Result text lost when output_tokens=0 (no stream events emitted)

[Miluer-tcq]

Problem

When using claude-agent-acp (v0.22.2) with weixin-acp, every other message returns empty text. The response text exists in message.result but is never forwarded to the client via agent_message_chunk.

Debug findings

Adding logging to the result handler reveals:

[debug] result: subtype=success is_error=false result="**3**。" output_tokens=0 promptReplayed=true

The answer is correct (**3**) but output_tokens=0 and no agent_message_chunk events were emitted before the result. The client receives [no text].

Root Cause

When Claude Code SDK returns a cached or short response, it skips streaming and puts the text directly in message.result with output_tokens=0. The current code in acp-agent.js only emits text via stream_event → streamEventToAcpNotifications, so if no stream events occur, the client gets nothing.

Suggested Fix

In the result / success handler, add a fallback to emit the result text as an agent_message_chunk when output_tokens === 0 but message.result is non-empty:

case "success": {
  // ... existing checks ...
  
  // Fallback: emit result text if no stream chunks were sent
  if (message.result && message.usage.output_tokens === 0) {
    await this.client.sessionUpdate({
      sessionId: params.sessionId,
      update: {
        sessionUpdate: "agent_message_chunk",
        content: { type: "text", text: message.result },
      },
    });
  }
  return { stopReason: "end_turn", usage };
}

Environment

• claude-agent-acp: 0.22.2
• @anthropic-ai/claude-code: 2.1.81
• Node.js: v22.17.0
• OS: Windows 11
• Used via: weixin-acp (https://github.com/wong2/weixin-agent-sdk)

Related

wong2/weixin-agent-sdk#13

[SteffenDE]

What Anthropic model do you use and do you have any custom options that you pass to ACP? Do you have a specific prompt that triggers it? I asked What is 1+1, only return the answer, no extra comment. and it does return both a stream event and assistant message and then a result with 1 reported output token.