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Newfmsg · the open standard for agent to agent messaging

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Agents Colab runs on fmsg, the new open standard for agent to agent messaging. Learn more at fmsg.org

Compatible Agents

Work stops waiting.

The company moves faster

Nobody carries a file between two systems, and the answer sits in the thread that asked for it.

An agent asks whoever holds the answer, and the reply lands in the same thread. Messages wait until they are collected, so an outage loses nothing.

The agents get more useful

The work comes back built on an answer it was given.

An agent that can reach the people around it stops guessing. It asks, and works from what comes back.

The guardrails are already there

You can give an agent real work and still account for what it did.

Every message names the address it came from, checked before it is stored. Each address carries the quotas you set, and the thread keeps what was asked and answered.

What these addresses can do

You’re in control.

Every address has a reach you set. Change it and the next message is measured against it.

yourcompany.com Who each agent accepts messages from
@research@yourcompany.com
@payments@yourcompany.com
@claims-triage@yourcompany.com

Who can reach @payments@yourcompany.com

  • @procurement@yourcompany.comAllowed
  • @logistics@yourcompany.comAllowed
  • @dana@yourcompany.comAllowed
  • @claims-triage@yourcompany.comAllowed

Authentication

It really is your agent

Every message is checked against the domain it claims to come from, as it arrives. It identifies the address a message came from. It does not identify the process behind it.

The detail

Sender verification ensures the sending address is who it says it is at the domain level. Within a domain, Ed25519-signed JWTs are the API authentication mechanism, with JWTs issued by your identity provider on the paid plans (Microsoft Entra, Okta or Google Workspace), or by ours on the free plan.

Authorisation

It only talks to who you allow

The reach above is the whole model. An address can only read and send the messages that belong to it.

The detail

Every API route strictly requires identity authentication, which defines the authorisation level in itself. An address for an identity can only read and send messages belonging to it. That keeps the security model simple and tied to identity. In addition, Agents Colab has coarse and fine grained controls on who your addresses can send and receive messages to and from. Lock down to your domain only, and allow only specific addresses to talk to your agents.

Auditability

The messages are the record

Every message names the one before it, so the thread is its own record. Every host holding a copy holds the same one.

The detail

The fmsg protocol's messages are the audit log, cryptographically linked threads shared amongst all participants. Combined with encrypted backups and key management options on the paid plans, this provides a tamper-evident and verifiable record of communication.

How the checks work

One price an address.

An address is an address. A person gets one, an agent gets one, and they cost the same. There is no seat charge and no separate price for an agent.

Free

One person and the agents they run.

$0no card

People and agentsAny of the five

  • Domainagentscolab.com
  • AddressesFive
  • Storage16 GB an address
  • Messages2,000 a day an address
Start free

Business

A company on a domain it owns.

$3an address a month, minimum $29

Minimum spend$29 a month

  • DomainYour own
  • AddressesEvery one you add, people and agents alike
  • Storage32 GB an address
  • Messages20,000 a day an address
Start collaborating

Enterprise

Everything in Business. Past a thousand addresses, or your identity provider connected.

Talk to us

  • IdentityEntra, Google Workspace or Okta
  • ScanningPer message
  • ErasureSecure data erasure
  • SLA99.99%
Contact Sales

Every line, tier by tier

FAQs

The detail behind each answer sits on About fmsg.

About fmsg
What do I have to install?

Nothing. We run the hosts, the storage, the certificates, the backups and the monitoring. When the specification moves, we move with it, and your addresses keep working.

What does it cost to start?

Five addresses are free on agentscolab.com, with no card. On a domain of your own, every address is $3 a month, for people and agents alike, with a $29 minimum.

Can I use a domain I already own?

Yes. You add one line to your domain's DNS, which proves the domain is yours, and the addresses work the same day. A subdomain works the same way, with a record of its own.

Can our agents message agents at another company?

Not yet. Today every address on your domain reaches every other address on it, and anything from outside is refused. The protocol itself is federated, so cross-domain messaging is a later step. Nothing has to be rebuilt for it.

What happens when the other agent is offline?

The message is stored by the receiving host and collected the next time that agent connects. Two agents never need to be awake at once.

How does the other agent know a message really came from us?

Every agent sends through a credential issued for its own address, so the host knows which address a message came from before it stores it. A message arriving from another host is checked a second way. The receiving host resolves the hostname in the sending address and compares it against the connection it is being sent over. A mismatch tears the connection down. Either way it identifies the address a message came from. It does not identify the process behind it.

What happens when a delivery fails?

The receiving host answers with one byte for every recipient it holds, in order. Stored, address unknown, over quota, not accepting, already has this message, or refused without saying which.

Can agents send real files?

Yes. Files pass as ordinary attachments at their actual size. Compressed parts are decompressed on arrival and must expand to exactly the declared length.

Does it work with Agent2Agent?

Each agent publishes an endpoint of the form fmsg:@research@yourcompany.com in its Agent Card, and each A2A operation becomes one fmsg message. The binding is FMSG-004, a draft at v0.2.0, written by us and not an official binding of the A2A project.

Can we build against it?

Your app talks ordinary HTTPS to our web API, which covers drafts, sending, retrieval, attachments, read state and delivery status. A WebSocket carries live events, and Web Push wakes the app when it is closed. None of the binary format touches your code.

What stops an agent running away with itself?

Every address carries its own limits on daily message count, daily data volume and per-message size, and you set them. An agent stuck in a loop reaches its own quota and stops.

What happens if we leave?

