fix(evm): buffer arbitrum gasPrice/maxFeePerGas to prevent base-fee-drift tx failures

[kaladinlight]

Problem

Arbitrum /fees was occasionally returning suspect data (identical slow/average/fast tiers, maxPriorityFeePerGas: 0), and clients were hitting transaction failures:

max fee per gas less than block base fee: maxFeePerGas: 20072000 baseFee: 20110000

Root cause

Arbitrum's sequencer orders transactions first-come-first-served and ignores priority fees, so eth_feeHistory reports 0 rewards at every percentile (confirmed against a live RPC — 0/100 blocks non-zero at percentiles 50/70/90). Two consequences in getGasFees:

1. maxPriorityFeePerGas falls back to minPriorityFee (0 for Arbitrum), collapsing all three tiers to identical values.
2. gasPrice = baseFee + 0 = baseFee — pinned to the bare base fee with zero headroom. Arbitrum's base fee jitters ~1% block-to-block, so any upward tick between estimation and inclusion rejects the tx. The rejected value 20072000 matches gasPrice exactly (not maxFeePerGas, which was baseFee × 2 ≈ 40M), confirming the client consumes the gasPrice field for Arbitrum.

This is a regression from the Blocknative → eth_feeHistory swap (#1281); Blocknative's price carried predictive headroom.

Fix

Make the base-fee and gas-price tier multipliers configurable per chain (same pattern as minPriorityFee), defaulting to the exact prior behavior so no other coinstack changes. Arbitrum opts into per-tier buffers:

• gasPriceMultiplier: [1.1, 1.2, 1.5] — modest headroom so legacy/gasPrice txs stay above the base fee as it drifts, without meaningfully overpaying (gasPrice is the amount actually paid).
• baseFeeMultiplier: [1.5, 2, 3] — larger cap-only headroom on maxFeePerGas (a cap, so no overpay) and restores tier differentiation.

In the failing scenario, slow.gasPrice becomes 22079200, comfortably above the 20110000 base fee that rejected the tx. Invariants verified: maxFeePerGas ≥ gasPrice, gasPrice > baseFee, and tiers monotonically increasing.

Scope

Validated against live RPCs that Optimism and Base are unaffected — both show non-zero priority fees at every percentile (100/100 blocks) and stable base fees, so gasPrice already has ample headroom there. Change is Arbitrum-only; all other chains keep their current behavior via the defaults.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements
  • Updated gas fee estimates to use tier-specific multipliers for slow, average, and fast transaction speeds.
  • Improved gas price and maximum fee calculations for Arbitrum transactions.
  • Arbitrum gas-related endpoints now provide more differentiated fee estimates across speed options.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 83eb6331-2c50-4d1b-bafa-4aae9d594388

📥 Commits

Reviewing files that changed from the base of the PR and between c246b14 and 07940ac.

📒 Files selected for processing (1)

• node/coinstacks/common/api/src/evm/moralisService.ts

📝 Walkthrough

Walkthrough

MoralisService now supports per-tier base-fee and gas-price multipliers, applies them to gas estimates, and allows the Arbitrum controller to provide chain-specific multiplier arrays.

Changes

Gas multiplier configuration

Layer / File(s)	Summary
Multiplier contract and initialization node/coinstacks/common/api/src/evm/moralisService.ts	Adds optional per-tier multiplier arguments, default arrays, persisted fields, and constructor initialization.
Tiered fee calculation and Arbitrum wiring node/coinstacks/common/api/src/evm/moralisService.ts, node/coinstacks/arbitrum/api/src/controller.ts	getGasFees applies slow, average, and fast multipliers; Arbitrum configures [1.5, 2, 3] base-fee and [1.1, 1.2, 1.5] gas-price multipliers.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• shapeshift/unchained#1281: Also modifies MoralisService gas-fee estimation and multiplier handling.

Poem

A rabbit hops where gas fees grow,
With slow, average, fast in a row.
Arbitrum brings its numbers bright,
Service turns tiers into speed and might.
Hop, hop—estimates take flight!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the Arbitrum gas-fee buffering change and its purpose of preventing base-fee-drift transaction failures.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/arbitrum-gas-fee-headroom

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@node/coinstacks/common/api/src/evm/moralisService.ts`:
- Around line 398-399: Add constructor-time validation in the relevant Moralis
service class to ensure the user-supplied baseFeeMultiplier and
gasPriceMultiplier arrays each have at least REWARD_PERCENTILES.length entries
before getGasFees() can index them; reject invalid configurations with a clear
error.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0f483718-d873-4c26-bd0a-3eff12de4ed6

📥 Commits

Reviewing files that changed from the base of the PR and between f5c7f66 and c246b14.

📒 Files selected for processing (2)

• node/coinstacks/arbitrum/api/src/controller.ts
• node/coinstacks/common/api/src/evm/moralisService.ts