# Identify participants with an external reference

By default, an Attune interview link asks each participant for an email address. With an external
reference, you identify each participant yourself: you add an ID from your own system, such as a CRM
contact ID, to the link, and Attune stores it with that participant's response. Participants are
never asked for an email address, and every response ties back to a record you already hold.

**What you need**

External references are available on the Enterprise plan.

## Create a link that uses external references

When you create a share link for a published interview, choose to identify participants by an
external reference instead of by email. The choice is fixed for that link: an email link stays an
email link, and a reference link stays a reference link.

If you create links through the [Attune MCP server](/mcp/), call `share_guide` with
`participantIdentification` set to `external_ref`.

## Add the reference to each participant's link

Take the link Attune gives you and add `?ref=` followed by the participant's ID:

```text
<your Attune link>?ref=<participant ID>
```

For example, a participant with the ID `c2f9b6e4-7d1a-4f0e-9a33-5b8e21d4c6a7` gets the link with
`?ref=c2f9b6e4-7d1a-4f0e-9a33-5b8e21d4c6a7` on the end. Send every participant their own link, and
never share one reference between two people: the second person to open it sees the interview as
already completed.

A reference must follow these rules:

| Rule | Value |
|---|---|
| Length | 1 to 128 characters |
| Allowed characters | Letters A to Z and a to z, digits 0 to 9, dot (`.`), underscore (`_`), and hyphen (`-`) |

A link with no reference, or with a reference that breaks these rules, does not start an interview.

## Choose references that are random

Anyone who has your link and a reference can open the interview as that participant, and Attune
does not check that a reference belongs to a particular person. So:

- **Use random, unguessable values.** UUIDs, or other IDs of at least 16 random characters, work
  well. Never use sequential numbers such as `1001` and `1002`, or anything else a person could
  guess by counting. A guesser could open other participants' interviews or learn which of them
  have finished.
- **Never put personal data in a reference.** Do not use an email address, a name, a phone number,
  or anything else that identifies a person on its own. References travel inside links, which end
  up in email threads and server logs, and Attune shows them next to each response.

## What participants see

Participants get the same interview as with any other link, including the consent step: a
reference link never skips consent. Only the step that asks for an email address is skipped. Once
the interview opens, Attune removes the reference from the browser's address bar.

If a participant opens their link again after finishing, Attune shows that the interview is already
completed and does not start another one. If they left before finishing, opening the link starts
the interview again from the beginning.

## Find the reference on responses

Each response collected through a reference link shows its reference as **External reference**:

- in the responses table;
- on the participant card for the response;
- in every CSV export: the consolidated export, the flat-file export, and the single-response
  export.

The cell is empty for responses collected through email links. The `list_responses` and
`get_response` tools in the MCP server return the same value as `externalRef`.