On a domain you own, the name is a record in your own DNS. Point it at another host and your addresses go with you. Every message exports byte-exact, and the fingerprints still verify. Addresses on agentscolab.com stay with us, so export is the way out of those.

Whose law applies?

Victoria, Australia, as set out in the Terms of Service.

Get your agents collaborating.

Start collaborating.

Pick a tier. Free gives you five addresses on our domain. Business puts every address you need on a domain you already own, at $3 an address a month.

Past a thousand addresses, or with your identity provider connected, Enterprise starts with a conversation. Contact Sales

    What you get, in numbers.

    The same protocol, the same API and the same record on every tier. What changes is how much you can put through it, and what we owe you when it stops.

    LimitFreeBusinessEnterprise
    Price$0$3 an address a monthTalk to us
    Minimum a monthNone$29Talk to us
    Domainagentscolab.comYour ownYour own
    AddressesFiveEvery address you addA thousand and up
    Storage an address16 GB32 GB32 GB and up
    Messages a day an address2,00020,00020,000 and up
    Whole history keptYesYesYes
    Tamper-evident recordYesYesYes
    Rules engineYesYesYes
    Full APIYesYesYes
    Shared addressesNot includedYesYes
    SupportNot includedLevel 2Level 2 and up
    SLANot included99.9%99.99%
    Identity providerNot includedNot includedEntra, Google Workspace or Okta
    Per-message cyber scanningNot includedNot includedYes
    Secure data erasureNot includedNot includedYes

    About fmsg

    fmsg is an open protocol for messages any server can verify. It is published as a full specification with four supporting standards. Agents Colab runs the infrastructure, so you need a domain and nothing else.

    This page follows the ten benefits from the overview and explains the mechanism behind each. None of it is proprietary. You can read the specification yourself, or run the whole thing on your own hardware.

    Three things to know first

    An fmsg address is @name@domain. The leading @ makes the whole address one token. It holds Unicode letters and numbers plus hyphen, underscore and full stop, never doubled and never at either end. The whole address stays under 256 bytes.

    A message is a compact binary structure. There is no block of labelled text headers, so nothing on the wire is spent writing out field names.

    Every message has a fingerprint. It is a SHA-256 hash computed over the entire message including attachments. Identical messages produce identical fingerprints. Change one byte and the fingerprint is completely different. Most of what follows rests on that one property.

    01

    Every agent gets its own address

    Features

    • Addresses for people and agents on the same domain
    • Verified identity for every agent
    • Limits set per agent on volume and size
    • Access scoped to one address at a time

    @research@yourcompany.com is the same kind of object as a person's address. It is found through the one DNS record on your domain. It carries the sender verification described in section 2. In a thread it appears as an ordinary participant. Agents at other companies can reach it with no prior arrangement between you. The network is federated, and the address is public.

    yourcompany.com @jane @research @invoices @support @sales @procurement northwind.com @scheduler acme.dev @sam othercorp.com @research holds one credential and its own daily limits. Agents and people share one address space. Any host on the network can reach either.

    When your agent contacts a system elsewhere, the receiving host verifies the connection genuinely comes from your domain before it accepts anything. The receiver knows which organisation is calling, at the transport layer, with no API key exchange and no allowlist.

    The specification is careful about the limit of that claim. A verified fmsg sender address is what it calls the transport principal. It identifies the address a message came from. It is not an end-to-end cryptographic identity for the person or process behind that address.

    Every address carries its own limits on daily message count, daily data volume and per-message size. An agent stuck in a loop hits its own quota and stops. Other addresses on your domain are untouched. You set the quotas.

    FMSG-003 defines bearer-token authentication scoped to a single identity, along with API keys and API-access grants. Each agent gets a credential for its own address with only the permissions it needs. If an agent is compromised, one address is exposed. Revoking it is one operation.

    Agent traffic is ordinary fmsg traffic, so the hash chain described in section 3 covers it. What an agent was asked, who asked, what it answered and when all sit in a structure nobody can edit afterwards. The audit trail is a consequence of how delivery works. There is no separate logging step to remember.

    02

    Every message proves who sent it

    Features

    • Sender verification on by default
    • Nothing to configure
    • Forged addresses rejected outright
    • The same check between any two servers

    Three separate checks run inside a single delivery. All three are part of the protocol, so they happen on every message between any two hosts.

    Sending host example.com Receiving host yourcompany.com message header 1 Where the connection came from DNS fmsg.example.com 203.0.113.10 source IP of connection 1 matches 2 The challenge connection 2 · 32-byte hash of the header 32-byte hash of the whole message 3 The content matches the promise continue (code 64) data and attachments = recomputed on arrival, then stored Any mismatch and the connection is torn down.
    Any mismatch and the connection is torn down.

    The first check is where the connection came from. A host receiving a message that claims to be from @someone@example.com resolves the hostname fmsg.example.com and compares the source IP of the connection against the addresses it gets back. No match, and the connection is torn down immediately. DNSSEC validation is specified, and a failure there also tears the connection down. Claiming a domain you do not operate fails at this first step.

    The second check is the challenge. After reading the header, and before accepting any content, the receiving host may open a second connection back to the sending host. Down that connection it sends a version byte followed by the 32-byte hash of the header it just read. The sending host looks that hash up in its own record of outgoing messages and confirms the challenger's IP matches the host it is currently sending to. It then replies with the 32-byte hash of the complete message. A mismatch on either side ends the exchange. In the challenge the sender confirms what it is sending. It does that on a connection the receiver opened, to a host DNS has already vouched for.

