fix(types): drop the empty-model arm codegen emits beside a constrained array

scripts/post_generate_fixes.py

@@ -4417,7 +4417,9 @@ def _require_identity_match_dimensions(self) -> TmpProviderRegistration1:
         print("  trusted_match/identity_match_response.py response class not found")
         return
     response_class = response_class_match.group(1)
-    source = source.rstrip() + f"""
+    source = (
+        source.rstrip()
+        + f"""
 
     @model_validator(mode='after')
     def _validate_tmpx_provider_ids(self) -> {response_class}:
@@ -4432,6 +4434,7 @@ def _validate_tmpx_provider_ids(self) -> {response_class}:
             raise ValueError('tmpx_providers keys must be valid provider_id values')
         return self
 """
+    )
     identity_match_response.write_text(source.rstrip() + "\n")
     print("  trusted_match/identity_match_response.py: added runtime validators")
 
@@ -5266,7 +5269,9 @@ def restore_flattened_contract_field_types() -> None:
 
     if "@model_validator(mode='before')" not in source:
         source = ensure_pydantic_import(source, "model_validator")
-        source = source.rstrip() + """
+        source = (
+            source.rstrip()
+            + """
 
     @model_validator(mode='before')
     @classmethod
@@ -5281,6 +5286,7 @@ def _reject_seller_bound_manifest_fields(cls, data: Any) -> Any:
                 )
         return data
 """
+        )
     representation_target.write_text(source)
     print("  core/creative_representation.py: restored canonical format contract")
 
@@ -5313,7 +5319,9 @@ def enforce_transformer_output_contract() -> None:
     if "class Transformer(" not in source:
         raise RuntimeError("transformer.py: Transformer class not found")
     source = ensure_pydantic_import(source, "model_validator")
-    target.write_text(source.rstrip() + """
+    target.write_text(
+        source.rstrip()
+        + """
 
     @model_validator(mode='after')
     def _require_output_format_declaration(self) -> Transformer:
@@ -5328,7 +5336,8 @@ def _require_output_format_declaration(self) -> Transformer:
                 'one of output_capability_ids or deprecated output_format_ids is required'
             )
         return self
-""")
+"""
+    )
     print("  core/transformer.py: enforced output declaration requirement")
 
 
@@ -6255,7 +6264,8 @@ def fix_verify_brand_claim_models() -> None:
     bulk_response = OUTPUT_DIR / "brand" / "verify_brand_claims_response.py"
 
     if request.exists() and "claim_type:" not in request.read_text():
-        request.write_text("""# generated by datamodel-codegen:
+        request.write_text(
+            """# generated by datamodel-codegen:
 #   filename:  brand/verify_brand_claim_request.json
 
 from __future__ import annotations
@@ -6287,11 +6297,13 @@ class VerifyBrandClaimRequest(AdcpVersionEnvelope):
         dict[str, Any],
         Field(description='Claim payload. Shape varies by claim_type.'),
     ]
-""")
+"""
+        )
         print("  brand/verify_brand_claim_request.py: restored claim fields")
 
     if response.exists() and "VerifyBrandClaimSuccessResponse" not in response.read_text():
-        response.write_text("""# generated by datamodel-codegen:
+        response.write_text(
+            """# generated by datamodel-codegen:
 #   filename:  brand/verify_brand_claim_response.json
 
 from __future__ import annotations
@@ -6350,7 +6362,8 @@ class VerifyBrandClaimErrorResponse(AdcpVersionEnvelope, ProtocolEnvelope):
 
 
 VerifyBrandClaimResponse = VerifyBrandClaimSuccessResponse | VerifyBrandClaimErrorResponse
-""")
+"""
+        )
         print("  brand/verify_brand_claim_response.py: restored response arms")
 
     response_schema = SCHEMA_DIR / "brand" / "verify-brand-claim-response.json"
@@ -6359,7 +6372,8 @@ class VerifyBrandClaimErrorResponse(AdcpVersionEnvelope, ProtocolEnvelope):
         and response_schema.exists()
         and '"signed_response"' in response_schema.read_text()
     ):
-        response.write_text("""# generated by datamodel-codegen:
+        response.write_text(
+            """# generated by datamodel-codegen:
 #   filename:  brand/verify_brand_claim_response.json
 
 from __future__ import annotations
@@ -6510,7 +6524,8 @@ class VerifyBrandClaimErrorResponse(AdcpVersionEnvelope, ProtocolEnvelope):
 
