---
title: "Firebase AI Logic in JavaScript SDK 13.0.0 turns Part into a discriminated union: 2026-10-07 | Harbyn"
description: "Firebase AI Logic in JavaScript SDK 13.0.0 turns Part into a discriminated union, announced Oct 7, 2026."
canonical: "https://harbyn.com/changes/firebase/2026-10-07-part-refactored-into-a-discriminated-union"
last-updated: 2026-10-07
---

# Firebase AI Logic in JavaScript SDK 13.0.0 turns Part into a discriminated union

Markdown version of <https://harbyn.com/changes/firebase/2026-10-07-part-refactored-into-a-discriminated-union>.

Breaking change - Firebase. medium severity. In effect.

Firebase JavaScript SDK 13.0.0 refactors Part into a discriminated union with explicit type properties, which are stripped before requests are sent. A new UnknownPart type covers raw wire and user-provided inputs, and chat history is deep-copied defensively.

## Facts

| Fact | Value |
| --- | --- |
| Vendor | [Firebase](https://harbyn.com/vendors/firebase.md) |
| Kind | Breaking change |
| Severity | medium |
| Status | In effect |
| Announced | Oct 7, 2026 |
| Added to Harbyn | Oct 7, 2026 |
| Lead time | No dated deadline |
| Symbols | `Part` |
| Review | Reviewed by a person, extracted by a model |

## Find it in your code

```sh
npx harbyn scan
```

This change names no identifier the scanner can match line by line, so the scan tells you whether your repository uses Firebase, from its packages and API hosts. Nothing leaves your machine.

## Sources

Where this event comes from: the page we read, when we read it, and the hash of that snapshot.

- <https://firebase.google.com/support/release-notes/js> (changelog, fetched 2026-10-07 23:09 UTC, sha256 `2407d40b1d1bbfdc5572e4c5640df949d51c50d74ee44b8e35cafa4951fb13c5`)

## Related changes

### More from Firebase

- [AI Logic in JavaScript SDK 13.0.0 exports ChatSession as an interface, not a class](https://harbyn.com/changes/firebase/2026-10-07-chatsession-class-replaced-by-interface.md) (Breaking change, announced Oct 7, 2026)
- [AI Logic in JavaScript SDK 13.0.0 makes gemini-3.5-flash-lite the default hybrid in-cloud model](https://harbyn.com/changes/firebase/2026-10-07-default-hybrid-in-cloud-model-changed.md) (Behavior change, announced Oct 7, 2026)
- [AI Logic in JavaScript SDK 13.0.0 removes previously deprecated APIs](https://harbyn.com/changes/firebase/2026-10-07-deprecated-apis-removed.md) (Breaking change, announced Oct 7, 2026)
- [Remote Config in JavaScript SDK 13.0.0 stops false CONFIG\_UPDATE\_STREAM\_ERROR events on backgrounding](https://harbyn.com/changes/firebase/2026-10-07-expected-background-connection-closes-handled-differently.md) (Behavior change, announced Oct 7, 2026)

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[All upcoming deadlines](https://harbyn.com/deadlines.md)