Anthropic published a new Messages API version, 2026-10-21, and the first SDK majors that use it. The change most likely to break existing integrations is small and absolute:
In
2026-10-21, a request that sendstemperature,top_p, ortop_kreturns a 400 error, even at the parameter's default value.
The older version, 2023-06-01, still accepts the parameters, but with an important caveat that predates this release: Claude 4.7 and later models accept only the default values. Anthropic's parameter deprecation table says earlier API versions return a 400 when these are set to a non-default value on those models.
What else changed in 2026-10-21
documentblocks placed directly in anassistantmessage now return 400. Send documents inusermessages.- A
textblock with a non-emptycitationsarray in ausermessage now returns 400. Citations on assistant text blocks you send back are still accepted. - The latest SDK majors and the CLI no longer expose the three sampling parameters at all.
Why this hits OpenAI-compatible gateways
OpenAI-style clients send temperature all the time. It is in tutorials, framework defaults, and almost every code sample. A gateway that translates OpenAI requests into Anthropic Messages requests and forwards temperature as-is will:
- fail every request on API version
2026-10-21, and - fail requests with a non-default value on Claude 4.7 and later on any API version.
What a request loses when the parameter is removed depends on the model. Without it, sampling uses the defaults, including a temperature of 1.0. On models that accept only the defaults, such as Claude 4.7 and later, behavior is unchanged. On models that accept other values, such as Claude Sonnet 4.6, a request that lowered temperature for more consistent output loses that effect, and Anthropic notes that no other setting replaces it.
What to do
- Decide per model, not per request. Omit
temperature,top_pandtop_kfor models that accept only defaults (Claude 4.7 and later). For older models, forward them only while you stay on2023-06-01. - Pin the API version deliberately. If any traffic depends on a lowered temperature on an older model, keep that traffic on
2023-06-01until it moves to a newer model. - Get consistency from the prompt. For newer models, Anthropic's guidance is to guide behavior through prompting rather than sampling parameters.
- Test the whole translation layer. Documents in assistant turns and citations on user text blocks also changed; replayed conversation histories are where these show up.
NeuronGate angle
NeuronGate accepts OpenAI-format requests. When it routes a Claude model directly to Anthropic, it pins API version 2023-06-01 and leaves temperature, top_p and top_k out for Claude 4.7 and later, the models that accept only the defaults, so OpenAI-style clients don't hit a 400. Older Claude models still receive the values you send. See the docs for the request format.
FAQ
Does setting temperature to 1.0 avoid the error?
Not on 2026-10-21: the parameter is rejected even at its default value. Leave it out.
Will my output change if I remove temperature?
On Claude 4.7 and later, no; they already sample with the defaults. On older models that accepted a lowered temperature, outputs may become less consistent.
