AI Transport conventions

AI Transport support is additive and default-off. Enable it with the ai-transport Cargo feature and runtime [ai_transport] config, then scope validation to channel prefixes:

[ai_transport]
enabled = true

[[ai_transport.channels]]
prefix = "private-ai-"

AI lifecycle events are plain channel events: ai-input, ai-output, ai-run-start, ai-run-suspend, ai-run-resume, ai-run-end, and ai-cancel. Server-side validation applies only on enabled AI channels. WebSocket clients may publish ai-input and ai-cancel when their token allows publish; agent events require trusted server-side HTTP publish until an explicit agent token marker is specified.

AI metadata is carried in the existing V2 extras envelope:

{
  "extras": {
    "ai": {
      "transport": {
        "run-id": "run-1",
        "status": "streaming",
        "role": "assistant"
      },
      "codec": {
        "content-type": "application/json"
      }
    }
  }
}

Each transport and codec tier is limited to 32 string keys. Keys must be at most 64 bytes. Transport keys match [a-z0-9-]+; codec keys match [A-Za-z0-9_-]+ for Ably/Vercel codec fields. Transport values and ordinary codec values must be at most 256 bytes; extras.ai.codec.providerMetadata may be up to 8 KiB because Vercel provider metadata is JSON-serialized there. Transport keys are validated against the wire protocol registry. Codec keys are opaque but still bounded. Ably-native regenerate publishes use extras.ai.transport.msg-regenerate as the codec-message-id of the assistant message being regenerated. Any *-client-id transport value on an untrusted WebSocket publish must match the verified signed-in client identity. Trusted app-key publishes may preserve empty run-client-id and step-client-id values as unknown-owner sentinels; derived identities treat those empty values as absent.

Existing idempotency_key behavior is unchanged. AI SDKs may additionally send message_id on HTTP publish for idempotent creates, and mutation requests may send op_id for no-op replay of append/update/delete operations. V2 publish and mutation acknowledgements expose message_serial, history_serial, delivery_serial, and version_serial when those serials are assigned.

Deployment matrix

AI Transport requires durable history and versioned messages. Startup rejects ai_transport.enabled = true unless both [history].enabled and [versioned_messages].enabled are true.

Under AI Transport, versioned-message delivery serials are leased in 128-position blocks per node and channel. The backing store still owns monotonic reservation authority; the lease cache reduces append-stream reservation contention without changing stored serials or recovery semantics.

Append rollup is egress-only and defaults to a 40 ms window. Streaming messages are tracked in the shared cache while their extras.ai.transport.status is streaming. Any node can claim a stale stream after ai_transport.rollup.orphan_ttl_ms, which defaults to 60000 ms, then append a normal versioned message.update with status=cancelled and error-code=orphan_timeout. The claim key prevents duplicate cancellation across nodes, and the latest version is re-read before mutation so active streams that advanced meanwhile are refreshed instead of cancelled.

Apps can subscribe to the webhook event type ai_stream_orphaned to observe synthetic orphan cancellation. The event payload includes channel, message_serial, and reason.

Scale-out verification

The repository includes a three-node Redis/PostgreSQL AI Transport stack and verification scripts:

docker compose -f docker-compose.ai-transport.yml up --build -d
node scripts/ai-transport-3node-bench.mjs
scripts/ai-transport-jepsen-lite.sh

scripts/ai-transport-3node-bench.mjs runs a same-stack baseline plus a cross-node workload, verifies every node reads the same final aggregate, and enforces the cross-node added append p99 budget. scripts/ai-transport-jepsen-lite.sh runs the same correctness check before, during, and after a docker pause/unpause partition of one node; steady state uses MAX_APPEND_ADDED_P99_MS and partition/heal phases use PARTITION_MAX_APPEND_ADDED_P99_MS.