SymmACP: Extending Zed's ACP to Support Composable Agents (smallcultfollowing.com)

SymmACP is a proposed extension to Zed’s Agent Client Protocol (ACP) that turns monolithic CLI/browser AI tools into composable building blocks. Instead of rebuilding whole agents to add features, SymmACP introduces an intermediary “proxy” layer and a small set of protocol extensions so independent repositories (e.g., datetime injection, persona/context, long-running research tasks, tangent/history mode, interactive walkthroughs) can be mixed-and-matched with any frontend or backend that speaks ACP. The idea is to enable interoperable plugins—think Unix pipes or browser extensions for agents—so UIs, TUIs, or IDEs can combine capabilities without mutual awareness. Technically, SymmACP builds on ACP’s JSON-RPC over stdin/stdout messaging and augments it with proxy-delivered tool invocations, unprompted agent-to-editor sends, and replay semantics for session editing. That lets a proxy decorate prompts (inject current time or a “yiasou” kickoff), execute asynchronous MCP tools and notify agents/editors when results complete, or snapshot/replay history for tangent mode. These changes address ACP limits (blocking tool calls, agent-owned history, no async notifications) while preserving simple message flows between editor, proxy, and agent. The proposal is currently a thought experiment with initial interest from Zed; the author plans to prototype in Symposium and invites collaborators on Zulip.