How to Give Claude Code, Cursor, and Codex the Same Context (Without Copy-Pasting)
One canonical context over MCP beats three copies of the same instructions
You run Claude Code in one terminal, Cursor in another, and Codex for the tasks each does better. Three agents, one you — and every one of them starts each session knowing nothing about how you work, what you're building, or the standards you hold everything to. So the same paragraph about your stack and conventions lives in three files, drifting out of sync at three different rates.
This isn't a discipline problem you can solve by being tidier. It's structural, and the usual fixes trade one kind of maintenance for another.
Three tools, three copies of you
Each coding agent keeps its own siloed context file, and they don't talk to each other. Claude Code reads CLAUDE.md. Cursor reads .cursorrules (and now AGENTS.md). Codex has its own config. The formats overlap in spirit but differ in detail, so the same information — your role, your stack, your conventions, the client you're building for — gets written three times and maintained three times.
Some of that content is genuinely project-specific and belongs in the repo: build commands, directory conventions, the reason behind an architecture choice. But a large share of it isn't project-specific at all. Who you are, how you communicate, the standards you apply across every codebase — that's stable context about you, and it's identical in all three files. Duplicating it is pure overhead, and the moment you update one copy, the other two are wrong.
With one tool, that's a minor annoyance. With three, it's a standing tax: every change to how you work means editing three files, and any file you forget silently feeds an agent stale instructions.
What people try — from copy-paste to custom pipelines
The fixes people build climb a ladder of sophistication, and the maintenance cost climbs right along with them.
Manual copy-paste and symlinks. The first instinct is to write the context once and paste it into each file, or symlink them so they share content. Copy-paste rots the instant one file changes and the others don't. Symlinks help only where tools accept an identical format — which they mostly don't, since CLAUDE.md, .cursorrules, and Codex config each expect their own structure.
Hand-syncing across formats. The next step is reconciling the files by hand — keeping CLAUDE.md, AGENTS.md, and the Codex config aligned whenever something changes. This works exactly as long as your attention holds, which is to say not long. It's the same duplication problem with more steps.
Automated self-built sync. The serious version: a script or git hook that regenerates each tool's file from one source you maintain — a canonical document fanned out to the formats each agent expects. This genuinely solves the rot. Edit the source, run the pipeline, every file updates. The cost moves rather than disappearing — you've built and now own a small piece of infrastructure that needs maintaining, and it breaks quietly when a tool changes its config format.
A real local-first brain — gBrain. At the top of the ladder is gBrain, Garry Tan's open-source, local-first "brain" for AI agents. It's a proper approach: Markdown plus Postgres, MCP-native, structured as a real knowledge graph rather than a pile of synced text files. Builders adopt it because it's genuinely good — context compounds over months as you feed it more. The honest tradeoff is the one every serious DIY option shares: gBrain is developer-operated. You run the CLI, manage embeddings and sync jobs, and accept that day-one value is limited because the payoff accrues over time. You get a system crafted to your needs, and you become the person who operates it.
File-sync vs. source-of-truth
The real problem isn't that your copies fall out of sync — it's that you have copies at all. Every solution above, from clipboard to knowledge graph, is a way of managing N derived copies fanned out from a source. Better tools make the fan-out automatic. None of them removes the fan-out.
That distinction matters because inconsistent context isn't a cosmetic issue. A July 2026 survey of 101 enterprises found 57% had experienced AI agents producing confident, wrong answers driven by missing or inconsistent context. The individual-builder version is the same failure: when Cursor is working from last month's version of your standards and Claude Code has this week's, at least one of them is confidently applying the wrong picture — and neither tells you which.
The cleaner model inverts the whole setup. Instead of one source you replicate into every tool, keep one canonical context that every tool reads directly. No copies to reconcile, no pipeline to run, no format to keep translating.
How MCP makes one canonical context possible
The reason this is newly practical is the Model Context Protocol. Anthropic open-sourced MCP in November 2024 as an open standard for connecting AI tools to external data and context. In roughly eighteen months it's gone from launch to de-facto standard — supported across Claude Code, Cursor, and a growing list of others, with thousands of community servers built on it.
What MCP changes for this problem: an MCP-compatible tool can pull structured context from an external source at session start, before you type anything. That means your context can live in one place and be served to every tool that speaks the protocol — read directly, not copied in. Update the source once, and Claude Code, Cursor, and Codex all see the change on their next session. The fan-out disappears because there's nothing to fan out.
Setting it up: project context vs. who-you-are context
The practical move is to split your context by how often it changes and where it belongs. Project rules — build commands, repo conventions, architecture notes — can stay in per-repo files like CLAUDE.md, because they're specific to that codebase and version naturally with it. The stable layer — who you are, how you work, the standards and client context that apply everywhere — belongs in one source every tool reads, not duplicated into each.
This is where Unabyss fits. It holds your personal context as a single structured vault and serves it to any MCP-compatible tool, so Claude Code, Cursor, and Codex all start each session with the same accurate picture of who you are — without you building or operating a sync pipeline. It's the same outcome a self-built brain gives you, minus the part where you're the one running it.
If you've already invested in a local-first setup, the two aren't mutually exclusive: Unabyss can mirror its context to a self-hosted gBrain, so you keep your own brain and skip the extraction-and-upkeep work of populating it.
The test is simple. If updating how you work means editing more than one place, you don't have a sync problem to automate — you have a source-of-truth problem to fix.
Set up your context once, use it in every tool →
