Response drift: polymarket — `markets[].feeSchedule` structure not accessed by normalizer

[realfishsam]

Exchange

polymarket

Severity

MEDIUM

What Our Normalizer Expects

mapMarketToUnified in utils.ts does not access any feeSchedule fields from raw market objects. The field is not declared in the PolymarketRawMarket TypeScript interface.

What The Live API Returns

The live Gamma API (GET https://gamma-api.polymarket.com/markets) returns a nested feeSchedule object on each market:

markets[].feeSchedule.exponent: integer
markets[].feeSchedule.rate: float
markets[].feeSchedule.takerOnly: boolean
markets[].feeSchedule.rebateRate: float

Impact

• Normalized polymarket markets carry no fee information
• Consumers cannot compute net expected value or accurate P&L without knowing taker fees and rebate rates
• takerOnly flag is especially important — maker orders on taker-only markets would fail or be repriced unexpectedly
• Fee schedule changes at the venue are invisible to PMXT

Suggested Fix

1. Add feeSchedule?: { exponent?: number; rate?: number; takerOnly?: boolean; rebateRate?: number } to the PolymarketRawMarket interface
2. Surface fee information on the normalized UnifiedMarket (e.g., in sourceMetadata.fees or a dedicated feeSchedule field)

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Found by automated response shape drift audit