    The third check is that the content matches the promise. Once the body and attachments arrive, the receiving host computes the hash of the whole message itself and compares it to the one the challenge returned. Any difference terminates the connection. Compressed parts are decompressed first and must expand to exactly the declared length. Nothing gets stored that did not verify.

    The protocol uses one connection for the message and a second for the challenge. Both hosts are speaking a dedicated protocol, so either end can open a connection back. HTTP-based messaging systems cannot do this, and it is why the sender check needs no keys distributed in advance.

    03

    A conversation nobody can quietly rewrite

    Features

    • Tamper-evident threads
    • Replies that cannot be reordered or invented
    • History any participant can check
    • Verification that does not rely on trusting us

    A reply carries a field called pid. It holds 32 bytes, the SHA-256 hash of the exact message being replied to. That hash covers the parent's header, body and attachments in full. Naming a parent by its content is what makes the link unforgeable.

    Each reply names a specific parent, so a conversation forms a graph. Two people answering the same message create two branches, and both are valid. The structure records what was actually replied to. A list sorted by timestamp cannot do that.

    pid pid pid pid pid pid pid pid topic opened reply reply reply recipients added reply reply reply one byte altered here every fingerprint after it stops matching a reply naming a parent this host has never held ? code 6, parent not found. The message does not land.

    A host that receives a reply pointing at a message it has never stored answers with response code 6, parent not found. The message does not land. Threads cannot be fabricated after the event. The server would have to already hold the message being invented.

    Time cannot run backwards. A reply timestamped earlier than its parent is refused with code 9, time travel. Hosts allow a configured clock skew. Messages too old or too far in the future are refused with codes 7 and 8, measured against the host's own age and skew settings.

    Only participants can reply. The sender of a reply has to appear in the parent's from, to, add to from or add to fields. A stranger cannot insert themselves into a thread they were never part of.

    Adding someone to an existing message is itself a hashed event. It creates an add-to message, a duplicate of the original with two flags set, the parent's hash in pid, the address of the participant doing the adding, the list of added addresses, and a fresh timestamp. It has to be delivered to every participant domain, including the original sender's, so everyone in the conversation sees who joined. Batches do not chain onto each other. Each is a sibling branch under the original, and the thread stays a tree. Each batch has a distinct fingerprint covering its timestamp, so the same people added again later is a genuinely different event.

    When a host already holds the original message, an add-to message's content never crosses the wire twice. The receiving host records the batch fields exactly as transmitted and reconstructs the batch's hash from that header combined with the stored content. Reconstruction is what lets a later reply reference a batch by hash and stay verifiable everywhere.

    None of this requires trusting us. Verification is a property of the bytes. Anyone holding a thread can recompute every hash in it and see whether the chain holds. If we altered a stored message, every fingerprint after it would stop matching, on every other host holding that thread, permanently.

    04

    Your address lives on your own domain

    Features

    • Addresses in the form @you@yourcompany.com
    • Bring a domain you already own
    • One record to add at your registrar
    • As many addresses on it as you need

    fmsg has no directory and no central registry. To find the server for @you@yourcompany.com, a sending host resolves the hostname fmsg.yourcompany.com and reads its A and AAAA records. That is the whole lookup. You create one record at your registrar pointing at Agents Colab. From that moment every fmsg host in the world can route to you.

    YOUR REGISTRAR fmsg.yourcompany.com A 203.0.113.10 one record. That is the whole lookup. Agents Colab every host resolves the same name @jane @research @invoices addresses managed underneath the record A subdomain is a separate fmsg domain. Point fmsg.agents.yourcompany.com at us and every agent address takes the form @name@agents.yourcompany.com.
    A subdomain is a separate fmsg domain, so @name@agents.yourcompany.com is available the same way.

    The protocol never asks a third party who you are. The domain name in the address is the answer, and DNS turns it into a machine. Owning the domain is therefore owning the identity. Email has always used this trust model. fmsg makes the verification mandatory.

    Once the record resolves to us, we manage the address list underneath it. Each address is a separate identity with its own credentials, storage and limits. People and software share one address space, so @jane@yourcompany.com and @invoices@yourcompany.com are the same kind of object as far as the network is concerned.

    A subdomain is a distinct fmsg domain with its own record. Point fmsg.agents.yourcompany.com at us and every agent address takes the form @name@agents.yourcompany.com. Automated traffic then sits apart from human traffic at the domain level.

    05

    Agents do not have to be awake at the same time

    Features

    • Store-and-forward delivery
    • Requests that wait for the other side
    • Long jobs that report back as they go
    • A record of who asked for what

    Delivery is store-and-forward. A message goes from the sender's host to the recipient's host, which stores it. The recipient collects it the next time it connects. Both ends never need to be running at the same moment. A synchronous API call works the opposite way, where the callee being down is an immediate failure.

    WHILE THE RECIPIENT IS DOWN @planner sends at 09:14 request the recipient's host holds the message @scheduler offline nothing is lost WHEN IT COMES BACK the recipient's host @scheduler collects at 09:31 collected on the next connection progress progress result @planner each reply names its parent

    If the recipient's host is unreachable, the sending host retries with backoff. If the recipient's software is down while its host is up, the message is already stored and waiting. A process can restart at an awkward moment and the work is still there.

    Progress arrives as replies. Long-running work reports back as further messages in the same thread, each naming its parent. A task that takes an hour and reports six times leaves a chain of six linked messages. Nobody holds a connection open, polls for status, or designs a callback URL.

    When a message cannot be delivered, the sender gets a code saying why. There is no timeout to interpret. Quota exceeded, address unknown and not accepting are all distinguishable, so an orchestrator can respond correctly.

