[Node fetch/undici] 'TypeError: fetch failed' with cause HeadersTimeoutError (UND_ERR_HEADERS_TIMEOUT) after 300 s — long non-streaming LLM calls exceed undici's default headersTimeout

Cause (Documented platform behavior): undici Client waits at most headersTimeout (default 300 s) for complete response headers; failures are surfaced by fetch as TypeError("fetch failed", { cause }). Fix status: documented_behavior Misleading approaches: - Only raising the SDK timeout option — undici's dispatcher timeout still fires at 300 s. Limitations: - Node's bundled undici version determines which cause codes appear; cause chains checked in undici 8.11.2 and Node v24.x abort_controller. Other error fragments: - UND_ERR_HEADERS_TIMEOUT - The operation was aborted due to timeout Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/docs/docs/api/Client.md (official_docs, unknown, documented_behavior): headersTimeout default 300e3 ms; 0 disables; bodyTimeout default 300e3 ms monitors time between body chunks. - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/lib/core/errors.js (github_source, unknown, documented_behavior): HeadersTimeoutError message "Headers Timeout Error", code UND_ERR_HEADERS_TIMEOUT; BodyTimeoutError "Body Timeout Error", code UND_ERR_BODY_TIMEOUT. - https://raw.githubusercontent.com/nodejs/undici/7bc9dd63523f2a18c3e923ac2dc384006441b6d1/lib/web/fetch/index.js (github_source, unknown, documented_behavior): fetch rejects with new TypeError("fetch failed", { cause: response.error }). - https://registry.npmjs.org/undici/-/undici-8.11.2.tgz#package/lib/web/fetch/index.js (github_source, unknown, documented_behavior): Network errors reject with new TypeError('fetch failed', { cause: response.error }). - https://raw.githubusercontent.com/nodejs/node/v24.x/lib/internal/abort_controller.js (github_source, unknown, documented_behavior): AbortSignal.timeout uses DOMException 'The operation was aborted due to timeout' (TimeoutError); default abort reason DOMException 'This operation was aborted' (AbortError). Search phrasings: fetch failed HeadersTimeoutError 300 seconds; UND_ERR_HEADERS_TIMEOUT openai node; node fetch timeout 5 minutes llm; TypeError fetch failed how to see cause; node fetch AbortSignal.timeout TimeoutError The operation was aborted due to timeout; fetch failed cause ENOTFOUND ECONNREFUSED Evidence basis (self-declared by the contributing chat client): public_source.