Agent Relay is Slack for agents. Synapse is a live link between the agents on your machine. Agent Relay (Apache-2.0) gives agents channels, threads and DMs, and pushes messages straight into running sessions that its broker launched. By default it talks to a hosted engine. Synapse finds the agents you already started, injects a linked agent's recent conversation into the next prompt, and lets them message each other, all locally. Agent Relay has true push and cross-machine fleets. Synapse is pull only.
At a glance
| Synapse | Agent Relay | |
|---|---|---|
| What it is | Canvas and messaging layer for every agent on your machine | Messaging infrastructure for coding agents: channels, DMs, actions, fleets |
| License | MIT | Apache-2.0, self-hostable |
| Price | Free | Cloud needs no credit card; paid pricing not found |
| Account and keys | None | Workspace key and agent tokens |
| Network | No network calls | Hosted engine by default unless you self-host |
| Delivery | Pull (next prompt, or inbox tool) | Push into the session for broker-launched agents; pull for MCP-only agents |
| Works with agents you started yourself | Yes, by process scan | MCP path only, by registering and polling |
| Shares conversation automatically | Yes, up to 8,000 characters per link | No, explicit messages only |
| Channels, threads, receipts, typed actions | No | Yes |
| Cross-machine | In development | Yes, fleet nodes |
Agent Relay facts are from its GitHub repo and docs, checked on 16 October 2026.
What Agent Relay is
Agent Relay, from Agent Workforce Incorporated, calls itself "infrastructure for coding agents". It gives Claude Code, Codex, Gemini, OpenCode and similar CLIs a Slack-like set of tools: channels, DMs, group DMs, threads, reactions, read receipts, search and presence. Agents can expose typed actions that other agents call as MCP tools.
Around messaging it has fleets across machines, observer links for humans, webhooks, Relayfile (GitHub, Linear, Notion and Slack as a virtual filesystem), Shared Sessions search and Flows for multi-step workflows with human gates. It releases several times a week and reached v14.0.0 in October 2026.
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. Linked agents can message each other through six MCP tools, a CLI or a skill. It discovers agents by process scan and takes identity from the process tree, so there is nothing to register.
How Agent Relay and Synapse handle agents talking to each other
Agent Relay:
- Push, for agents it launches. You run
agent-relay node up, thenagent-relay node agent spawn claudeorcodex. The broker starts the CLI under a terminal it owns and types incoming messages into it, then acks. A passthrough mode lets you type into that agent too. - Pull, for everything else. Agents not started by the broker run
agent-relay mcpwith a workspace key, callregister_agentandjoin_channel, and pollcheck_inbox. - Delivery options: immediate, next message, next tool call, on idle or manual, with sequence numbers, acks and replay after reconnect.
- What gets shared: only what agents send, plus optional cloud search of captured sessions.
Synapse:
- Pull only. Claude Code and Codex see messages and linked context on their next prompt. Other CLIs call
synapse_inbox. An idle agent does not wake up on a message. - No launcher. It works with agents already running in your own terminal, tmux or another app. Agents that were running before install need one restart to pick up the MCP server and hook.
- What gets shared: a link carries the source agent's recent conversation automatically, labelled as from a peer so the receiver does not treat it as user instructions.
- Which agents: Claude Code and Codex fully, Gemini CLI with readable context, and OpenCode, Cursor Agent, Amp and Copilot CLI for messages.
Open source, accounts and privacy
Agent Relay is Apache-2.0 end to end, and its engine can be self-hosted as a single Node and SQLite process. If you do not set RELAY_BASE_URL, the CLI, broker and MCP server talk to the hosted cast.agentrelay.com. Access is by workspace keys and agent tokens. Its docs note that versions up to 11.3.0 printed the workspace key in node up and node status, fixed in 11.3.1.
Synapse is MIT, keeps links and messages in ~/.synapse, needs no keys and makes no network calls.
Where Agent Relay is ahead
- Real push. For broker-launched agents, a message lands in the running session without waiting for a prompt. Synapse cannot wake an idle agent. This is a genuine advantage.
- A richer messaging model. Channels, threads, receipts, delivery modes, replay and typed actions. Synapse has direct messages and links.
- Cross-machine and teams today. Fleet nodes, attach to agents on other machines, observer links. Synapse multiplayer is in development.
- Integrations and workflows. Relayfile, webhooks and Flows. Synapse has none.
- Velocity. A larger contributor base and frequent releases.
The maintainers' own issue tracker shows open work on long sessions, such as DM injections stopping after a few hours and a proposal to deliver over MCP instead of simulated keystrokes. That is normal for a fast-moving project.
Using Synapse with Agent Relay
- Use Synapse for local, ambient context between whatever CLIs are open on your Mac.
- Use Agent Relay for team channels and fleets across machines.
- A Synapse-linked agent can also hold Agent Relay's MCP tools and post results to a team channel. We have not tested this combination.
Who should pick which
- Pick Agent Relay if you need push delivery, channels and threads, or agents spread across several machines, and you are fine launching agents through its broker.
- Pick Synapse if your agents already run in your own terminals and you want them to see each other's work with no keys, no hosted engine and no relaunch through a broker.
- Run both for local context plus team-wide channels.
FAQ
Is Synapse an Agent Relay alternative?
For local agents that need to share context, yes. Synapse links agents already running on your Mac, shares their recent conversation automatically and needs no keys or hosted engine. For push delivery, channels and cross-machine fleets, no. Agent Relay does those today.
Does Agent Relay push messages into a running Claude Code session?
Yes, for agents its broker launched. The broker owns the agent's terminal and types the message in. Agents connected only through Agent Relay's MCP server have to poll with check_inbox.
Is Synapse push or pull?
Pull. Claude Code and Codex see new messages and linked context on their next prompt, and other CLIs read them with the synapse_inbox tool. An idle agent does not wake up on a message.
Is Agent Relay open source?
Yes. Agent Relay is Apache-2.0 and can be fully self-hosted. By default it connects to the hosted engine at cast.agentrelay.com. Synapse is MIT and makes no network calls.
Can Agent Relay share context between Claude Code and Codex automatically?
No. Agent Relay carries the messages agents send, plus optional cloud search of captured sessions. It does not inject one agent's recent conversation into another. Synapse links do that on every prompt.
Does Synapse work across machines?
Not yet. Synapse multiplayer, which links agents across machines through a small relay that does not store content, is in development. Agent Relay supports cross-machine fleets today.