Skip to content
Hire an AI employee
AI EmployeeSuper-AgentsAgent-to-AgentBenchmarksPricingBlog
Insights 04 Multi-agent & AI organizations

Anthropic API 2026-10-21: Breaking Changes and SDK 2.0

Nitish Garg · Founder & CEO, CellCog · 21 October 2026 · 5 min read · 1,146 words
At a glanceQuick answers
What changed?
A new Claude API version, 2026-10-21, with three Messages API breaking changes and renamed Admin API fields, plus new major SDK versions that send it by default.
Who has to act?
Anyone upgrading to the new SDKs, anyone sending temperature, top_p or top_k, and anyone running a proxy that reads the X-Api-Key header.
Do existing integrations break today?
No. Requests on 2023-06-01 and the old SDK versions keep working; the breaks apply once you move to 2026-10-21.
Data illustration on a near-white ground: two API request cards, 2023-06-01 with temperature, top_p and top_k dials and a teal check, 2026-10-21 with the same dials crossed out in amber and a 400, beside a large 8 for seven SDKs and the CLI
Fig 0Same request, new version, a 400. Made by CellCog's image agent, running GPT Image 2.5.

Anthropic released Claude API version 2026-10-21 on October 21, 2026, together with new major versions of seven client SDKs and the ant CLI that send it by default. It is not a drop-in upgrade. Requests that send temperature, top_p or top_k now return a 400 error, and the new SDKs change how your API key travels.

This page reads Anthropic’s release notes and its migration guide for 2026-10-21. Nothing breaks until you opt in: requests on 2023-06-01 and the current SDK versions keep working.

On this page · 6 sectionsOpen
  1. What breaks in the API
  2. The new SDK versions
  3. Two days of platform changes
  4. How to upgrade without surprises
  5. What it means for teams running agents
  6. What to watch
Key points5 · 5 min full read
  1. A calendar with a teal version tag.
    Anthropic released Claude API version 2026-10-21 on October 21, 2026, the successor to 2023-06-01.
  2. Three dials crossed out in amber.
    Requests on 2026-10-21 that send temperature, top_p or top_k return a 400 error, even at the default value.
  3. Eight package boxes in a grid.
    Seven SDKs and the ant CLI got new major versions that send 2026-10-21 by default, among them Python 2.0.0 and TypeScript 1.0.0.
  4. A key moving from one header card to another.
    The new SDKs send your API key as Authorization: Bearer instead of X-Api-Key, which can break proxies and gateways.
  5. A document with a balance scale on it.
    From these versions the SDKs and the CLI are licensed under Apache 2.0 instead of MIT.

§ 01What breaks in the API

Change Affects you if What to do
Sampling parameters removed You send temperature, top_p or top_k in Messages or Batch requests Remove them; requests sample at the defaults, including a temperature of 1.0
Documents in assistant turns rejected You place a document block directly in an assistant message Send documents in user messages
Citations in user turns rejected You send text blocks with non-empty citations in user messages Remove the citations from those blocks
Old field names dropped You read roles, group_id, name, group_type, workspace_id, actor.email or summaries from the Admin or Analytics API Read role_ids, rbac_group_id, display_name, group.type, scope, actor.email_address and data
Table 1Breaking changes in API version 2026-10-21 (Anthropic migration guide)

The sampling change is the one most code will hit. The guide says a request with any of the three parameters fails “even at the parameter’s default value”, so a hard-coded temperature=1 breaks too. Models that already accept only the defaults, Claude 4.7 and later, sample as before. On a model like Claude Sonnet 4.6, code that lowered temperature for steadier output loses that effect.

Animation: one request with temperature 0.2 returns 200 on API version 2023-06-01 and 400 on 2026-10-21; with the temperature line struck out it returns 200, labelled samples at the default
Fig 1The same request: 200 on 2023-06-01, 400 on 2026-10-21, 200 again without temperature

On Amazon Bedrock, 2026-10-21 works only for Claude 4.7 and later. Teams calling older models there stay on their current SDK.

