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API and webhooks

Every reply, every label, every workspace, over an API

Inbox, prospects, labels, views, workflows, workspaces, reply agents and the sequencer integrations: 150 or so endpoints under one key, with webhooks for the events you would otherwise poll for. Built for agencies wiring Master Inbox into their own portal and teams feeding replies to their own database.

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Free 7-day POC. Unlimited users on every plan. API access is included.

API keys in Master Inbox settings: named keys with a link to the API docs

What the API covers

Grouped the way the reference groups them. Every group is the same key and the same base URL.

Inbox

Threads, messages, drafts, replies, folders, reminders.

Prospects

Search, create, update, labels per prospect, lists.

Labels

Create labels, label stats per campaign, sync logs, bulk relabel.

Views

Create and update saved views with the same filters the inbox uses.

Workflows

List, create, update, pause and delete, plus the live catalog of triggers and actions.

Workspaces

Create, update, clone and switch workspaces. Members and channels alongside.

Reply agents

Create and update agents, drafts, human-in-the-loop state.

Integrations

Smartlead, Instantly, EmailBison, HeyReach, PlusVibe, QuickMail, SalesRobot campaigns and lists by id.

Calendar, signatures, templates

Bookings across Cal.com, Calendly, HubSpot, GoHighLevel, Chili Piper and Google Calendar.

A workflow, created in one request

The same payload the two-click builder on the workflows page produces. Validation runs before anything is written, and a rejected request lists every problem with the field it applies to.

POST https://api.masterinbox.com/api/api-webhook/v1/api/workflows/create-workflow
{
  "name": "Notify Slack when a hot lead replies",
  "trigger_type": "add_label",
  "trigger_config": { "label_ids": [5] },
  "action_configs": [
    { "action_type": "add_labels", "label_ids": [5] },
    { "action_type": "slack_notify", "slack_webhook_url": "https://hooks.slack.com/services/..." }
  ]
}

Fetch /workflows/filter-fields first: it returns the live triggers, actions and filter fields from the same catalog the validator enforces, so nothing you build can drift from what the API accepts.

Webhooks for the events you would otherwise poll for

Register a URL in Settings or over the API, pick the events, and Master Inbox posts each one as it happens.

  • message_incoming_outgoing A reply landed or a reply was sent, on email or LinkedIn.
  • label_changed A label was added, removed or replaced, by a human, the AI or a sync.
  • prospect_list_changed A thread moved between lists.
  • agent_draft_created A reply agent drafted a reply and is waiting for review.
  • reminder_notification A reminder came due for the thread owner.
  • email_disconnected A mailbox lost its connection, before replies go missing.
  • slack_integration Slack-side events for the Master Inbox Slack app.

Each workflow can also expose its own incoming webhook URL, so another tool can start a workflow with a payload of its own.

How agencies use it

The asks that come up on calls, and the endpoints that answer them.

  • A client portal on top of Master Inbox. Create a workspace per client, clone it for the next one, create a view of positives only, and read threads and labels into your own UI.
  • Replies into your own database. Subscribe to message and label events, or pull prospects with their labels on a schedule, instead of parsing sequencer webhooks.
  • Provisioning at scale. Workspace, members, channels and labels created by script when a new client signs, with workflows added one call each.
  • Campaign hygiene. Look up Smartlead, Instantly, EmailBison, HeyReach or PlusVibe campaigns by id, then add, remove, pause or unsubscribe prospects from them.

Full reference: app.masterinbox.com/api-docs.html. The workflow section is also published as plain markdown for coding agents at public-workflow-api.md.

Explore more: Workflows, integrations, Client workspaces, pricing

Questions we get

How do I get a key?
Settings, APIs, Create new key. Keys are per account, named, and revocable. Keep them out of client-side code; if one leaks, delete it and create another.
Is the API included in every plan?
Yes. API access, webhooks and workflows are included. There is no per-call metering.
Is there rate limiting?
Yes, sensible per-key limits. Bulk needs, such as backfilling a large workspace, are best done through the bulk endpoints for labels and prospects rather than one call per record.
Can I manage everything the app can?
Nearly. Two workflow triggers, Incoming Webhook and Scheduled View Report, and a handful of actions that need ids the API cannot look up (Google Sheets, Close) are app-only. The reference marks each one.
Do you have a Zapier or n8n node?
Not yet. Outbound webhooks plus the REST API cover both today, and workflows remove most of the need. A node is on the roadmap.
Is there an SDK?
Not yet. The reference is OpenAPI, so generated clients work, and the workflow section is published as markdown so a coding agent can build against it directly.

See it on your own inboxes

Book a demo with a developer on the call and we will walk the endpoints against your workspace.

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