This situation feels like a +1 for Agent Client Protocol (ACP) [1].

In ACP, you auth directly to the underlying agent (eg Claude Code SDK) rather than a third-party tool (eg OpenCode) that then calls an inference endpoint on your behalf. If you're logged into Claude Code, you're already logged into Claude Code through any ACP client.

[1] https://agentclientprotocol.com/overview/agents