Swarmail

[Swarmail]

[CI status] [Latest release] [MIT license] [Swarmail on Glama]

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.

[Claude Code and Cursor sessions on one repository. The person asks Claude Code to hand the README install section to the other agent. Claude Code mails the Cursor session, which wakes, makes the edit and replies. Claude Code wakes on the reply and checks the change.]

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 mailbetween coding-agent sessions on one machine, served over MCP. Read itsREADME and docs/usage.md, then look at how I run agents here: which hosts Iuse, 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, Windows, 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 Iadopt 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).

Install

Requires Bun 1.4.2 or newer on Linux with systemd or on Windows 10 or 11. WSL 2 with systemd enabled counts as Linux, and step 3 can register Windows-side hosts against the server running in WSL. Run Swarmail either in WSL or natively on Windows, not both: both servers use port 18765, which mirrored WSL networking shares with Windows. macOS is not supported yet.

Native Windows is a preview. The test suite runs on Windows in CI, and the hooks have been run through Git Bash, PowerShell 5.1, pwsh 7, cmd and Cursor's PowerShell form against a real server. On Windows 11, headless claude -p and cursor-agent -p sessions have registered through the hooks and used the MCP tools. An idle interactive Claude Code session on Windows 11 woke when mail arrived and replied with no prompt, with its hooks talking to a server that ran in WSL. Not yet verified on Windows: that wake against a server running natively on Windows, Cursor waking on mail, and the logon task starting a server at a real logon. If something fails there, please open an issue.

1. Clone and install the dev tools:

   bash

   git clone https://github.com/bompus/swarmail.gitcd swarmailbun install

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

   bash

   scripts/enable.sh

   On Windows, build ~\.local\bin\swarmail.exe and start the server in a scheduled task named Swarmail, which runs it hidden at each logon and needs no administrator:

   powershell

   bun scripts/enable-windows.ts

   Rerun either script after pulling; it stops the running server and starts the new build.

   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:

   bash

   bun scripts/configure-mcp.ts

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

   An MCP client that installs servers from a registry, or speaks only stdio, can run npx -y swarmail-mcp instead. That relay forwards each request to the server from step 2 and installs nothing itself.

4. Install the hooks: the register hook for every installed host, the wake hook for Claude Code and Cursor, and the Swarmail mod for Claude Code. A host counts as installed when its config directory exists. Add --dry-run to preview the edits. With --no-claude-mod, new Claude Code sessions don't load the mod (one installed before included) and use the wake hook.

   bash

   bun scripts/configure-hooks.ts

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

   On Windows, hosts run each hook through Git Bash, PowerShell or cmd, so the hooks name the binary as one unquoted path with forward slashes. If your profile path has a space or a non-ASCII letter, the hooks use its 8.3 short name, and the installer stops with an error on a volume that has short names turned off. Cursor passes hook input through Windows PowerShell 5.1, which turns non-ASCII characters into ?, so a repository path with such characters reaches the Cursor register hook mangled.

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, scripts/enable-windows.ts, 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. On Windows the scheduled task reads them from your user environment (setx SWARMAIL_PORT 18865) at the next logon.

On Windows, Stop-ScheduledTask Swarmail ends only the task's console host, and the server keeps running. To stop the server for good, run Unregister-ScheduledTask Swarmail and end swarmail.exe in Task Manager.

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. Claude Code and Cursor also run it when a session starts. It registers the session under its working directory's repository and tells the agent its name, so the agent uses that name instead of registering a second one. 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 prompt or edit. State lives in ~/.local/state/swarmail-register/, under your profile on Windows. After upgrading, run bun scripts/configure-hooks.ts again to add the session-start hook.

Waking sessions

Claude Code sessions wake through the Swarmail mod (src/claude-wake-mod.js), which waits for mail for as long as the session runs. The installer copies it to ~/.local/share/swarmail/claude-plugin and adds that directory to env.CLAUDE_CODE_PLUGIN_DIRS in ~/.claude/settings.json, so every Claude Code session loads it, including ones an app starts through the Agent SDK. When mail arrives at an idle session, the mod starts a turn with a one-line hint naming the recipient and sender, urgent mail first. During a turn the hint goes with the next tool result, or starts the next turn if the turn ends first. The mod sets SWARMAIL_WAKE_MOD=1, and the wake hooks exit at once where they see it. It was tested with Claude Code 2.1.288. A Claude Code without mods never sets the variable, so the hooks keep waking it.

Cursor, and Claude Code without the mod, wake through the wake hook, which long-polls the server after each turn: about 23 days in Claude Code, 8 hours in Cursor. When mail arrives, it starts a new turn with the hint. 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, 250 seed messages, 1,560 messages by the end), each server started on empty storage. Startup is hyperfine's mean of 20 runs; every other number is the median of three rounds, with latency from Tinybench and requests per second from oha. The multipliers are computed from those values. docs/benchmarks.md has the method, a sixth server, each tool's raw output and a feature comparison with seven other local agent-mail servers.

[Bar charts comparing Swarmail with mcp_agent_mail_rust, mcp_agent_mail, agentbus and Project Relay. Send p50: 0.46 ms, 39 ms, 70 ms, 2.3 ms and 2.5 ms. Search p50: 0.65 ms, 56 ms and 12 ms; agentbus and Project Relay have no search tool. Startup: 38 ms, 1.5 s, 0.86 s, 408 ms and 123 ms. Idle memory: 32 MiB, 190 MiB, 154 MiB, 81 MiB and 121 MiB.]

* 5,240 calls on the servers with search and 3,930 on agentbus and Project Relay, so their CPU multipliers compare fewer calls with Swarmail's 5,240.

Startup as hyperfine reports it. Each run starts the server, waits for its health check, then kills it; wrapper only is the harness without a server, and hyperfine's Relative column compares against that row:

hyperfine timed all six servers, so agent-inbox appears here but not in the table above. Its inbox pages hold 50 messages instead of 20, its roster is global and its search matches substrings, so its fetch, list and search do different work. docs/benchmarks.md has its full column.

mcp_agent_mail commits each send to a Git archive before it returns, so these numbers don't compare durability. Requests per second varied by up to 29% between rounds of the same server.

Development

bun installbun testbun 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.

Sponsoring

Swarmail is built and maintained by one person. If it saves you time, you can support it on Ko-fi.

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.