[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