PostLake
Direct messages through one integration, on the two networks that actually permit it. The rest of this page explains why that number is two and not nine, because everyone else quietly implies otherwise.
Straight from the same registry the publish path validates against, so nothing here can disagree with what the API does.
A cross-network DM inbox is the most requested feature in this category and the least deliverable, so here is the honest position rather than a roadmap promise.
We looked at building the unified inbox and dropped it, because the honest version of it covers two networks and the marketed version covers nine. Declaring the gap is more useful than a feature that fails on seven.
The reason this works as one integration rather than 2 is that every network answers in the same normalised shape. Your code does not branch per platform, and an agent does not need a mental model of each one. Where a network genuinely differs, the difference is declared in GET /v1/platforms rather than discovered from a rejection.
Pages listed here cover only the networks that genuinely support this. Where a network does not, it is absent rather than represented by a page explaining that it cannot.