diff --git a/scripts/post_generate_fixes.py b/scripts/post_generate_fixes.py
index 134623d68..13d068cb7 100644
--- a/scripts/post_generate_fixes.py
+++ b/scripts/post_generate_fixes.py
@@ -799,6 +799,260 @@ def _first_subscript_arg(node: ast.Subscript) -> ast.AST | None:
     return slice_node
 
 
+# ---------------------------------------------------------------------------
+# #624: widen documented extension-point list[X] fields to Sequence[X].
+#
+# Adopters who follow Critical Pattern #1 (subclass a library response type
+# and override the parent's list field with a more specific element type)
+# hit `# type: ignore[assignment]` on every override under mypy --strict —
+# list is invariant in its element type. Sequence is covariant, so a
+# Sequence[Parent] parent permits list[Child] override cleanly.
+#
+# Scope is intentionally narrow: only fields the SDK documents as
+# extension points (response payloads adopters routinely subclass, plus
+# request bodies that compose extendable sub-records like packages and
+# creatives). Internal scalars stay as list.
+#
+# Allowlist format: (class_name, field_name). datamodel-codegen emits
+# bundled response files that each inline copies of subordinate types
+# (Placement, TargetingOverlay, etc.); the rewriter walks every generated
+# .py file and applies the substitution to every emission of the named
+# (class, field) pair so all copies stay consistent.
+
+_SEQUENCE_EXTENSION_POINTS: list[tuple[str, str]] = [
+    # Response payloads adopters subclass to add internal-only fields.
+    # `UpdateMediaBuySuccessResponse` is the success variant of the
+    # `UpdateMediaBuyResponse` discriminated union — emitted as
+    # `UpdateMediaBuyResponse1` (v3.0) and `UpdateMediaBuyResponse3`
+    # (v3.0.6 bundled).
+    ("UpdateMediaBuyResponse1", "affected_packages"),
+    ("UpdateMediaBuyResponse3", "affected_packages"),
+    ("GetMediaBuyDeliveryResponse", "media_buy_deliveries"),
+    ("GetCreativeDeliveryResponse", "creatives"),
+    ("Signal", "deployments"),
+    ("GetSignalsResponse", "signals"),
+    ("GetMediaBuysResponse", "media_buys"),
+    ("ListCreativesResponse", "creatives"),
+    # Request bodies that carry extendable sub-records — adopters subclass
+    # the inner record type and need to override the list element type.
+    ("PackageRequest", "creatives"),
+    ("CreateMediaBuyRequest", "packages"),
+    ("UpdateMediaBuyRequest", "packages"),
+    # Cross-cutting record types referenced from multiple responses; each
+    # bundled response file inlines its own copy. The walker rewrites
+    # every emission.
+    ("Placement", "format_ids"),
+    ("TargetingOverlay", "geo_countries_exclude"),
+    ("TargetingOverlay", "geo_regions_exclude"),
+    ("TargetingOverlay", "geo_metros_exclude"),
+    ("TargetingOverlay", "geo_postal_areas_exclude"),
+]
+
+
+def widen_extension_point_lists_to_sequence():
+    """Rewrite ``list[X]`` to ``Sequence[X]`` on documented extension-point fields.
+
+    Walks every generated ``.py`` file under :data:`OUTPUT_DIR`. For each
+    file, applies every ``(class, field)`` pair in
+    :data:`_SEQUENCE_EXTENSION_POINTS` that matches a class declaration
+    in that file. The same ``(class, field)`` pair commonly appears in
+    multiple files because bundled response emission inlines copies of
+    subordinate types — every emission is rewritten so all paths stay
+    consistent. Each rewritten file gets ``from collections.abc import
+    Sequence`` added if it isn't already present.
+
+    Pairs that produce zero rewrites across the whole tree emit a WARN
+    so allowlist drift surfaces fast (a renamed field or removed class
+    means the override pattern this entry was protecting no longer
+    exists).
+
+    See `adcp-client-python#624 <https://github.com/adcontextprotocol/adcp-client-python/issues/624>`_
+    for the design rationale and the spike that validated the Pydantic
+    plugin accepts ``Sequence[Parent]`` parent + ``list[Child]`` child
+    override under mypy --strict.
