Slack-style multi-tenant login: resolve the tenant, then send (or fake) a passwordless code

[dan]

We're building a multi-tenant app where each customer is a FusionAuth tenant, similar to how Slack has one "workspace" per customer. We'd like two entry points, like Slack has:

• A known-tenant URL (acme.ourapp.com) that takes returning users straight to their workspace's login.
• A generic entry point (ourapp.com/login) where someone just types an email — if it doesn't match any tenant, show "No account found"; if it does, send them a one-time code.

We'd also like to match Slack's exact behavior on the known-tenant URL: even if the email typed in isn't a member of that workspace, still show the same "enter your code" challenge screen. The "no account" information should only ever reach the person by email, never through the UI — that's what lets someone try a few email addresses if they forget which one they used for a given workspace.

What's the current best practice for doing this with FusionAuth?

[dan]

Both entry points, and especially the anti-enumeration behavior on the known-tenant path, are best built by calling FusionAuth's passwordless APIs directly from your own UI rather than relying on the hosted login pages. Here's more on the difference.

Using the API is the only way to get full control over what the UI shows on a failure.

Known-tenant entry (acme.ourapp.com)

• Maintain your own slug/domain → tenantId table (outside FusionAuth), populated when you provision each tenant via the Tenant API. FusionAuth has no built-in hostname-to-tenant mapping. You can learn more in the multi-tenant guide.
• At request time, resolve the subdomain to a tenantId from that table (outside FusionAuth). If nothing matches, show "Workspace not found" — this never touches FusionAuth.
• Build your own "enter your email" screen for this workspace (not FusionAuth's hosted page — you need to control what's shown on failure, which the hosted page won't let you do).
• When the user submits an email, your backend calls /api/passwordless/start with that tenantId + your Universal Application id + the email as loginId. Universal Application docs here.
• Branch on the response, entirely inside your backend:
  • 200, code sent → hang onto the returned code value, respond to your frontend with a generic "we sent you a code" message, and show your own "enter the code" screen.
  • 404, no user found → this is the real signal Slack is hiding. FusionAuth returns a 404 with an empty body when no user matches the tenant. Instead of surfacing that: send your own "no account with this email in this workspace" notice through your own transactional email provider (outside FusionAuth or using the send email API), then respond to your frontend with the exact same "we sent you a code" message. Show the identical "enter the code" screen either way.
• Completing the challenge:
  • Real branch: verify the code the user types via /api/passwordless/login (tenantId + code), which returns tokens; pair this with the Hosted Backend / BFF if it's a SPA. Alternatively, redirect the browser to /oauth2/passwordless/{code}?tenantId=<tenantId>&client_id=<id> and let FusionAuth's hosted page take over the code-entry step. This is the URL FusionAuth's own passwordless email template constructs for the "click to log in" link, so redirecting there yourself just skips the email round-trip.
  • Fake branch: there is no FusionAuth code to hand off, so there's nothing to redirect to on FusionAuth's hosted domain — you have to own the "enter code" screen yourself here. Whatever the user types, respond with the same generic "invalid or expired code" error you'd give a real wrong code; never a distinct message. If you're using the hosted-page redirect for the real branch, don't try to fake that redirect for this branch — just render your own equivalent screen and always reject, rather than attempting a look-alike hosted destination.
  • Pad the fake branch's response time to roughly match a real Start call, so the two paths don't diverge on timing, which would leak the same thing you're hiding in the UI.

Generic entry (ourapp.com/login)

• Call /api/user/search with a global (not tenant-scoped) API key and no tenantId filter, searching by the submitted email. Each matching User record already includes its own tenantId, so this single call tells you every workspace the email belongs to. This pattern (global key, /api/user/search?queryString=<email>, results include per-tenant matches) is answered on the forum. Keep the key server-side only.
  • If you'd rather not run a global-key search on every anonymous request (latency, or wanting the key confined to a webhook receiver instead of a public endpoint), you can maintain your own email → tenantId[] table fed by FusionAuth's user.create / user.update / user.delete webhooks instead. Docs at https://fusionauth.io/docs/extend/events-and-webhooks/. Either is fine — this is a trade-off, not a hard requirement.
• Branch on the result (outside FusionAuth):
  • No match → "No account found," stop there.
  • One match → continue with that tenant.
  • Multiple matches → show a workspace picker, then continue with the chosen one. This is currently functionality you have to build yourself, though there is an open issue.
• Once a tenant is resolved, it's the same steps as section 1 from "build your own enter-email screen" onward — including the fake-challenge branch, if you want the same anti-enumeration behavior here too (Slack's generic "find your workspaces" entry also avoids confirming or denying a match directly, for the same reason).

A few things worth flagging

• FusionAuth users are tenant-scoped records — the same person needs a separate User object per workspace they belong to.
• API key scope matters: the cross-tenant search in section 2 needs a global key; the tenant-specific calls in section 1 can safely use a tenant-scoped key. [Docs](Docs at https://fusionauth.io/docs/apis/api-keys).
• The SSO session cookie isn't tenant-aware across hostnames — FusionAuth doesn't currently support true multi-tenant SSO through the hosted pages, so the user behavior when switching tenants requires another login. Always pass tenantId explicitly on every call.
• Passwordless API reference for the exact Start/Login request and response shapes: