A protocol spreads on the side that was not selling anything

Alex Hancock of Block, who works on the Goose harness and the MCP Rust SDK, gave a talk published September 9 with a complaint that is easy to follow. Every harness has its own client, sometimes only one, and the analogy offered is a web where each site required its own browser. The proposed fix is ACP, the Agent Client Protocol, which came out of the Zed and JetBrains editor teams: JSON-RPC sessions where a client sends a task and the agent streams back text, tool call notices, and permission requests, with an underscore convention for custom methods so the ecosystem can see what everyone adds. Block contributed an HTTP transport so the harness can live on another machine. The demo is one Goose agent driven from Zed, then from a terminal client, same implementation, any front end.

The radar the same week shows the appetite. Jackalope on Product Hunt puts Codex, Claude Code, Grok, and OpenCode in one shared workspace. A thread on r/LocalLLaMA asks whether we are missing a benchmark for agent runtimes rather than models. The harness is now the thing people compare.

Which is exactly the problem for a universal remote. Hancock’s best line is that the most powerful thing about MCP is not anything in MCP, it is that everyone uses it. My working theory about why everyone did: MCP standardized the tool side, and nobody’s business was the tool interface. It made generic a side with no vendor to object. ACP asks harness vendors to become interchangeable, and those are the companies with the most to lose from it.

The vault’s protocol notes are cautious for a different reason. The name is overloaded, one ACP being IBM’s agent communication protocol now folded into the A2A lineage and the other this editor protocol, and the posture is to treat it by context and not bet on it unless a specific workflow needs it today. The layer map on that page has six rows and no winner.

Concede that Zed, JetBrains, Goose, and a Poolside client is not nothing, and that the editors have a real itch.

Still, a standard gets adopted by whoever benefits from the other side being generic. I would watch which harness vendor implements ACP first. It will be the one that is not winning.