+    """
+    print("Widening extension-point list[X] fields to Sequence[X] (#624)...")
+
+    # Track total rewrites per (class, field) — a pair with zero hits is
+    # a stale allowlist entry and surfaces as a WARN.
+    # Track per-pair state across all files:
+    #   rewrites: how many list[X] sites were rewritten this run
+    #   already_widened: how many sites are already in Sequence[X] form
+    # A pair with rewrites == 0 AND already_widened == 0 is genuinely stale
+    # (field renamed/removed) and warrants a WARN. A pair with already_widened
+    # > 0 is silent — that's the steady-state idempotent run.
+    rewrites_per_pair: dict[tuple[str, str], int] = {pair: 0 for pair in _SEQUENCE_EXTENSION_POINTS}
+    already_per_pair: dict[tuple[str, str], int] = {pair: 0 for pair in _SEQUENCE_EXTENSION_POINTS}
+    files_touched = 0
+    total_widened = 0
+
+    for file_path in sorted(OUTPUT_DIR.rglob("*.py")):
+        original = file_path.read_text()
+        content = original
+        widened_in_file = 0
+
+        for class_name, field_name in _SEQUENCE_EXTENSION_POINTS:
+            # Quick filter — skip files that don't declare this class.
+            if f"class {class_name}(" not in content and f"class {class_name}:" not in content:
+                continue
+            new_content, did_widen = _widen_field_annotation(content, class_name, field_name)
+            if did_widen:
+                content = new_content
+                widened_in_file += 1
+                rewrites_per_pair[(class_name, field_name)] += 1
+            elif _field_already_widened(content, class_name, field_name):
+                already_per_pair[(class_name, field_name)] += 1
+
+        if widened_in_file == 0:
+            continue
+
+        content = _ensure_sequence_import(content)
+        file_path.write_text(content)
+        files_touched += 1
+        total_widened += widened_in_file
+        print(f"  ✓ {file_path.relative_to(OUTPUT_DIR)}: widened {widened_in_file} field(s)")
+
+    stale = [
+        pair
+        for pair in _SEQUENCE_EXTENSION_POINTS
+        if rewrites_per_pair[pair] == 0 and already_per_pair[pair] == 0
+    ]
+    for class_name, field_name in stale:
+        print(
+            f"  WARN: {class_name}.{field_name} — neither list[X] nor Sequence[X] "
+            "found in any generated file (field renamed or removed?)"
+        )
+
+    if total_widened == 0:
+        print("  No extension-point fields to widen")
+    else:
+        print(
+            f"  ✓ Widened {total_widened} extension-point field(s) "
+            f"across {files_touched} file(s)"
+        )
+
+
+def _widen_field_annotation(content: str, class_name: str, field_name: str) -> tuple[str, bool]:
+    """Rewrite ``list[X]`` → ``Sequence[X]`` in one field's annotation.
+
+    Locates ``class {class_name}(...):`` then walks forward to the first
+    ``    {field_name}:`` line at class-body indentation, **bounded to the
+    target class** so a same-named field on a later class in the same
+    file cannot mis-match. Within the AnnAssign's annotation block (which
+    may span multiple lines for ``Annotated[..., Field(...)]``), replaces
+    the first ``list[`` with ``Sequence[``. Idempotent — a second pass
+    over already-widened content is a no-op.
+    """
+    # Anchor on the class definition.
+    class_pattern = re.compile(rf"^class {re.escape(class_name)}\b", re.MULTILINE)
+    class_match = class_pattern.search(content)
+    if class_match is None:
+        return content, False
+
+    # Bound the search region to the current class body. Scanning past the
+    # next `^class ` would let `re.search` mis-target a same-named field
+    # on a sibling class in the same file (the lookahead in
+    # field_start_pattern terminates a *match*, but `re.search` is free to
+    # scan past the first class's boundary looking for a hit).
+    class_body_start = class_match.end()
+    next_class = re.compile(r"^class ", re.MULTILINE).search(content, class_body_start)
+    region_end = next_class.start() if next_class is not None else len(content)
+    region = content[class_body_start:region_end]
+
+    # The annotation block runs from the field name to the next class-body
+    # statement at 4-space indentation (next field, model_config, or method).
