opencode-claude-tailguard

An OpenCode plugin that prevents This model does not support assistant message prefill. errors caused by the deprecated assistant message prefill in Claude Opus 4.6, Sonnet 4.6.

Due to a bug in OpenCode, the conversation array can end with an assistant message, which the Claude 4.6 API rejects. This plugin fixes the message array before it is sent to the API.

Setup

~/.config/opencode/opencode.json:

{
  "plugins": [
    "opencode-claude-tailguard",
  ]
}

Configuration

Set the OPENCODE_CLAUDE_TAILGUARD_MODE environment variable to control behavior:

Value	Behavior
removal (default)	Remove all trailing assistant messages regardless of content
transform	Remove empty trailing assistants; append synthetic "Continue." if a content-bearing assistant remains

Set via shell or .env file in your project directory:

OPENCODE_CLAUDE_TAILGUARD_MODE=transform opencode

Notes

The transformation is applied only when a Claude 4.6 model is in use. Other models are not affected.