openclaw - 💡(How to fix) Fix [Bug]: ACPX model forwarding to Claude Code triggers prompt cache invalidation on every turn [1 comments, 1 participants]

Bug type

Behavior bug

Beta release blocker

No

Summary

When ACPX drives a Claude Code session via ACP, the Anthropic prompt cache is invalidated on the majority of turns, forcing full cache recreation (~90K+ tokens) instead of cache reads. The issue does not occur when using Claude Code CLI directly on the same session.

Steps to reproduce

1. Have an ACPX-bound agent configured (e.g. Discord binding with claude-agent-acp)
2. Send several messages to the agent through the bound channel
3. Inspect the Claude Code session JSONL at ~/.claude/projects/<project>/<session-id>.jsonl
4. Compare cache_creation_input_tokens vs cache_read_input_tokens across turns

Expected behavior

After the initial turn creates the cache, subsequent turns should show high cache_read_input_tokens and near-zero cache_creation_input_tokens — same as when using Claude Code CLI directly.

Actual behavior

24 out of 57 turns show full cache recreations (~90K tokens each). The session JSONL shows all ACPX-driven turns hitting the API as claude-sonnet-4-6. When the user switched to direct CLI interaction on the same session (last ~30 turns), the model reported as claude-opus-4-6 and cache stabilized immediately — no more recreations.

Two observations:

1. ACPX appears to be driving Claude Code with Sonnet rather than the user's intended model (Opus). Whether this is a /model default command being injected or the _meta.claudeCode.options.model forwarding mechanism is unclear from the session logs alone.
2. Every ACPX-driven turn injects <local-command-stdout>Set model to claude-sonnet-4-6</local-command-stdout> into the conversation, which may be mutating the message prefix and invalidating the cache even when the model doesn't actually change.

OpenClaw version

2026.4.5

Operating system

Ubuntu (Linux 6.8.0-107-generic)

Install method

npm global

Model

anthropic/claude-opus-4-6 (intended); claude-sonnet-4-6 (what ACPX forwarded)

Provider / routing chain

openclaw -> acpx -> @agentclientprotocol/claude-agent-acp@^0.25.0 -> Claude Code CLI -> Anthropic API

Additional provider/model setup details

ACPX session state shows:

• agentCommand: "npx -y @agentclientprotocol/claude-agent-acp@^0.25.0"
• protocolVersion: 1
• Agent capabilities include claudeCode.promptQueueing: true

The agent's model config in OpenClaw is set to use Opus, but the session JSONL shows all ACPX-driven API calls going to claude-sonnet-4-6.

Logs, screenshots, and evidence

See attached cache-invalidation-log.txt for the full per-turn breakdown.

Key pattern — ACPX-driven turns (sonnet, frequent recreations):

    Time                   Model   CacheCreate     CacheRead  Notes
    19:04:09  claude-sonnet-4-6        89,771        11,027   CACHE RECREATED
    19:12:06  claude-sonnet-4-6        90,110        11,027   CACHE RECREATED
    19:12:51  claude-sonnet-4-6        90,628        11,027   CACHE RECREATED
    19:13:24  claude-sonnet-4-6        90,891        11,027   CACHE RECREATED
    19:13:52  claude-sonnet-4-6        91,088        11,027   CACHE RECREATED
    19:14:33  claude-sonnet-4-6        91,285        11,027   CACHE RECREATED
    19:15:09  claude-sonnet-4-6        91,543        11,027   CACHE RECREATED
    19:15:48  claude-sonnet-4-6        91,723        11,027   CACHE RECREATED

Direct CLI turns (opus, cache stable):

    19:28:12  claude-opus-4-6        102,431        11,299   model switch (expected recreation)
    19:28:41  claude-opus-4-6             96       113,730   cache stable
    19:29:45  claude-opus-4-6            129       113,826   cache stable
    19:30:52  claude-opus-4-6            211       113,955   cache stable
    ... (all subsequent turns stable)

The session JSONL also shows <local-command-stdout>Set model to claude-sonnet-4-6</local-command-stdout> injected as user message content on every ACPX-driven turn.

Attached files:

• cache-invalidation-log.txt — readable per-turn summary
• claude-session-c3f6ae80.jsonl — full Claude Code session with API usage, model strings, and /model injections
• acpx-session.json — ACPX-side session state showing agent config and message flow

Impact and severity

• Affected: Users running agents through ACPX → Claude Code ACP binding
• Severity: High — silent token/cost multiplier with no user-visible indication
• Frequency: Observed on every ACPX-driven turn in this session
• Consequence: Significant unnecessary cache creation costs across the session. In this 57-turn session, 24 turns had full ~90K cache recreations that should have been cache reads.

Additional information

• Related: #58488 (ACPX model forwarding uses Claude-specific _meta convention)
• The root cause may be in ACPX, claude-agent-acp, or Claude Code itself — the logs don't definitively pinpoint which layer is responsible for the model mismatch or the /model command injection
• Not tested on 2026.4.7 or 2026.4.8 yet; release notes for those versions don't mention a fix for this specific issue