Add to ChromeDesign Mode shipped its first version with a single handoff button: Copy Prompt. It dumped a Markdown export of your changes into the clipboard, you pasted it into Claude, Cursor, or whatever, and an agent picked it up from there. It worked. But it was clunky — context-switch, paste, switch back.
When Anthropic released Model Context Protocol in late 2024, it was obvious in retrospect what the right surface was. MCP is a standard for letting agents call tools. A design edit is a tool call. Match made.
Why MCP and not a custom protocol
We could have built a bespoke WebSocket protocol and an SDK per agent. We didn't, because MCP solved three problems for free: every major coding agent already speaks it, the auth and transport story is settled, and the user's mental model is the same regardless of which agent they pick.
MCP is also one of the rare protocols designed by an AI lab that respects the principle of least privilege. The agent only gets the six tools we expose. It doesn't get filesystem, it doesn't get the network, it doesn't get to do anything we didn't sign up for.
Three connection modes
We launched Local mode first — a stdio MCP server you run on your laptop. Zero network egress, lowest latency, best for power users. But Local mode has a steep on-ramp: you have to be comfortable with terminal commands and your agent has to be on the same machine.
Cloud mode came next. It's a hosted SSE relay at mcp.designmode.app. Your agent dials it over HTTPS with a bearer token. Nothing persists; payload bodies are dropped within ~60 seconds. The on-ramp is one paste of a config block — no install required.
Self-hosted mode is the same Cloud relay code (packages/mcp-cloud) deployed on infrastructure you operate. For teams who want Cloud ergonomics but their own infra.
What changed in the loop
Before MCP, sending edits felt like a tax. After MCP, it became the natural end of a design iteration. You tweak something in the side panel; you hit Send to Agent; the agent writes the production code. The loop stopped being three tools and three context-switches.
If you're building any kind of agent-facing tool, MCP is now the answer. The standard is settled enough to bet on, and the user experience compounds across every agent your users already have installed.