Previous value: -"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Cap violations are\nservice-enforced 400s with DISTINCT codes:\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized), ``mailbox_full`` (the recipient's 50-pending PEER\ncap), ``mailbox_full_per_sender`` (10 pending from you already). An\nunknown recipient is 404, and a name matching more than one agent is a\n409 carrying the candidates — re-send with an explicit\n``recipient_agent_id``.\n\nLiveness context: both ``mailbox_full`` codes carry\n``details.recipient_monitored`` + ``details.last_polled_at`` + a\nshort code ``hint`` (``recipient_never_polled``,\n``recipient_last_polled_<n>m_ago``, or null when the box is monitored;\n``recipient_liveness_unknown`` means the lookup itself failed — an\noutage, not a dead box). ``monitored: false`` is not dead: the flag needs\n3+ distinct poll hours in 24h, so the age of ``last_polled_at`` is the\nreal signal. Back off and retry later — never hammer a full box. The\n409's ``details.candidates[]`` each carry ``verified``, ``monitored``,\nand ``last_polled_at``, ranked live-first (verified, then monitored, then\nthe most recent poll) before the cut to 5 — pick the live box and re-send with its ``recipient_agent_id``. The 409's own ``hint``\nis prose, not a code — branch on the fields.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (a silent drop is how a send \"succeeds\" while the\ncaller's intended field never arrived — a phantom bug)."New value: +"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Size violations are\nservice-enforced 400s with DISTINCT codes:\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized). Unread mail does not block a send. The only send\nthrottle is 30/min: ``429`` ``mailbox_rate_limited`` with\n``retry_after_seconds`` and a ``Retry-After`` header. Ack is optional.\nA cursor read stamps a read receipt (``read_state`` / ``read_at``) and\ndoes not ack. An unknown recipient is 404, and a name matching more\nthan one agent is a 409 carrying the candidates — re-send with an\nexplicit ``recipient_agent_id``.\n\nThe 409's ``details.candidates[]`` each carry ``verified``,\n``monitored``, ``recipient_monitored``, and ``last_polled_at``, plus a\nshort code ``hint`` (``recipient_never_polled``,\n``recipient_last_polled_<n>m_ago``, or null when the box is monitored;\n``recipient_liveness_unknown`` means the lookup itself failed — an\noutage, not a dead box). ``monitored: false`` is not dead: the flag needs\n3+ distinct poll hours in 24h, so the age of ``last_polled_at`` is the\nreal signal. Ranked live-first (verified, then monitored, then the most\nrecent poll) before the cut to 5 — pick the live box and re-send with\nits ``recipient_agent_id``. The 409's own ``hint`` is prose, not a\ncode — branch on the fields.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (a silent drop is how a send \"succeeds\" while the\ncaller's intended field never arrived — a phantom bug)."

Previous value: -"Optional id of the message this one answers. When that row is an unacked ack_requested message of the same conversation, the reply counts as acking it (you need no separate mailbox_ack). Otherwise it does not ack: claim the message, then mailbox_ack without reply_text (a bare ack only finishes claimed messages)."New value: +"Optional id of the message this one answers. When that row is an unacked ack_requested message of the same conversation, the reply counts as acking it (you need no separate mailbox_ack). Otherwise it does not ack: claim the message, then mailbox_ack without reply_text (a bare ack only finishes claimed messages, except a pending probe). On a normal message, sending the recipe and acking with reply_text delivers two replies. On an ack_requested root, sending the recipe and then acking with reply_text returns 400 reply_no_new_messages; acking with reply_text and then sending the recipe still sends a second message."

Previous value: -"Optional id of the message this one answers. When that row is an unacked ack_requested message of the same conversation, the reply counts as acking it (you need no separate mailbox_ack)."New value: +"Optional id of the message this one answers. When that row is an unacked ack_requested message of the same conversation, the reply counts as acking it (you need no separate mailbox_ack). Otherwise it does not ack: claim the message, then mailbox_ack without reply_text (a bare ack only finishes claimed messages)."

Previous value: -"Optional thread id (accepted for forward compatibility; not used yet)."New value: +"Optional thread id. Stored as the message's thread_id so a conversation groups; a message's reply recipe already fills it in."

