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Four channel types, one envelope. Every message arrives wrapped in: 
{"type": "data", "channel": "dist:fomc-2026-04-29", "data": { ... }}

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Channel types

ChannelSubscribe toDescriptionTier
distdist:fomc-2026-04-29Cross-venue probability distribution for all outcomesFree
bookbook:fomc-2026-04-29:holdNormalized orderbook updates for one outcomePro
tradetrade:fomc-2026-04-29:holdTrade executions for one outcomePro
snapshotsnapshot:fomc-2026-04-29Full state reset every 60s for all outcomesPro

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Distribution

Shows the market-implied probability for every outcome in an event, combining data from both venues. 
{
  "event_key": "fomc-2026-04-29",
  "seq": 312,
  "published_ts": 1773892530947,
  "outcomes": [
    {
      "outcome": "hold",
      "label": "Fed maintains rate",
      "kalshi_prob": 0.945,
      "poly_prob": 0.955,
      "prob": 0.9479,
      "kalshi_depth_usd": 4714197.04,
      "poly_depth_usd": 1965879.48
    }
  ],
  "total_kalshi_depth_usd": 4784397.57,
  "total_poly_depth_usd": 2104564.65
}
FieldDescription
probCombined probability, weighted by each venue’s liquidity. Kalshi at 94.5% with 4.7M USD depth + Polymarket at 95.5% with 2.0M USD depth = 94.79%.
kalshi_prob / poly_probEach venue’s YES mid-price (best bid + best ask) / 2. Null if that venue has no data.
kalshi_depth_usd / poly_depth_usdTotal USD within +/-5c of mid on each venue. Shows how much liquidity backs each probability.

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Book update

Normalized orderbook updates from both venues. Prices are contract prices (0-1), sizes are absolute quantities. Derived fields like best bid/ask, mid, spread, and depth are pre-computed on every update. 
{
  "event_key": "fomc-2026-04-29",
  "outcome": "hold",
  "venue": "polymarket",
  "token": "yes",
  "venue_id": {
    "condition_id": "0x36e8ca2...",
    "token_id": "63586620628..."
  },
  "seq": 4217,
  "exchange_ts": 1773892530351,
  "received_ts": 1773892530360,
  "published_ts": 1773892530363,
  "update_type": "delta",
  "levels": [
    {"side": "bid", "price": "0.955", "size": 8300.00},
    {"side": "ask", "price": "0.965", "size": 0}
  ],
  "best_bid": "0.955",
  "best_ask": "0.965",
  "mid": "0.960",
  "spread": "0.010",
  "bid_depth_usd": 47141.97,
  "ask_depth_usd": 23456.78
}
FieldDescription
venue_idIdentifiers to execute on the venue. Kalshi: market ticker. Polymarket: condition ID + token ID.
update_type”snapshot” = full book replacement. “delta” = only changed levels. Size of 0 = level removed.
levelsPrice levels with side (bid/ask), price (0-1), and size (absolute quantity after this update).
best_bid / best_ask / mid / spreadPre-computed from full cross-token book state, not just the levels in this update.
bid_depth_usd / ask_depth_usdTotal USD within +/-5c of mid on bid and ask sides.

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Trade

Every trade execution from both venues, including the specific contract traded, price, quantity, and a venue-provided trade ID. 
{
  "event_key": "fomc-2026-04-29",
  "outcome": "hold",
  "venue": "kalshi",
  "token": "no",
  "action": "buy",
  "venue_id": {"market_ticker": "KXFEDDECISION-26APR-H0"},
  "seq": 89,
  "exchange_ts": 1773892530000,
  "received_ts": 1773892530008,
  "published_ts": 1773892530010,
  "price": "0.06",
  "qty": 136.00,
  "trade_id": "a4d77927-dafe-5bde-53ed-80f6129cfd19"
}

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Timestamps

Every book and trade message carries four timestamps (Unix ms) for end-to-end latency decomposition.
FieldDescription
exchange_tsWhen the exchange says the event occurred. Null if not provided by the venue.
received_tsWhen we read the raw message off the venue WebSocket.
published_tsWhen the normalized message was published internally.
gateway_tsWhen the gateway sent the message to your WebSocket.