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Eric Marchand
Eric Marchand

Posted on • Originally published at phimage.github.io

ACP: Copilot in Xcode, without the plugin 🔌

#ai #agents #xcode #acp

There's a new standardization layer in the agentic-coding stack, and it's a good
one: the Agent Client Protocol (ACP). After MCP standardized what tools an
agent can reach, ACP standardizes how editors and agents talk to each other —
local or remote — so any ACP agent drops into any ACP editor with no bespoke,
per-vendor glue. It's developed in the open (JetBrains and Zed), and there's a
clean introduction if
you want the protocol view.

The reason this matters today: Xcode 27 speaks ACP. (It's in the developer
beta, and it's already turning up in the
26.6 RC.)

MCP vs ACP, in one line

Xcode 27 actually has two protocol layers, and it's worth keeping them straight:

  • MCP decides what an agent can do inside Xcode — read files, build, run tests, pull diagnostics.
  • ACP decides which agents are even allowed to connect in the first place.

So ACP is the front door, MCP is the toolbox once you're inside. Apple shipped
GitHub and Figma as launch plugin partners — but because the door is a standard,
you don't actually need their plugin. Any ACP-speaking agent can knock.

The trick: register Copilot CLI as a raw ACP agent

Copilot CLI can run as an ACP server, so you can wire it straight into Xcode
instead of installing the official plugin. In Xcode → Settings → Intelligence →
Coding Intelligence → Add an Agent…:

Xcode's

  • Name: Copilot
  • Executable: /opt/homebrew/bin/copilot (wherever Homebrew put it — which copilot)
  • Arguments: --acp

That's the whole registration. Xcode notes that ACP agents get full use of Xcode
and your project context, and it asks for consent before a new agent connects —
the protocol is permissioned by design, not a backdoor.

Pick your model with an env var

The fun part: you're not locked to one model. Add an Environment Variable to
the agent and pin whichever model you want — for example:

COPILOT_MODEL=claude-sonnet-4.6
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So you get Copilot's harness and auth, driving the model of your choice, inside
Apple's IDE. "Use any LLM in Xcode" is barely an exaggeration.

Under the hood

If you want to see what Xcode is launching, the CLI flag is documented. --acp
defaults to stdio mode (ideal for an IDE spawning the process); add --port
for TCP instead:

copilot --acp --stdio      # stdio (default when --acp is passed)
copilot --acp --port 3000  # TCP
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One caveat worth flagging: ACP support in Copilot CLI is still public preview,
so flags and behavior may shift. Tool-filtering and reasoning-effort flags, if you
pass them, apply per session started by the client.

Worth reading

The bigger picture: the N×M integration problem — every editor needing a custom
connector for every agent — collapses to N+M once both sides speak a shared
protocol. MCP did it for tools; ACP is doing it for the agents themselves. Xcode
adopting it this fast is the surprise. 🔌

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