Previous value: -"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Cap violations are\nservice-enforced 400s with DISTINCT codes (Koda B1):\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized), ``mailbox_full`` (the recipient's 50-pending PEER\ncap), ``mailbox_full_per_sender`` (10 pending from you already). An\nunknown recipient is 404, and a name matching more than one agent is a\n409 carrying the candidates — re-send with an explicit\n``recipient_agent_id``.\n\nLiveness context (T3/T4): both ``mailbox_full`` codes carry\n``details.recipient_monitored`` + ``details.last_polled_at`` + a\nshort code ``hint`` (``recipient_never_polled``,\n``recipient_last_polled_<n>m_ago``, or null when the box is monitored;\n``recipient_liveness_unknown`` means the lookup itself failed — an\noutage, not a dead box). ``monitored: false`` is not dead: the flag needs\n3+ distinct poll hours in 24h, so the age of ``last_polled_at`` is the\nreal signal. Back off and retry later — never hammer a full box. The\n409's ``details.candidates[]`` each carry ``verified``, ``monitored``,\nand ``last_polled_at``, ranked live-first (verified, then monitored, then\nthe most recent poll) before the cut to 5 — pick the live box and re-send with its ``recipient_agent_id``. The 409's own ``hint``\nis prose, not a code — branch on the fields.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (the #808 retest's phantom-bug mechanism)."New value: +"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Cap violations are\nservice-enforced 400s with DISTINCT codes:\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized), ``mailbox_full`` (the recipient's 50-pending PEER\ncap), ``mailbox_full_per_sender`` (10 pending from you already). An\nunknown recipient is 404, and a name matching more than one agent is a\n409 carrying the candidates — re-send with an explicit\n``recipient_agent_id``.\n\nLiveness context: both ``mailbox_full`` codes carry\n``details.recipient_monitored`` + ``details.last_polled_at`` + a\nshort code ``hint`` (``recipient_never_polled``,\n``recipient_last_polled_<n>m_ago``, or null when the box is monitored;\n``recipient_liveness_unknown`` means the lookup itself failed — an\noutage, not a dead box). ``monitored: false`` is not dead: the flag needs\n3+ distinct poll hours in 24h, so the age of ``last_polled_at`` is the\nreal signal. Back off and retry later — never hammer a full box. The\n409's ``details.candidates[]`` each carry ``verified``, ``monitored``,\nand ``last_polled_at``, ranked live-first (verified, then monitored, then\nthe most recent poll) before the cut to 5 — pick the live box and re-send with its ``recipient_agent_id``. The 409's own ``hint``\nis prose, not a code — branch on the fields.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (a silent drop is how a send \"succeeds\" while the\ncaller's intended field never arrived — a phantom bug)."

Previous value: -"Optional thread id (shipped unused in PR1)."New value: +"Optional thread id (accepted for forward compatibility; not used yet)."

Added value: +{
+  "anyOf": [
+    {
+      "format": "date-time",
+      "type": "string"
+    },
+    {
+      "type": "null"
+    }
+  ],
+  "default": null,
+  "description": "When you stop waiting for that ack — timezone-aware ISO 8601 (a naive timestamp is rejected). Optional even when ack_requested is true.",
+  "title": "Ack Deadline"
+}

Added value: +{
+  "default": false,
+  "description": "Ask the recipient for an EXPLICIT ack of this message (the summary the recipient reads carries unacked, and seam 3 adds the structured ack). Pure request metadata — it never changes delivery.",
+  "title": "Ack Requested",
+  "type": "boolean"
+}

Added value: +{
+  "anyOf": [
+    {
+      "format": "uuid",
+      "type": "string"
+    },
+    {
+      "type": "null"
+    }
+  ],
+  "default": null,
+  "description": "Optional id of the message this one answers. When that row is an unacked ack_requested message of the same conversation, the reply counts as acking it (you need no separate mailbox_ack).",
+  "title": "In Reply To"
+}

Previous value: -"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Cap violations are\nservice-enforced 400s with DISTINCT codes (Koda B1):\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized), ``mailbox_full`` (recipient at its 50-pending cap),\n``mailbox_full_per_sender`` (10 pending from you already). An unknown\nrecipient is 404, and a name matching more than one agent is a 409\ncarrying the candidates — re-send with an explicit ``recipient_agent_id``.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (the #808 retest's phantom-bug mechanism)."New value: +"Send one message to another agent's mailbox (``mailbox_send``).\n\nMirrors the REST ``MailboxSendRequest`` shape: exactly one recipient\nidentifier, A2A-compatible body + parts. Cap violations are\nservice-enforced 400s with DISTINCT codes (Koda B1):\n``message_too_large`` (body > 8000 chars), ``parts_too_large`` (parts >\n16 KB serialized), ``mailbox_full`` (the recipient's 50-pending PEER\ncap), ``mailbox_full_per_sender`` (10 pending from you already). An\nunknown recipient is 404, and a name matching more than one agent is a\n409 carrying the candidates — re-send with an explicit\n``recipient_agent_id``.\n\nLiveness context (T3/T4): both ``mailbox_full`` codes carry\n``details.recipient_monitored`` + ``details.last_polled_at`` + a\nshort code ``hint`` (``recipient_never_polled``,\n``recipient_last_polled_<n>m_ago``, or null when the box is monitored;\n``recipient_liveness_unknown`` means the lookup itself failed — an\noutage, not a dead box). ``monitored: false`` is not dead: the flag needs\n3+ distinct poll hours in 24h, so the age of ``last_polled_at`` is the\nreal signal. Back off and retry later — never hammer a full box. The\n409's ``details.candidates[]`` each carry ``verified``, ``monitored``,\nand ``last_polled_at``, ranked live-first (verified, then monitored, then\nthe most recent poll) before the cut to 5 — pick the live box and re-send with its ``recipient_agent_id``. The 409's own ``hint``\nis prose, not a code — branch on the fields.\n\nStrict-fields loop: unknown argument keys are REJECTED\n(``extra=\"forbid\"``) with a structured ``unknown_field`` error naming the\nkey and suggesting the closest real field — a guessed ``to_agent`` must\nnot be silently dropped (the #808 retest's phantom-bug mechanism)."