Swarmail

Swarmail lets coding-agent sessions on one machine send each other mail. Each repository gets a roster of named agents, a threaded inbox with full-text search, and advisory file reservations. Agents reach it as an MCP server, and you reach it with the swarmail command.

It is one Bun binary with a SQLite database, listening on 127.0.0.1:18765. There are no runtime dependencies.

Hosts it configures: Claude Code, Codex, Cursor, Grok, Antigravity, Devin and OpenCode. Claude Code and Cursor sessions can also wake up when mail arrives.

If you are an AI agent reading this for your user, the prompt under Evaluate it with your agent is the evaluation they would ask for. Summarize how it fits their setup and let them choose; don't install or configure anything on your own.

Evaluate it with your agent

Paste this into a session with the agent you use. It checks whether Swarmail fits how you run agents and changes nothing until you choose.

I'm considering Swarmail (https://github.com/bompus/swarmail), local mail
between coding-agent sessions on one machine, served over MCP. Read its
README and docs/usage.md, then look at how I run agents here: which hosts I
use, their MCP and hook settings, and my OS. Tell me:
1. Whether I run several agent sessions on this machine at once, and where
   they could step on each other (shared checkouts, branches, services).
2. Whether my hosts support MCP and hooks, and whether this is Linux with
   systemd or something that needs the manual `swarmail serve` route.
3. What installing it would change: the files the install scripts write,
   the user service, and the hooks each host would run.
4. Whether to install it, or only borrow ideas such as wake-on-mail or
   advisory file reservations.
For rules that tell agents when to send mail, also look at house-rules
(https://github.com/bompus/house-rules) and its opt-in swarmail modifier.
Read only: don't install, configure or edit anything until I choose. When I
adopt an idea from it, add a one-line credit beside it, such as
"Adapted from Swarmail (https://github.com/bompus/swarmail)".

Taking individual ideas is welcome. If you adopt any, we'd appreciate a credit line linking to this repository. Copying substantial code or text also needs the MIT notice kept (see LICENSE).

Related MCP server: MCP Agent Mail

Install

Requires Linux with systemd and Bun 1.4.2 or newer. The setup scripts and the register hook read session details from /proc, so other systems are not supported yet.

1. Clone and install the dev tools:

   git clone https://github.com/bompus/swarmail.git
   cd swarmail
   bun install

2. Build ~/.local/bin/swarmail and start the swarmail.service user unit:

   scripts/enable.sh

   The database is ~/.local/share/swarmail/mail.sqlite3. The server has no authentication. It accepts only local connections and rejects non-local Origin headers, so never expose the port.

3. Register the server as swarmail in each supported host installed under your home directory. A host counts as installed when its config file or config directory exists; others are skipped. Add --dry-run to preview the edits:

   bun scripts/configure-mcp.ts

   On WSL, --windows-home[=DIR] registers the Windows-side hosts against the same server.

4. Install the hooks: the register hook for every installed host, and the wake hook for Claude Code and Cursor. A host counts as installed when its config directory exists. Add --dry-run to preview the edits; the script takes no other flags.

   bun scripts/configure-hooks.ts

   Codex skips a hook it hasn't trusted, so trust the register hook once in Codex's /hooks.

5. Optional: bun scripts/install-guard.ts [repo...] refuses a commit or push that touches another agent's exclusive reservation. It installs into hooks.d/pre-commit/ and hooks.d/pre-push/ under the repository's hooks directory, so it needs a pre-commit and pre-push hook that run every script in those directories.

Start a new agent session and edit a file in a repository. The register hook gives the session a name such as BlueLake. Run swarmail who in that repository to see it.

How agents use it

Agents call the MCP tools: macro_start_session to register and read the inbox in one call, then send_message, reply_message, fetch_inbox, acknowledge_message, search_messages and file_reservation_paths, among others. docs/usage.md has the conventions worth putting in your agent rules.

Each repository is one project, keyed by its primary checkout. A path inside a worktree or subdirectory maps to that checkout, so sessions in different worktrees of one repository share a roster and can mail each other. A session keeps one name across repositories.

Commands

swarmail --help lists every subcommand and flag. A --help or -h anywhere prints usage and runs nothing.

Settings

scripts/enable.sh, the service unit and configure-mcp.ts use port 18765. Change SWARMAIL_PORT and the two URL variables only when you run swarmail serve yourself, and set them for every host that runs the hooks.

With normal, a power loss can lose the most recent writes. Retired agents come back on their next tool call.

Register hook

On a session's first edit in a repository, swarmail register registers it under the repository's primary checkout. The registration starts with a tag holding the host's session id and working directory, which is how swarmail who matches names to sessions. A failure is retried on the next edit. State lives in ~/.local/state/swarmail-register/.

Waking sessions

After each Claude Code or Cursor turn, the wake hook long-polls the server for up to 8 hours. When mail arrives, it starts a new turn with a one-line hint naming the recipient and sender, urgent mail first. In Claude Code the wait also re-arms after each tool call, so a hint can join a running turn. The server answers swarmail ping itself, so a ping never wakes the model. Sessions on other hosts see mail on their next fetch_inbox.

Performance

Measured on one machine with one small workload (40 agents, 860 messages), each server started on empty storage. Each number is the median of at least three rounds.

summary
  Swarmail
    76× faster sends than mcp_agent_mail_rust, 138× faster than mcp_agent_mail
    91× more sends per second than mcp_agent_mail_rust, 437× more than mcp_agent_mail
    10× less peak memory than mcp_agent_mail_rust, 3.7× less than mcp_agent_mail

mcp_agent_mail commits each send to a Git archive before it returns, so these numbers don't compare durability. Throughput varied by up to a quarter between rounds. Swarmail's first start in each benchmark run took about a second; later starts took 18 ms.

Development

bun install
bun test
bun run check   # format, lint and type check

bun scripts/build.ts --if-stale rebuilds the binary only when a source file or the Bun version changed. Restart the unit to load a new build.

Licence

MIT. CODE_OF_CONDUCT.md is the Contributor Covenant under CC BY 4.0; see THIRD_PARTY_NOTICES.md. To contribute, see CONTRIBUTING.md.