."""
+
+    url: str | None = None
+    name: str | None = None
+
+    model_config = {"extra": "allow"}
+
+
 class Event(BaseModel):
     """A Kalshi event (container for one or more markets)."""
 
@@ -31,6 +40,12 @@ class Event(BaseModel):
     # too (#385), so this is no longer a spec deviation — kept nullable to
     # match server reality.
     product_metadata: dict[str, Any] | None = None
+    # Spec-required on EventData but `nullable: true` (added to the live
+    # v3.21.0 spec post-#449, #451). NullableList coerces a JSON null -> [] so
+    # the key-present contract holds while callers always see a list. Same
+    # nullable shape as Series.settlement_sources; EventMetadata's is a plain
+    # list because its spec field isn't nullable.
+    settlement_sources: NullableList[SettlementSource]
     last_updated_ts: AwareDatetime | None = None
     markets: NullableList[Market] = []
 
@@ -75,15 +90,6 @@ class MarketMetadata(BaseModel):
     model_config = {"extra": "allow"}
 
 
-class SettlementSource(BaseModel):
-    """A settlement source for an event."""
-
-    url: str | None = None
-    name: str | None = None
-
-    model_config = {"extra": "allow"}
-
-
 class EventMetadata(BaseModel):
     """Metadata for an event including images and settlement sources."""
 
diff --git a/kalshi/ws/models/market_lifecycle.py b/kalshi/ws/models/market_lifecycle.py
index 1a92473..672ebbb 100644
--- a/kalshi/ws/models/market_lifecycle.py
+++ b/kalshi/ws/models/market_lifecycle.py
@@ -41,6 +41,13 @@ class MarketLifecyclePayload(BaseModel):
     # price_level_structure for `price_level_structure_updated` (or `created`).
     additional_metadata: dict[str, Any] | None = None
     floor_strike: DollarDecimal | None = None
+    # metadata_updated only (added to the live asyncapi spec post-#449).
+    # strike_type decides how floor_strike/cap_strike are read ("between" uses
+    # both, "greater" floor only, "less" cap only); custom_strike carries
+    # structured strikes. All optional — present only on metadata_updated.
+    strike_type: str | None = None
+    cap_strike: DollarDecimal | None = None
+    custom_strike: dict[str, Any] | None = None
     price_level_structure: str | None = None
     yes_sub_title: str | None = None
     model_config = {"extra": "allow", "populate_by_name": True}
diff --git a/specs/asyncapi.yaml b/specs/asyncapi.yaml
index 3015a5d..f7b137c 100644
--- a/specs/asyncapi.yaml
+++ b/specs/asyncapi.yaml
@@ -81,7 +81,22 @@ info:
     | 27 | Too many requests | The subscription exceeded its command rate limit
     |
 
-    | 28 | Markets not found | Unknown tickers were not subscribed |
+
+    ## Terminal Errors
+
+
+    The following channel errors are terminal and the user must resubscribe.
+
+
+    | Code | Error |
+
+    |------|-------|
+
+    | 10 | Channel error |
+
+    | 17 | Internal error |
+
+    | 25 | Subscription buffer overflow |
   contact:
     name: Kalshi Support
     url: https://kalshi.com
@@ -1391,8 +1406,6 @@ components:
 
         | 27 | Too many requests | The subscription exceeded its command rate
         limit |
-
-        | 28 | Markets not found | Unknown tickers were not subscribed |
       contentType: application/json
       payload:
         $ref: '#/components/schemas/errorResponsePayload'
@@ -1694,7 +1707,9 @@ components:
             msg:
               market_ticker: KXBTC-25APR30-T0915-B95000
               event_type: metadata_updated
+              strike_type: between
               floor_strike: 95000
+              cap_strike: 95250
         - name: metadataUpdatedSubtitle
           summary: Market metadata updated event (subtitle)
           payload:
@@ -2471,10 +2486,8 @@ components:
 
