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.

Find Swarmail on Glama and MCPRush.

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. Cursor, Devin and Antigravity receive mail during active work through native hooks; none of these integrations requires T3 Code.

Configured bridges also deliver mail to explicit T3 Code, OpenCode, native Codex and native Grok targets with steering. They are opt-in foreground processes; installation does not start them. Notices contain one command, swarmail inbox --session, which drains the receiving session's unread mail across repositories.

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, 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 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).

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:

   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

   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:

   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:

   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.

   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. Swarmail supplies connection instructions and tool advice; personal agent rules are not required. swarmail --help shows the same briefing. docs/usage.md explains the details and client refresh limits.

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

Command	What it does
swarmail serve	Run the server
swarmail who [repo]	Agent names, session titles and edit checkouts, live sessions first
swarmail inbox, send, search	Read, send or search mail as this session
swarmail thread <id>	One thread's messages, oldest first
swarmail inbox --session	Drain this session's unread mail across repositories
swarmail wake-bridge <config.json>	Deliver notices to one explicitly configured local target
swarmail send <to> <subject> <body> --notification-policy quiet	Store normal/low informational mail without an automatic inbox hint
swarmail ping <agent>	Exit 0 if that agent's wake hook is waiting
swarmail register	The register hook; --tag prints the tag for a manual registration; --host <host> selects the host explicitly
swarmail hook wake <host>	The Claude Code and Cursor wake hook
swarmail hook context <cursor|devin|agy> [stop]	Check mail at a native context point without cancelling the task
swarmail hook rearm	The Claude Code re-arm on Windows, which has no POSIX shell to run the Linux one
swarmail guard	The git guard
swarmail version	The source hash the binary was built from
swarmail updates --session	Approved update targets and loaded attestations

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

Settings

Variable	Default	Effect
SWARMAIL_DB	~/.local/share/swarmail/mail.sqlite3	Database path
SWARMAIL_PORT	18765	Server port
SWARMAIL_URL	http://127.0.0.1:18765/mcp/	MCP endpoint for the command, the register hook and the swarmail-mcp relay
SWARMAIL_WAKE_URL	http://127.0.0.1:18765	Server base URL for the wake hook
SWARMAIL_SYNCHRONOUS	normal	full syncs every commit, at about 3 ms per send instead of 0.5 ms
SWARMAIL_ENABLE_MUTATIONS	0	Enable sender withdrawal/priority edits only after all readers are qualified; rollout and rollback requirements
SWARMAIL_RETIRE_DAYS	7	Retire idle agents and drop projects whose checkout is gone; 0 keeps both
SWARMAIL_GUARD	block	warn only reports, off skips
SWARMAIL_AGENT	from hook state	Name used by inbox, send, ping, guard and who
SWARMAIL_LIVE_ROOM	unset	A JSON heartbeat file (heartbeatAt, plus agentName, hostSessionId or t3Thread); while its heartbeat is under 5 minutes old, who flags the session it names
SWARMAIL_UPDATE_TARGETS	unset	Absolute approved-target manifest; quiet session checks and loaded attestations
SWARMAIL_UPDATE_HOLD	unset	1 holds update hints and attestations for a frozen session context

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. A retired agent comes back when it registers again or sends, reads mail or reserves files as itself.

Maintenance and retention

The server runs cleanup at startup and hourly. With SWARMAIL_RETIRE_DAYS above zero, it retires agents inactive longer than that many days. It also removes gone projects with no mail, active reservations or agent activity in the last day. Paths outside the user home are not judged gone.

Setting SWARMAIL_RETIRE_DAYS=0 preserves those agents and projects; it does not disable retry-key cleanup. Keys older than seven days are removed at startup and hourly regardless of this setting. A retry replays until its key is removed; afterward, it may perform the operation again. These sweeps do not delete message bodies. Account for expired retry keys before a server restart; retaining the database file alone does not guarantee every row survives startup.

Read-only MCP requests do not suspend cleanup. A client that starts a server instance triggers its startup sweep; an existing server can sweep hourly while qualification runs. Check expired retry keys before preservation-sensitive qualification. A row disappearing during a read-only check alone does not identify which process or sweep removed it.

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.

Host	Session id in the shell
Claude Code	$CLAUDE_CODE_SESSION_ID
Codex	$CODEX_THREAD_ID
Cursor	$CURSOR_CONVERSATION_ID
OpenCode, Devin	none; their hooks carry it

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 short instruction to read the session inbox. 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. Hosts without a wake integration see mail on their next fetch_inbox.

