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0016. Combo ids: the pmc: namespace, legs keyed by Gamma position ids, eligibility from Gamma

• Status: Proposed
• Date: 2026-10-01

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Context

• The RFQ keys legs by Protocol V2 position ids, Gamma’s positionIds, not by CLOB token ids (clobTokenIds) or condition ids.
• The Data API’s leg_condition_id on combo positions is not always the Gamma conditionId (the fixtures show a 0x01… form).
• Two sources say which markets can be legs: Gamma comboStatus on each market, and the public RFQ catalog (combos-rfq-api.polymarket.com/v1/rfq/combo-markets).

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Decision

• Namespace pmc: for the Combos venue: pmc:<combo conditionId> for a combo market and pmc:<combo YES positionId> for a combo position, which is also the Accept order’s token. Legs stay pm:<tokenId>, the outcomes the rest of the app already shows, prices and links to.
• Legs are keyed by Gamma position id only. The metadata cache stores each outcome’s positionIds entry in outcomes.position_id ('' when Gamma sends none), indexed. The browser sends leg token ids; engine/orders maps each to its position id through the catalog, and the adapter sends them as canonical decimals in ascending order. A leg the venue reports (a combo position’s legs) maps back to an outcome by position id: from the cache, or, for a closed market the cache does not hold, from Gamma by position_ids with closed=true, which is then cached. A leg is never matched by condition id or by the Data API’s leg_condition_id.
• Eligibility comes from Gamma comboStatus only, refreshed with the rest of the market metadata into markets.combo_eligible: enabled is eligible; pending, disabled and any unknown value are not. A market with an outcome whose position id is missing is not eligible either. The public RFQ catalog is never read.
• engine/risk refuses a quote request with a leg whose market is not eligible, or is unknown to the catalog, with combo_ineligible, and a closed market with market_closed. Two outcomes of one market, or of one neg-risk event, are combo_contradictory_legs.
• The adapter checks that the venue’s quote echoes the request (legs by position id, side, size, wallet) and names a combo condition before the quote is shown; a mismatch is no_quote with combo_quote_mismatch.

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Consequences

• Leg prices, links and names come from the existing pm: outcomes and books; the combo itself has no book and no price but the quote (ADR 0007 and PLAN §3.7).
• Combo positions and combo trades are told apart from Predictions ones by the pmc: prefix; Data API trade rows with is_combo are mapped to pmc: ids.
• Eligibility is as fresh as the catalog’s Gamma refresh. A market the venue enables shows as eligible after the next refresh; one it disables in between is refused by the venue on the quote request. The app does not fill the gap from the catalog endpoint.
• A leg the cache has never seen needs one Gamma request before a combo position can show its names.
• Open: security finding P8-05. Gamma sends outcomes, clobTokenIds and positionIds as separate arrays, and the cache pairs them by index. The proposed fix marks a market ineligible unless its position ids are consecutive from the first, and compares the filled position’s legs with the Accept’s; the live run checks it (row 16.42).

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Alternatives considered

• Key legs by CLOB token id and let the adapter translate at the edge. Rejected: the translation still needs the position id, and storing it once in the cache keeps one mapping.
• Match legs by condition id. Rejected: the Data API’s leg condition ids do not match Gamma’s.
• Read the RFQ catalog for eligibility, or merge it with Gamma. Rejected: two sources for one signal; when one goes stale the merged result still looks right.
• Reuse pm: for combos. Rejected: combo positions would look like Predictions positions to every consumer of pm: ids, including the books and the portfolio.