[BUG] Anthropic doesnt like DCP -- still having this issue regularly (assistant message prefill)

[caleb-reyes-op]

I just ran into this issue as well, using anthropic; here is the json from the ERROR log entry (redacted the messages, system, and tools fields for security && brevity):

{
  "error": {
    "name": "AI_APICallError",
    "url": "https://api.anthropic.com/v1/messages",
    "requestBodyValues": {
      "model": "claude-sonnet-4-6",
      "max_tokens": 32000,
      "stream": true
    },
    "statusCode": 400,
    "responseHeaders": {
      "anthropic-organization-id": "18aa8ab8-a2eb-4e1f-8865-87023f2a0f7f",
      "cf-cache-status": "DYNAMIC",
      "cf-ray": "9eded6e1cfd4dcfe-SJC",
      "connection": "keep-alive",
      "content-encoding": "gzip",
      "content-security-policy": "default-src 'none'; frame-ancestors 'none'",
      "content-type": "application/json",
      "date": "Fri, 17 Apr 2026 22:24:20 GMT",
      "request-id": "req_011CaA8K4i7kBdnE9ZcQcTgn",
      "server": "cloudflare",
      "server-timing": "x-originResponse;dur=43",
      "strict-transport-security": "max-age=31536000; includeSubDomains; preload",
      "transfer-encoding": "chunked",
      "vary": "Accept-Encoding",
      "x-envoy-upstream-service-time": "42",
      "x-robots-tag": "none",
      "x-should-retry": "false",
      "set-cookie": "_cfuvid=jfz1zQxCIrGW9VuquDDC14GJzvhlKp0c7CpgUl7LFT4-1776464660.7643719-1.0.1.1-khLHScW8Kj9zCTmZgOjBTe33xWmwSTD.Lwz.eftBiqk; HttpOnly; SameSite=None; Secure; Path=/; Domain=api.anthropic.com"
    },
    "responseBody": "{\"type\":\"error\",\"error\":{\"type\":\"invalid_request_error\",\"message\":\"This model does not support assistant message prefill. The conversation must end with a user message.\"},\"request_id\":\"req_011CaA8K4i7kBdnE9ZcQcTgn\"}",
    "isRetryable": false,
    "data": {
      "type": "error",
      "error": {
        "type": "invalid_request_error",
        "message": "This model does not support assistant message prefill. The conversation must end with a user message."
      }
    }
  }
}

ERROR 2026-04-17T22:24:20 +98ms service=llm providerID=anthropic modelID=claude-sonnet-4-6 sessionID=ses_262915ac3ffeb0h3F56QYx3GDA small=false agent=build mode=primary

Originally posted by @caleb-reyes-op in #470

[ktwbc]

Same issue, using sonnet 4.5 via github copilot, displayed at end of conversation and doesn't finish cleanly (but intermittent, not sure what the trigger is)

Evaluate the conversation for compressible ranges.
If any messages are cleanly closed and unlikely to be needed again, use the compress tool on them.
If direction has shifted, compress earlier ranges that are now less relevant.
The goal is to filter noise and distill key information so context accumulation stays under control.
Keep active context uncompressed.
Compressed block context:
- Active compressed blocks in this session: 0 (none)
- If your selected compression range includes any listed block, include each required placeholder exactly once in the summary using (bN).
</parameter>
</invoke>
This model does not support assistant message prefill. The conversation must end with a user message.