---
title: "Anthropic Claude Haiku 5.5 rejects manual extended thinking and returns thinking blocks by default: 2026-10-07 | Harbyn"
description: "Anthropic Claude Haiku 5.5 rejects manual extended thinking and returns thinking blocks by default, announced Oct 7, 2026."
canonical: "https://harbyn.com/changes/anthropic/2026-10-07-claude-haiku-5-5-behavior-can-break-code-written-for-claude-haiku-4-5"
last-updated: 2026-10-07
---

# Anthropic Claude Haiku 5.5 rejects manual extended thinking and returns thinking blocks by default

Markdown version of <https://harbyn.com/changes/anthropic/2026-10-07-claude-haiku-5-5-behavior-can-break-code-written-for-claude-haiku-4-5>.

Behavior change - Anthropic. medium severity. In effect.

Code written for Claude Haiku 4.5 can break when moved to Claude Haiku 5.5 (claude-haiku-5-5): manual extended thinking with budget\_tokens returns a 400 error, adaptive thinking is on by default so a response can begin with thinking blocks, and the same text counts as more tokens.

## Facts

| Fact | Value |
| --- | --- |
| Vendor | [Anthropic](https://harbyn.com/vendors/anthropic.md) |
| Kind | Behavior change |
| Severity | medium |
| Status | In effect |
| Announced | Oct 7, 2026 |
| Added to Harbyn | Oct 7, 2026 |
| Lead time | No dated deadline |
| Model IDs | `claude-haiku-5-5` |
| Review | Reviewed by a person, extracted by a model |

## Find it in your code

```sh
npx harbyn scan
```

The scanner reads your repository on your machine and reports every file and line that uses the identifiers above. 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> (changelog, fetched 2026-10-07 23:09 UTC, sha256 `000ddce5ae0fe544e3297b52e05c34f5b63753f09ebbbcd31426476260f0991f`)

## Related changes

### More from Anthropic

- [Limited networking applies allowed\_hosts to web tools](https://harbyn.com/changes/anthropic/2026-10-07-limited-networking-applies-allowed-hosts-to-web-tools.md) (Behavior change, announced Oct 7, 2026)
- [Claude Sonnet 4.5 model: retirement on 2026-11-30](https://harbyn.com/changes/anthropic/2026-09-30-claude-sonnet-4-5-model-retirement.md) (Retirement, effective Nov 30, 2026)
- [Claude Sonnet 5.5 rejects five Claude Sonnet 5 request patterns](https://harbyn.com/changes/anthropic/2026-09-28-claude-sonnet-5-5-changes-compatibility-with-claude-sonnet-5-code.md) (Behavior change, announced Sep 28, 2026)
- [Compliance Activity Feed no longer returns names or titles](https://harbyn.com/changes/anthropic/2026-09-24-compliance-activity-feed-no-longer-returns-names-or-titles.md) (Behavior change, announced Sep 24, 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)