Halfnine MCP Server

Halfnine

A small, calm PostgreSQL client for macOS and Windows, light enough to leave open all day.

It is early: ROADMAP.md says where things stand.

Install

Download the installer from Releases. The builds aren't signed yet, so each system asks once before opening them:

• macOS (Apple silicon, macOS 13 or later): open the .dmg and drag Halfnine to Applications. Open it once, then choose System Settings › Privacy & Security › Open Anyway. Or run xattr -dr com.apple.quarantine /Applications/Halfnine.app.
• Windows (x64): run the -setup.exe. At the SmartScreen warning choose More info › Run anyway. It installs for the current user, without administrator rights.

When a connection saves its password, macOS asks once before Halfnine first reads it, and again after each update until the app is signed. Choose Always Allow.

Docker databases and backups

Turn on Docker databases in Settings, or at the first launch, and the Postgres containers on this computer show under Docker on the connections screen. Double-click one to connect; Halfnine reads its user and password from the container. The Docker page makes new ones (PostgreSQL 14 to 18, optionally from a dump or an SQL file), copies, starts, stops and deletes them. It works with Docker Desktop, OrbStack, Colima and Rancher Desktop.

With backups on, as they are unless you turn them off in Settings, dropping or emptying a table, dropping a column, or deleting a Docker database backs it up first and acts at once, with Undo at the foot of the window. Backups stay on the Docker page until you delete them. A table bigger than the limit in Settings, or one Halfnine can't back up, asks for its name instead.

Use with AI agents

Halfnine is also an MCP server, so Claude Code, Codex, Cursor, VS Code and Claude Desktop can use your connections through it. Open Agents, at the foot of the sidebar or the connections screen, choose Connect next to your tool, and Halfnine shows what it will change before it adds itself. Or add it by hand; on macOS:

claude mcp add --scope user --transport stdio halfnine -- /Applications/Halfnine.app/Contents/MacOS/halfnine mcp
codex mcp add halfnine -- /Applications/Halfnine.app/Contents/MacOS/halfnine mcp

For Claude Desktop, install Halfnine first, then open the .mcpb file from the same release to add it as an extension. It is also listed in the official MCP Registry as io.github.notinferred/halfnine.

Each connection sets what agents may do there, under Agents in its settings: Off, Read, Read with approved writes, or Full. Red-labelled connections start at Off and every other at Read. At Read with approved writes, an agent's change opens in Halfnine as staged rows or SQL and waits for your answer. Everything agents do is listed on the Agents page, where one switch pauses them all. What an agent reads goes to its AI provider.

With Docker databases on, an agent can make its own databases, or copy one before risky work. Those start at Full and are marked with the agent's name, and it gets their URL for programs it runs. It deletes one made for it without asking; deleting any other asks you. While backups are on, what an agent drops or empties is backed up first, and run_sql won't drop tables, schemas or databases.

Develop

You need Node 24 or later, pnpm (the version in package.json is fetched automatically), Rust through rustup (rust-toolchain.toml picks the version), and on macOS the Xcode Command Line Tools. With Homebrew, brew install rustup, then put /opt/homebrew/opt/rustup/bin on your PATH; Homebrew leaves it off.

To check the app or run the Postgres tests on a Mac without a database, start a throwaway one in Docker. It deletes itself when stopped:

docker run -d --rm --name halfnine-check -e POSTGRES_PASSWORD=check -p 127.0.0.1:55432:5432 postgres:18
docker exec -i halfnine-check psql -U postgres -q < crates/pg/tests/fixture.sql
# connect the app to postgres://postgres:check@localhost:55432/postgres
# or: HN_TEST_DATABASE_URL=postgres://postgres:check@localhost:55432/postgres cargo test --workspace
docker stop halfnine-check

Layout

• apps/desktop: the UI, in React and Vite. Design tokens are in src/ui/tokens.css.
• crates/desktop: the Tauri app that hosts the UI, with its window, permissions and icons.
• crates/core, crates/pg and crates/docker: the types every crate shares, the Postgres adapter, and the Docker engine adapter.
• docs/: architecture, decisions and brand.

License

Halfnine is source-available under the PolyForm Shield License 1.0.0. You may use it for any purpose, at work included, and change it, but not to sell it or to provide a product that competes with it. Copyright 2026 notInferred.