ACP: Copilot in Xcode, without the plugin 🔌
Xcode 27 speaks the Agent Client Protocol — so you can register the Copilot CLI directly as an agent, pick the model with an env var, and skip the official plugin entirely.
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200–1000 m · the twilight zone
Eric Marchand
Open-source software writer by night. Swift, AI tooling, and notes from where the sunlight runs out.
Three handles, one diver
Where I surface
Sunlit · day job @e-marchand
Engineer at 4D — developer tools and components for the 4D language ecosystem (AIKit, View Pro, dependencies).
github.com/e-marchand ↗ Twilight · experiments @mesopelagiqueOpen-source 4D experiments. OTP, Mark4Down, ChatGPT4D, and a heap of small sharp tools.
github.com/mesopelagique ↗ Midnight · by night @phimageSwift and AI/ML, written after dark. Contributor across OAuthSwift, IBAnimatable, IBDecodable.
github.com/phimage ↗Instruments
What I build with
SwiftPythonC++JavaPHP4DPyTorchHugging Facescikit-learnLLMsRAGAgents
Field notes
Writing
macOS 27's AI dev ecosystem — and pointing 4D AIKit at a local model 🍎
macOS 27 ships the fm CLI (Apple Foundation Models from the terminal), a Python SDK, and Core AI. The kicker: fm serve is an OpenAI-compatible endpoint, so 4D AIKit can drive an on-device model with no API key and no cloud bill.
Loop Engineering — the newest way to burn tokens 🔥
After prompt, context, and harness engineering comes loop engineering — designing a system that keeps guiding and correcting an LLM until the task is actually done.
Spending fewer tokens with Claude Code and GitHub Copilot 🪙
An improvised, battle-tested guide to burning fewer tokens in your IDE assistants — compress terminal output, freeze build scripts, script mass edits, throw away long sessions, slim your CLAUDE.md, clean your MCP servers, and let the AI be terse.
AI and design: a workspace, and a contract 🎨
Two design-with-AI updates landed together. Claude Design is a workspace you design inside; Google's DESIGN.md is a file your agents read to stay on-brand. One's a destination, the other's a contract — and both touch generating 4D forms.
AGENTS.md for 4D — auto-generating a README for your AI agents
AGENTS.md is a README for AI agents: a small markdown file at the repo root that hands an LLM the project's shape so it doesn't burn tokens exploring. Here's one auto-generated for 4D projects — plus the Claude Code import trick.
Reranking in RAG — the precise second pass
Embedding search is a fast first pass that's only roughly right. A reranker is the cheap precision fix: a cross-encoder that re-scores the top candidates by reading query and document together. Here's why it works and what's out there.
Xcode 26.3 as an MCP server — letting agents drive Xcode
Most MCP servers give your agent access to something external. Xcode 26.3 flips it: Xcode exposes itself as a server, so an agent in VS Code (or anywhere) can discover, build, test, and preview a real Xcode project through Xcode's own machinery.
Chrome's built-in AI, and the CEF catch 🌐
Chrome ships on-device AI (Gemini Nano) behind Web-standard APIs — translate, summarize, prompt, all local and private. The asterisk: it leans on Google-internal code that isn't in upstream Chromium, so embedded browsers (CEF) don't get it for free.
Copilot SDK in technical preview — and why a 4D one is within reach
The Copilot SDK gives four languages programmatic access to the Copilot CLI. Under the hood it's the same trick as MCP, LSP, and ACP — spawn the CLI, talk JSON-RPC — which means any app can host an agent.
Agent Skills in VS Code — capabilities that load only when needed
Agent Skills landed in VS Code: folders of instructions and scripts an agent loads on demand. The clever part is a three-level progressive loading scheme, so you can keep dozens installed and pay tokens only for the one that's relevant.
MCP, in practice: GitHub in VS Code — checked in for the team
Eight months after wondering whether anyone had tried MCP, here it is running for real: the GitHub MCP server in VS Code, wired to our issues, projects, and test cases — with the config committed to the repo so the whole team gets it on checkout.
Context engineering — yes it's a buzzword, but this one's real
Context engineering is the step past prompt-writing: assembling everything the model needs — instructions, data, code, memory, tools — in the right format at the right time. A rebrand, sure, but one that points at where the real leverage is.
Apple's Foundation Models — an on-device LLM in a few lines of Swift 🧠
At WWDC 2025 Apple opened the on-device model behind Apple Intelligence to developers via the Foundation Models framework. The API is tiny — and I wrapped it in a CLI and an MCP server to see how far it goes.
Has anyone tried MCP? — and what it could mean for 4D
March 2025: MCP is only a few months old and not yet a standard, but people are already building real things on it. Here's the pitch — and a first sketch of what an MCP server for 4D could look like.