diff --git a/SCHEMA_DELTAS.md b/SCHEMA_DELTAS.md
index d3ef8d1bf..0f0db1c28 100644
--- a/SCHEMA_DELTAS.md
+++ b/SCHEMA_DELTAS.md
@@ -1,6 +1,3 @@
 # Generated-types delta
 
-## Field changes
-
-- `extensions/extension_meta.py`
-  - `AdcpExtensionFileSchema`: `-field_id`
+_No field-shape changes detected._
diff --git a/examples/sales_proposal_mode_seller/src/platform.py b/examples/sales_proposal_mode_seller/src/platform.py
index c673c3f8b..61d95b01c 100644
--- a/examples/sales_proposal_mode_seller/src/platform.py
+++ b/examples/sales_proposal_mode_seller/src/platform.py
@@ -155,7 +155,7 @@ def create_media_buy(
             "media_buy_id": media_buy_id,
             "buyer_ref": getattr(req, "buyer_ref", None),
             "status": "active",
-            "confirmed_at": datetime.now(timezone.utc).isoformat(),
+            "confirmed_at": datetime.now(timezone.utc).isoformat().replace("+00:00", "Z"),
             "proposal_id": str(proposal_id) if proposal_id else None,
             "packages": [
                 {
diff --git a/examples/sales_proposal_mode_seller/src/proposal_manager.py b/examples/sales_proposal_mode_seller/src/proposal_manager.py
index 190db2a9d..6e60f2240 100644
--- a/examples/sales_proposal_mode_seller/src/proposal_manager.py
+++ b/examples/sales_proposal_mode_seller/src/proposal_manager.py
@@ -269,7 +269,7 @@ async def finalize_proposal(
             if pid in firm_cpm:
                 entry["firm_cpm"] = firm_cpm[pid]
         expires_at = datetime.now(timezone.utc) + timedelta(hours=24)
-        committed_payload["expires_at"] = expires_at.isoformat()
+        committed_payload["expires_at"] = expires_at.isoformat().replace("+00:00", "Z")
         return FinalizeProposalSuccess(
             proposal=committed_payload,
             expires_at=expires_at,
diff --git a/src/adcp/server/helpers.py b/src/adcp/server/helpers.py
index 206a23102..648c339dc 100644
--- a/src/adcp/server/helpers.py
+++ b/src/adcp/server/helpers.py
@@ -402,7 +402,7 @@ def cancel_media_buy_response(
         "media_buy_id": media_buy_id,
         "status": "canceled",
         "canceled_by": canceled_by,
-        "canceled_at": canceled_at or datetime.now(timezone.utc).isoformat(),
+        "canceled_at": canceled_at or datetime.now(timezone.utc).isoformat().replace("+00:00", "Z"),
         "valid_actions": [],
         "sandbox": sandbox,
     }
diff --git a/src/adcp/server/proposal.py b/src/adcp/server/proposal.py
index 40cb7beaa..3ccda4ed5 100644
--- a/src/adcp/server/proposal.py
+++ b/src/adcp/server/proposal.py
@@ -301,7 +301,7 @@ def build(self) -> dict[str, Any]:
         if self._brief_alignment:
             proposal["brief_alignment"] = self._brief_alignment
         if self._expires_at:
-            proposal["expires_at"] = self._expires_at.isoformat()
+            proposal["expires_at"] = self._expires_at.isoformat().replace("+00:00", "Z")
         if self._budget_guidance:
             proposal["total_budget_guidance"] = self._budget_guidance
         if self._ext:
diff --git a/src/adcp/server/responses.py b/src/adcp/server/responses.py
index aa00ab7fd..bb2976d2e 100644
--- a/src/adcp/server/responses.py
+++ b/src/adcp/server/responses.py
@@ -28,6 +28,20 @@ async def get_products():
 
 from adcp.server.helpers import valid_actions_for_status
 
+
+def _rfc3339_now() -> str:
+    """Current UTC time as an RFC 3339 timestamp with ``Z`` suffix.
+
+    Python's :meth:`datetime.isoformat` emits ``+00:00`` for UTC, but
+    several strict schema validators in the AdCP ecosystem — notably
+    the ``zod.string().datetime()`` check that the AdCP storyboard
+    runner uses — reject the offset form by default. Normalizing to
+    the Zulu form (``...Z``) keeps response timestamps acceptable to
+    every common validator without losing precision.
+    """
+    return datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
+
+
 _logger = logging.getLogger("adcp.server")
 