                 - 27: Too many requests - The subscription exceeded its command
                 rate limit
-
-                - 28: Markets not found - Unknown tickers were not subscribed
               minimum: 1
-              maximum: 28
+              maximum: 27
             msg:
               type: string
               description: Human-readable error message
@@ -2484,11 +2497,6 @@ components:
             market_ticker:
               type: string
               description: Market ticker if error is market-specific (optional)
-            market_tickers:
-              type: array
-              items:
-                type: string
-              description: Market tickers if error is market-specific (optional)
     listSubscriptionsCommandPayload:
       type: object
       required:
@@ -3165,11 +3173,28 @@ components:
                 - linear_cent
                 - deci_cent
                 - tapered_deci_cent
+            strike_type:
+              type: string
+              description: >-
+                Optional - This key will ONLY exist for metadata_updated events.
+                Determines how floor_strike / cap_strike are interpreted (e.g.
+                "between" uses both, "greater" uses floor_strike only, "less"
+                uses cap_strike only)
             floor_strike:
               type: number
               description: >-
                 Optional - This key will ONLY exist for metadata_updated events.
-                The updated floor strike value for the market
+                The floor (lower bound) strike value for the market
+            cap_strike:
+              type: number
+              description: >-
+                Optional - This key will ONLY exist for metadata_updated events.
+                The cap (upper bound) strike value for the market
+            custom_strike:
+              type: object
+              description: >-
+                Optional - This key will ONLY exist for metadata_updated events
+                with a custom or structured strike type
             yes_sub_title:
               type: string
               description: >-
@@ -3985,6 +4010,3 @@ x-error-codes:
     - code: 27
       name: Too many requests
       description: The subscription exceeded its command rate limit
-    - code: 28
-      name: Markets not found
-      description: Unknown tickers were not subscribed
diff --git a/specs/openapi.yaml b/specs/openapi.yaml
index 8486382..72c714d 100644
--- a/specs/openapi.yaml
+++ b/specs/openapi.yaml
@@ -8072,6 +8072,7 @@ components:
         - collateral_return_type
         - mutually_exclusive
         - available_on_brokers
+        - settlement_sources
       properties:
         event_ticker:
           type: string
@@ -8123,6 +8124,12 @@ components:
           x-omitempty: true
           description: Additional metadata for the event.
           x-go-type-skip-optional-pointer: true
+        settlement_sources:
+          type: array
+          nullable: true
+          items:
+            $ref: '#/components/schemas/SettlementSource'
+          description: The official sources used for the determination of markets within this event. Methodology is defined in the rulebook.
         last_updated_ts:
           type: string
           format: date-time
diff --git a/tests/_model_fixtures.py b/tests/_model_fixtures.py
index 900b78b..c268bc3 100644
--- a/tests/_model_fixtures.py
+++ b/tests/_model_fixtures.py
@@ -147,6 +147,7 @@ def event_dict(**overrides: Any) -> dict[str, Any]:
         "collateral_return_type": "self_collateralized",
         "available_on_brokers": False,
         "product_metadata": {},
+        "settlement_sources": [],
     }
     base.update(overrides)
     return base
diff --git a/tests/test_models.py b/tests/test_models.py
index e8fd146..4c44f6b 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -10,7 +10,7 @@
 from pydantic import BaseModel, ValidationError
 
 from kalshi.models.communications import RFQ, Quote
-from kalshi.models.events import Event, EventMetadata
+from kalshi.models.events import Event, EventMetadata, SettlementSource
 from kalshi.models.historical import Trade
 from kalshi.models.incentive_programs import IncentiveProgram
 from kalshi.models.markets import Market
@@ -437,6 +437,40 @@ def test_parses_when_server_omits_product_metadata(self) -> None:
         assert e.product_metadata is None
 
 
+class TestEventSettlementSources:
+    """#451 spec drift: ``EventData.settlement_sources`` (required, nullable).
+
+    Typed ``NullableList[SettlementSource]`` so a JSON ``null`` coerces to ``[]``
+    while the key-present contract still holds (same shape as
+    ``Series.settlement_sources``).
+    """
+
+    def test_null_coerces_to_empty_list(self) -> None:
+        e = Event.model_validate(event_dict(settlement_sources=None))
+        assert e.settlement_sources == []
+
+    def test_parses_list_of_sources(self) -> None:
+        e = Event.model_validate(
+            event_dict(
+                settlement_sources=[
+                    {"url": "https://example.com", "name": "NWS"},
+                    {"url": None, "name": None},
+                ]
+            )
+        )
+        assert len(e.settlement_sources) == 2
+        assert all(isinstance(s, SettlementSource) for s in e.settlement_sources)
+        assert e.settlement_sources[0].url == "https://example.com"
+        assert e.settlement_sources[0].name == "NWS"
+
+    def test_field_is_required(self) -> None:
+        """Spec marks the field required — the key must be present."""
+        data = event_dict()
+        data.pop("settlement_sources")
+        with pytest.raises(ValidationError):
+            Event.model_validate(data)
+
+
 class TestEventMetadataV3180Fields:
     """v3.18.0 backfill (issue #160): 2 new optional fields on ``EventMetadata``."""
 
diff --git a/tests/ws/test_models.py b/tests/ws/test_models.py
index 884b696..42ee73a 100644
--- a/tests/ws/test_models.py
+++ b/tests/ws/test_models.py
@@ -1548,6 +1548,42 @@ def test_floor_strike_rejects_bool(self) -> None:
             MarketLifecycleMessage.model_validate(raw)
 
 
+class TestMarketLifecycleStrikeFields:
+    """#451 spec drift: ``metadata_updated`` lifecycle payloads gained
+    ``strike_type`` / ``cap_strike`` / ``custom_strike`` (all optional)."""
+
+    def test_parses_between_strike(self) -> None:
+        """The asyncapi 'between' example: strike_type + floor + cap together."""
+        raw = {
+            "type": "market_lifecycle_v2",
+            "sid": 1,
+            "msg": {
+                "event_type": "metadata_updated",
+                "market_ticker": "T",
+                "strike_type": "between",
+                "floor_strike": 95000,
+                "cap_strike": 95250.5,
+                "custom_strike": {"team": "A"},
+            },
+        }
+        msg = MarketLifecycleMessage.model_validate(raw)
+        assert msg.msg.strike_type == "between"
+        assert isinstance(msg.msg.cap_strike, Decimal)
+        assert msg.msg.cap_strike == Decimal("95250.5")
+        assert msg.msg.custom_strike == {"team": "A"}
+
+    def test_strike_fields_default_to_none(self) -> None:
+        raw = {
+            "type": "market_lifecycle_v2",
+            "sid": 1,
+            "msg": {"event_type": "created", "market_ticker": "T"},
+        }
+        msg = MarketLifecycleMessage.model_validate(raw)
+        assert msg.msg.strike_type is None
+        assert msg.msg.cap_strike is None
+        assert msg.msg.custom_strike is None
+
+
 class TestWsPayloadsRejectNaiveDatetime:
     """#270 Item 1: WS payloads with datetime fields must reject naive RFC3339 strings.