+    field_start_pattern = re.compile(
+        rf"^(    {re.escape(field_name)}: )(.*?)(?=^    [a-zA-Z_]|\Z)",
+        re.MULTILINE | re.DOTALL,
+    )
+    field_match = field_start_pattern.search(region)
+    if field_match is None:
+        return content, False
+
+    annotation_block = field_match.group(2)
+    # Replace the first list[ inside the annotation only. Generated
+    # annotations always have `list[X]` as the outer container; the
+    # narrow scope of the allowlist (no `dict[str, list[X]]` entries)
+    # makes this safe in practice. If a future entry has nested list,
+    # this needs to anchor on the outer container explicitly.
+    new_annotation = re.sub(r"\blist\[", "Sequence[", annotation_block, count=1)
+    if new_annotation == annotation_block:
+        return content, False
+
+    # Stitch back. .start()/.end() are relative to `region`; convert to
+    # absolute offsets in `content`.
+    abs_start = class_body_start + field_match.start(2)
+    abs_end = class_body_start + field_match.end(2)
+    new_content = content[:abs_start] + new_annotation + content[abs_end:]
+    return new_content, True
+
+
+def _field_already_widened(content: str, class_name: str, field_name: str) -> bool:
+    """Return True when the named field's annotation is already ``Sequence[X]``.
+
+    Used to silence the WARN on idempotent re-runs: a pair that's already
+    widened is the steady state, not allowlist drift.
+    """
+    class_match = re.search(rf"^class {re.escape(class_name)}\b", content, re.MULTILINE)
+    if class_match is None:
+        return False
+    class_body_start = class_match.end()
+    next_class = re.compile(r"^class ", re.MULTILINE).search(content, class_body_start)
+    region_end = next_class.start() if next_class is not None else len(content)
+    region = content[class_body_start:region_end]
+    field_match = re.search(
+        rf"^(    {re.escape(field_name)}: )(.*?)(?=^    [a-zA-Z_]|\Z)",
+        region,
+        re.MULTILINE | re.DOTALL,
+    )
+    if field_match is None:
+        return False
+    return "Sequence[" in field_match.group(2)
+
+
+def _ensure_sequence_import(content: str) -> str:
+    """Add ``from collections.abc import Sequence`` if not already present.
+
+    Inserts after the ``from __future__ import annotations`` line so the
+    import sits with sibling stdlib imports rather than landing at the top
+    of the file.
+    """
+    if "from collections.abc import Sequence" in content:
+        return content
+    # If `collections.abc` is already imported, extend the import line.
+    extend_pattern = re.compile(r"^from collections\.abc import ([^\n]+)$", re.MULTILINE)
+    match = extend_pattern.search(content)
+    if match is not None:
+        existing = match.group(1)
+        # Maintain alphabetical order if the existing import is sorted.
+        names = sorted({*[n.strip() for n in existing.split(",")], "Sequence"})
+        new_line = f"from collections.abc import {', '.join(names)}"
+        return content[: match.start()] + new_line + content[match.end() :]
+
+    # Otherwise insert after the typing imports block. Codegen always emits
+    # ``from typing import Annotated`` near the top, so anchor on it.
+    typing_pattern = re.compile(r"^from typing import [^\n]+$", re.MULTILINE)
+    match = typing_pattern.search(content)
+    if match is not None:
+        return (
+            content[: match.end()]
+            + "\nfrom collections.abc import Sequence"
+            + content[match.end() :]
+        )
+
+    # Fallback: prepend after the `from __future__` line.
+    future_pattern = re.compile(r"^from __future__ import annotations$", re.MULTILINE)
+    match = future_pattern.search(content)
+    if match is not None:
+        return (
+            content[: match.end()]
+            + "\n\nfrom collections.abc import Sequence"
+            + content[match.end() :]
+        )
+
+    return "from collections.abc import Sequence\n\n" + content
+
+
 def main():
     """Apply all post-generation fixes."""
     print("Applying post-generation fixes...")
