openclaw - 💡(How to fix) Fix [Bug]: pi-ai anthropic adapter does not merge consecutive same-role messages — causes 400 errors with custom proxies [1 participants]

Bug type

Missing message normalization in @mariozechner/pi-ai anthropic provider adapter.

Summary

The convertMessages() function in @mariozechner/pi-ai/dist/providers/anthropic.js does not merge consecutive messages with the same role before sending to the Anthropic API. The Anthropic Messages API requires strict user/assistant alternation — violating this causes 400 Bad Request.

This is especially triggered when using custom Anthropic-compatible proxies (e.g., corporate gateways, ccproxy) that enforce alternation more strictly than the native Anthropic endpoint. But even without proxies, upstream message transforms (tool result batching, system event injection, aborted responses) can produce back-to-back messages with the same role.

Reproduction

1. Configure a custom provider with api: "anthropic-messages" pointing to an Anthropic-compatible proxy
2. Have a session where upstream transforms produce consecutive user or assistant messages (e.g., multi-tool-result + system event, or aborted assistant turn followed by retry)
3. Send a message
4. Expected: Messages are normalized to strict alternation before API call
5. Actual: Raw messages with consecutive same-role entries are sent, proxy returns 400

Root Cause

In @mariozechner/pi-ai/dist/providers/anthropic.js, the convertMessages() function (around line 880, before return params) does not check for or merge consecutive same-role messages. The OpenClaw wrapper layer (provider-stream-*.js) has its own convertAnthropicMessages() but it also lacks this normalization.

Workaround

We apply a local patch that merges consecutive same-role content blocks and strips trailing assistant messages:

// After message conversion, before return params:
for (let i = params.length - 1; i > 0; i -= 1) {
    const cur = params[i], prev = params[i - 1];
    if (cur.role !== prev.role) continue;
    const toBlocks = (c) => typeof c === "string" ? [{ type: "text", text: c }] : Array.isArray(c) ? c : [];
    prev.content = [...toBlocks(prev.content), ...toBlocks(cur.content)];
    params.splice(i, 1);
}
while (params.length > 0 && params[params.length - 1].role === "assistant") { params.pop(); }

Related Issues

• #17728 (MiniMax consecutive same-role — closed, but only fixed for MiniMax provider)
• #17124 (MiniMax consecutive same-role 400 error — closed)
• #58567 (trailing assistant message causes prefill error loop)
• #72865 (4.25 regression: Bedrock prefill error)

Environment

• OpenClaw: 2026.4.25 (aa36ee6)
• pi-ai: bundled version in openclaw 2026.4.25
• Provider: custom anthropic-messages via corporate proxy
• OS: macOS (Darwin 25.4.0)