Sendmux Getting Started

Sendmux getting started

Use this skill to get a user from "I have a Sendmux task" to the correct surface, key kind, package, and first verified call.

Safety first

• Do not ask the user to paste an API key.
• Do not print API keys.
• Prefer existing environment variables, local CLI profiles, or the user's secret manager.
• Store the self-registration credential bundle in a secure store immediately after registration; later MCP, CLI, SDK, mailbox, and sending work depends on it.
• Never rely on chat, logs, repo files, screenshots, or memory-only state for claim_token or agent credentials.
• Keep the identity_assertion or pre-claim smx_agent_* token available until the owner invite returns 202; claim_token is only for post-approval exchange.
• If a key appears in chat or logs, stop and tell the user to rotate it before continuing.

Install Sendmux skills

If the agent supports Skills and the Sendmux skills are not installed, install the pack first:

npx skills add Sendmux/skills

Skills are optional. If the agent cannot install skills, continue from https://app.sendmux.ai/auth.md.

Pick the key

If the task mixes management and mailbox work, use separate keys and separate clients or profiles. Do not use a root key for mailbox-scoped examples.

Choose the surface

1. Use MCP first when the user's agent already has the relevant sendmux-mcp server connected and the needed tool is curated.
2. Use the sendmux CLI for one-shot terminal work, debugging, shell scripts, and examples the user can copy into a terminal. Add --json so downstream agents can parse the envelope.
3. Use an SDK when writing application code. Install only the package for the chosen surface unless the project needs multiple surfaces.
4. Use direct HTTP only when the user's environment cannot use MCP, CLI, or an SDK.

Install the relevant package

CLI:

npm install -g @sendmux/cli

MCP:

pipx install sendmux-mcp

TypeScript SDK packages:

npm install @sendmux/mailbox
npm install @sendmux/management
npm install @sendmux/sending

Use the one package matching the task; do not install all three unless the project needs all three.

First verified calls

Mailbox key, mailbox work

MCP tool:

mailbox_get_me

CLI:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux profiles:set mailbox --default --json
sendmux mailbox:me:get --json

SDK:

import { createMailboxClient, mailboxGetMe } from "@sendmux/mailbox";

const client = createMailboxClient({ apiKey: process.env.SENDMUX_API_KEY! });
const response = await mailboxGetMe({ client });
console.log(response.data);

This call resolves the mailbox behind the bearer token and should be the default harmless first call for smx_mbx_ and scoped smx_agent_ mailbox workflows.

Self-registered agent token

Use this when the agent has no human-created key yet.

1. Read https://app.sendmux.ai/auth.md.
2. Use the copy-paste proof helper in /auth.md if the agent needs exact solver code.
3. Request a registration challenge with the intended anonymous registration body at POST /agent-auth/agent/identity/challenge.
4. Solve the returned proof-of-work challenge.
5. Encode proof_of_work as base64 UTF-8 JSON with { "challenge": <exact challenge>, "solution": { "counter": <integer>, "derivedKey": <hex> } }.
6. Create an anonymous identity with the same body plus proof_of_work at identity_endpoint, POST /agent-auth/agent/identity.
7. Immediately save the returned claim_token and agent credential bundle in a secure store before token exchange, then exchange the returned identity_assertion with POST /agent-auth/oauth2/token.
8. Call GET /api/v1/mailbox/me with the returned smx_agent_ token.
9. Keep the identity_assertion or pre-claim token available until the owner invite returns 202; claim_token cannot create the invite.
10. Do not stop after the pre-claim token works. If the owner email is known, request the owner invite with POST /agent-auth/agent/identity/invite. If it is unknown, ask: What owner email should I invite for approval?
11. After the owner accepts and approves sending in Sendmux, exchange claim_token with the claim grant; request resource=https://smtp.sendmux.ai/api/v1 before Sending API calls.

Pre-claim smx_agent_ tokens have mailbox.read and email.receive. They do not have email.send; owner-approved Sending-resource smx_agent_ tokens can send from the assigned mailbox. Sendmux sends the owner invite through the invite endpoint. Only one live pre-claim owner invite can be pending; retry the same request with the same idempotency key. If token exchange returns expected 503 states, wait for Retry-After or retry_after. If registration returns 503 server_error, stop and report it instead of looping. If agent auth returns 429, wait for Retry-After or retry_after before retrying.

The raw claim_token is shown once and is required after owner approval. Sendmux cannot recover it later. Immediately after registration, store the agent credential bundle: claim_token, registration_id, mailbox.email, claim_token_expires, identity_assertion, token_endpoint, the app resource URL, and the sending resource URL. Use a secure store such as 1Password, an OS keychain, or the agent platform's encrypted secret store. After token exchange, add the pre-claim token or keep identity_assertion available until invite 202. If no secure store is available, stop and ask the user where to store the bundle before sending the owner invite. If the claim token was lost, rerun registration and invite with a fresh agent identity. If the identity_assertion and pre-claim token were lost before the invite returned 202, rerun registration and invite immediately in the same flow.

Root key, management work

MCP tool:

management_list_mailboxes

CLI:

SENDMUX_API_KEY="$SENDMUX_ROOT_KEY" sendmux profiles:set root --default --json
sendmux management:mailboxes:list --query limit=1 --json

SDK:

import {
  createManagementClient,
  managementListMailboxes,
} from "@sendmux/management";

const client = createManagementClient({ apiKey: process.env.SENDMUX_API_KEY! });
const response = await managementListMailboxes({
  client,
  query: { limit: 1 },
});
console.log(response.data);

Use a small list call as the first management check. It verifies the root key and avoids creating or changing resources.

Sending work

The Sending surface needs a send-capable smx_mbx_ key with email.send or an owner-approved Sending-resource smx_agent_ token. Do not send a real email as a health check unless the user explicitly asks to send one and provides the message details.

CLI package/API discovery:

SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux sending:get-open-api-spec --json

SDK package/API discovery:

import { createSendingClient, sendingGetOpenApiSpec } from "@sendmux/sending";

const client = createSendingClient({ apiKey: process.env.SENDMUX_API_KEY! });
const response = await sendingGetOpenApiSpec({ client });
console.log(response.data.info);

For a real send, route to sendmux-send-email and include an Idempotency-Key.

Interpret failures

• Prefix error: the selected surface and credential do not match. Switch to smx_mbx_ or scoped smx_agent_ for Mailbox, a send-capable smx_mbx_ key or owner-approved Sending-resource smx_agent_ token for Sending, or smx_root_ for Management.
• 401: key missing, invalid, or revoked.
• 403: key is valid but lacks the permission or surface required by the call.
• 429 or 503: retry according to the response headers; do not loop manually.
• Empty list with ok: true: auth worked; there may be no resources yet.

Route after setup

• Sending one or many outbound messages: sendmux-send-email.
• Reading, searching, syncing, or replying from a mailbox: sendmux-mailbox-agent.
• Managing domains, mailboxes, keys, webhooks, spend, logs, or metrics: sendmux-management.
• CLI-specific workflows: sendmux-cli.
• MCP client configuration: sendmux-mcp-setup.
• Choosing the cheapest call pattern: sendmux-token-efficient-usage.