@@ -816,6 +1070,7 @@ def main():
     fix_reuse_model_discriminator_bug()
     restore_format_category_deprecation_shim()
     inject_literal_discriminator_defaults()
+    widen_extension_point_lists_to_sequence()
 
     print("\n✓ Post-generation fixes complete\n")
 
diff --git a/src/adcp/types/generated_poc/bundled/creative/get_creative_delivery_response.py b/src/adcp/types/generated_poc/bundled/creative/get_creative_delivery_response.py
index 8bd6f86b2..1a8337e9a 100644
--- a/src/adcp/types/generated_poc/bundled/creative/get_creative_delivery_response.py
+++ b/src/adcp/types/generated_poc/bundled/creative/get_creative_delivery_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum, IntEnum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, Field, RootModel, StringConstraints
@@ -3612,7 +3613,7 @@ class GetCreativeDeliveryResponse(AdCPBaseModel):
     ]
     reporting_period: Annotated[ReportingPeriod, Field(description='Date range for the report.')]
     creatives: Annotated[
-        list[Creative], Field(description='Creative delivery data with variant breakdowns')
+        Sequence[Creative], Field(description='Creative delivery data with variant breakdowns')
     ]
     pagination: Annotated[
         Pagination | None,
diff --git a/src/adcp/types/generated_poc/bundled/creative/list_creatives_response.py b/src/adcp/types/generated_poc/bundled/creative/list_creatives_response.py
index 5119d6dcf..df2422ecf 100644
--- a/src/adcp/types/generated_poc/bundled/creative/list_creatives_response.py
+++ b/src/adcp/types/generated_poc/bundled/creative/list_creatives_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum, IntEnum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel, StringConstraints
@@ -3810,7 +3811,7 @@ class ListCreativesResponse(AdCPBaseModel):
         ),
     ]
     creatives: Annotated[
-        list[Creative], Field(description='Array of creative assets matching the query')
+        Sequence[Creative], Field(description='Array of creative assets matching the query')
     ]
     format_summary: Annotated[
         dict[Annotated[str, StringConstraints(pattern=r'^[a-zA-Z0-9_-]+$')], int] | None,
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_request.py b/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_request.py
index 255f1b192..cf4104588 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_request.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_request.py
@@ -6,6 +6,7 @@
 
 from enum import Enum, IntEnum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel, StringConstraints
@@ -2576,7 +2577,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -2590,7 +2591,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -2604,7 +2605,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2618,7 +2619,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -5030,7 +5031,7 @@ class CreateMediaBuyRequest(AdCPBaseModel):
         ),
     ] = None
     packages: Annotated[
-        list[Package] | None,
+        Sequence[Package] | None,
         Field(
             description="Array of package configurations. Required when not using proposal_id. When executing a proposal, this can be omitted and packages will be derived from the proposal's allocations.",
             min_length=1,
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_response.py b/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_response.py
index 299d94f98..026298b80 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_response.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/create_media_buy_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel
@@ -2540,7 +2541,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -2554,7 +2555,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -2568,7 +2569,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2582,7 +2583,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/get_media_buy_delivery_response.py b/src/adcp/types/generated_poc/bundled/media_buy/get_media_buy_delivery_response.py
index d9d1e5b37..5a4e31784 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/get_media_buy_delivery_response.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/get_media_buy_delivery_response.py
@@ -6,6 +6,7 @@
 
 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
@@ -2400,7 +2401,7 @@ class GetMediaBuyDeliveryResponse(AdCPBaseModel):
         ),
     ] = None
     media_buy_deliveries: Annotated[
-        list[MediaBuyDelivery],
+        Sequence[MediaBuyDelivery],
         Field(
             description='Array of delivery data for media buys. When used in webhook notifications, may contain multiple media buys aggregated by publisher. When used in get_media_buy_delivery API responses, typically contains requested media buys.'
