opencode-anthropic-auth

[!WARNING] This plugin comes with no guarantees. You might be banned for breaking the TOS, you might not be. I don't work at Anthropic, nor am I an attorney.

Use your best judgment and don't abuse your subscription.

Fork of ex-machina-co/opencode-anthropic-auth, with caching improvements inspired by cortexkit/anthropic-auth.

An OpenCode plugin that provides Anthropic OAuth authentication, enabling Claude Pro/Max users to use their subscription directly with OpenCode.

Install

Add to your OpenCode config (~/.config/opencode/opencode.json):

{
  "plugin": ["@sahiljassal/opencode-anthropic-auth"]
}

Authentication Methods

• Claude Pro/Max — OAuth flow via claude.ai. Uses your existing subscription at no additional API cost.
• Create an API Key — OAuth flow via console.anthropic.com that creates an API key on your behalf.
• Manually enter API Key — Standard API key entry.

Prompt Caching

This fork applies hybrid 1-hour ephemeral prompt caching on every request, placing up to 4 breakpoints strategically:

| Breakpoint | Behaviour | |---|---| | System anchor | Last system block after the identity block (skipped when bridge occupies the slot) | | messages[0] | Magic-context split: anchors block[0] + block[1] when stable prefix and volatile delta are merged; otherwise anchors the last cacheable block | | messages[1] / bridge | Last cacheable block of messages[1]; replaced by a bridge anchor when the block count between two user anchors (counting ALL block types across every role) exceeds Anthropic's 20-block lookback window | | Rolling latest | Most recent user message beyond index 1, keeping cache hot across long sessions |

Additional behaviours:

• System tail coalescing — plugin-added system blocks beyond the primary prompt are merged into one block before placing the system anchor, preventing cache busts when block layout changes between requests
• Trailing assistant strip — assistant messages at the tail of the request are removed before forwarding (OAuth rejects assistant prefill)
• Thinking block guard — thinking and redacted_thinking blocks are excluded from cache anchor placement; messages containing only thinking blocks receive no cache_control (avoids Anthropic 400)
• SSE retryable errors — transient server errors (api_error, overloaded_error, server_error) emitted inside HTTP 200 streams are detected and thrown as connection-reset errors so OpenCode auto-retries
• Buffered stream rewriting — tool name stripping buffers partial "name" tokens across chunk boundaries to avoid corruption

Configuration

| Variable | Description | | -------------------- | ---------------------------------------------------------------------------------------- | | ANTHROPIC_BASE_URL | Override API endpoint URL (e.g. for proxying). Must be a valid HTTP(S) URL. | | ANTHROPIC_INSECURE | Set to 1 or true to skip TLS verification. Only effective with ANTHROPIC_BASE_URL. |

License

MIT