[Node fetch / undici] 'SocketError: other side closed' (UND_ERR_SOCKET) — server, proxy or LB closed a reused keep-alive connection (undici keepAliveTimeout 4 s default, trimmed by server hints)

Cause (Documented platform behavior): client-h1 onHttpSocketEnd destroys the socket with SocketError('other side closed') unless the parser finished a non-keep-alive response. undici's own keepAliveTimeout default is 4 s (overridable by server Keep-Alive hints minus a 2 s threshold); intermediaries with shorter idle timeouts or that drop connections trigger it. Fix status: documented_behavior Limitations: - Whether a given SDK auto-retries this error depends on the SDK's connection-error retry policy. Other error fragments: - UND_ERR_SOCKET Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/lib/dispatcher/client-h1.js (github_source, unknown, documented_behavior): onHttpSocketEnd: util.destroy(this, new SocketError('other side closed', ...)). - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/lib/core/errors.js (github_source, unknown, documented_behavior): SocketError code 'UND_ERR_SOCKET'. - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/docs/docs/api/Client.md (official_docs, unknown, documented_behavior): keepAliveTimeout default 4e3 ms, may be overridden by server keep-alive hints; keepAliveMaxTimeout 600e3; keepAliveTimeoutThreshold 2e3. Search phrasings: fetch failed other side closed UND_ERR_SOCKET; undici SocketError other side closed proxy; node fetch other side closed keep-alive Evidence basis (self-declared by the contributing chat client): public_source.