         ),
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/get_media_buys_response.py b/src/adcp/types/generated_poc/bundled/media_buy/get_media_buys_response.py
index ed4ba83cf..8123eec69 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/get_media_buys_response.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/get_media_buys_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel
@@ -1590,7 +1591,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -1604,7 +1605,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -1618,7 +1619,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -1632,7 +1633,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2143,7 +2144,7 @@ class GetMediaBuysResponse(AdCPBaseModel):
         extra='allow',
     )
     media_buys: Annotated[
-        list[MediaBuy],
+        Sequence[MediaBuy],
         Field(
             description='Array of media buys with status, creative approval state, and optional delivery snapshots'
         ),
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/get_products_response.py b/src/adcp/types/generated_poc/bundled/media_buy/get_products_response.py
index 9395f6fd3..fd210fce5 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/get_products_response.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/get_products_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel
@@ -203,7 +204,7 @@ class Placement(AdCPBaseModel):
         ),
     ] = None
     format_ids: Annotated[
-        list[FormatId] | None,
+        Sequence[FormatId] | None,
         Field(
             description='Format IDs supported by this specific placement. Can include: (1) concrete format_ids (fixed dimensions), (2) template format_ids without parameters (accepts any dimensions/duration), or (3) parameterized format_ids (specific dimension/duration constraints).',
             min_length=1,
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/package_request.py b/src/adcp/types/generated_poc/bundled/media_buy/package_request.py
index ceb3a6b34..3fd9368da 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/package_request.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/package_request.py
@@ -6,6 +6,7 @@
 
 from enum import Enum, IntEnum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel, StringConstraints
@@ -2163,7 +2164,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -2177,7 +2178,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -2191,7 +2192,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2205,7 +2206,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -4400,7 +4401,7 @@ class PackageRequest(AdCPBaseModel):
         ),
     ] = None
     creatives: Annotated[
-        list[Creative] | None,
+        Sequence[Creative] | None,
         Field(
             description='Upload new creative assets and assign to this package (creatives will be added to library). Use creative_assignments instead for existing library creatives.',
             max_length=100,
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_request.py b/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_request.py
index 8d1a4a291..113b820f8 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_request.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_request.py
@@ -6,6 +6,7 @@
 
 from enum import Enum, IntEnum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel, StringConstraints
@@ -2568,7 +2569,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -2582,7 +2583,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -2596,7 +2597,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2610,7 +2611,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -7596,7 +7597,7 @@ class UpdateMediaBuyRequest(AdCPBaseModel):
         AwareDatetime | None, Field(description='New end date/time in ISO 8601 format')
     ] = None
     packages: Annotated[
-        list[Package] | None,
+        Sequence[Package] | None,
         Field(description='Package-specific updates for existing packages', min_length=1),
     ] = None
     invoice_recipient: Annotated[
diff --git a/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_response.py b/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_response.py
index 3a208556d..d6795fa80 100644
--- a/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_response.py
+++ b/src/adcp/types/generated_poc/bundled/media_buy/update_media_buy_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, EmailStr, Field, RootModel
@@ -1871,7 +1872,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -1885,7 +1886,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -1899,7 +1900,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -1913,7 +1914,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -2388,7 +2389,7 @@ class UpdateMediaBuyResponse3(AdCPBaseModel):
         ),
     ] = None
     affected_packages: Annotated[
