openclaw - 💡(How to fix) Fix ACP session replies delivered as 'Background task done' without content after upgrade to 2026.4.2 [1 participants]

Description

After upgrading from OpenClaw 2026.3.28 to 2026.4.2, ACP sessions (via acpx) stop delivering reply text to Telegram. Instead of the agent's actual response, Telegram only shows:

Background task done: oscar-1775307657 (run 10504).

The ACP session spawns and the agent starts successfully (startup announce message is delivered), but subsequent user messages result in empty "background task done" notifications with no content.

Downgrading back to 2026.3.28 immediately resolves the issue — replies are delivered normally.

Environment

• OS: Ubuntu 24.04 (Linux 6.8.0-106-generic)
• Node: v22.22.0
• OpenClaw: 2026.4.2 (d74a122) — regression vs 2026.3.28 (f9b1079)
• acpx: Tested with both bundled 0.4.0 and local extension 0.3.1 — same behavior
• Channel: Telegram
• ACP backend: acpx, mode: persistent
• ACP agent: claude (Claude Code 2.1.92)

Agent config (simplified)

{
  "id": "oscar",
  "runtime": {
    "type": "acp",
    "acp": {
      "agent": "claude",
      "backend": "acpx",
      "mode": "persistent",
      "cwd": "/home/jack/Openclaw-Oscar"
    }
  },
  "model": {
    "primary": "kimi-coding/k2p5",
    "fallbacks": ["anthropic/claude-sonnet-4-6"]
  }
}

Reproduction steps

1. Upgrade from 2026.3.28 to 2026.4.2 (npm update -g openclaw)
2. Restart gateway
3. Send a message to the ACP-backed agent via Telegram
4. Observe: agent startup announce works, but user message replies only show "Background task done: ... (run XXXX)."

Observed behavior in logs

On every user message, the ACP session repair loop fires:

[plugins] acpx ensureSession repairing dead named session by resuming backend session:
  session=agent:claude:acp:58702d6c-... status=dead summary=queue owner unavailable

[plugins] acpx ensureSession dead-session resume repair failed; retrying with fresh session owner:
  error=-32002: Resource not found: <resumeSessionId>

The new dead-session repair mechanism in 2026.4.2 (ACPX changelog entries #58669, related fix for stale session IDs) attempts to resume with resumeSessionId, but the backend session no longer exists (-32002: Resource not found). After the repair loop, no reply text is delivered — only the task completion notification.

Startup identity reconcile always fails:

[gateway] acp startup identity reconcile (renderer=v1): checked=209 resolved=0 failed=209

All 209 existing ACP sessions fail reconciliation on every restart.

When ACP dispatch fails entirely, fallback to embedded runner also fails:

[agent/embedded] embedded run agent end: isError=true model=k2p5 provider=kimi-coding
  error=429 rate_limit_error

But even when the embedded runner doesn't error, the "Background task done" message appears without content.

Suspected cause

The new unified background task control plane introduced in 2026.3.31/2026.4.2 (changelog: "turn tasks into a real shared background-run control plane") appears to wrap ACP runs as background tasks. The task completion notification is sent to Telegram, but the actual agent reply text is not being relayed through the task delivery path.

Workaround

Downgrade to 2026.3.28:

npm install -g [email protected]
# Restore local acpx extension (0.3.1) if previously backed up
systemctl --user restart openclaw-gateway