diff --git a/scripts/generate_ergonomic_coercion.py b/scripts/generate_ergonomic_coercion.py
index ae69b243..9a2d6482 100644
--- a/scripts/generate_ergonomic_coercion.py
+++ b/scripts/generate_ergonomic_coercion.py
@@ -95,22 +95,26 @@ def get_base_type(annotation: Any) -> Any:
 
 
 def is_list_of(annotation: Any, item_check) -> tuple[bool, Any]:
-    """Check if annotation is list[X] where X passes item_check.
+    """Check if annotation is list[X] or Sequence[X] where X passes item_check.
 
-    Handles both list[X] and list[X] | None.
+    Handles both T[X] and T[X] | None (where T is list or collections.abc.Sequence).
     """
-    # First check if the annotation itself is a list
+    from collections.abc import Sequence as AbcSequence
+
+    _list_origins = (list, AbcSequence)
+
+    # First check if the annotation itself is a list/Sequence
     origin = get_origin(annotation)
-    if origin is list:
+    if origin in _list_origins:
         args = get_args(annotation)
         if args and item_check(args[0]):
             return True, args[0]
 
-    # Then check if it's Optional[list[X]] (i.e., list[X] | None)
+    # Then check if it's Optional[list[X]] or Optional[Sequence[X]]
     base = get_base_type(annotation)
     if base is not None and base is not annotation:
         origin = get_origin(base)
-        if origin is list:
+        if origin in _list_origins:
             args = get_args(base)
             if args and item_check(args[0]):
                 return True, args[0]
@@ -369,12 +373,15 @@ def _find_success_variant() -> type[_PydBaseModel]:
         "5. FieldModel (enum) lists accept string lists",
         "",
         "Note: List variance issues (list[Subclass] not assignable to list[BaseClass])",
-        "are a fundamental Python typing limitation. Users extending library types",
-        "should use Sequence[T] in their own code or cast() for type checker appeasement.",
+        "are a fundamental Python typing limitation. Response-only container fields",
+        "(affected_packages, media_buys, packages, media_buy_deliveries) already use",
+        "Sequence[T] in their generated base class. For other fields not yet migrated,",
+        "adopters should use Sequence[T] in their own code or cast() for appeasement.",
         '"""',
         "",
         "from __future__ import annotations",
         "",
+        "from collections.abc import Sequence",
         "from typing import Annotated, Any",
         "",
         "from pydantic import BeforeValidator",
@@ -555,11 +562,18 @@ def _find_success_variant() -> type[_PydBaseModel]:
                 )
                 lines.append("    )")
             elif c["type"] == "subclass_list":
+                from collections.abc import Sequence as AbcSequence
+
                 target = c["target_class"].__name__
-                # Check if the field is required (no | None)
                 field_info = cls.model_fields[field]
                 is_optional = "None" in str(field_info.annotation)
-                type_str = f"list[{target}] | None" if is_optional else f"list[{target}]"
+                # Preserve Sequence[T] when the field already uses it (covariant
+                # inheritance, set by post_generate_fixes.rewrite_response_list_to_sequence).
+                ann = field_info.annotation
+                base_ann = get_base_type(ann)
+                is_seq = get_origin(base_ann if base_ann is not None else ann) is AbcSequence
+                container = "Sequence" if is_seq else "list"
+                type_str = f"{container}[{target}] | None" if is_optional else f"{container}[{target}]"
                 lines.append("    _patch_field_annotation(")
                 lines.append(f"        {type_name},")
                 lines.append(f'        "{field}",')
diff --git a/scripts/post_generate_fixes.py b/scripts/post_generate_fixes.py
index 69e64bac..b5c2c623 100644
--- a/scripts/post_generate_fixes.py
+++ b/scripts/post_generate_fixes.py
@@ -547,6 +547,67 @@ def add_rootmodel_getattr_proxy():
         print("  No RootModel union types needed __getattr__ proxy")
 
