[diegosouzapw]

Summary

#4252 reports frequent 502 bursts from ProxyFetch Undici dispatcher failed, falling back to native fetch: fetch failed where curl and native fetch() work fine. The reason it's been undiagnosable: proxyFetch.ts logged only the bare "fetch failed" message and discarded err.cause — so the real reason (ECONNRESET / UND_ERR_CONNECT_TIMEOUT / ENETUNREACH / Happy-Eyeballs AggregateError) was invisible. When the native fallback also failed, the job sat in the concurrency queue until the 30s semaphore timeout, then tripped the circuit breaker — the cascade the report describes.

What this does (and does NOT do)

Does not fix the underlying network failure (environment-specific, not reproducible locally). Ruled out during investigation: autoSelectFamily (the custom Agent already inherits net.getDefaultAutoSelectFamily()=true, undici 8.5.0) and stale keep-alive socket (repro recovered 3/3).

Makes the failure diagnosable + fast-propagating:

• describeFetchCause(err) flattens the cause chain + AggregateError sub-errors into one line (code/syscall/errno/address:port + truncated message). Never a stack trace (Rule fix(ui): fix Select dropdown dark theme inconsistency #12). Pure + testable.
• The dispatcher-failure warn now appends describeFetchCause(dispatcherError).
• When undici AND native both fail: log both causes + tag the propagated error with proxyFetchDetail so the combo executor sees a diagnosable error immediately, not a bare "fetch failed".

Validation (TDD — Rule #18)

4 new tests RED→GREEN (cause-chain flatten, AggregateError expansion, String() fallback, both-fail propagation). 8/8 in the file, sibling proxyFetch suites 18/18, typecheck:core + eslint + check:file-size (incl. #4273 test-file cap) clean.

Next step (gated on this): once a reporter's logs show the real err.cause, target the actual network fix. Tracked in #4252.

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[gemini-code-assist]

Code Review

This pull request introduces the describeFetchCause utility to flatten and format fetch error cause chains (including AggregateError sub-errors) into a single diagnostic line, and integrates it into patchedFetch to provide detailed logs when both Undici and native fetch fallbacks fail. The feedback is highly constructive and aims to make the error parsing more robust. Specifically, it suggests handling primitive values in the error cause chain and AggregateError sub-errors, as well as replacing the instanceof Error check with a more inclusive object check to ensure diagnostic details are attached to non-standard or environment-specific exceptions.

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[gemini-code-assist]

If cur is a primitive value (such as a string or number), typeof cur !== 'object' will be true. Currently, the loop does not handle primitive causes, meaning that if an error has a string as its cause (e.g., new Error('fail', { cause: 'auth failed' })), the cause is completely discarded and not included in the diagnostics. We can fix this by checking if cur is a primitive, appending its string representation, and breaking the loop.

  for (let depth = 0; cur && depth < 5 && !seen.has(cur); depth++) {
    seen.add(cur);
    if (typeof cur !== "object") {
      parts.push(String(cur));
      break;
    }
    const e = cur as Record<string, unknown>;

[gemini-code-assist]

If e.errors contains primitive values (like strings) instead of structured objects, they will be silently ignored because they don't have properties like code, syscall, or message. We should handle primitive sub-errors by appending them directly to parts.

      for (const sub of (e.errors as unknown[]).slice(0, 4)) {
        if (sub && typeof sub !== "object") {
          parts.push("↳ " + String(sub));
          continue;
        }
        const s = (sub ?? {}) as Record<string, unknown>;

[gemini-code-assist]

Using nativeError instanceof Error might fail to match non-standard error objects, custom exception classes, or certain environment-specific exceptions (like some DOMExceptions) that do not inherit from the global Error class. To ensure the diagnostic proxyFetchDetail is always attached when possible, we should check if nativeError is a non-null object instead.

Suggested change

	if (nativeError instanceof Error) {
	(nativeError as Error & { proxyFetchDetail?: string }).proxyFetchDetail = detail;
	}
	if (nativeError && typeof nativeError === "object") {
	(nativeError as Record<string, unknown>).proxyFetchDetail = detail;
	}