Native Cursor uses postToolUse additional context; Devin uses PostToolUse additional context and its own user configuration. Antigravity uses a PreInvocation user message. Checks are bounded and use GET /wait?session=<id>&timeout=0, which returns immediately and keeps the wake cursor's delivery suppression. Message bodies and read/ack receipts stay in Swarmail. Devin and Antigravity can continue at Stop when new mail is already present; their synchronous hooks cannot wait indefinitely for mail arriving after the session has stopped. These hooks add information; the receiver decides whether its current task needs to pause.

Quiet mail remains available through inbox and search. Automatic wake paths and mailbox snapshots exclude it; see notification policy.

Adapters can revalidate a queued hint with GET /wait/peek?session=<id>. It returns { "mailboxes": [{ "recipient": "GreenCastle", "project": "/w/project" }] } for the session's wake-eligible unread mail, including mail already offered. High and urgent mailboxes come first. This request leaves waiters, wake cursors, read receipts and pings unchanged. Unknown or retired sessions return an empty list. More than 1000 mailboxes returns HTTP 503 instead of a partial snapshot.

Feature comparison

These are the six servers measured below. Features describe the pinned source versions in the detailed comparison, which also covers runtimes, platforms, licences and two additional servers.

Server	Roster scope	Search	Threads and receipts	File reservations	Wakes an idle session
Swarmail	Per repository; worktrees share it	Full text (FTS5)	Threads; read, acknowledged and delivery receipts	Advisory, with a git guard	Claude Code and Cursor
mcp_agent_mail_rust	Per project path	Full text (Tantivy)	Threads; read, acknowledged and delivery receipts	Advisory, with a git guard	No; Claude Code hooks check the inbox during a turn
mcp_agent_mail	Per project path	Full text (FTS5)	Threads; read and acknowledged	Advisory, with a git guard that is off by default	No; Claude Code, Codex and Factory hooks check the inbox during a turn
agent-inbox	One per hub	Substring, at most 25 results	Threads; read state	None	Claude Code (opt-in hook), opencode, omp
agentbus	One per server	None	No threads; reading moves a cursor	None	No; an agent waits in recv for up to 600 s
Project Relay	Per project	None	Replies; acknowledged per recipient	Advisory leases	No; an inbox read waits for up to 25 s

An inbox read that waits for mail needs an agent already running. It does not wake an idle session.

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. 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.

Each chart ranks its metric from best to worst. Lower values are better in all four charts. The tables below put servers in rows and units in headings. Rust Agent Mail and Python Agent Mail are mcp_agent_mail_rust and mcp_agent_mail.

Latency

Median latency in milliseconds; lower is better. Rows are sorted by send latency. Search has a different ranking. A dash means no search tool.

Server	Send ↓	Inbox ↓	Search ↓
Swarmail	0.46	0.50	0.65
agentbus	2.3	1.1	—
Project Relay	2.5	1.4	—
Rust Agent Mail	39	12	56
Python Agent Mail	70	23	12

Throughput

Requests per second with eight clients; higher is better. Rows are sorted by send throughput. Search has a different ranking.

Server	Send ↑	Inbox ↑	Search ↑
Swarmail	5,500	6,300	4,300
agentbus	876	1,600	—
Project Relay	482	1,100	—
Rust Agent Mail	50	416	90
Python Agent Mail	9.7	27	44

Memory

Resident memory in MiB; lower is better. Rows are sorted by idle memory. Peak memory has a different ranking.

Server	Idle ↓	Peak under load ↓
Swarmail	32	67
agentbus	81	96
Project Relay	121	296
Python Agent Mail	154	258
Rust Agent Mail	190	688

CPU usage, seed-loading times and the full ratios are in the detailed results.

Startup

Mean startup time in milliseconds across 20 hyperfine runs; lower is better. Rows are sorted by mean time. Each run starts the server, waits for its health check, then stops it.

Server	Mean ± SD ↓	Min	Max
Swarmail	37.7 ± 0.7	37.1	39.3
Project Relay	122.9 ± 5.3	113.8	132.7
agent-inbox*	263.0 ± 11.4	252.0	297.9
agentbus	408.2 ± 15.6	385.1	443.3
Python Agent Mail	857.7 ± 38.7	803.5	925.7
Rust Agent Mail	1530.3 ± 14.9	1509.6	1562.8

The harness without a server takes 7.2 ± 0.6 ms (min 6.6, max 8.5). Hyperfine's raw Relative column compares against that harness, not Swarmail; see the startup output.

* agent-inbox appears only in startup here. 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. The detailed comparison 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

Archived validator research records the generated/runtime comparison, TypeBox compatibility findings and the limits of the measurements for the tested mail contracts.

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, the dependency manifest or lockfile, 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 sponsor it monthly or once through GitHub Sponsors, or leave a tip 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.