> For the complete documentation index, see [llms.txt](https://docs.tryterra.co/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tryterra.co/faq/help-topics/data-api-sdk/historical-data-and-backfill/s3-reference-id-and-backfill-completion.md).

# How do I detect backfill completion with S3 destinations?

**`reference_id` is only populated if you pass it when connecting users** (in the widget session or the `generateAuthToken` call); otherwise it is `null`.

You can still map `user_id` to `reference_id` from the [auth event](https://docs.tryterra.co/health-and-fitness-api/user-authentication/handling-authentication-events) files. Every S3 file is named `{user_id}-{start_time}.json` and carries `user.user_id` in the body.

For S3 there is **no explicit backfill-completion signal** (the `terra-reference` chunk count only applies to webhook destinations). Treat historical files as done once the `start_time` values in the filenames catch up to the present.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.tryterra.co/faq/help-topics/data-api-sdk/historical-data-and-backfill/s3-reference-id-and-backfill-completion.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.