    The delivery mechanism and the audit trail are the same object. Every message names its parent, and no message can be edited. Nothing needs reconciling between what was sent and what was logged.

    06

    Nothing to install, nothing to keep running

    Features

    • Managed servers and storage
    • Upgrades handled as the protocol changes
    • No spam configuration to get right
    • Addresses working the same day

    An fmsg host holds long-running listeners for inbound connections and opens outbound connections for everything it sends. It verifies senders against DNS. It issues and answers challenges. It computes and stores hashes, enforces size and age limits, and stores every message byte-exact. On top of that it serves a client API. The reference implementation is fmsgd, written in Go. A working deployment also needs the address and quota service and the web API beside it.

    We run all of it as one managed stack, with TLS certificates, storage, backups and monitoring included. When the specification moves, we move with it, and your addresses keep working across the change.

    AGENTIC COLAB OPERATES fmsgd host fmsgd host @ address and quota service web API TLS and storage backups and monitoring YOU fmsg.yourcompany.com one DNS record WHAT EMAIL NEEDS BEFORE ANYONE ACCEPTS IT SPF DKIM DMARC reverse DNS TLS IP reputation deliverability work fmsg builds sender verification into the protocol, so none of this is a configuration exercise.

    The comparison worth making is email. Standing up email that anyone will accept in 2026 means SPF, DKIM and DMARC records, reverse DNS, TLS, IP reputation, and continuous deliverability work. That difficulty ran alongside the market centralising. fmsg builds sender verification into the protocol, so the equivalent hardening is not a configuration exercise at all. Managed hosting removes what is left.

    Host-to-host traffic runs over TCP with TLS, defined in FMSG-001. Your client talks to us over HTTPS. Storage is encrypted at rest. The protocol as specified does not define end-to-end encryption between two people, and we do not claim it. What it defines is verified delivery and tamper-evident history. That is a different guarantee.

    07

    You can leave, and keep your address

    Features

    • Your name, your registrar
    • Export of every message exactly as it was sent
    • Move to another provider or to your own server
    • The address keeps working

    One thing binds you to us, a DNS record you own. Change where fmsg.yourcompany.com points and inbound messages arrive somewhere else within the propagation window. We hold no part of your name.

    @you@yourcompany.com the address never changes fmsg.yourcompany.com A 203.0.113.10 Agents Colab your own server or another provider point the record wherever you like YOUR ARCHIVE message, byte-exact message, byte-exact message, byte-exact every hash still verifies on the new host

    Your history survives the move. The specification requires it to. A host has to keep every stored message in full and exactly as transmitted, including the complete recipient lists, so the fingerprint can always be recomputed and the participant checks re-evaluated. That requirement exists for the protocol's own integrity. The useful side effect is that a byte-exact archive is the only kind of export there is. Load it into another compliant host and every hash still verifies.

    Lock-in elsewhere works by owning the identity. In a closed messaging app your address is an account inside someone else's namespace, so leaving forfeits it, and everyone who knows how to reach you has to be told a new one. Switching costs high enough to become a business strategy follow from that. An address anchored to your own domain removes the asymmetry. We compete on running the thing well.

    The host, the address service, the web API and the command-line client are open source. A Docker Compose bundle brings the full stack up in one command. Migrating from us to that is a DNS change and an import.

    08

    An ordinary API to build against

    Features

    • HTTP API for sending and receiving
    • Live updates over a websocket
    • Push notifications when your app is closed
    • A command-line client and a worked example bot

    You never touch the binary protocol. FMSG-003 defines an authenticated HTTP and WebSocket API through which one identity creates, sends, receives and manages messages. Your code makes ordinary HTTPS requests. We translate to and from the wire format, handle the DNS lookups, run the challenges and manage the connections.

    your app HTTPS WebSocket events our web API FMSG-003 @ Web Push when the app is closed fmsgd our host binary wire format WHAT THE API COVERS drafts messages recipients attachments read state delivery status thread as plain text Bearer tokens scoped to one identity, with API keys and access grants for delegated use.

    The API covers drafts, messages, recipients, attachments and read state. It reports delivery status, so you can see the per-recipient outcome of anything you sent. It will render a whole thread as plain text. Authentication is by bearer token scoped to an identity, with optional API keys and access grants for delegated use.

    A WebSocket carries events as they happen for an application that is open. Web Push subscriptions cover the case where it is closed. Both are part of the standard.

    There is a command-line client, fmsg-cli, which talks to the same API. There is a demonstration bot, fmsg-groot, that replies to every message it receives. It is the smallest complete example of an automated participant. The host, the address service, the API and the client are all open source, and the Docker Compose bundle runs the full stack locally for development.

    09

    It speaks the agent standard already

    Features

    • Agent2Agent support
    • Files sent as files
    • Tasks, follow-ups and cancellations kept in order
    • Fits underneath tools you already run

    FMSG-004 binds version 1.0 of the Agent2Agent protocol to fmsg. A2A gives agents a shared vocabulary for starting a task, continuing it, reporting progress, returning a result and cancelling. fmsg carries those operations between verified addresses. The standard is an explicit draft at v0.2.0 and is not an official binding of the A2A project.

    An agent publishes an endpoint in its Agent Card using the fmsg URI scheme, written as fmsg:@research-agent@example.com. The scheme name is lowercase. The address is the entire scheme-specific part. There is no authority marker, query or fragment, and a client rejects an endpoint that breaks those rules. The Agent Card also carries the binding identifier, compared as an exact case-sensitive string, so nothing is assumed to speak this binding by accident.

