Intermittent 400 Bad Request with empty body from HTTPS proxy during concurrent requests (SSE streaming + POST)

Environment

• portless 0.15.6
• portless proxy start --port 1355 --https --skip-trust + portless run nuxt dev (Nuxt 4 / Nitro)
• Client: browser fetch (HTTP/1.1 keep-alive) via Nuxt's $fetch (ofetch)

Symptom

A POST to the proxied backend intermittently fails with HTTP 400 and an EMPTY response body, even though the exact same request succeeds most of the time. It is not reproducible with plain sequential or parallel requests — it correlates with concurrent traffic: a long-lived SSE stream on the same host (e.g. an AI chat /chat streaming endpoint) is open while the POST fires.

Client error (ofetch FetchError):

FetchError: [POST] "/api/items": 400
  statusCode: 400
  data: ''            // empty body
  responseStatus: 400

Evidence that the proxy, not the backend, emits the 400

• The backend (Nuxt) handler is wrapped by defineApiHandler, so every handler error is returned as HTTP 200 with a JSON error body; unauthenticated requests return 401 with a JSON body. The backend cannot produce a real 400-with-empty-body.
• In failing cases the request never reaches the backend (no backend access log), i.e. the proxy rejects it before forwarding.
• Hitting the backend directly (bypassing portless) never shows this 400; sequential and concurrent POSTs against the proxy (120× keep-alive) all passed with the correct status.
• proxy.log shows no 400/error entry for the failing requests.

Expected behavior

Backend responses (2xx/4xx, including bodies) pass through unchanged. The proxy should never emit a bare 400 with an empty body for a well-formed HTTP request.

Notes / possible cause

• Failure is intermittent and timing-dependent; could not build a deterministic reproducer (120 sequential + concurrent keep-alive POSTs all passed).
• Likely a connection/keep-alive race where Node's http server clientError path responds 400 Bad Request with no body on a reused/half-closed connection during concurrent streaming + POST traffic on the HTTPS proxy path.

Best-effort reproduction

1. Run a backend with an SSE endpoint and a JSON POST endpoint (e.g. AI chat stream + item create).
2. From a browser, keep the SSE stream open and fire the POST repeatedly/concurrently.
3. Observe intermittent 400 with an empty body on the POST.