Support plaintext agent messages

[jif-oai]

Why

Multi-agent v2 send_message deliveries already reach the receiving model as typed agent_message items with encrypted content. Child-completion notifications are generated by Codex itself, so their content is plaintext and previously fell back to a serialized JSON envelope inside an assistant message.

With plaintext input_text supported for agent_message, both delivery paths can use the same model-visible type while preserving explicit author and recipient metadata.

What changed

• add plaintext input_text support to AgentMessageInputContent and regenerate the affected app-server schemas
• preserve InterAgentCommunication as structured mailbox input instead of converting it to assistant text
• record delivered communications as typed agent_message history items
• persist a dedicated rollout item so local delivery metadata such as trigger_turn remains available without leaking into the Responses request
• reconstruct typed agent messages on resume and preserve fork-turn truncation behavior
• remove request-time assistant-content parsing
• preserve plaintext and encrypted inter-agent deliveries in stage-one memory inputs
• normalize and link plaintext and encrypted agent messages in rollout traces without treating inbound messages as child results
• cover the real MultiAgent V2 child-completion path end to end with deterministic mailbox synchronization

Verification

• just test -p codex-core plaintext_multi_agent_v2_completion_sends_agent_message
• just test -p codex-core input_queue_drains_mailbox_in_delivery_order record_initial_history_reconstructs_typed_inter_agent_message fork_turn_positions_use_inter_agent_delivery_metadata
• just test -p codex-memories-write serializes_inter_agent_communications_for_memory
• just test -p codex-rollout-trace agent_messages_preserve_routing_and_content sub_agent_started_activity_creates_spawn_edge
• just test -p codex-rollout-trace agent_result_edge_falls_back_to_child_thread_without_result_message
• just test -p codex-protocol -p codex-rollout -p codex-app-server-protocol

[jif-oai]

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[jif-oai]

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If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".