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## What
A client that declares the AIR capability `loadReplayEnd` gets one extra
`session/update` after the history replay of `session/load`:
```json
{
"sessionId": "<loaded session>",
"update": {
"sessionUpdate": "session_info_update",
"_meta": { "jetbrains": { "air": { "version": 1, "loadReplayEnd": true } } }
}
}
```
The client declares the capability in
`clientCapabilities._meta.jetbrains.air.capabilities`, as it declares
the other AIR extensions. The agent does not advertise it.
## Why
ACP has no signal that the history replay of `session/load` is over,
other than the response. This adapter replays the transcript while the
Claude Code CLI starts, and the CLI start takes most of the load time.
So the history is whole long before the response. With this event, the
client can show the history at once.
The protocol side is the RFD
https://github.com/agentclientprotocol/agent-client-protocol/pull/2407.
## Design change
The first version of this PR marked each replayed notification with an
index (`loadSessionReplayIndex`) and the last one with
`loadSessionEndOfRecordedHistory`. To know the last one, it held the
latest update and queued every other outbound message of the session
behind it. That design is replaced with a capability-gated single end
event. The replay sends as before. Only one event is added at its end.
There are no indexes, no held update and no queue.
## When the event is sent
- After every replayed update, the subagent history that the parent
replay sends to the child sessions included, and before the
`session/load` response.
- Also for an empty history.
- For `session/load` of a session that starts while the replay runs, for
`session/load` of a session that is already live, and for the ACP v2
`session/resume` with `replayFrom: {type: "start"}`.
It is not sent:
- When the replay stops (the CLI start failed during the replay) or
fails.
- For a live update.
- To a client without the capability.
The update has no `title` and no `updatedAt`. A `null` there would clear
the value. The load can still fail after the event, for example when the
CLI start fails after the replay. The event means that the recorded
history arrived whole.
## Client status
JetBrains Air shows the history before the load answer only after the
Kotlin SDK delivers the updates of a loading session before the answer.
An SDK PR for this is in progress.
## Tests
- `create-session-options.test.ts`: the event comes once after the
replay and before the response; before the CLI start finishes; for an
empty history; for an already live session; not when the CLI start fails
part way; not when the replay fails; not without the capability. The
event has no `title` and no `updatedAt` key.
- `acp-agent.test.ts`: the subagent history from its own transcript file
comes before the event.
- `acp-v2.test.ts`: v2 `session/resume` with a replay ends with the
event before the response; a resume without a replay sends none.
- `docs/air-extensions.md`: a new "Load replay end" section, and the
capability and key rows.