New forwarding methods on GatewayService for Anthropic platform groups:
- ForwardAsResponses: accept Responses body → convert to Anthropic →
forward to upstream → convert response back to Responses format.
Supports both streaming (SSE event-by-event conversion) and buffered
(accumulate then convert) response modes.
- ForwardAsChatCompletions: chain CC→Responses→Anthropic for request,
Anthropic→Responses→CC for response. Streaming uses dual state machine
chain with [DONE] marker.
Both methods reuse existing GatewayService infrastructure:
buildUpstreamRequest, Claude Code mimicry, cache control enforcement,
model mapping, and return UpstreamFailoverError for handler-level retry.
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