 
@@ -312,7 +326,7 @@ def media_buy_response(
         "media_buy_id": media_buy_id,
         "packages": _serialize(packages),
         "revision": revision if revision is not None else 1,
-        "confirmed_at": confirmed_at or datetime.now(timezone.utc).isoformat(),
+        "confirmed_at": confirmed_at or _rfc3339_now(),
         "sandbox": sandbox,
     }
     if buyer_ref is not None:
@@ -407,7 +421,7 @@ def delivery_response(
         currency: ISO 4217 currency code.
         sandbox: Whether this is simulated data.
     """
-    now = datetime.now(timezone.utc).isoformat()
+    now = _rfc3339_now()
     return {
         "reporting_period": reporting_period or {"start": now, "end": now},
         "media_buy_deliveries": media_buy_deliveries,
@@ -465,14 +479,14 @@ def list_creatives_response(
     Timestamp defaults: every Creative item in the spec requires
     ``created_date`` and ``updated_date`` (ISO 8601 UTC). For any dict
     item that omits either field, this helper fills it with the current
-    UTC timestamp (``datetime.now(timezone.utc).isoformat()``). Both
-    fields default to the same value when neither is provided, which
-    matches the intuitive meaning for a freshly-listed item. Explicit
-    caller-provided values are always preserved. Pydantic model items
-    are passed through ``_serialize`` unchanged — callers using typed
-    Creative models should set timestamps on the model.
+    UTC timestamp via :func:`_rfc3339_now` (Zulu suffix, RFC 3339).
+    Both fields default to the same value when neither is provided,
+    which matches the intuitive meaning for a freshly-listed item.
+    Explicit caller-provided values are always preserved. Pydantic
+    model items are passed through ``_serialize`` unchanged — callers
+    using typed Creative models should set timestamps on the model.
     """
-    now = datetime.now(timezone.utc).isoformat()
+    now = _rfc3339_now()
     filled: list[Any] = []
     for item in creatives:
         if isinstance(item, dict):
diff --git a/src/adcp/types/_ergonomic.py b/src/adcp/types/_ergonomic.py
index 37f22b6ff..6fbbf22f6 100644
--- a/src/adcp/types/_ergonomic.py
+++ b/src/adcp/types/_ergonomic.py
@@ -256,7 +256,7 @@ def _apply_coercion() -> None:
         PackageRequest,
         "creatives",
         Annotated[
-            list[CreativeAsset] | None,
+            Sequence[CreativeAsset] | None,
             BeforeValidator(coerce_subclass_list(CreativeAsset)),
         ],
     )
@@ -281,7 +281,7 @@ def _apply_coercion() -> None:
         CreateMediaBuyRequest,
         "packages",
         Annotated[
-            list[PackageRequest] | None,
+            Sequence[PackageRequest] | None,
             BeforeValidator(coerce_subclass_list(PackageRequest)),
         ],
     )
@@ -383,7 +383,7 @@ def _apply_coercion() -> None:
         ListCreativesResponse,
         "creatives",
         Annotated[
-            list[Creative],
+            Sequence[Creative],
             BeforeValidator(coerce_subclass_list(Creative)),
         ],
     )
diff --git a/src/adcp/types/_generated.py b/src/adcp/types/_generated.py
index 1a649a252..5adaaea9f 100644
--- a/src/adcp/types/_generated.py
+++ b/src/adcp/types/_generated.py
@@ -10,7 +10,7 @@
 DO NOT EDIT MANUALLY.
 
 Generated from: https://github.com/adcontextprotocol/adcp/tree/main/schemas
-Generation date: 2026-05-08 18:05:19 UTC
+Generation date: 2026-05-19 21:40:28 UTC
 """
 
 # ruff: noqa: E501, I001
diff --git a/src/adcp/types/capabilities.py b/src/adcp/types/capabilities.py
index 6927ddba8..1f0c1b4b8 100644
--- a/src/adcp/types/capabilities.py
+++ b/src/adcp/types/capabilities.py
@@ -32,6 +32,8 @@
 
 from __future__ import annotations
 
+from typing import get_args as _get_args
+
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
     A2ui,
     Adcp,
@@ -88,6 +90,9 @@
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
     Account as CapabilitiesAccount,
 )
+from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
+    Adcp as _Adcp,
+)
 
 # ``Capabilities`` (line 580 of the generated module) is the SI-block's
 # inner ``capabilities`` field type — modalities / components / commerce
@@ -109,31 +114,49 @@
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
     Creative as CapabilitiesCreative,
 )
-
-# ``Features2`` is the codegen name for the ``Signals.features`` type
-# (numbered because ``Features`` already names the media_buy features
-# block at line 142 of the generated module). Surface under a stable
-# adopter-facing name so signals declarations read cleanly.
-from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
-    Features2 as SignalsFeatures,
-)
-
-# ``Idempotency`` ships as a ``oneOf`` on the wire (``IdempotencySupported``
-# vs ``IdempotencyUnsupported``) — the codegen names them ``Idempotency``
-# and ``Idempotency3`` (with the numbered variant covering the
-# ``supported: false`` arm). Surface the union halves under stable
-# semantic names so adopters can construct either side without remembering
-# which numbered variant is which.
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
     Idempotency as IdempotencySupported,
 )
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
-    Idempotency3 as IdempotencyUnsupported,
+    MediaBuy as CapabilitiesMediaBuy,
 )
 from adcp.types.generated_poc.bundled.protocol.get_adcp_capabilities_response import (
-    MediaBuy as CapabilitiesMediaBuy,
+    Signals as _Signals,
 )
 
