Personal IndexCustody systems eventually run into the same question: who is allowed to move this asset, under what conditions, and who decides. A policy engine is the piece that turns that question into something enforceable at the point a transaction is created, not after it has already been signed.
What a policy needs to reason about
A transaction policy usually isn't a single rule. It's a combination of:
- the user or role initiating the transaction
- the asset and network involved
- the destination address, and whether it's known or new
- the amount, often relative to recent activity rather than a fixed threshold
- the contract call itself, for anything beyond a simple transfer
- historical behavior — velocity limits, cumulative spend, anomaly signals
Any one of these, evaluated in isolation, is easy. The difficulty is composing them into rules that stay legible as more get added.
Evaluation has to happen before signing
Policy checks belong before a transaction is handed to the signing layer, not after. Once MPC signing has produced a valid signature, there's no way to "un-approve" a transaction that already satisfies the network's rules. The policy engine's job is to gate what reaches that stage in the first place — approve, require additional approval, or reject.
Rules as data, not code paths
Hardcoding policy logic into the transaction pipeline works until the second or third exception shows up. Representing policies as structured, versioned data — evaluated by a generic engine — makes it possible to change authorization behavior without redeploying the systems that build and broadcast transactions. It also makes policies auditable: you can point to exactly which rule allowed or blocked a specific transaction.
This is still an evolving part of the system. The interesting edges are less about the individual rules and more about how they compose, how they fail closed, and how much context is available at evaluation time.