§ 02The new SDK versions

Package New version Note
Python anthropic 2.0.0 One credential per client
TypeScript @anthropic-ai/sdk 1.0.0 Move the Bedrock, Vertex and Foundry packages to v1 too
C# Anthropic 13.0.0
Go 2.0.0 Module path ends in /v2
Java anthropic-java 3.0.0
PHP anthropic-ai/sdk 1.0.0 composer require, not composer update
Ruby anthropic 2.0.0
ant CLI 2.0.0 Go install path ends in /v2
Table 2SDK and CLI versions that send 2026-10-21 by default

These majors also carry changes that do not depend on the API version:

  • The key moves to a Bearer header. The SDKs send Authorization: Bearer <key> instead of X-Api-Key. A proxy or gateway that reads X-Api-Key stops seeing the key. “The Claude API accepts a key in either header, so a program that talks to the API directly needs no change.”
  • One credential per client. In Python, passing both api_key and auth_token now raises ValueError; v1 sent both.
  • Text Completions is gone from the TypeScript, Go, Java and Ruby SDKs and the CLI.
  • The license changes. The new versions are Apache 2.0. “Earlier versions remain under the MIT License.”
  • A quiet Go trap. Installing the CLI from the old module path gives you v1 without complaint: “The old path keeps installing the latest v1 release, with no error or warning.”
Animation: a request carrying Authorization Bearer stops at a gateway that reads X-Api-Key and is blocked; the gateway switches to reading Authorization and the request reaches the Claude API
Fig 2The new SDK sends a Bearer header; a gateway reading X-Api-Key blocks it

§ 03Two days of platform changes

From one API version to the next, with the Oct 20 platform additionsTimeline: API version 2023-06-01, then on Oct 20 2026 user profiles, organization webhooks and inference hooks for Enterprise, then on Oct 21 2026 API version 2026-10-21 with new SDK majors, highlightedJun 1, 2023API version 2023-06-01Oct 20, 2026User profiles, org webhooksOct 21, 20262026-10-21 and SDK majorsFrom one API version to the next, with the Oct 20 platform additionsTimeline: API version 2023-06-01, then on Oct 20 2026 user profiles, organization webhooks and inference hooks for Enterprise, then on Oct 21 2026 API version 2026-10-21 with new SDK majors, highlightedJun 1, 2023API version 2023-06-01Oct 20, 2026User profiles, org webhooksOct 21, 20262026-10-21 and SDK majors
Fig 3From one API version to the next, with the Oct 20 platform additions

The day before, Anthropic added user profiles, so a platform that resells API access can tag each request with its end customer and a safety action lands on that customer instead of the whole organization. It also opened organization webhooks in beta that fire when one of those profiles is restricted or banned, and moved inference hooks out of beta for Claude Enterprise.

§ 04How to upgrade without surprises

  1. Search your code for temperature, top_p and top_k in Claude calls, including Message Batches, and delete them.
  2. Check any proxy, gateway or logging layer that reads X-Api-Key.
  3. Find code that reads the renamed Admin API fields, especially workspace_id on API keys, which becomes scope.
  4. Bump the SDK to the new major and run your tests against 2026-10-21. Anthropic’s advice on order: “Release the upgrade together with your fixes from the next step.”

§ 05What it means for teams running agents

Agent stacks are where the sampling change will surface first, because temperature settings tend to live in config files nobody has opened in months. The safe move is to pin the current SDK major, fix the four items above in one branch, and upgrade once. The license switch matters only if you track dependency licenses; the code does not change.

§ 06What to watch

  • Whether Anthropic sets an end date for API version 2023-06-01.
  • How the major frameworks and gateways handle the Bearer header switch.
  • More endpoints for user profiles as resellers adopt them.
Frequently asked5 questions

Q1What is Claude API version 2026-10-21?

It is the new version of Anthropic’s Claude API, released October 21, 2026. You opt in by setting the anthropic-version header or by upgrading to the new major SDK versions.

Q2Does temperature still work in the Claude API?

Not on 2026-10-21: sending temperature, top_p or top_k returns a 400 error, even at the default. Requests sample at the defaults, including a temperature of 1.0. Requests on 2023-06-01 are unchanged.

Q3Which Anthropic SDK versions use API version 2026-10-21?

Python 2.0.0, TypeScript 1.0.0, C# 13.0.0, Go 2.0.0, Java 3.0.0, PHP 1.0.0, Ruby 2.0.0 and the ant CLI 2.0.0.

Q4Why did my proxy break after upgrading the Anthropic SDK?

The new SDKs send the key as Authorization: Bearer instead of X-Api-Key. A gateway that reads X-Api-Key must read Authorization, or you can send the key in X-Api-Key yourself.

Q5Can an AI employee handle this upgrade?

A CellCog AI software engineer can find every place your code sends sampling parameters or reads renamed fields, prepare the SDK upgrade and test it, and every deploy waits for your approval.

Published 21 October 2026 All Multi-agent & AI organizations →