LangGraph

Use this page to load Sendmux tools into a LangGraph agent. The LangChain MCP adapters convert MCP tools into LangChain tools, so anything that accepts a tool list will accept them.

Replace placeholder keys and tokens before running any snippet. Do not commit smx_root_, smx_mbx_, or private HTTP bearer tokens to version control.

Requirements

Python 3.10 or newer.
A Sendmux key for the surface you are calling. Mailbox work needs an smx_mbx_ key or a scoped smx_agent_ token.

Install

pip install langchain-mcp-adapters langgraph sendmux-mcp

Connect over local stdio

The adapter launches the server and passes your key through the subprocess environment. Values written as ${VAR} are expanded from the current environment, so the key never appears in source.

client.py

from langchain_mcp_adapters.client import MultiServerMCPClient

client = MultiServerMCPClient(
    {
        "sendmux": {
            "transport": "stdio",
            "command": "sendmux-mcp-mailbox",
            "args": [],
            "env": {"SENDMUX_API_KEY": "${SENDMUX_MAILBOX_API_KEY}"},
        }
    }
)

Run more than one surface by switching to the sendmux-mcp entry point and naming the surfaces:

"sendmux": {
    "transport": "stdio",
    "command": "sendmux-mcp",
    "args": [],
    "env": {
        "SENDMUX_MCP_SURFACES": "mailbox,sending",
        "SENDMUX_MAILBOX_API_KEY": "${SENDMUX_MAILBOX_API_KEY}",
        "SENDMUX_SENDING_API_KEY": "${SENDMUX_SENDING_API_KEY}",
    },
}

Connect over private HTTP

Use this when your graph runs somewhere it cannot spawn a process. Start the server yourself:

SENDMUX_API_KEY=smx_mbx_... \
SENDMUX_MCP_HTTP_BEARER_TOKEN=local-mcp-token \
sendmux-mcp-mailbox --transport http --host 127.0.0.1 --port 8765

Then connect with the streamable HTTP transport and send the bearer token:

client.py

from langchain_mcp_adapters.client import MultiServerMCPClient

client = MultiServerMCPClient(
    {
        "sendmux": {
            "transport": "streamable_http",
            "url": "http://127.0.0.1:8765/mcp",
            "headers": {"Authorization": "Bearer local-mcp-token"},
        }
    }
)

Build the agent

get_tools() returns LangChain tools you can hand to a prebuilt agent or bind to a model inside your own graph.

agent.py

import asyncio

from langchain.agents import create_agent

from client import client


async def main() -> None:
    tools = await client.get_tools()

    agent = create_agent(
        "openai:gpt-4.1",
        tools,
        prompt=(
            "You triage the mailbox you have been granted. "
            "Search before you read, and quote the message you acted on."
        ),
    )

    result = await agent.ainvoke(
        {"messages": "What arrived overnight that needs a reply?"}
    )
    print(result["messages"][-1].content)


asyncio.run(main())

Set tool_name_prefix=True on MultiServerMCPClient when you connect Sendmux alongside other MCP servers. It prefixes tool names with the server name and prevents collisions.

Give each user their own mailbox

Mailbox-scoped keys are the isolation boundary. Build a client per tenant rather than sharing one across the graph, so the agent working one tenant structurally cannot read another tenant’s mail.

def client_for(mailbox_key: str) -> MultiServerMCPClient:
    return MultiServerMCPClient(
        {
            "sendmux": {
                "transport": "stdio",
                "command": "sendmux-mcp-mailbox",
                "args": [],
                "env": {"SENDMUX_API_KEY": mailbox_key},
            }
        }
    )

Sending stays gated

A durable smx_agent_ token includes mailbox.read and email.receive, not email.send. After a named human owner accepts the invite and approves sending, exchange the durable token for a one-hour Sending-resource token and pass that as SENDMUX_SENDING_API_KEY.

Troubleshooting

get_tools returns an empty list

Confirm the surfaces the server started with. sendmux-mcp requires SENDMUX_MCP_SURFACES; the single-surface entry points do not.

The subprocess exits immediately

Check the key prefix. Mailbox accepts smx_mbx_ or a scoped smx_agent_, Sending accepts a send-capable smx_mbx_ or an owner-approved Sending-resource smx_agent_, and Management requires smx_root_.

Tool errors stop the graph

Set handle_tool_errors=True on MultiServerMCPClient to return errors as tool messages the model can react to instead of raising.

You granted more than one mailbox

Start the workflow with mailbox_list_granted_mailboxes and pass the returned mailbox_id to tools that act on one mailbox.

Next steps

Agent frameworks

Compare connections across frameworks.

MCP

Review environment variables and tool discovery.

Mailbox API guides

Use search and batch operations efficiently.

Python SDK

Call Sendmux directly when you want full endpoint coverage.

Getting started

Developer tools

AI integrations

Sending

Receiving

Domains

Monitoring

Webhooks

Use cases

Integrations

Account and billing

Requirements

Install

Connect over local stdio

Connect over private HTTP

Build the agent

Give each user their own mailbox

Sending stays gated

Troubleshooting

Next steps

Agent frameworks

MCP

Mailbox API guides

Python SDK

​Requirements

​Install

​Connect over local stdio

​Connect over private HTTP

​Build the agent

​Give each user their own mailbox

​Sending stays gated

​Troubleshooting

​Next steps

Agent frameworks

MCP

Mailbox API guides

Python SDK

Requirements

Install

Connect over local stdio

Connect over private HTTP

Build the agent

Give each user their own mailbox

Sending stays gated

Troubleshooting

Next steps