 
 VerifyBrandClaimResponse = VerifyBrandClaimSuccessResponse | VerifyBrandClaimErrorResponse
-""")
+"""
+        )
         print("  brand/verify_brand_claim_response.py: restored signed response fields")
 
     bulk_response_schema = SCHEMA_DIR / "brand" / "verify-brand-claims-response.json"
@@ -6519,7 +6534,8 @@ class VerifyBrandClaimErrorResponse(AdcpVersionEnvelope, ProtocolEnvelope):
         and bulk_response_schema.exists()
         and '"signed_response"' in bulk_response_schema.read_text()
     ):
-        bulk_response.write_text("""# generated by datamodel-codegen:
+        bulk_response.write_text(
+            """# generated by datamodel-codegen:
 #   filename:  brand/verify_brand_claims_response.json
 
 from __future__ import annotations
@@ -6702,7 +6718,8 @@ class VerifyBrandClaimsErrorResponse(AdcpVersionEnvelope, ProtocolEnvelope):
 
 
 VerifyBrandClaimsResponse = VerifyBrandClaimsResponseBulk | VerifyBrandClaimsErrorResponse
-""")
+"""
+        )
         print("  brand/verify_brand_claims_response.py: restored signed response fields")
 
     if bulk_response.exists():
@@ -7234,7 +7251,9 @@ def fix_legacy_purchase_accepted_losses() -> None:
     )
     validator_marker = "    def _accepted_losses_match_schema("
     if validator_marker not in fixed and "field_validator" in fixed:
-        fixed = fixed.rstrip() + """
+        fixed = (
+            fixed.rstrip()
+            + """
 
 
     @field_validator('selected_product_ids')
@@ -7262,11 +7281,103 @@ def _accepted_losses_match_schema(
             raise ValueError('accepted_losses must include the required compatibility losses')
         return values
 """
+        )
     if fixed != source:
         target.write_text(fixed)
         print("  media_buy/legacy_purchase_continuation_input.py: narrowed accepted_losses")
 
 
+def drop_empty_model_arms_from_array_fields() -> None:
+    """Strip the empty-model arm codegen emits beside a constrained array (#1616).
+
+    A schema array that carries an ``allOf`` of ``contains``/``if``-``then``
+    constraints (``losses`` on the legacy purchase continuation, ``refinements``
+    on refine_proposals_request) has no Python shape codegen can give those
+    keywords, so datamodel-code-generator renders the composition as an empty
+    object model and widens the field to ``list[Item] | Empty``. Pydantic then
+    applies the array's ``min_length``/``max_length`` to the whole union, and a
+    dict on the wire reaches the model arm, where the constraint raises a raw
+    ``TypeError`` (``object of type 'Losses' has no len()``) instead of a
+    ``ValidationError``. Every schema-valid ``McpWebhookPayload`` and
+    ``AsyncResponseData`` document that visits that arm while the union is
+    being resolved crashes the same way.
+
+    The protocol contract is the list; the model arm is an artifact. Drop the
+    arm from every field whose union is a ``list[...]`` plus an empty
+    ``AdCPBaseModel`` subclass, in either order, and leave the class itself in
+    place: it is a public ``adcp.types`` name and removing it would be a 9.x
+    surface change for nothing. Idempotent: a field with no empty arm is
+    untouched.
+    """
+    print("Dropping empty-model arms from constrained array fields...")
+    for path in sorted(OUTPUT_DIR.rglob("*.py")):
+        source = path.read_text()
+        tree = ast.parse(source)
+        empty_models = {
+            node.name
+            for node in tree.body
+            if isinstance(node, ast.ClassDef)
+            and len(node.bases) == 1
+            and isinstance(node.bases[0], ast.Name)
+            and node.bases[0].id == "AdCPBaseModel"
+            and len(node.body) == 1
+            and isinstance(node.body[0], ast.Pass)
+        }
+        if not empty_models:
+            continue
+        replacements: list[tuple[ast.expr, ast.expr]] = []
+        for node in ast.walk(tree):
+            if not isinstance(node, ast.AnnAssign):
+                continue
+            for union in ast.walk(node.annotation):
+                if not (isinstance(union, ast.BinOp) and isinstance(union.op, ast.BitOr)):
+                    continue
+                arms = _flatten_union_arms(union)
+                kept = [
+                    arm
+                    for arm in arms
+                    if not (isinstance(arm, ast.Name) and arm.id in empty_models)
+                ]
+                has_list = any(
+                    isinstance(arm, ast.Subscript)
+                    and isinstance(arm.value, ast.Name)
+                    and arm.value.id == "list"
+                    for arm in kept
+                )
+                if len(kept) == len(arms) or not kept or not has_list:
+                    continue
+                replacements.append((union, _rebuild_union(kept)))
+                break
+        if not replacements:
+            continue
+        lines = source.splitlines(keepends=True)
+        for union, rebuilt in sorted(
+            replacements, key=lambda pair: (pair[0].lineno, pair[0].col_offset), reverse=True
+        ):
+            assert union.end_lineno is not None and union.end_col_offset is not None
+            start = sum(len(line) for line in lines[: union.lineno - 1]) + union.col_offset
+            end = sum(len(line) for line in lines[: union.end_lineno - 1]) + union.end_col_offset
+            source = source[:start] + ast.unparse(rebuilt) + source[end:]
+            lines = source.splitlines(keepends=True)
+        path.write_text(source)
+        print(f"  {path.relative_to(OUTPUT_DIR)}: dropped {len(replacements)} empty-model arm(s)")
+
+
+def _flatten_union_arms(node: ast.expr) -> list[ast.expr]:
+    """The arms of a ``A | B | C`` expression, left to right."""
+    if isinstance(node, ast.BinOp) and isinstance(node.op, ast.BitOr):
+        return _flatten_union_arms(node.left) + _flatten_union_arms(node.right)
+    return [node]
+
+
+def _rebuild_union(arms: list[ast.expr]) -> ast.expr:
+    """``arms`` joined back into a ``|`` union (a single arm stands alone)."""
+    rebuilt = arms[0]
+    for arm in arms[1:]:
+        rebuilt = ast.BinOp(left=rebuilt, op=ast.BitOr(), right=arm)
+    return rebuilt
+
+
 def preserve_request_signing_operation_strings() -> None:
     """Keep request-signing operation lists source-compatible with beta.5.
 