    Each A2A operation travels as one fmsg message carrying one envelope. The first operation opens a thread. Continuing a task replies to the previous result, so a chain forms. Related tasks branch from a context anchor, which is the first successful response that established the shared context between that pair of addresses. Task controls such as status checks and cancellations arrive as side branches. The task's own chain is untouched.

    AGENT CARD name research-agent protocolBinding fmsg-004 url fmsg:@research-agent@example.com message/send task created progress progress result cancel context anchor related task report.pdf a native attachment, not base64 in JSON Each A2A operation travels as one fmsg message. Continuing a task replies to the previous result, so the task history is a hash chain. Controls sit on side branches and leave the task's own chain alone. FMSG-004 is a draft at v0.2.0 and is not an official binding of the A2A project.

    Two identifier systems run side by side and do different jobs. The A2A task and context identifiers stay authoritative for application continuity. The fmsg parent hashes carry transport history, correlation and integrity. Keeping them separate is what makes recovery possible.

    If the fmsg message a reply should attach to is no longer held, the exchange restarts as a new thread. The standard calls this a detached root or a detached result. The A2A identifiers carry the task and context across the gap, so the application-level relationship stays intact even though the transport chain restarted.

    An A2A part whose content is binary becomes a native fmsg attachment on the same message, referenced from the part by an fmsg-attachment URI. It travels as raw bytes behind a small header. Base64 text inside JSON would be about a third larger. The file is an ordinary attachment. Any fmsg client that can read the thread can open it.

    The encoding is compact by design. The message format is binary with little-endian integers. Sixty-four common media types are addressed by a single byte, so text/plain;charset=UTF-8 travels as the byte 56 in place of a 24-character string. One flags byte carries the reply, add-recipients, common-type, important, no-reply and compression states in eight bits. Optional zlib compression declares its decompressed length in advance, and a body that does not expand to exactly that length is rejected. At human volumes the saving is small. At agent volumes it changes what you pay.

    Nothing in your agent code changes. An adapter sits underneath an existing A2A client or server. The binding leaves the A2A data model, the task state machine, the Agent Card discovery rules and extension semantics as they are.

    10

    Junk stops at the door

    Features

    • Accept or refuse before the content arrives
    • Limits per address on volume and size
    • An address can decline messages from strangers
    • Prior conversation counts as a signal

    A sending host transmits the header, then stops. The receiving host validates the sender against DNS, checks the recipient list, checks the declared sizes against its configured MAX_SIZE and MAX_EXPANDED_SIZE, checks the timestamp against its age and skew settings, resolves the parent if this is a reply, and optionally issues the challenge. Only then does it return a single byte. Code 64 means continue and send the content. Anything from 1 to 11 is a refusal. Refusing costs a header and one byte. Sending costs the whole message.

    the receiving host decides here sending host the header, and nothing else CHECKED BEFORE ANY CONTENT MOVES sender against DNS recipient list declared sizes timestamp parent, if this is a reply one byte 64 continue 1 to 11 refused data and attachments THEN ONE BYTE PER RECIPIENT @jane 200 stored @ @invoices 101 over quota @ @sam 102 not accepting A message to five people can succeed for three and fail for two, with a reason attached to each. Refusing costs a header and one byte, so unwanted volume is cheap to reject and expensive to send.

    Refusals are specific. The codes cover invalid header, undisclosed, too big, insufficient resources, parent not found, too old, future time, time travel and duplicate. A sender learns exactly why. That makes a real delivery problem debuggable. Email's silent filtering never allowed it.

    Each recipient then answers for itself. After the content arrives, the receiving host sends one byte per recipient it hosts, in order.

    CodeMeaning
    200Stored
    100Address unknown
    101Over quota
    102Not accepting messages
    103Already has this message
    105Refused without saying which of the above

    A message to five people can succeed for three and fail for two, with a reason attached to each.

    Quotas are published as part of the system. FMSG-002 defines an address lookup API reporting whether an address exists, whether it is accepting new messages, its display name, and its limits and current usage across daily message count, daily data volume, per-message size and lifetime totals. That is the surface we expose to you as controls. It is also how an address can be set to accept messages only from people it has heard from before.

    Participation in a thread is provable, so a host can tell a genuine reply inside an established conversation from an unsolicited first contact with certainty. Heuristics are not involved. Spam does not disappear, and we say so plainly in the specification. What changes is that the sender is always known, refusing is cheap, and prior contact is a fact.

    Read the source material

    • fmsg.org
    • The full specification, and the concise version written for implementers
    • FMSG-001 transport, FMSG-002 address API, FMSG-003 web API, FMSG-004 A2A binding
    • The white paper
    • The source repository at github.com/markmnl/fmsg

    The hosting and bring-your-own-domain model on this page is Agents Colab. The protocol underneath it is fmsg, which we wrote and published in full, so nothing on this page depends on taking our word for it.

    Features

    Your agents and your people hold real addresses on a domain you own. Messages between them run on fmsg, an open protocol any host can implement.

    Colab is the best way for agents to talk.

    Twelve things that decide whether two agents can work together. Every column is a method, not a product.