 
+# Response-only list fields changed to Sequence[T] so adopters can narrow the
+# element type without type: ignore[assignment] under strict mypy.  Only
+# response-side fields (received, never mutated) are safe to change; request-
+# side list fields (packages/creatives on request types) stay as list[T]
+# because adopters call .append() on them.  See issue #624.
+RESPONSE_SEQUENCE_FIELDS: list[tuple[str, str]] = [
+    ("media_buy/update_media_buy_response.py", "affected_packages"),
+    ("media_buy/get_media_buys_response.py", "media_buys"),
+    ("media_buy/get_media_buys_response.py", "packages"),
+    ("media_buy/get_media_buy_delivery_response.py", "media_buy_deliveries"),
+]
+
+
+def rewrite_response_list_to_sequence() -> None:
+    """Change list[T] → Sequence[T] on response-only container fields.
+
+    list[T] is invariant so ``affected_packages: list[MyPkg]`` on a subclass
+    triggers mypy[assignment] against the parent's ``list[Pkg]``.  Sequence[T]
+    is covariant, removing the error for adopters who extend element types.
+    """
+    print("Rewriting response list fields to Sequence for covariant inheritance...")
+
+    for rel_path, field_name in RESPONSE_SEQUENCE_FIELDS:
+        target = OUTPUT_DIR / rel_path
+        if not target.exists():
+            print(f"  {rel_path}: not found (skipping)")
+            continue
+
+        content = target.read_text()
+
+        # Idempotency: skip if field already uses Sequence
+        if re.search(rf"{re.escape(field_name)}: Annotated\[\s+Sequence\[", content):
+            print(f"  {rel_path}: {field_name} already uses Sequence (skipping)")
+            continue
+
+        new_content = re.sub(
+            rf"({re.escape(field_name)}: Annotated\[\s+)list\[",
+            r"\1Sequence[",
+            content,
+        )
+
+        if new_content == content:
+            print(f"  {rel_path}: {field_name} — list[ pattern not found (skipping)")
+            continue
+
+        # Add Sequence import from collections.abc in stdlib block.
+        # Anchor on the first stdlib import line (enum or typing) so Sequence
+        # lands in correct alphabetical position (c < e < t).
+        if "from collections.abc import Sequence" not in new_content:
+            new_content = re.sub(
+                r"^(from (?:enum|typing) import .+)$",
+                r"from collections.abc import Sequence\n\1",
+                new_content,
+                count=1,
+                flags=re.MULTILINE,
+            )
+
+        target.write_text(new_content)
+        print(f"  {rel_path}: {field_name} → Sequence[...]")
+
+
 def fix_list_field_shadowing():
     """Fix models where a field named 'list' shadows the builtin list type.
 
@@ -1151,6 +1212,7 @@ def main():
     unwrap_rootmodel_unions()
     add_rootmodel_getattr_proxy()
     fix_list_field_shadowing()
+    rewrite_response_list_to_sequence()
     fix_reuse_model_discriminator_bug()
     restore_format_category_deprecation_shim()
     inject_literal_discriminator_defaults()
diff --git a/src/adcp/types/_ergonomic.py b/src/adcp/types/_ergonomic.py
index f5133f54..37f22b6f 100644
--- a/src/adcp/types/_ergonomic.py
+++ b/src/adcp/types/_ergonomic.py
@@ -25,12 +25,15 @@
 5. FieldModel (enum) lists accept string lists
 
 Note: List variance issues (list[Subclass] not assignable to list[BaseClass])
-are a fundamental Python typing limitation. Users extending library types
-should use Sequence[T] in their own code or cast() for type checker appeasement.
+are a fundamental Python typing limitation. Response-only container fields
+(affected_packages, media_buys, packages, media_buy_deliveries) already use
+Sequence[T] in their generated base class. For other fields not yet migrated,
+adopters should use Sequence[T] in their own code or cast() for appeasement.
 """
 
 from __future__ import annotations
 
+from collections.abc import Sequence
 from typing import Annotated, Any
 
 from pydantic import BeforeValidator
@@ -500,7 +503,7 @@ def _apply_coercion() -> None:
         GetMediaBuyDeliveryResponse,
         "media_buy_deliveries",
         Annotated[
-            list[MediaBuyDelivery],
+            Sequence[MediaBuyDelivery],
             BeforeValidator(coerce_subclass_list(MediaBuyDelivery)),
         ],
     )
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 256b6267..c2ffd065 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
@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Sequence
 from enum import Enum
 from typing import Annotated, Any
 from collections.abc import Sequence
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 0d405c0f..cd6f720d 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
@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Sequence
 from enum import Enum
 from typing import Annotated
 from collections.abc import Sequence
@@ -334,7 +335,7 @@ class MediaBuy(AdCPBaseModel):
         ),
     ] = None
     packages: Annotated[
-        list[Package],
+        Sequence[Package],
         Field(
             description='Packages within this media buy, augmented with creative approval status and optional delivery snapshots'
         ),
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 1b57f77a..eed31c44 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
@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Sequence
 from typing import Annotated
 from collections.abc import Sequence