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 >= 4.6 — any variant (Opus, Sonnet, Haiku, Fable, ...), including Bedrock IDs such as global.anthropic.claude-sonnet-5 and global.anthropic.claude-fable-5.

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 in place before it is sent to the API.

Setup

~/.config/opencode/opencode.json:

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

How it works

When an assistant turn is persisted as [tool, step-start, text, ...], the embedded step-start makes the AI SDK split it into assistant(tool_use) -> tool(result) -> assistant(text), leaving the request ending with an assistant turn. The plugin truncates that trailing step-start + text block so the request ends with tool(result) — exactly what the model asked for via stop_reason: tool_use. No context is removed beyond the trailing narration text, and no synthetic message is injected, so a finished agent is never prodded into a runaway loop.

Mutations are applied in place: OpenCode (~1.3+) discards the hook's return value and keeps using its own messages reference, so rebinding output.messages would be silently ignored.

Configuration

Set the OPENCODE_CLAUDE_TAILGUARD_MODE environment variable to control behavior:

Value	Behavior
surgical (default)	Truncate the trailing step-start + text block of the last API-visible assistant message (see above). No entries removed, nothing injected
removal (legacy)	Remove all trailing assistant messages. WARNING: in agent loops the whole session tail after the last user message is a run of assistant messages, so this can strip nearly the entire conversation
transform (legacy)	Remove trailing empty/errored assistants; append synthetic "Continue." only when the tail is a mid-loop turn (finish === "tool-calls"). A genuine end turn (finish === "stop") is dropped instead — appending "Continue." there caused infinite runaways

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

OPENCODE_CLAUDE_TAILGUARD_MODE=transform opencode

Notes

• Provider-independent: the check looks only at the model ID string, so it works across Anthropic first-party (claude-opus-4-6-20260101), Amazon Bedrock (global.anthropic.claude-sonnet-5), OpenRouter (anthropic/claude-opus-4.6), OpenCode gateways (go/zen) and other routers. Any ID containing "claude" plus a version >= 4.6 matches, regardless of variant name (Opus, Sonnet, Haiku, Fable, ...). Dated aliases parse correctly: claude-sonnet-4-20250514 (Claude 4.0, prefill still supported) is not matched, claude-opus-4-6-20260101 (4.6) is. Claude <= 4.5 and non-Claude models still support (or tolerate) trailing assistant turns and are not affected. Aliases whose ID does not contain "claude" cannot be matched.
• Errored or empty trailing assistant messages are skipped by OpenCode's own message conversion and never reach the API; the plugin walks past them to the last message the API will actually see. This also makes a stuck session recoverable: a plain retry after the 400 error now produces a valid request.