---
title: "Anthropic Tool choice restriction on Fable 5.1 and Mythos 5.1: 2026-09-01 | Harbyn"
description: "Anthropic Tool choice restriction on Fable 5.1 and Mythos 5.1, announced Sep 1, 2026."
canonical: "https://harbyn.com/changes/anthropic/2026-09-01-tool-choice-restriction-on-fable-5-1-and-mythos-5-1"
last-updated: 2026-09-21
---

# Anthropic Tool choice restriction on Fable 5.1 and Mythos 5.1

Markdown version of <https://harbyn.com/changes/anthropic/2026-09-01-tool-choice-restriction-on-fable-5-1-and-mythos-5-1>.

Behavior change - Anthropic. medium severity. Announced.

On Claude Fable 5.1 and Claude Mythos 5.1, tool\_choice types any and tool are unsupported and return a 400 error.

## Facts

| Fact | Value |
| --- | --- |
| Vendor | [Anthropic](https://harbyn.com/vendors/anthropic.md) |
| Kind | Behavior change |
| Severity | medium |
| Status | Announced |
| Announced | Sep 1, 2026 |
| Added to Harbyn | Sep 21, 2026 |
| Lead time | No dated deadline |
| Symbols | `claude-fable-5-1`, `claude-mythos-5-1` |
| Review | Automated, 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 Anthropic, 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://platform.claude.com/docs/en/release-notes/overview.md> (changelog, fetched 2026-09-21 22:00 UTC, sha256 `1341e592634fe9ba5dccb842bd5ca4f82bf86e59b168e63120ef72c7cb7c8b00`)

## Related changes

### More from Anthropic

- [Claude Enterprise Admin, Analytics and Compliance API requests should send the anthropic-version header](https://harbyn.com/changes/anthropic/2026-09-01-enterprise-admin-and-compliance-api-requests-must-send-api-version-header.md) (Behavior change, announced Sep 1, 2026)
- [SDK beta headers removal for files and skills](https://harbyn.com/changes/anthropic/2026-08-27-sdk-beta-headers-removal-for-files-and-skills.md) (Deprecation, announced Aug 27, 2026)
- [Fingerprint storage requires the diagnostics object](https://harbyn.com/changes/anthropic/2026-09-09-fingerprint-storage-requires-the-diagnostics-object.md) (Behavior change, announced Sep 9, 2026)
- [Cache diagnostics is out of beta](https://harbyn.com/changes/anthropic/2026-09-23-cache-diagnostics-is-out-of-beta.md) (Behavior change, announced Sep 23, 2026)

### Changes at other vendors

- [Amazon Selling Partner API: ORDER\_CHANGE notifications add PreFulfillmentStatusChange for Amazon Autos](https://harbyn.com/changes/amazon-sp-api/2026-10-14-sp-api-updates-new-listings-items-api-ivp-values-in-eu5-jp-and-ca-stores-oct-3.md) (effective Oct 28, 2026)
- [BigCommerce: 301 redirects will apply only to storefront channels](https://harbyn.com/changes/bigcommerce/2026-10-07-301-redirects-now-apply-to-storefront-channels-only.md) (effective Nov 10, 2026)
- [GitHub: Secret scanning audit log business field changes to enterprise slug](https://harbyn.com/changes/github/2026-10-20-updates-to-github-secret-scanning-7.md) (effective Nov 12, 2026)
- [Jira / Atlassian Cloud: New IP address ranges in use from January 1, 2027](https://harbyn.com/changes/atlassian-jira/2026-10-16-atlassian-cloud-adds-new-ip-address-ranges.md) (effective Jan 1, 2027)

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