Match @ai-sdk/anthropic exactly: baseURL is the full API prefix, so
/messages is appended as-is instead of unconditionally inserting /v1,
which broke configs where the baseURL already ends in /v1 (the common
form OpenCode itself passes to @ai-sdk/anthropic).
Addresses review feedback from @btriapitsyn on PR #3000.
The Anthropic small-model dispatch (session-assist recap/suggestion,
walkthrough, etc.) hardcoded https://api.anthropic.com/v1/messages,
ignoring provider.anthropic.options.baseURL — the same override every
other provider branch already respects. Any setup routing the anthropic
provider through a local proxy (e.g. Meridian for Claude Max / Vertex)
had the small-model call bypass the proxy and hit the real Anthropic API
with whatever placeholder credential the proxy config uses, failing with
401 invalid x-api-key on every call.
Matt Peter