Fix streaming handler causing duplicate non-streaming requests

[callanjfox]

Summary

The streaming response handler in handleStreamingResponse() was modifying the response in two ways that caused Claude Code to send duplicate non-streaming replay requests for every streaming request:

1. Dropped upstream headers: Only 3 hardcoded headers (Content-Type, Cache-Control, Connection) were sent to the client. All Anthropic response headers (request-id, server-timing, rate-limit metadata, anthropic-organization-id, etc.) were stripped.

2. Filtered the SSE stream: Only data: lines were forwarded. event: type prefixes and blank-line separators were dropped, altering the SSE format the client receives.

Claude Code detects the incomplete/altered streaming response and fires a non-streaming replay with identical message content to recover the full metadata. This means every turn recorded in requests.db appears twice — once streaming, once non-streaming — doubling the apparent request count.

How we confirmed this

We ran the same Claude Code prompt through three setups and counted requests:

Proxy	Requests	Details
This proxy (before fix)	4	2 streaming + 2 non-streaming replays
Minimal Python proxy (forwards headers + full SSE)	2	2 streaming only
This proxy (after fix)	2	2 streaming only

The non-streaming replays have identical message bodies (differing only by the 14-byte "stream":true, field) and arrive 2-5 seconds after the streaming request completes. They get independent API responses (different msg_id, sometimes different output token counts) but near-100% cache hits since the prompt is already cached.

Impact

• Request count in DB drops by ~50% for real usage — the non-streaming replays were artifacts, not organic Claude Code behavior
• Any cache analysis or cost tracking built on this proxy's data was counting double
• No functional change to how the proxy stores data — it still captures full response metadata for the DB via the existing data: line parsing

Changes

• Forward all upstream response headers to the client before setting SSE essentials
• Forward the complete SSE byte stream (all lines including event: prefixes and blank separators) while still parsing only data: lines for DB storage

Test plan

• Start proxy, run ANTHROPIC_BASE_URL=http://localhost:3001 claude -p "What is 2+2?"
• Verify requests.db contains 1 request (streaming), not 2
• Run a tool-use prompt to verify multi-turn still works correctly
• Verify DB still captures full response metadata (usage, message ID, etc.)

🤖 Generated with Claude Code