Matching a bridge transfer, or admitting you cannot
Cross-chain tracing succeeds or fails on whether the destination transaction can be matched. The matching criteria we use, and what we report when none of them hold.
How is a cross-chain bridge transfer traced?
A bridge transfer is traced by matching a destination transaction to its source using amount, timing and route. Matching succeeds only where those criteria identify a unique candidate. Where several transactions fit, or none do, the honest report is that the trail ends at the bridge rather than presenting a guess as a finding.
Cross-chain tracing is frequently described as though following funds through a bridge were routine. It is not. It succeeds when the destination transaction can be matched to the source transaction, and it fails otherwise — and the honest thing to do when it fails is to say so.
The matching criteria
We treat a bridge transfer as matched when the following hold together:
- Value correspondence. The destination amount matches the source amount after documented bridge fees.
- Temporal proximity. The destination transaction falls inside the bridge's normal settlement window.
- Uniqueness. No other destination transaction in that window fits equally well. This is the criterion that most often fails.
- Protocol evidence. Where the bridge emits a correlatable identifier, it matches.
When uniqueness fails
On a busy bridge, a round-number transfer during a busy window may have a dozen equally plausible destination candidates. There is no analytical technique that resolves this; picking the most convenient candidate is not analysis, it is invention.
In that situation the trace is reported as ending at the bridge, with the candidate set recorded so that a later corroborating fact — a subsequent report, an exchange response — can resolve it if one arrives.
Why an unmatched bridge is still useful
It establishes that value left the chain, when, and through which service. Bridges are operated by identifiable parties more often than the addresses either side of them, and that party may hold records the chain does not.
An unmatched bridge is the end of the analytical trail, not necessarily the end of the matter.