fix(llm): surface the real error when retries are exhausted, cap OSError backoff

[atulya-singh]

Three bugs in LLMClient._call_with_retry, all on the path where retries run out. They're separate symptoms but they live in the same ~40 lines and the fix for one is the fix for the others, so I've kept them together.

The error gets thrown away when retries are exhausted

This is the one that actually bites users. On a persistent retryable status (429, 503, 529…), the loop runs out and raises a bare RuntimeError, dropping the HTTPError on the floor.

That matters because preflight() classifies failures by pulling the HTTPError back out of RuntimeError.__cause__:

except RuntimeError as e:
    # chat() wraps errors in RuntimeError; extract original HTTPError
    cause = e.__cause__
    if isinstance(cause, urllib.error.HTTPError):
        ...

The cause was never set, so that lookup always missed. With a rate-limited key you got:

All models failed: All models failed. Last error: LLM call failed after 3 retries for model gpt-test

when the code has a perfectly good message sitting right there for exactly this case:

Rate limited - try again in a moment

Same story for 401/403/404 behind a retry. doctor and preflight both go through this, which I suspect is some of what was going on in #292 and #241.

The final attempt now re-raises the original exception instead of falling out of the loop. The trailing RuntimeError is unreachable for any sane max_retries now, but I left it as a guard for max_retries <= 0 and chained it with from last_exc so it can't swallow context either.

Heads up: part of this is a regression

The diagnostics here were already fixed once in #234 (8ec230d, "track last error across retries"). The v0.5.0 release commit (12d3fd8) reverted it — silently, as far as I can tell, since it's a 318-file +42k/−8.6k commit and nothing in it mentions the retry path. git log -S"last_err" -- researchclaw/llm/client.py shows it going in and right back out.

I went a bit further than #234 did: it tracked the error as a formatted string for the message, which reads better but still leaves __cause__ unset, so preflight() stays broken. Keeping the exception object fixes both.

Might be worth a look at what else went out in that release commit.

It sleeps after the last attempt

The HTTPError branch had no "is this the final attempt?" guard, so it slept the full backoff after the last try and then failed anyway. With the 300s ceiling plus 30% jitter that's up to ~390s of dead waiting per model, and chat() walks the whole fallback chain, so you pay it per model.

The URLError and OSError branches already guarded this with if attempt < self.config.max_retries - 1; HTTPError just never got it. N attempts now means N−1 sleeps.

The backoff ceiling wasn't applied to connection errors

_MAX_BACKOFF_SEC = 300 is described as a "5-minute ceiling for retry delays", and the HTTPError and URLError branches both min() against it. The TimeoutError/OSError branch didn't — it just did retry_base_delay * (2**attempt) and let it rip. Measured delays at max_retries=12:

1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024

That's a 17-minute sleep on a flaky connection, from the branch that catches ConnectionResetError and friends — i.e. exactly the transient case you'd want to recover from quickly. Now capped like the other two.

Why the tests didn't catch any of this

The existing preflight tests patch chat() directly:

with patch.object(client, "chat", side_effect=err):
    ok, msg = client.preflight()

So test_preflight_429_rate_limited passes while the real 429 path is broken — it never enters the retry loop at all. The new tests patch _raw_call instead, so they go through _call_with_retry for real, and they stub time.sleep to assert on the delays rather than actually waiting.

Added five: the HTTPError and URLError re-raise cases, preflight reporting a rate limit through the real path, the sleep count, and the backoff cap.

Testing

pytest tests/ → 2802 passed, 56 skipped. Same as before the change; the one warning is a pre-existing unawaited-coroutine thing in test_servers.py, unrelated.