Claude API 2026-10-21: Temperature Is Gone, and Even the Default Breaks You

Anthropic shipped API version 2026-10-21 yesterday — its first new versioned release since 2023-06-01, a gap of over three years. Seven SDK languages picked up major version bumps simultaneously: Python 2.0, TypeScript 1.0, Go 2.0, Java 3.0, C# 13.0, PHP 1.0, Ruby 2.0. The headline change is the most dangerous one: temperature, top_p, and top_k are gone. Not soft-deprecated. Gone. Send any of them — even at their default values — and the API returns a 400 error.

The Temperature Trap Is Subtler Than It Looks

Removing temperature sounds simple: just delete the parameter. The problem is that many codebases pass it explicitly as a form of documentation, setting temperature=1.0 to make intent clear. On the new API version, that call breaks too. Anthropic is unambiguous: a request on 2026-10-21 that sends temperature, top_p, or top_k returns a 400 error, even at the parameter’s default value.

There is no replacement. Anthropic says no other setting replaces it. If you need non-default sampling behavior, you stay on 2023-06-01. That version is not deprecated — pinning is the official recommendation.

The architectural reason matters: modern reasoning models run multi-pass internal verification — multiple chain-of-thought branches and self-correction loops before producing output. External temperature controls collapse those reasoning paths into a single greedy line. This is not unique to Anthropic. OpenAI already removed temperature controls on o1, o3, and GPT-5. Google flagged it as harmful on Gemini 3. The industry is converging on the view that frontier models manage their own sampling.

One more thing to audit: evaluation harnesses. If your evals used temperature=0 to get deterministic outputs for comparison, those runs will now either fail on the new version or produce more variable results. Re-baseline before you trust the numbers.

The Silent Proxy Breaker

The second change is less visible but equally dangerous. New SDK versions send credentials as Authorization: Bearer sk-ant-... instead of the legacy X-Api-Key header. Any proxy or API gateway reading the key from X-Api-Key will receive requests with no visible authentication credential. The failure is silent at the proxy layer — the key just disappears.

If you have internal gateways routing Anthropic API calls, check your header handling before upgrading. If you need to preserve the old header, the SDK lets you override:

const client = new Anthropic({
  defaultHeaders: {
    "X-Api-Key": process.env.ANTHROPIC_API_KEY,
    Authorization: null
  }
});

SDK Changes That Will Catch You

The temperature removal is the most common break, but each SDK has its own gotchas.

Python 2.0: MessageParam is now a union type. Constructing it as MessageParam(role="user", content="Hello") raises a TypeError. Use a plain dict instead: {"role": "user", "content": "Hello"}. Passing both api_key and auth_token to the client now raises ValueError.

TypeScript 1.0: The temperature field has been removed from the type definitions entirely, so the compiler will catch this at build time rather than runtime — actually useful. The betas parameter on models.list() is also gone; move to client.beta.models.list().

Go 2.0: The module path changes to github.com/anthropics/anthropic-sdk-go/v2. Leave the old import path in your go.mod and you silently install v1 while wondering why nothing changed.

If you use Bedrock, Vertex, or Foundry platform SDKs, upgrade them simultaneously with the core SDK. Mismatched versions cause package managers to install duplicate SDK copies.

Bedrock Developers: Check Your Model Version First

API version 2026-10-21 only works with Claude 4.7 or later on Amazon Bedrock. If you run older Claude models via Bedrock, stay on current SDK versions until you upgrade the model. Upgrading the SDK without upgrading the model will cause every request to fail.

Migration Checklist

1. Search your codebase for temperature, top_p, and top_k — including batch jobs, test fixtures, and prompt templates
2. Check any API gateway or proxy for X-Api-Key header dependencies
3. Review Admin API usage for renamed fields (workspace_id → scope, roles → role_ids, name → display_name)
4. Move document blocks out of assistant message turns into user messages
5. Upgrade all SDK packages together — coordinate platform SDK packages if using Bedrock/Vertex/Foundry
6. Verify Bedrock models are Claude 4.7+ before upgrading on Bedrock
7. Re-run and re-baseline eval harnesses that depended on fixed temperatures

The official Anthropic migration guide covers every field rename and SDK-specific detail. The old 2023-06-01 API version remains supported with no deprecation timeline announced, so you can take this migration at your own pace — just do not auto-upgrade your SDK without auditing your codebase first.