Expose Claude Code mid-turn message injection (typed-ahead / `/btw`) over ACP

[Seluj78]

Summary

Expose Claude Code's mid-turn message injection over ACP, so an ACP client can deliver a message to a turn that is already running (steering), the way the Claude Code CLI does with typed-ahead input and the /btw ("by the way") command.

Background

claude-agent-acp bridges the Claude Agent SDK to ACP. In the Claude Code CLI you can:

• Type a message while the agent is mid-turn; it is picked up and incorporated into the running turn.
• Use /btw to inject a "by the way" note into the in-flight turn so the model course-corrects without the turn ending.

The claude binary carries this natively (findBtwTriggerPositions, getBtwHistory, createBtwHistoryState, etc.).

The gap over ACP

There is no way to deliver input to a turn that is already running:

• session/prompt starts or queues a separate turn. The adapter runs a mid-turn session/prompt only after the current turn settles: activateTurn / ensureActiveTurn promote the queued turn once the active one ends, and input is echoed/run FIFO (dist/acp-agent.js).
• agentCapabilities._meta.claudeCode.promptQueueing: true advertises queue-next, not steering.
• The only mid-turn levers are session/cancel and interrupt.

So an ACP client can queue a follow-up turn or cancel, but it cannot do what the CLI does: add to the turn in flight. Clients that want "send while working" have to fake it by holding the message and re-sending it as a new turn after Stopped, which is a different behavior (the agent finishes first, then reads the message, instead of course-correcting mid-work).

Ask

1. A way to inject a message into the running turn (steering), equivalent to the CLI's typed-ahead input, so the model incorporates it mid-turn without ending the turn.
2. Ideally a dedicated /btw-style semantic (a "by the way" note distinct from a full follow-up prompt) surfaced over ACP.

Possible shapes

• A new ACP method or notification (for example session/steer, or session/prompt with an "append to current turn" flag) that the adapter forwards to the SDK's in-turn input / btw mechanism.
• Or a _meta.claudeCode extension on an existing request that routes to the same SDK path.

If this needs a protocol-level primitive rather than a Claude-specific extension, it may belong in agentclientprotocol/agent-client-protocol; happy to move or cross-file.

Context

Filed from the agent-of-empires project, which drives claude-agent-acp and today can only offer queue-after (a follow-up turn) in its structured view, not the CLI's mid-turn behavior.