@@ -7392,7 +7503,9 @@ def _require_portable_bound(self) -> {class_name}:
         )
         class_start = source.find("class MediaBuyChangeTerm(AdCPBaseModel):")
         if class_start >= 0 and "def _validate_constraint_action" not in source[class_start:]:
-            source = source.rstrip() + """
+            source = (
+                source.rstrip()
+                + """
 
     @model_validator(mode='after')
     def _validate_constraint_action(self) -> MediaBuyChangeTerm:
@@ -7412,7 +7525,9 @@ def _validate_constraint_action(self) -> MediaBuyChangeTerm:
         if action not in allowed.get(kind, set()):
             raise ValueError('constraint kind is incompatible with action')
         return self
-""" + "\n"
+"""
+                + "\n"
+            )
             term_path.write_text(source)
             print("  media_buy/change_term.py: restored constraint/action compatibility")
 
@@ -7425,7 +7540,9 @@ def _validate_constraint_action(self) -> MediaBuyChangeTerm:
         )
         class_start = source.find("class CommercialTerms(AdCPBaseModel):")
         if class_start >= 0 and "def _validate_change_term_set" not in source[class_start:]:
-            source = source.rstrip() + """
+            source = (
+                source.rstrip()
+                + """
 
     @model_validator(mode='after')
     def _validate_change_term_set(self) -> CommercialTerms:
@@ -7474,7 +7591,9 @@ def _validate_change_term_set(self) -> CommercialTerms:
             ):
                 raise ValueError('change-term earliest effective time exceeds latest time')
         return self
-""" + "\n"
+"""
+                + "\n"
+            )
             terms_path.write_text(source)
             print("  media_buy/commercial_terms.py: restored change-term set invariants")
 
@@ -8417,6 +8536,7 @@ def main(argv: list[str] | None = None) -> int:
         fix_product_fields_item_reference,
         fix_audience_evidence_attestation_subject,
         fix_legacy_purchase_accepted_losses,
+        drop_empty_model_arms_from_array_fields,
         preserve_request_signing_operation_strings,
         enforce_change_term_runtime_constraints,
         inject_literal_discriminator_defaults,

scripts/post_generation_manifest.json

@@ -36,6 +36,7 @@
     "fix_product_fields_item_reference": true,
     "fix_audience_evidence_attestation_subject": true,
     "fix_legacy_purchase_accepted_losses": true,
+    "drop_empty_model_arms_from_array_fields": true,
     "preserve_request_signing_operation_strings": true,
     "enforce_change_term_runtime_constraints": true,
     "inject_literal_discriminator_defaults": true,