-        list[AffectedPackage] | None,
+        Sequence[AffectedPackage] | None,
         Field(description='Array of packages that were modified with complete state information'),
     ] = None
     valid_actions: Annotated[
diff --git a/src/adcp/types/generated_poc/bundled/signals/get_signals_response.py b/src/adcp/types/generated_poc/bundled/signals/get_signals_response.py
index 94c40b42b..116d78a7e 100644
--- a/src/adcp/types/generated_poc/bundled/signals/get_signals_response.py
+++ b/src/adcp/types/generated_poc/bundled/signals/get_signals_response.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, Field, RootModel
@@ -419,7 +420,7 @@ class Signal(AdCPBaseModel):
         float, Field(description='Percentage of audience coverage', ge=0.0, le=100.0)
     ]
     deployments: Annotated[
-        list[Deployments | Deployments3], Field(description='Array of deployment targets')
+        Sequence[Deployments | Deployments3], Field(description='Array of deployment targets')
     ]
     pricing_options: Annotated[
         list[PricingOption],
@@ -538,7 +539,7 @@ class GetSignalsResponse(AdCPBaseModel):
     model_config = ConfigDict(
         extra='allow',
     )
-    signals: Annotated[list[Signal], Field(description='Array of matching signals')]
+    signals: Annotated[Sequence[Signal], Field(description='Array of matching signals')]
     errors: Annotated[
         list[Error] | None,
         Field(
diff --git a/src/adcp/types/generated_poc/core/placement.py b/src/adcp/types/generated_poc/core/placement.py
index 581aef4dd..ca7ebf037 100644
--- a/src/adcp/types/generated_poc/core/placement.py
+++ b/src/adcp/types/generated_poc/core/placement.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import ConfigDict, Field
@@ -35,7 +36,7 @@ class Placement(AdCPBaseModel):
         ),
     ] = None
     format_ids: Annotated[
-        list[format_id.FormatReferenceStructuredObject] | None,
+        Sequence[format_id.FormatReferenceStructuredObject] | None,
         Field(
             description='Format IDs supported by this specific placement. Can include: (1) concrete format_ids (fixed dimensions), (2) template format_ids without parameters (accepts any dimensions/duration), or (3) parameterized format_ids (specific dimension/duration constraints).',
             min_length=1,
diff --git a/src/adcp/types/generated_poc/core/targeting.py b/src/adcp/types/generated_poc/core/targeting.py
index 92187c3a6..e63e7bcf1 100644
--- a/src/adcp/types/generated_poc/core/targeting.py
+++ b/src/adcp/types/generated_poc/core/targeting.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated, Any
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import ConfigDict, Field, RootModel
@@ -393,7 +394,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_countries_exclude: Annotated[
-        list[GeoCountriesExcludeItem] | None,
+        Sequence[GeoCountriesExcludeItem] | None,
         Field(
             description="Exclude specific countries from delivery. ISO 3166-1 alpha-2 codes (e.g., 'US', 'GB', 'DE').",
             min_length=1,
@@ -407,7 +408,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_regions_exclude: Annotated[
-        list[GeoRegionsExcludeItem] | None,
+        Sequence[GeoRegionsExcludeItem] | None,
         Field(
             description="Exclude specific regions/states from delivery. ISO 3166-2 subdivision codes (e.g., 'US-CA', 'GB-SCT').",
             min_length=1,
@@ -421,7 +422,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_metros_exclude: Annotated[
-        list[GeoMetrosExcludeItem] | None,
+        Sequence[GeoMetrosExcludeItem] | None,
         Field(
             description='Exclude specific metro areas from delivery. Each entry specifies the classification system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
@@ -435,7 +436,7 @@ class TargetingOverlay(AdCPBaseModel):
         ),
     ] = None
     geo_postal_areas_exclude: Annotated[
-        list[GeoPostalAreasExcludeItem] | None,
+        Sequence[GeoPostalAreasExcludeItem] | None,
         Field(
             description='Exclude specific postal areas from delivery. Each entry specifies the postal system and excluded values. Seller must declare supported systems in get_adcp_capabilities.',
             min_length=1,
diff --git a/src/adcp/types/generated_poc/creative/get_creative_delivery_response.py b/src/adcp/types/generated_poc/creative/get_creative_delivery_response.py
index 3c8c09dbf..f7f1682b5 100644
--- a/src/adcp/types/generated_poc/creative/get_creative_delivery_response.py
+++ b/src/adcp/types/generated_poc/creative/get_creative_delivery_response.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
@@ -103,7 +104,7 @@ class GetCreativeDeliveryResponse(AdCPBaseModel):
     ]
     reporting_period: Annotated[ReportingPeriod, Field(description='Date range for the report.')]
