Show HN: Brain.md – A persistent memory layer for your coding agents

A persistent memory layer for your coding agents.

An open, agent-agnostic standard for capturing a project's durable knowledge as plain Markdown — read and written through one small CLI. It lives in your repo and travels across agents, machines, and models.

Why · Quick start · See it work · CLI · How it works

This repository is the toolkit, not a brain itself. Install it once, then run brain-setup

inside any project: it scaffolds a BRAIN.md protocol file and a

brain/

directory into your repo. From then on, any coding agent — Claude Code, Codex, anything that reads files — learns to use that brain just by reading the project's

BRAIN.md

. The brain is plain Markdown, lives in the repo, and outlives every session.A coding agent's knowledge lives nowhere durable. The reasons behind a decision, the constraints you agreed on, the path not taken — they sit in chat logs and in your head, and they vanish the moment the session ends. The next agent starts from zero.

A brain fixes that. It is the project's persistent memory: the durable decisions, requirements, and constraints, written down as plain Markdown next to the code.

Repo-native— Markdown that lives in your project and travels in git, with or without a runtime on top.** Agent-agnostic**— the contract is a file (BRAIN.md

). Any agent that can read it can use the brain.Correct by construction— every write goes through thebrain

CLI, so the brain's invariants can't be broken by a malformed edit. There is no validator because none is needed.

The test for what belongs in it: will this still matter in six months, and is it hard to reconstruct from the code itself? If yes, it goes in the brain. Pure implementation details and anything readable straight from the code and git history stay where they are.

1. Install the tools once (global):

./setup        # symlinks skills/ into every detected agent (~/.claude/skills, ~/.codex/skills, …)
./uninstall    # reverses it cleanly; never touches any project's brain data

2. Initialize a project — run the brain-setup skill in it. It scaffolds BRAIN.md

the brain skeleton, wires the chosen agents' config files, and can install a pre-commit hook.

3. Seed real knowledge — run the brain-bootstrap skill. On an existing project it reads the code, docs, and git log

to draft the root pages and capture key decisions; on a near-empty one it interviews you. (Setup leaves the brain empty on purpose — seeding is a separate, deliberate step.)

4. Work as usual. The agent reads and writes the brain only through the brain

CLI, following BRAIN.md

— brain files are never hand-edited.

The point of a brain is what happens across sessions — a decision made today is still there, with its rationale, weeks later:

You    Let's store config as Markdown, not SQLite — easier to diff and migrate.

Agent  Capturing that as a decision so it outlives this session.
       $ brain create-page --id config-as-markdown --category decision \
             --title "Store config as Markdown, not SQLite"
       ✓ page created · indexed

— three weeks and a fresh session later —

You    Why aren't we using a database for config?

Agent  $ brain read-page config-as-markdown
       We chose Markdown for diff-ability and zero migrations. Here's the original call
       and the trade-offs we weighed …

Reading and writing the brain both go through one zero-dependency Node CLI (run with node

):

brain() { node skills/brain-page/bin/brain.mjs "$@"; }

brain brain-dir                              # where is the brain?
brain list-pages                             # list pages
brain read-page my-decision                  # read a page
brain create-page --id my-decision --category decision --title "Use X over Y"
echo "the new understanding" | brain update-truth --id my-decision --summary "why it changed"
brain append-timeline --id my-decision --kind evidence --summary "benchmark confirmed it"
echo "## Overview …" | brain update-root architecture
brain wire --agent claude-code,codex,opencode         # wire CLAUDE.md / AGENTS.md to BRAIN.md
brain reindex && brain lint-links

A page carries a rewritable compiled_truth (the current best understanding) plus an append-only timeline (the chain of evidence). update-truth

rewrites the truth and appends its timeline entry in one atomic write — so the understanding can never change without a trace.

Three design choices keep the brain durable and tamper-evident:

Correct by construction, no validator. The CLI is the only writer. Frontmatter is always generated, andupdate-truth

rewrites understanding + records why in a single atomic write. The two things a validator used to guard are now structurally impossible.Exactly one brain, location-independent. It defaults to./brain

, but a project can redirect it viabrainRoot

in.mindmux/preferences.json

(e.g. an external sidecar). Every command resolves the location itself — tools never create a second, shadow brain.Pure files, portable. The brain is Markdown plus one Node script — it lives in your repo and travels in git, and runtimes (MindMux over MCP, more to come) layer on top of the same files.

The skills that drive it all:

brain.md is led and incubated by MindMux — the standalone open-source landing of MindMux's Brain Spec + coding-agent adapter. The specification layer uses neutral naming so it can be adopted widely; stewardship and maintenance belong to MindMux. Licensed under Apache-2.0.