src/adcp/types/domains/media_buy/refine_proposals_request.py

@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/refine_proposals_request.json
-#   timestamp: 2026-10-21T11:05:54+00:00
+#   timestamp: 2026-10-27T00:03:14+00:00
 
 from __future__ import annotations
 
@@ -69,7 +69,7 @@ class RefineProposalsRequest(AdcpRequest, AdcpVersionEnvelope):
         ),
     ]
     refinements: Annotated[
-        list[proposal_refinement.ProposalRefinement] | Refinements,
+        list[proposal_refinement.ProposalRefinement],
         Field(
             description='Proposal operations to apply, with at most 25 entries per request. revise creates a draft successor from a draft, committed, or accepted source. finalize MUST target a draft, reserves inventory, and creates a committed successor whose expires_at is the hold deadline. A batch containing finalize MUST contain only finalize entries and is atomic. proposal_id values MUST be unique; results preserve request order.',
             max_length=25,

src/adcp/types/domains/media_buy/refine_proposals_response.py

@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/refine_proposals_response.json
-#   timestamp: 2026-10-25T19:32:54+00:00
+#   timestamp: 2026-10-27T00:03:14+00:00
 
 from __future__ import annotations
 
@@ -596,7 +596,7 @@ class RefineProposalsResponse1(AdcpResponse, AdCPBaseModel):
         NoExplicitNull,
     ] = None
     results: Annotated[
-        list[Results8] | Results,
+        list[Results8],
         Field(
             description="Ordered results. If any result is finalized, every result MUST be finalized; a finalize batch either creates every requested hold or none. Every returned proposal carries parent_proposal_id equal to the result's source_proposal_id, making negotiation lineage reconstructible from the proposals alone.",
             min_length=1,
@@ -631,7 +631,7 @@ class RefineProposalsResponse2(AdcpResponse, CompactTaskSubmitted):
         NoExplicitNull,
     ] = None
     results: Annotated[
-        list[Results14] | Results | None,
+        list[Results14] | None,
         Field(
             description="Ordered results. If any result is finalized, every result MUST be finalized; a finalize batch either creates every requested hold or none. Every returned proposal carries parent_proposal_id equal to the result's source_proposal_id, making negotiation lineage reconstructible from the proposals alone.",
             min_length=1,

src/adcp/types/domains/media_buy/request_proposals_response.py

@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  media_buy/request_proposals_response.json
-#   timestamp: 2026-10-25T19:32:54+00:00
+#   timestamp: 2026-10-27T00:03:14+00:00
 
 from __future__ import annotations
 
@@ -136,7 +136,7 @@ class PurchaseContinuation1(AdCPBaseModel):
         ),
     ]
     losses: Annotated[
-        list[Loss] | Losses,
+        list[Loss],
         Field(
             description='Guarantees that the established create_media_buy continuation cannot provide. The coordinator MUST fail before mutation unless the caller explicitly accepts every listed loss.',
             min_length=2,

tests/conformance/_round_trip.py

@@ -176,6 +176,16 @@ def describe(self) -> str:
         return f"{where}: {self.kind} {self.before!r} -> {self.after!r}"
 
 
+#: Exceptions a generated model has been seen to raise from inside validation
+#: instead of reporting a ``ValidationError``: a constraint applied to an arm
+#: that cannot carry it (#1616). A refusal recorded under one of these class
+#: names is a model defect, never a schema disagreement.
+RAISED_OUTSIDE_VALIDATION: tuple[type[Exception], ...] = (TypeError, ValueError)
+RAISED_OUTSIDE_VALIDATION_NAMES: frozenset[str] = frozenset(
+    exc.__name__ for exc in RAISED_OUTSIDE_VALIDATION
+)
+
+
 @dataclass(frozen=True)
 class Refusal:
     """A schema-valid document the model does not accept."""
@@ -704,9 +714,10 @@ def round_trip(
             frozenset(error["type"] for error in exc.errors()),
             "; ".join(f"{'/'.join(map(str, e['loc']))}: {e['msg']}" for e in exc.errors()[:3]),
         )
-    except (TypeError, ValueError) as exc:
+    except RAISED_OUTSIDE_VALIDATION as exc:
         # A model whose validator raises rather than refuses is a defect of
-        # its own; the sweep records it and moves on.
+        # its own (#1616); the sweep records it under the exception's class
+        # name, which ``test_wire_round_trip`` forbids, and moves on.
         return [], Refusal(
             document.schema, document.name, frozenset({type(exc).__name__}), str(exc)[:200]
         )