MCP Agent Mail is email for agents. Synapse is a live link between them. Agent Mail gives each agent a registered identity, an inbox, threaded messages and advisory file leases, stored in SQLite and a Git archive. Synapse finds the agents already running on your machine and, when you link two, injects one agent's recent conversation into the other's next prompt, with messaging on top. Agent Mail is stronger for big swarms in one repo. Synapse needs no registration, tokens or server to manage.
At a glance
| Synapse | MCP Agent Mail | |
|---|---|---|
| What it is | Canvas and messaging layer for every agent on your machine | MCP server with agent identities, inboxes, threads and file reservations |
| License | MIT | "MIT with OpenAI/Anthropic Rider", not OSI approved |
| Price | Free | Free (a commercial Companion stack is mentioned, pricing not public) |
| Account | None | None, local bearer token |
| Runs as | Local canvas plus hooks and MCP, no network calls | Local HTTP server on 127.0.0.1:8765 |
| Setup per agent | None, found by process scan | Each agent registers with a project key |
| Shares conversation automatically | Yes, up to 8,000 characters per link per prompt | No, only what an agent writes into a message |
| Delivery | Pull (next prompt, or inbox tool) | Pull, with optional hooks that remind about unread mail |
| File leases and commit guard | No | Yes |
| Durable, searchable history | No | Yes, SQLite search plus Git archive |
| Platforms | macOS first (core is plain Node 18) | Linux, macOS, Windows, Docker |
MCP Agent Mail facts are from its Rust repo and the original Python repo, checked on 16 October 2026.
What MCP Agent Mail is
MCP Agent Mail, by Jeffrey Emanuel, calls itself "Gmail for your coding agents". It is an MCP server that gives agents memorable identities like GreenCastle or BlueLake, then lets them send each other threaded messages with subjects, CC, importance and read receipts. Everything is stored in SQLite and a Git audit archive, so you can search it later.
It is large. The Rust version lists 45 MCP tools and 25 read-only resources, a 16-screen terminal console, a web UI at /mail, and a "Human Overseer" composer for messaging agents as yourself. It works with Claude Code, Codex CLI, Gemini CLI, Copilot CLI and any MCP client.
In 2026 the project was rewritten in Rust. The Python README now says it is superseded by mcp_agent_mail_rust, which ships near-weekly releases. It sits alongside Beads, a task tool from the same author.
What Synapse is
Synapse puts every live coding agent on your Mac on one canvas. Drag a line from agent A to agent B, and on every prompt B gets what A has said since last time, up to 8,000 characters per link. Any two linked agents can also message each other through six MCP tools, a CLI or a skill.
It finds Claude sessions from Claude Code's own session registry and every other CLI by scanning processes. Identity comes from the process tree, so there is nothing to register.
How MCP Agent Mail and Synapse handle agents talking to each other
MCP Agent Mail:
- Setup: each agent calls
ensure_projectandregister_agentwith an absolute repo path. The README ships a blurb for AGENTS.md or CLAUDE.md so agents remember. Messaging across repos needs a contact handshake. - What gets shared: only what an agent writes. Messages live in the archive, deliberately not in agent context windows, which keeps token use low.
- Delivery: pull. An agent sees mail when it calls
fetch_inbox. Optional hook scripts for Claude Code and Codex check every 120 seconds by default and inject an "unread mail" reminder. They need the project, agent name and a registration token in the environment. - Identity: self-declared at registration. The docs say it does not authenticate the model, provider or process.
Synapse:
- Setup:
node install.mjsonce. It wires an MCP server into each agent CLI it finds, plus a prompt hook for Claude Code and Codex. Agents already running need one restart. - What gets shared: a link carries the source agent's recent conversation automatically. Nobody has to write a summary. Shared context is labelled as coming from peers, and agents are told not to treat it as user instructions.
- Delivery: pull. Claude Code and Codex see messages on their next prompt; other CLIs call
synapse_inbox. An idle agent does not wake up. - Which agents: Claude Code and Codex fully, Gemini CLI with readable context, and OpenCode, Cursor Agent, Amp and Copilot CLI for messages.
The short version: Agent Mail is deliberate and structured. Synapse is ambient and automatic.
Open source, accounts and privacy
Both run locally and need no account.
MCP Agent Mail's license is MIT text plus a rider that grants no rights to OpenAI, Anthropic, their affiliates or anyone acting for them, and forbids use in their training or eval data. GitHub reports it as NOASSERTION. Because of the rider it is not an OSI-approved open source license. If your company has a strict license policy, check it first.
Synapse is plain MIT with a zero-dependency Node core. Links and messages live in ~/.synapse. There is no server port or token to manage, and Synapse makes no network calls.
Where MCP Agent Mail is ahead
- File leases. Agents reserve globs with a time limit, and pre-commit and pre-push hooks block commits that touch another agent's exclusive lease. This is real conflict avoidance for many agents in one repo. Synapse has nothing like it.
- Durable, searchable history. Threads, acks, receipts and search over a Git archive. Synapse keeps no message archive like that.
- Structure for big swarms. Build slots, a cross-repo "product bus", Beads integration, an operator console and web UI.
- Platforms. Linux, macOS and Windows binaries today. Synapse is macOS first.
- Maturity. Agent Mail has thousands of GitHub stars and steady releases. Synapse is a new project with a small community.
To be fair about rough edges too: the Rust repo has open issues about SQLite corruption under heavy swarm load, and the Python version had registration and timeout issues. The project ships fixes often.
Using Synapse with MCP Agent Mail
Both are MCP servers, so you can install them side by side in Claude Code and Codex.
- Use Agent Mail for durable task threads and file leases in long multi-agent runs.
- Use Synapse for live context: link the implementer to the reviewer so the reviewer always sees the latest work without anyone writing a message.
- Use
synapse sendfor quick questions, and Agent Mail threads for anything you want on the record.
Who should pick which
- Pick MCP Agent Mail if you run many agents in one repo, need file locking, and want an auditable message history.
- Pick Synapse if you want agents to know what each other is doing with no registration, tokens or AGENTS.md blurbs, and you want a plain MIT license.
- Run both if you have a swarm that needs leases and agents that need live context.
FAQ
Is Synapse an MCP Agent Mail alternative?
For live context between running agents, yes. Synapse shares a linked agent's recent conversation automatically and needs no per-agent registration. For file reservations, threaded mail and searchable history, no. MCP Agent Mail does those and Synapse does not.
Is MCP Agent Mail open source?
Its code is public and free, but the license is MIT plus a rider that excludes OpenAI, Anthropic and their affiliates. Because of that restriction it is not an OSI-approved open source license. Synapse is plain MIT.
Does MCP Agent Mail share context between Claude Code and Codex automatically?
No. MCP Agent Mail only carries what an agent writes into a message, and agents must check their inbox. Optional hooks remind agents about unread mail. Synapse links inject one agent's recent conversation into another's next prompt.
Does MCP Agent Mail have file locking?
Yes. Agents can reserve files by glob with a time limit, and pre-commit and pre-push guards block commits that touch another agent's exclusive lease. Synapse has no file locking.
What is mcp_agent_mail_rust?
It is the Rust rewrite of MCP Agent Mail, started in February 2026. The original Python repo now says it is superseded by the Rust version, which is recommended for new installs.
Is Synapse free?
Yes. Synapse is free and MIT licensed, with no account and no paid tier.