     creatives: Annotated[
-        list[Creative], Field(description='Creative delivery data with variant breakdowns')
+        Sequence[Creative], Field(description='Creative delivery data with variant breakdowns')
     ]
     pagination: Annotated[
         Pagination | None,
diff --git a/src/adcp/types/generated_poc/creative/list_creatives_response.py b/src/adcp/types/generated_poc/creative/list_creatives_response.py
index dcf3dc263..c8a0e290f 100644
--- a/src/adcp/types/generated_poc/creative/list_creatives_response.py
+++ b/src/adcp/types/generated_poc/creative/list_creatives_response.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field, StringConstraints
@@ -190,7 +191,7 @@ class ListCreativesResponse(AdCPBaseModel):
     ]
     pagination: pagination_response.PaginationResponse
     creatives: Annotated[
-        list[Creative], Field(description='Array of creative assets matching the query')
+        Sequence[Creative], Field(description='Array of creative assets matching the query')
     ]
     format_summary: Annotated[
         dict[Annotated[str, StringConstraints(pattern=r'^[a-zA-Z0-9_-]+$')], int] | None,
diff --git a/src/adcp/types/generated_poc/media_buy/create_media_buy_request.py b/src/adcp/types/generated_poc/media_buy/create_media_buy_request.py
index 88ccb79c5..0060530a6 100644
--- a/src/adcp/types/generated_poc/media_buy/create_media_buy_request.py
+++ b/src/adcp/types/generated_poc/media_buy/create_media_buy_request.py
@@ -6,6 +6,7 @@
 
 from enum import Enum
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AnyUrl, AwareDatetime, ConfigDict, Field
@@ -170,7 +171,7 @@ class CreateMediaBuyRequest(AdCPBaseModel):
         ),
     ] = None
     packages: Annotated[
-        list[package_request.PackageRequest] | None,
+        Sequence[package_request.PackageRequest] | None,
         Field(
             description="Array of package configurations. Required when not using proposal_id. When executing a proposal, this can be omitted and packages will be derived from the proposal's allocations.",
             min_length=1,
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 cf7ae5a33..256b62674 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
@@ -6,6 +6,7 @@
 
 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
@@ -556,7 +557,7 @@ class GetMediaBuyDeliveryResponse(AdCPBaseModel):
         ),
     ] = None
     media_buy_deliveries: Annotated[
-        list[MediaBuyDelivery],
+        Sequence[MediaBuyDelivery],
         Field(
             description='Array of delivery data for media buys. When used in webhook notifications, may contain multiple media buys aggregated by publisher. When used in get_media_buy_delivery API responses, typically contains requested media buys.'
         ),
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 20f963924..0d405c0f9 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
@@ -6,6 +6,7 @@
 
 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
@@ -346,7 +347,7 @@ class GetMediaBuysResponse(AdCPBaseModel):
         extra='allow',
     )
     media_buys: Annotated[
-        list[MediaBuy],
+        Sequence[MediaBuy],
         Field(
             description='Array of media buys with status, creative approval state, and optional delivery snapshots'
         ),
diff --git a/src/adcp/types/generated_poc/media_buy/package_request.py b/src/adcp/types/generated_poc/media_buy/package_request.py
index 7bca5b9a6..68c889ce5 100644
--- a/src/adcp/types/generated_poc/media_buy/package_request.py
+++ b/src/adcp/types/generated_poc/media_buy/package_request.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
@@ -113,7 +114,7 @@ class PackageRequest(AdCPBaseModel):
         ),
     ] = None
     creatives: Annotated[
-        list[creative_asset.CreativeAsset] | None,
+        Sequence[creative_asset.CreativeAsset] | None,
         Field(
             description='Upload new creative assets and assign to this package (creatives will be added to library). Use creative_assignments instead for existing library creatives.',
             max_length=100,
diff --git a/src/adcp/types/generated_poc/media_buy/update_media_buy_request.py b/src/adcp/types/generated_poc/media_buy/update_media_buy_request.py
index 20115716c..cd600b18e 100644
--- a/src/adcp/types/generated_poc/media_buy/update_media_buy_request.py
+++ b/src/adcp/types/generated_poc/media_buy/update_media_buy_request.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated, Literal
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
@@ -66,7 +67,7 @@ class UpdateMediaBuyRequest(AdCPBaseModel):
         AwareDatetime | None, Field(description='New end date/time in ISO 8601 format')
     ] = None
     packages: Annotated[
-        list[package_update.PackageUpdate] | None,
+        Sequence[package_update.PackageUpdate] | None,
         Field(description='Package-specific updates for existing packages', min_length=1),
     ] = None
     invoice_recipient: Annotated[
