fix(proxy): missing proxyRes error handler + SSE corruption on error after headers

[aattaran]

Summary

Two production bugs in proxy/model-proxy.js surfaced by an audit run against aattaran/Jarvis2 (shared proxy ancestry — both fixes apply verbatim).

Bug 1 — missing proxyRes error handler (process crash)

The three response paths inside the httpsRequest callback —

• proxyRes.pipe(norm).pipe(clientRes) (SSE)
• the data/end JSON collector
• the passthrough proxyRes.pipe(clientRes)

— never attach an 'error' handler to proxyRes. If the upstream connection drops mid-response (TCP RST, TLS abort, server crash), Node emits an unhandled 'error' on the response stream and the entire proxy process exits, killing every concurrent in-flight Claude Code session.

Fix: attach a single proxyRes.on('error', ...) immediately after the response is opened, before any of the branch-specific piping. Mirrors the existing proxyReq.on('error') shape: 502+JSON if headers haven't gone out, otherwise destroy the response.

Bug 2 — SSE corruption when proxyReq errors after headers sent

Current proxyReq.on('error') unconditionally calls:

clientRes.end(JSON.stringify({ error: { message: 'Upstream connection error' } }));

If headers already went out as text/event-stream (the common SSE case for /v1/messages), that JSON gets injected mid-stream. Claude Code's SSE parser then sees a non-event payload and either silently truncates the assistant turn or throws — depending on where in the stream the upstream died.

Fix: clientRes.destroy(err) once clientRes.headersSent is true, so the client sees a clean transport-level abort instead of a corrupted event stream.

Files

• proxy/model-proxy.js — added proxyRes error handler; gated proxyReq error handler's end() on !headersSent.

Note on the third audit finding

The same audit also flagged the /_proxy/mode Origin-prefix CSRF/DNS-rebinding issue. That's already covered by PR #16 (open, from aaronjmars), so it is intentionally not in this PR.

Test plan

• Local repro: kill upstream mid-SSE (iptables -A OUTPUT -p tcp --dport 443 -j REJECT after first chunk) → before patch: process crashes / SSE garbage; after patch: clean client error, proxy survives.
• Existing happy path unchanged: streaming + JSON /v1/messages still flow through UsageNormalizer and produce correct /_proxy/cost.
• /_proxy/status, /_proxy/mode, /_proxy/cost still respond correctly.

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