[ACP] dsh-acp sends agent_thought_chunk and agent_message_chunk with the same messageId, so clients merge reasoning into the reply

[geoqiao]

What happens

dsh-acp gives the reasoning and the reply of one assistant message the same messageId. An ACP client that groups chunks by messageId treats both as one message, and the reasoning text appears inside the reply.

Seen in Paseo (0.9.2 and 0.10.x) through the DeepSeek Harness provider plugin: the full reasoning is shown again in the reply body.

Where

@deepseek-ai/dsh-acp lib/index.js, assistantUpdates (0.2.0-rc.1, also 0.1.7-rc.2):

if (block.type === "reasoning") {
  updates.push({
    sessionUpdate: "agent_thought_chunk",
    messageId: event.data.message.id,   // same id
    content: { type: "text", text: block.text },
  });
  continue;
}
updates.push({
  sessionUpdate: "agent_message_chunk",
  messageId: event.data.message.id,     // same id
  content,
});

Why this is a problem

ACP v1, Prompt Turn → Message IDs:

The Agent MAY include an opaque, unique messageId on message chunks. Chunks with the same messageId belong to the same message; a changed messageId indicates a new message.

A thought and a reply are different messages, but here they share one ID, so a spec-following client is told they are the same message.

Suggested fix

Give thought chunks their own ID, for example ${message.id}:thought, or omit messageId on agent_thought_chunk.

Workaround on the client side

The Paseo plugin now rewrites the thought chunk ID before passing it on (geoqiao/paseo-stuff#21). We will remove that once dsh-acp sends distinct IDs.