[bokelley]

Summary

AdCP 3.3: buyers MUST NOT send packages[].start_requires_all_creatives_approved: true on control_media_buy to a seller that does not declare media_buy.features.creative_start_policy: true. #1872 added the seller half. This PR adds the buyer half to ADCPClient.

On a session negotiated at 3.3 or later, the client checks its cached get_adcp_capabilities response. That is the same cache version negotiation already fills. When the seller doesn't declare the feature, the client returns a failed TaskResult before anything is sent:

result.success      # False
result.dispatched   # False: refused locally, nothing was sent
result.adcp_error == {
    "code": "UNSUPPORTED_FEATURE",
    "message": "Seller does not support: creative_start_policy",
    "field": "packages[1].start_requires_all_creatives_approved",
    "details": {
        "feature": "media_buy.features.creative_start_policy",
        "fields": ["packages[1].start_requires_all_creatives_approved", "packages[3].start_requires_all_creatives_approved"],
        "wire_response_received": False,
    },
}

The code, field, details.feature and details.fields match the seller's refusal from #1872. The client doesn't add mutation_outcome, because that is the seller's claim; dispatched=False already says nothing was sent. client.supports("creative_start_policy") answers the question before the call.

Not blocked

• Capabilities unknown: a pinned server_version skips discovery, so the client sends and the seller decides. A conforming seller refuses with the same error.
• false and omission: always sent.
• Sessions below 3.3: unchanged. The field doesn't exist there, and version conversion refuses true exactly as it did before (a test pins this).

How

• Precedent: this follows the existing version-preflight path. _convert_outgoing_request raises the typed ADCPFeatureUnsupportedError(dispatched=False) right after negotiating the target version. _prepare_versioned_task turns that into the failed TaskResult documented in docs/cross-version-conversion.md, and that result carries wire_response_received: false and the idempotency key.
• ADCPFeatureUnsupportedError: gains optional field and details (additive), and _prepare_versioned_task copies them into adcp_error. Existing preflight errors don't set them, so their shape doesn't change. unsupported_features is ["creative_start_policy"], the same name client.supports() / client.require() use.
• Module move: the shared helpers move from adcp.server._creative_start_policy to adcp._creative_start_policy, so the client doesn't import the server package. Both halves now live in one module.

Not marked breaking

The only requests now refused are ones AdCP 3.3 forbids buyers to send. A conforming seller, including this SDK's server since #1872, refuses them with the same code, and the client only refuses when it already knows the seller lacks the feature. The public change is additive: two optional keyword arguments. There is still a MIGRATION_v9_to_v10.md row because the behaviour is visible to users.

TypeScript parity

adcontextprotocol/adcp-client#3649 covers the server side only. I've added a comment there noting buyer-side parity as a follow-up.

Docs

• docs/media-buy-action-rights.md: new "Creative start policy" subsection on buyer behaviour, including the unknown-capabilities case.
• docs/adcp-3.3-sdk-inventory.md: the row now records the client half.
• MIGRATION_v9_to_v10.md: new row.

Test plan

• Buyer-side tests in tests/test_control_media_buy_creative_start_policy.py:
  • Refused when not declared: MCP and A2A, with no feature and with the feature set to false, across several packages; the wire is never called.
  • Allowed when declared: MCP and A2A.
  • Allowed when capabilities are unknown: MCP and A2A.
  • false and omission: always sent.
  • 3.2 session: unchanged.
  • End to end: a client that discovers a real in-memory MCP seller refuses without calling the seller.
• The feat(server)!: refuse an undeclared creative start policy on control_media_buy #1872 MCP test now sends a raw tools/call, because the SDK client would refuse first. It pins the seller's full wire error, including mutation_outcome.
• Neighbouring client preflight suites (test_version_client_boundaries, test_list_proposals_collections, test_control_request_intent, test_account_change_feed): 133 passed.
• make typecheck-all
• make lint
• Full suite (local run in progress)

Closes #1876

🤖 Generated with Claude Code

[aao-secretariat]

Ladon verdict: Approve

Approve — buyer-side creative start policy gate (#1876), clean implementation with full test coverage.

What I checked:

• No critical/high findings (row 1 clear).
• gated_paths: false (row 2 n/a).
• high_risk: true only from src/adcp/server/mcp_tools.py (modified), but with zero medium-or-higher findings the modification is presumed safe — row 5 requires a medium finding, which is absent. No (deleted) reasons (row 3 clear). No (added)-only concern.
• No medium findings at all (rows 4, 8 clear).
• No no-auto-approve team match (row 7 clear).
• No prior decision (row 6 n/a).

The gate is correctly placed after capability discovery in _convert_outgoing_request, version-gated by is_adcp_version_at_least(target, "3.3-beta.0"), and the additive field/details on ADCPFeatureUnsupportedError are backward compatible (no semver break). The DR-0032/0033 "Refused" pathway is covered with tests across MCP/A2A, declared/undeclared/false/omitted, 3.2 sessions, and unknown capabilities — satisfying the mandatory pathway-plus-test rule.

Falls through to row 9 → approve.