Most agent protocol coverage focuses on MCP (tool access) and A2A (agent discovery and delegation). A narrower one, X42, is built specifically for the moment an agent built on one platform needs to invoke an agent or capability running on another. The scenario is common enough now to need its own plumbing: a sales agent triggering a workflow at a partner's system, or a personal agent invoking a third-party service's capability. Each of those crossings raises the same three questions — who is this caller, what are they allowed to do, and what actually happened. X42 wraps the invocation in signed requests, scoped tokens, and execution logs, producing a record of what was called, by whom, and under what authorization. That's a narrower claim than a full identity layer or a governance framework — it's closer to a permission envelope than a trust protocol. It doesn't try to establish who an agent *is* in any durable sense (that's identity-layer territory) or arbitrate disputes after the fact (that's more attestation's job). It just makes sure that every cross-boundary call carries its own proof of scope and leaves a log entry behind. The interesting part is what this implies about the shape of the stack. As of mid-2026, builders are reportedly stitching together six distinct protocols — MCP, A2A, AG-UI, A2UI, AP2, and X42 — to cover tool access, agent-to-agent coordination, UI streaming, state sync, payments, and now cross-platform execution permissions. That's not consolidation; it's specialization. Each new protocol picks off one narrow slice of the cross-boundary problem rather than one winner absorbing the whole stack. Whether X42 survives as a named standard or gets folded into A2A's authorization model over time is an open question — it's newer and less battle-tested than MCP or A2A. But the underlying need it names is real: agents crossing organizational or platform lines can't rely on implicit trust, and someone has to define what a signed, scoped, logged cross-boundary call actually looks like before enterprises will let agents make them unsupervised. This is a small, plumbing-level development, not a governance breakthrough. But it's a concrete instance of the industry answering the accountability question one narrow interface at a time rather than with a single unifying framework.