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This model does not support assistant message prefill / Github Copilot with Opus 4.6 #13768

@sbrunecker

Description

@sbrunecker

Description

Working with Opus 4.6, opencode frequently stops with the message "This model does not support assistant message prefill. The conversation must end with a user message."

This is from the session export:

    {
      "info": {
        "role": "assistant",
        "time": {
          "created": 1771187182207,
          "completed": 1771187184086
        },
        "error": {
          "name": "APIError",
          "data": {
            "message": "This model does not support assistant message prefill. The conversation must end with a user message.",
            "statusCode": 400,
            "isRetryable": false,
            "responseHeaders": {
              "cache-control": "no-cache",
              "content-security-policy": "default-src 'none'; sandbox",
              "content-type": "text/plain; charset=utf-8",
              "date": "Sun, 15 Feb 2026 20:26:23 GMT",
              "strict-transport-security": "max-age=31536000",
              "transfer-encoding": "chunked",
              "x-content-type-options": "nosniff",
              "x-github-backend": "Kubernetes",
              "x-github-request-id": "EAAC:14EF1C:253883:31B9E3:69922BEC",
              "x-request-id": "00000-ce7fe972-c41c-43ec-b02b-3d2299740ab9"
            },
            "responseBody": "{\"error\":{\"message\":\"This model does not support assistant message prefill. The conversation must end with a user message.\",\"code\":\"invalid_request_body\"}}\n",
            "metadata": {
              "url": "https://api.githubcopilot.com/chat/completions"
            }
          }
        },
        "parentID": "msg_c62f2be7600192uMJUKpSsJ0qr",
        "modelID": "claude-opus-4.6",
        "providerID": "github-copilot",
        "mode": "build",
        "agent": "build",
        "path": {
          "cwd": "/home/stefan/projects/odoo18",
          "root": "/home/stefan/projects/odoo18"
        },
        "cost": 0,
        "tokens": {
          "input": 0,
          "output": 0,
          "reasoning": 0,
          "cache": {
            "read": 0,
            "write": 0
          }
        },
        "id": "msg_c62fb9a7f001Rl3UkWhXmVKLb7",
        "sessionID": "ses_39d0d418cffeIz9g6wg7wg1Caj"
      },
      "parts": []
    }

Plugins

none

OpenCode version

1.2.4

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