Know which context called you

Goal: in a cross-context on call handler, read which context made the call.

When one context consumes another’s services, the call goes over a Cloudflare Service Binding — an internal, platform-dispatched channel that is not reachable from the outside. Bynk trusts that channel (the Internal scheme: “the channel itself is the assertion”), and the prelude actor Caller yields the calling context’s name as its identity.

context billing

service charges {
  on call by c: Caller (amount: Int) -> Effect[Result[String, String]] {
    -- c.identity : the qualified name of the context that called in,
    -- e.g. "shop.orders"
    Ok(c.identity)
  }
}

on call defaults to Caller, so you only write the by clause when you want to capture the name. A handler that omits it (or omits the binder) verifies the channel but reads no caller — and is unchanged from before.

How it works

The caller’s name is stamped by the compiler at the call site — a compile-time constant, not something the application sets — and travels beside the (unchanged) request body.
The callee reads it at the boundary and threads it into the handler as c.identity. A call that does not identify its caller is rejected fail-closed.
Verification is static / channel-based — no crypto. This establishes identity, not authorisation: it tells you who called, not whether they may.

Caller is admissible only on on call (cron takes Scheduler, queue takes Producer).