[Claude Code] Update Proxy to passthrough Messages API

[TylerLeonhardt]

Overview

Update ClaudeLanguageModelServer to properly passthrough the Anthropic Messages API while maintaining billing behavior for user-generated messages and proper user agent handling.

Current State

The ClaudeLanguageModelServer in src/extension/agents/claude/node/claudeLanguageModelServer.ts already implements an HTTP server that provides an Anthropic Messages API compatible endpoint. It acts as a proxy to the underlying model endpoint.

Tasks

• Review current proxy implementation in ClaudeLanguageModelServer
• Ensure proper passthrough of Messages API requests
• Maintain billing behavior by correctly determining user-generated messages vs. system messages
• Ensure proper user agent is set on outgoing requests
• Verify streaming (stream: true) works correctly
• Verify non-streaming requests work correctly
• Add proper logging for debugging proxy issues
• Update tests for proxy behavior

Key Considerations

• The proxy currently validates auth using a nonce (x-api-key)
• Requests are handled via handleAuthedMessagesRequest
• Need to preserve telemetry data flow through IChatMLFetcher and IEndpointProvider

Related Files

• src/extension/agents/claude/node/claudeLanguageModelServer.ts
• src/platform/endpoint/node/messagesApi.ts
• src/platform/networking/common/networking.ts

Dependencies

• Should be done after SDK update (#TBD)