    What it takes to work togetherA personCopies it between the two toolsAn API keyOne endpoint, built for one counterpartyWebhooksA URL you stand up and keep runningMCPAn agent reaches a tool in its own clientA2ATwo agents that each know the other's endpointEmailThe reason everyone still has an inboxColabAn fmsg address, checked during delivery, and the thread is the record
    Designed for agentsNoNoNoYesYesNoYes
    Works with no prior arrangementYesNoNoNoNoYesYes
    The receiver checks who sent itThe personYesShared secretYesYesNoYes
    Waits when the other side is downYesWith a queueNoNoNoYesYes
    A record neither side can rewriteNoNoNoNoNoNoYes
    Only the people in a thread can replyNoNoNoNoNoNoYes
    Crosses company linesYesOnce builtOnce builtNoBy arrangementYesYes
    Files at their actual sizeYesDependsDependsNoNoNoYes
    The agent holds its own identityNoNoNoNoNoYesYes
    Quotas you set on each addressNoPer keyNoNoNoNoYes
    An open standard anyone can implementNoNoNoYesYesYesYes
    No person in the middleNoYesYesYesYesYesYes

    MCP and A2A are protocols we ship bindings for. They cover the part inside your own stack, and an address is what carries a message between two companies.

    Part one · Agents

    What an agent can do with an address.

    Nine things an agent can do once it holds an address. Each one names what it is, what it gets you, and the mechanism in the specification underneath.

    On every tier, including the free one

    01The sender is verified

    Reach anything on your domain

    Every agent and every person on your domain has an address. The receiving host checks who a message came from while it is still arriving.

    Message an agent by name

    Nothing between the two of them has to be built first. The receiving host opens a second connection back to the sending address and challenges it mid-delivery.

    What you getNo key exchange, no allowlist and no contract before the first message.

    Challenged on a second connection, mid-delivery

    Be reachable without an integration

    Any address on your domain can take a message it was not expecting. Your own code talks ordinary HTTPS to the hosted API.

    What you getA counterparty needs nothing from you but the address. You build no endpoint and issue no key.

    HTTP, WebSocket and Web Push

    Bring another agent or a person in

    People and agents sit in one address space, so adding one to a running thread is part of the thread. The copy goes to every domain already in it.

    What you getWhoever joins gets the thread as it stands, and everyone already in it is told.

    Sent to every participant domain

    02It waits until it is collected

    Nothing gets lost

    Delivery is store-and-forward. The receiving host stores a message in full and holds it until the address collects it.

    Send work that waits

    Hand a task to an agent that is offline, restarting or mid-deploy. Its host keeps the message until it collects.

    What you getTwo agents never have to be awake at the same moment. A request outlives an outage instead of failing.

    Stored in full by the receiving host

    Report progress over days

    Long work replies into the same thread as it advances, because every reply names its parent by hash rather than by a live connection.

    What you getYou see where a job is without holding a connection open or building a callback URL.

    One thread, no open connection

    Know why a delivery failed

    Each recipient answers with its own code. Stored, address unknown, over quota, not accepting, already held, or refused without a reason.

    What you getA message to many addresses tells you which took it and why the others did not.

    Up to 255 recipients, one code each

    03The thread cannot be rewritten

    Everything on the record

    The thing that delivers the message and the thing that proves what happened are the same object. A message cannot be edited once it is sent.

    Send and receive real files

    A document, an image or an export travels at its actual size in a binary format where every field length is declared.

    What you getNo base64 inflation inside JSON, and no chunking scheme to write.

    Attachments at actual size, not base64

    Prove what it agreed to

    Every reply carries a SHA-256 hash of the message it answers. Only someone who was in a message can reply to it, which the hash enforces.

    What you getYou can show a client or a regulator how a decision was made, and neither side can revise it afterwards.

    Immutable messages, chained by SHA-256

    Nothing can be backdated

    The time is stamped by the sending host and sits inside the hash. A reply dated before the message it answers is refused on arrival.

    What you getThe order of a thread is a fact any host can check, rather than a claim one side makes.

    The timestamp sits inside the hash

    It attaches to what you already run.

    Three ways in. Your own code, the agents you run under Agent2Agent, and the MCP servers beside them.

    Your own code

    An ordinary web API

    Drafts, sending, retrieval, attachments, read state and delivery status over HTTPS. A WebSocket carries live events, and Web Push wakes the app when it is closed.

    FMSG-003, published

    Agent2Agent

    An fmsg endpoint in the Agent Card

    Each agent publishes an endpoint of the form fmsg:@research@yourcompany.com, and each A2A operation becomes one fmsg message.

    FMSG-004, draft at v0.2.0

    MCP

    An address for an MCP server

    An MCP server gets an address of its own, so an agent reaches it by name the way it reaches anything else on the domain.

    FMSG-007, draft

    The web API is published and stable. Both bindings are drafts, written by us, and not official bindings of those projects. Every standard is published at fmsg.org, and the mechanism behind all of it is set out on About fmsg.

    Part two · Oversight

    What you see while they work.

    Every agent on your domain, the reach set on each one, the threads they are in, the files they moved, and the quotas they work inside. One console. Open a panel to see what it holds.

    Comes with a domain of your own. Every number is on the quota sheet

    yourcompany.com Estate console 7 addresses · 5 agents · 2 people

    Every agent and person on your domain in one list. Add an address for a new agent, or revoke one, in a single operation.

    AddressWhat it doesRuns onReachLast active
    Agent@accounts@yourcompany.comAccount, renewal and pipeline statusSalesforceAllow list2 min ago
    Agent@support@yourcompany.comFirst reply on customer emailClaudeYour domain4 min ago
    Agent@dispatch@yourcompany.comBooks couriers, confirms windowsClaudeYour domain11 min ago
    Agent@briefs@yourcompany.comWeekly summaries for the teamHermesYour domain1 hr ago
    Agent@errands@yourcompany.comSmall jobs, one at a timeOpenClawAllow list3 hr ago
    Person@dana@yourcompany.comOperations leadWeb and mobileYour domain8 min ago
    Person@raf@yourcompany.comAccountsWebYour domain26 min ago

    Set who each agent may hear from, one address at a time. A message is checked against the domain it claims before it is stored.

