fix(refiller): don't fail the USDG sweep when Paxos rejects it as below minimum

[droplet-rl]

Why

zion-across-usdg-refiller has been failing every run since 2026-08-15T09:55Z, paging through the serverless hub (Some spoke calls returned errors, errorOutputs: zion-across-usdg-refiller).

The trigger was benign: the refiller's mainnet USDG balance (137.911345) crossed its MIN_USDG_SWEEP_AMOUNT of 100 for the first time, so sweepMainnetUsdgToRobinhood started actually calling Paxos. Paxos rejects the order quote with HTTP 400:

Order amount is below the minimum of $160.93 (160924200 base units). Please increase your offer amount.

Their per-order minimum floats with execution costs and currently sits well above both MIN_USDG_SWEEP_AMOUNT and the $5 floor hardcoded in PAXOS_TRANSIT_MINIMUMS. The error propagated out of runRefiller, so a "wait for the balance to grow" condition was reported as a bot failure — and repeated every 5 minutes. Verified against the live API: the same request at 200 USDG returns 200 with a valid quote, so the route, key and endpoint are healthy.

Two things also made it hard to diagnose: the message never reached the logs (HttpError: [object Object]) because the SDK's baseFetch only lifts a string error key off the body, and Paxos returns { error: { code, message, status } }; and the client retried the rejected request twice for the same answer.

What

• Refiller.sweepMainnetUsdgToRobinhood treats a below-minimum rejection as a skip and logs the live minimum. Every other error still throws.
• PaxosTransitClient throws a typed PaxosTransitApiError (extends the SDK's HttpError, adds apiStatus) so the message and status survive into the logs. Modelled on AcrossApiHttpError in AcrossApiBaseClient.ts, which exists for the same reason.
• getWithRetry only retries transient failures (transport errors, 408/425/429, 5xx).
• src/refiller/README.md: the "$5 minimum per order" line was misleading — that's the documented floor, not the live value.

There is no machine-readable discriminator for the below-minimum case in Paxos's response, so classification matches on the message; if that wording changes the failure mode is the current behavior (a throw), not a silent skip.

Testing

test/PaxosTransitClient.ts covers the error shape, the retry policy, and below-minimum classification against the verbatim body Paxos returned. yarn typecheck, yarn lint, test/PaxosTransitClient.ts, test/generic-adapters/PaxosTransitBridge.ts and test/Refiller.ts pass (21 passing).

Note for operators

This stops the paging but does not itself move the funds: the 137.91 USDG sweeps on the first run after the balance clears Paxos's minimum. Raising MIN_USDG_SWEEP_AMOUNT above it in bot-configs would avoid the doomed API calls in the meantime, but is not required.

🤖 Generated with Claude Code