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 949437e90..1b57f77a4 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
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import AwareDatetime, ConfigDict, Field
@@ -58,7 +59,7 @@ class UpdateMediaBuyResponse1(AdCPBaseModel):
         ),
     ] = None
     affected_packages: Annotated[
-        list[package.Package] | None,
+        Sequence[package.Package] | None,
         Field(description='Array of packages that were modified with complete state information'),
     ] = None
     valid_actions: Annotated[
diff --git a/src/adcp/types/generated_poc/signals/get_signals_response.py b/src/adcp/types/generated_poc/signals/get_signals_response.py
index 442777f65..8a078fb22 100644
--- a/src/adcp/types/generated_poc/signals/get_signals_response.py
+++ b/src/adcp/types/generated_poc/signals/get_signals_response.py
@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from typing import Annotated
+from collections.abc import Sequence
 
 from adcp.types.base import AdCPBaseModel
 from pydantic import ConfigDict, Field
@@ -67,7 +68,7 @@ class Signal(AdCPBaseModel):
         float, Field(description='Percentage of audience coverage', ge=0.0, le=100.0)
     ]
     deployments: Annotated[
-        list[deployment.Deployment], Field(description='Array of deployment targets')
+        Sequence[deployment.Deployment], Field(description='Array of deployment targets')
     ]
     pricing_options: Annotated[
         list[vendor_pricing_option.VendorPricingOption],
@@ -82,7 +83,7 @@ class GetSignalsResponse(AdCPBaseModel):
     model_config = ConfigDict(
         extra='allow',
     )
-    signals: Annotated[list[Signal], Field(description='Array of matching signals')]
+    signals: Annotated[Sequence[Signal], Field(description='Array of matching signals')]
     errors: Annotated[
         list[error.Error] | None,
         Field(
diff --git a/tests/type_checks/extend_response_with_sequence.py b/tests/type_checks/extend_response_with_sequence.py
new file mode 100644
index 000000000..343d2d335
--- /dev/null
+++ b/tests/type_checks/extend_response_with_sequence.py
@@ -0,0 +1,66 @@
+"""Adopter pattern: extend a library response type with a more-specific element.
+
+Critical Pattern #1 — subclass a library type, override a parent's
+``list[X]`` field with ``list[ChildX]`` where ``ChildX`` carries
+extra adopter-internal fields excluded from the wire.
+
+Before #624 this required ``# type: ignore[assignment]`` on every override
+because ``list[T]`` is invariant in T. After #624 the SDK ships the
+parent annotation as ``Sequence[T]`` (covariant), so the override
+typechecks under mypy --strict with zero ignores while keeping
+``.append()`` ergonomics on the child class.
+
+**Scope this test pins down:** Optional list fields where the parent's
+element type matches the public alias (``adcp.types.X``). The widening
+also applies to required-list fields, but a separate codegen issue
+(``X`` emitted multiple times across response files; the public alias
+resolves to one emission while ``ResponseFoo.bar: Sequence[X]``
+references a *different* local emission with the same name) means
+required-field overrides hit a type-identity mismatch *before* variance
+matters. That's tracked separately — Sequence widening is necessary
+but not sufficient for those cases.
+
+**Documented limitation: Optional widening is rejected** — overriding
+a required parent (``Sequence[X]``) with an optional child
+(``list[Y] | None``) is a genuine Liskov-incompatibility (parent
+promises present; child weakens to ``None``-allowed). Adopters must
+keep optionality identical between parent and child.
+"""
+
+from __future__ import annotations
+
+from pydantic import Field
+
+from adcp.types import Package
+from adcp.types.generated_poc.media_buy.update_media_buy_response import (
+    UpdateMediaBuyResponse1,
+)
+
+# --- Optional parent → Optional child (narrower element) ---
+
+
+class _InternalPackage(Package):
+    """Adopter extension — carries fields excluded from the wire envelope."""
+
+    internal_state: str | None = Field(default=None, exclude=True)
+
+
+class _ExtendedUpdateMediaBuyResponse(UpdateMediaBuyResponse1):
+    """Adopter override — narrower element type on the response field.
+
+    Library declares ``affected_packages: Sequence[Package] | None``.
+    Adopter declares ``list[_InternalPackage] | None`` here, which is a
+    valid subtype under Sequence's covariance — no ``# type: ignore``.
+    """
+
+    affected_packages: list[_InternalPackage] | None = None
+
+
+# Construction + .append() prove runtime ergonomics survive the widening.
+resp = _ExtendedUpdateMediaBuyResponse(
+    media_buy_id="mb_1",
+    affected_packages=[_InternalPackage(package_id="p1", internal_state="active")],
+)
+assert resp.affected_packages is not None
+resp.affected_packages.append(_InternalPackage(package_id="p2"))
+assert len(resp.affected_packages) == 2