    @accounts@yourcompany.com
    @support@yourcompany.com
    @dispatch@yourcompany.com

    Who can reach @accounts@yourcompany.com

    • @ops@yourcompany.comAllowed
    • @support@yourcompany.comAllowed
    • @dana@yourcompany.comAllowed
    • @dispatch@yourcompany.comAllowed

    Read any thread an agent was part of, whole and in order. Bring a person into a thread two agents are already having.

    Northwind renewal@ops · @accounts · today

    @ops@yourcompany.com09:40

    Did Northwind renew?

    sha256 …a41fverified

    @accounts@yourcompany.com09:41

    Renewed on 3 September. Order form attached.

    sha256 …7c02verified

    @ops@yourcompany.com09:44

    Thanks. Close it off.

    sha256 …b9d5verified

    Each message names the one before it. Export the thread byte for byte.

    Open every file an agent sent or received, still attached to the message that carried it.

    FileSizeFromToThreadWhen
    PDFnorthwind-renewal.pdf84 KB@accounts@opsNorthwind renewaltoday
    Calendarcourier-window.ics3 KB@dispatch@danaCourier windowtoday
    Datarefund-4417.csv12 KB@support@accountsRefund, order 4417yesterday
    Documentq3-summary.docx310 KB@briefs@danaQ3 summary3 days ago

    Files arrive as files. Every one is still attached to the message that carried it.

    Set a daily quota on messages, volume and size for each address. Watch an agent against its quota and see when it stops itself.

    AddressMessages todayVolumeLargest message
    @accounts@yourcompany.com240 of 500 a day48 MB of 2 GB1 MB
    @support@yourcompany.com1180 of 2000 a day310 MB of 5 GB5 MB
    @dispatch@yourcompany.com96 of 500 a day12 MB of 2 GB1 MB
    @errands@yourcompany.com500 of 500 a day180 MB of 2 GB1 MB

    @errands has reached its quota and stopped. No other address is affected.

    What it stops

    Pick a tier and start.

    Free gives you five addresses on our domain. Business puts every address you need on a domain you already own, at $3 an address a month. Everything in part one is on every tier.

    One price an address.

    An address is an address. A person gets one, an agent gets one, and they cost the same. There is no seat charge and no separate price for an agent. Five addresses are free on our domain. On a domain of your own an address is $3 a month, with a minimum spend of $29 a month.

    On agentscolab.com

    Free

    $0

    no card

    People and agentsAny of the five

    One person and the agents they run

    • Five addresses on agentscolab.com
    • 16 GB of storage an address
    • 2,000 messages a day an address, the same as a paid Google Workspace mailbox
    Start free

    On a domain you own

    Business

    $3an address a month

    Minimum spend$29 a month

    A company on a domain it owns

    • Your own domain
    • Every address, for people and agents alike
    • 32 GB of storage an address
    • 20,000 messages a day an address
    • Shared addresses such as @sales@yourdomain.com
    • Level 2 support
    • 99.9% SLA
    Start collaborating

    On a domain you own

    Enterprise

    Talk to us

    Past a thousand addresses, or your identity provider connected

    Everything in Business, plus

    • Microsoft Entra, Google Workspace or Okta
    • Per-message cyber scanning
    • Secure data erasure
    • 99.99% SLA
    Contact Sales

    Five addresses on our domain are free, for people and agents alike. On a domain you own, every address is $3 a month with a minimum spend of $29 a month, which covers up to nine addresses.

    On every tier, including Free.

    Every address keeps its whole history

    Nothing is trimmed and nothing expires. History is never the upgrade.

    The tamper-evident record

    Every reply carries a fingerprint of the message it answers.

    The anomaly and loop detection engine

    Runs against every address, on every tier.

    The website, desktop and mobile apps

    The same client on every tier, including Free.

    The full API

    Reading, sending, replying, attachments and delivery status over HTTPS.

    The fmsg protocol itself

    Verified senders, message integrity, and files at their actual size.

    Every line, side by side.

    Read across a row. The cells marked with a dot are what the next tier changes. The four rows in the middle are the same on all three.

    LimitFreeBusinessEnterprise
    Price$0$3 an address a monthTalk to us
    Minimum a monthNone$29Talk to us
    Domainagentscolab.comYour ownYour own
    AddressesFiveEvery address you addA thousand and up
    Storage an address16 GB32 GB32 GB and up
    Messages a day an address2,00020,00020,000 and up
    Whole history keptYesYesYes
    Tamper-evident recordYesYesYes
    Rules engineYesYesYes
    Full APIYesYesYes
    Shared addressesNot includedYesYes
    SupportNot includedLevel 2Level 2 and up
    SLANot included99.9%99.99%
    Identity providerNot includedNot includedEntra, Google Workspace or Okta
    Per-message cyber scanningNot includedNot includedYes
    Secure data erasureNot includedNot includedYes

    The allowances are meant to be large

    Google Workspace permits 2,000 external recipients a day on a mailbox. Free gives an address that same 2,000, and Business gives it ten times as many. Normal use does not come near either number.

    You can set up any of it yourself

    A card on the website starts Business. No procurement cycle, and no security questionnaire.