+# ``Signals.features`` and the unsupported arm of the ``Adcp.idempotency``
+# discriminated union are inline schemas the codegen materializes under
+# numbered class names (``Features<N>`` / ``Idempotency<N>``). Those
+# numbers are not stable across regens: ``datamodel-code-generator``
+# 0.56.1 assigns them from a global counter whose traversal order shifts
+# with both upstream schema layout AND filesystem-iteration order
+# (APFS-on-macOS vs ext4-on-Linux), so the same pinned generator produces
+# ``Features1`` in one environment and ``Features2`` in another. Reach
+# the classes via their parents' field annotation — both ``Signals`` and
+# ``Adcp`` are stable wire-spec class names, and the field annotations
+# carry the union arm types directly.
+_signals_features_arms = [
+    arm for arm in _get_args(_Signals.model_fields["features"].annotation) if arm is not type(None)
+]
+if len(_signals_features_arms) != 1:
+    raise RuntimeError(
+        "capabilities: Signals.features annotation lost its concrete type "
+        f"(got {_signals_features_arms!r})"
+    )
+SignalsFeatures: type = _signals_features_arms[0]
+
+_idempotency_arms = [
+    arm
+    for arm in _get_args(_Adcp.model_fields["idempotency"].annotation)
+    if arm is not IdempotencySupported and arm is not type(None)
+]
+if len(_idempotency_arms) != 1:
+    raise RuntimeError(
+        "capabilities: expected exactly one non-supported Idempotency arm, "
+        f"got {_idempotency_arms!r}"
+    )
+IdempotencyUnsupported: type = _idempotency_arms[0]
+
 __all__ = [
     "A2ui",
     "Adcp",
diff --git a/src/adcp/types/generated_poc/enums/daast_tracking_event.py b/src/adcp/types/generated_poc/enums/daast_tracking_event.py
index 293bc3f73..60c67ea25 100644
--- a/src/adcp/types/generated_poc/enums/daast_tracking_event.py
+++ b/src/adcp/types/generated_poc/enums/daast_tracking_event.py
@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  enums/daast_tracking_event.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
diff --git a/src/adcp/types/generated_poc/enums/daast_version.py b/src/adcp/types/generated_poc/enums/daast_version.py
index 36b24df93..cf2ad375b 100644
--- a/src/adcp/types/generated_poc/enums/daast_version.py
+++ b/src/adcp/types/generated_poc/enums/daast_version.py
@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  enums/daast_version.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
diff --git a/src/adcp/types/generated_poc/enums/forecastable_metric.py b/src/adcp/types/generated_poc/enums/forecastable_metric.py
index bf70f8a50..41a8bec22 100644
--- a/src/adcp/types/generated_poc/enums/forecastable_metric.py
+++ b/src/adcp/types/generated_poc/enums/forecastable_metric.py
@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  enums/forecastable_metric.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
diff --git a/src/adcp/types/generated_poc/media_buy/get_media_buy_delivery_response.py b/src/adcp/types/generated_poc/media_buy/get_media_buy_delivery_response.py
index c2ffd0652..64fb536d7 100644
--- a/src/adcp/types/generated_poc/media_buy/get_media_buy_delivery_response.py
+++ b/src/adcp/types/generated_poc/media_buy/get_media_buy_delivery_response.py
@@ -1,13 +1,12 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/get_media_buy_delivery_response.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
 from collections.abc import Sequence
 from enum import Enum
 from typing import Annotated, Any
-from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
diff --git a/src/adcp/types/generated_poc/media_buy/get_media_buys_response.py b/src/adcp/types/generated_poc/media_buy/get_media_buys_response.py
index cd6f720d6..e7803fbfc 100644
--- a/src/adcp/types/generated_poc/media_buy/get_media_buys_response.py
+++ b/src/adcp/types/generated_poc/media_buy/get_media_buys_response.py
@@ -1,13 +1,12 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/get_media_buys_response.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
 from collections.abc import Sequence
 from enum import Enum
 from typing import Annotated
-from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
diff --git a/src/adcp/types/generated_poc/media_buy/update_media_buy_response.py b/src/adcp/types/generated_poc/media_buy/update_media_buy_response.py
index eed31c44e..ee714a354 100644
--- a/src/adcp/types/generated_poc/media_buy/update_media_buy_response.py
+++ b/src/adcp/types/generated_poc/media_buy/update_media_buy_response.py
@@ -1,12 +1,11 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/update_media_buy_response.json
-#   timestamp: 2026-05-02T19:36:29+00:00
+#   timestamp: 2026-05-19T21:40:22+00:00
 
 from __future__ import annotations
 
 from collections.abc import Sequence
 from typing import Annotated
-from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field