Bank Alpha received the report and traced the money through its own ledger until the trail left the bank. Each panel below is one institution's view, and no institution can see inside another. Meridian and Union answered under a group-size threshold — they returned totals, never rows.
Alpha cannot see into its peers, and they will not simply hand over data. So its diplomat proposes terms for a joint investigation, and each peer's policy engine answers. The verdict is deterministic code, never a model, and the strictest terms on the table win.
The signed agreement compiles itself into a temporary clean room. Each bank contributes totals over its own accounts, the chain is joined, and the room is dissolved. What survives is the picture below and the audit trail.
Append-only, and the only thing that outlives the clean room. Every agent action, policy verdict and perimeter check is here with its actor and timestamp.