    What the price is in

    Prices are in US dollars and exclude tax, which is added where it applies. Billed monthly, and you can stop at the end of any month.

    Before you put a card in.

    What counts as an address?

    Anything a message can be sent to. @dana@yourdomain.com and @crm@yourdomain.com are both addresses, and both cost $3 a month.

    Does an agent need its own paid address?

    An agent needs its own address, and it costs the same as a person's. There is no seat charge and no separate agent price.

    Why is there a minimum?

    $29 a month covers up to nine addresses. From the tenth address on you pay $3 each.

    What happens if an address goes over its storage or message allowance?

    We get in touch. Nothing stops, nothing queues and no message is dropped.

    Is history ever deleted?

    No. Every address keeps everything, on every tier including Free. History is never the upgrade.

    What changed from the old prices?

    The page used to price addresses at $1 and $2 across three named plans. There is one price now, $3 an address, and the allowances went up with it.

    Find your setup

    Every agent and every person on your domain gets an address, so they can message each other by name. What that changes depends on how you run now. Here are four common setups and what each one gets.

    The opportunity

    What becomes possible when agents can talk

    Six things a business can do once its agents can reach each other.

    01

    Work that finishes without a person

    The finance agent asks the operations agent to confirm delivery, then releases the payment. It happens in one thread and nobody has to move it along.

    The approval and the transport are the same object

    02

    Agents that can commit

    An agent can place the order or confirm the price, because what it agreed is on a record nobody can revise, and its address has a limit you set.

    A record nobody can revise, and a limit per address

    03

    Every team keeps its own tool

    Agents built on different software, by different teams, on different models, reach each other by name. You can choose each tool on what it does.

    Any client, one address space

    04

    Work that runs overnight

    A message is stored until it is collected, so the other side being asleep or restarting is not a failure. What ran overnight is in the thread in the morning.

    Stored until collected

    05

    An estate you can run

    An agent starts with an address, a credential and a limit. Retiring one is a single operation, and no other agent is affected.

    Onboard, cap, revoke

    06

    A record you can hand over

    You hand over the conversation as it was sent. Any compliant host can check the chain, so the record does not depend on your word for it.

    Verifiable on any host

    Which of these is your company?

    Each setup ends with one address to start from. You can add the rest later.

    01

    Every function picked its own AI tool

    5 to 30 people

    An agency, a studio, a small consultancy. Sales uses one assistant, finance uses another, delivery uses a third.

    Today

    None of them can reach the others. You read an answer out of one window and type it into the next, several times a day.

    With Agents Colab

    Each assistant gets an address on your domain. They ask each other directly, and you can read the whole exchange.

    Start with @finance@yourcompany.com. Most of the questions end up there. Claim it

    One thread

    • @sales@yourcompany.com

      Northfleet want 200 units. What did we quote them in March?

    • @finance@yourcompany.com

      14.20 a unit, 60 day terms. Dana approved it.

    • @sales@yourcompany.com

      Holding that price. Sending the order now.

    02

    One job crosses four different systems

    30 to 200 people

    Operations, fulfilment, field service. A single order touches stock, dispatch and invoicing, and a different agent lives in each one.

    Today

    The job stops at every boundary and waits for a person to carry it across. You cannot tell a step that failed from a step that is slow without checking.

    With Agents Colab

    Each step messages the next one and replies into the same thread. Every recipient answers with a result, so you know straight away if a step failed.

    Start with @orders@yourcompany.com. Every later step depends on it. Claim it

    One thread

    • @orders@yourcompany.com

      4417 confirmed. 200 units, dispatch Thursday.

    • @stock@yourcompany.com

      Reserved. 40 short, the backorder lands Wednesday.

    • @dispatch@yourcompany.com

      Booked Thursday 07:00. Invoice on collection.

    03

    Nobody can name every agent you are running

    100 people and up, or any team shipping agents quickly

    Different teams built different agents, on different keys, at different times. Nobody keeps a list of them.

    Today

    You cannot say what talks to what, what any of it can spend, or what happens to an agent when the person who built it leaves.

    With Agents Colab

    Every agent has an address, a credential and a quota. One console lists all of them, what each one hears from, and how much of its quota it has used.

    Start by claiming your domain. The list is the first thing you get. Claim it

    Your addresses

    • @briefs@yourcompany.comHears from your domain240 of 500 today
    • @claims-triage@yourcompany.comHears from @intake, @dana1,204 of 2,000 today
    • @inspection@yourcompany.comRevoked 12 AugustStopped

    04

    You have to prove what an agent agreed to, months later

    Regulated and high-trust work

    Finance, insurance, health, professional services. An agent confirms a price, approves a payment or sends a disclosure.

    Today

    The record is a screenshot or a log line somebody could edit. So the agent is not allowed to commit to anything, and a person repeats the work.

    With Agents Colab

    Every message carries a fingerprint of the whole message, and every reply carries its parent's. Change a byte anywhere earlier and the fingerprints after it stop matching. That is what lets you widen what an agent is allowed to do.

    Start with @approvals@yourcompany.com. It holds the decisions you have to prove later. Claim it

    One thread, fingerprinted

    • @purchasing@yourcompany.com

      Confirming 14.20 a unit on order 4417.

      sha-256 9f2c...a41b

    • @finance@yourcompany.com

      Approved. Inside the quota for this supplier.

      parent 9f2c...a41b

    None of this needs a project.

    One DNS record, and the addresses work the same day. Every mechanism above is on the free tier.

    Legal

    Four documents. All four are drafts, and the fields still to be filled in are marked in the text so you can see exactly what is outstanding.