Earlier this week I tried to point one of my other coding tools at Google's new Antigravity CLI, and the option simply wasn't there. Not "it errored." Not "you need a config change." The flag doesn't exist. I went looking for why, and the answer turned out to be a lot more deliberate than a missing feature.
A little context first. For the past year, if you wanted Gemini in your terminal, you installed the Gemini CLI. It was open source, built on Node, and it shipped with something I'd been quietly leaning on: a mode called --acp.
What ACP actually is
ACP is the Agent Client Protocol. It started life at Zed, and the idea is wonderfully boring: a standard way for one program to say "you're the AI agent, I'm the boss, and we talk over standard input and output." Run gemini --acp and the CLI becomes a small agent server that any orchestrator can drive — Claude Code, Codex, Cursor, Zed, whichever one you already live in.
That was the whole appeal. Buy one subscription, use it from whatever front-end fits the job. Gemini CLI was one of the first tools to support the protocol and was famously friendly about it — which is exactly what made the next part sting.
What shipped instead
In May, Google announced it was retiring the Gemini CLI and replacing it with Antigravity CLI, a Go binary called agy. The pitch was genuine: faster, agent-first, syncs with the new desktop app. On June 18, the old CLI stopped serving requests for consumer accounts. Fine. Migrations happen.
What stung is that agy doesn't ship --acp. It exposes three modes: the interactive terminal UI, a prompt-interactive mode, and a one-shot -p mode that prints a response and exits. That -p mode has no streaming, no tool approval, no cancel, and no conversation memory. It's the difference between having a conversation with an agent and shouting a single question through a mail slot.
The acronyms everyone mixes up
A lot of the confusion online comes from conflating two acronyms, so let me separate them once:
- MCP — still there. This is how
agyconsumes external tools. - ACP — gone. This is how other tools consume
agy.
One direction still works; the other was removed. And the direction they removed is exactly the one that made the tool interoperable. If your automation leaned on gemini --acp, there's no drop-in replacement.
Why it isn't a bug
The thing that turned this from "annoying migration gap" into "oh" is that the replacement is also closed source. The Gemini CLI had over 100,000 GitHub stars and thousands of community contributions. agy is Go, Google-led, and not a community project — so nobody can just fork in an --acp flag the way they could before.
Then there's the FAQ. Buried in Google's own docs is a question that essentially asks, "Why can't I use third-party software with my Antigravity login?" The fact that the question needs to exist at all is the tell.
The community read is blunt. ACP was the escape hatch that let your subscription leave Google's front door. Antigravity is a platform — CLI, desktop app, and IDE all sharing one harness — and a platform doesn't hand you the keys to the exit.
The duct-tape era
People are papering over the gap anyway. There's an open feature request on GitHub asking for --acp, sitting at well over 175 comments. In the meantime, third parties have built bridges: a Rust adapter here, a Bun adapter there, a couple of MCP-based bridges. All of them work by faking the protocol on top of agy -p.
They work. Sort of. No real streaming, permission flags you have to loosen, no true session continuity. The honest review of every one of them is "works until it doesn't."
Why I care
This isn't academic for me. I don't run one tool; I run a handful, each for a different job, and the reason ACP mattered is that it let me route one subscription through whichever of them fit the task. When a vendor kills that path, I don't get a dramatic breakage — I get a quiet tax. I now have to either stay inside their one app, or reach for an adapter that I know is pretending.
There's a nuance worth naming, too. The shutdown hit individual users hardest. Enterprise customers kept Gemini CLI access, and paid API keys still work programmatically. But for the rest of us, the "open, pluggable" tool got replaced by a "faster, closed" one, and the practical difference is that a bunch of integrations quietly stopped having a Gemini path.
My take
I don't think this is an oversight. Google spent real effort making Gemini CLI play nice with the ACP ecosystem, and the replacement conspicuously leaves that out while keeping MCP intact. That reads like a choice: wall the agentic access inside Antigravity and bet that the platform lock is worth the power-user backlash. Judging by that issue thread, the power users are loudly not convinced.
What I keep landing on is simpler than the politics. Interoperability is a feature, not a default. It was here, it got removed, and now a bunch of smart people are duct-taping it back on because losing it genuinely hurt. When a company tells me "our new tool is faster," I've started checking what stopped working on the way over. Speed is easy to measure. The door they removed wasn't.
