refusals-and-fallback changedbuild-with-claude/refusals-and-fallback
Nearest release: v2.1.309, published an hour before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+10added
Lines−10removed
From line
15
where the diff opens
First seen
14 Aug 2026
this site's first read of the page
Recorded edits27to this page, all time
The whole hunk
from line 15, old and new numbered
/
from line 15
1515* [SDK middleware](https://platform.claude.com/docs/en/cli-sdks-libraries/middleware): the SDK helper that wraps all of this
1616* [Fallback and billing recipe](https://platform.claude.com/cookbook/fable-5-fallback-billing-guide): a worked end-to-end example
1717
18The simplest setup, in beta on the Claude API: set `fallbacks` to `"default"`, and the API retries a declined request on the fallback model Anthropic recommends for its refusal category. For categories with no recommended fallback, the refusal stands. Claude Haiku 5.5 has no server-side fallback, so for it use [client-side fallback with the SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) or [write the retry yourself](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#manual-retry).
18The simplest setup, in beta on the Claude API and on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed): set `fallbacks` to `"default"`, and the API retries a declined request on the fallback model Anthropic recommends for its refusal category. For categories with no recommended fallback, the refusal stands. Claude Haiku 5.5 has no server-side fallback, so for it use [client-side fallback with the SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) or [write the retry yourself](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#manual-retry).
1919
2020<CodeGroup>
2121 ```bash cURL
from line 205
205205
206206## How refusals are billed
207207
208These billing rules apply on every platform: the Claude API, Amazon Bedrock, Claude Platform on AWS, Google Cloud, and Microsoft Foundry.
208These billing rules apply on every platform.
209209
210210**Refusals before any output:** To disrupt attempts to circumvent Anthropic's safeguards at scale, a refusal that arrives before any output is billed when its `stop_details.category` is `"bio"`, `"frontier_llm"`, or `"reasoning_extraction"`. These are the categories where Anthropic measures low volumes of false positives, as of September 2026. These refusals are billed like any other request, at the rates of the model that ran it. A refusal before any output in any other category, or with a `null` category, is not billed. Either way, `content` is empty and token counts appear in `usage`. The request still counts against your rate limits.
211211
from line 219
219219
220220There are three ways to retry a refused request on another model. The right one depends on where you are running and how much control you need.
221221
222| Your situation | Use | Why |
223| ------------------------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------- |
224| Claude API, simplest setup | [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback) | One request, one response. The API handles the retry. |
225| Any platform, using an Anthropic SDK | [The SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) | Configure once on the client. Retries happen automatically. |
226| Raw HTTP or custom retry logic | [A manual retry](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#manual-retry) with [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) | Full control. Fallback credit keeps the cost down. |
222| Your situation | Use | Why |
223| ------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------- |
224| Claude API or Claude Models on Google Cloud (Anthropic managed), simplest setup | [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback) | One request, one response. The API handles the retry. |
225| Any platform, using an Anthropic SDK | [The SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) | Configure once on the client. Retries happen automatically. |
226| Raw HTTP or custom retry logic | [A manual retry](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#manual-retry) with [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) | Full control. Fallback credit keeps the cost down. |
227227
228228Server-side fallback and the SDK middleware apply fallback credit for you. You only need the [Fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) page when you build the retry yourself. Claude Haiku 5.5 has neither server-side fallback nor fallback credit, so use the SDK middleware or a manual retry.
229229
from line 232
232232Server-side fallback retries a refused request inside a single API call. In the default mode, when the primary model declines and the refusal category has a recommended fallback, the API runs the same request on the model Anthropic recommends for that category. You can instead [name up to three fallback models of your own](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#naming-your-own-fallback-models). Either way, you get back one response that names the model that answered, so your user gets an answer in one round trip.
233233
234234<Note>
235 Server-side fallback is in beta on the Claude API. The `fallbacks` parameter is not supported on the [Message Batches API](https://platform.claude.com/docs/en/build-with-claude/batch-processing) (a batch item that includes it comes back as an errored result) and is not available on Amazon Bedrock, Google Cloud, or Microsoft Foundry. On those platforms, use [client-side fallback with the SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) instead. Claude Haiku 5.5 has no server-side fallback: with `fallbacks: "default"`, a declined request stays declined, and a list of fallback models returns a 400 error.
235 Server-side fallback is in beta on the Claude API and on [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed). On platforms without server-side fallback, use [client-side fallback with the SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) instead. The `fallbacks` parameter is not supported on the [Message Batches API](https://platform.claude.com/docs/en/build-with-claude/batch-processing) (a batch item that includes it comes back as an errored result). Claude Haiku 5.5 has no server-side fallback: with `fallbacks: "default"`, a declined request stays declined, and a list of fallback models returns a 400 error.
236236</Note>
237237
238238### Making the request
from line 1100
11001100
11011101* Retries walk your fallback list in order. A fallback model that itself refuses passes the request to the next entry.
11021102* When every model in the list has declined, the middleware returns the final refusal (the last model's refusal response) rather than raising an error.
1103* Thinking blocks from Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, Claude Haiku 5.5, or Claude Fable 5 pass through unchanged. Each retry re-sends your original request body, and the only blocks the middleware removes from conversation history on later requests are the `fallback` boundary blocks it added itself. The fallback model can't read Claude Fable 5.1 blocks, which only [Claude Fable 5.1 and Claude Mythos 5.1 read](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model), so the API drops them. The API also drops Claude Opus 5.5 blocks for every fallback model except Claude Fable 5.1 and Claude Mythos 5.1 (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). It drops Claude Sonnet 5.5 blocks too, for every fallback model except Claude Opus 5.5 on the Claude API and Google Cloud. Claude Opus 5.5 and Claude Sonnet 5.5 read Claude Haiku 5.5 blocks on the Claude API and Google Cloud, and the API drops them for a fallback model that can't read them.
1103* Thinking blocks from Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, Claude Haiku 5.5, or Claude Fable 5 pass through unchanged. Each retry re-sends your original request body, and the only blocks the middleware removes from conversation history on later requests are the `fallback` boundary blocks it added itself. The fallback model can't read Claude Fable 5.1 blocks, which only [Claude Fable 5.1 and Claude Mythos 5.1 read](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model), so the API drops them. The API also drops Claude Opus 5.5 blocks for every fallback model except Claude Fable 5.1 and Claude Mythos 5.1 (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). It drops Claude Sonnet 5.5 blocks too, for every fallback model except Claude Opus 5.5 on the Claude API and Claude Models on Google Cloud (Google managed). Claude Opus 5.5 and Claude Sonnet 5.5 read Claude Haiku 5.5 blocks on the Claude API and Claude Models on Google Cloud (Google managed), and the API drops them for a fallback model that can't read them.
11041104* Responses served through the middleware include a `fallback` content block at each model boundary, the same as server-side fallback responses. The middleware manages those blocks for you on later requests.
11051105* The model that accepted is recorded in `BetaFallbackState` (go: `betafallback.BetaFallbackState`), so follow-up requests that share the state stay pinned to it rather than re-asking a model that refused.
11061106
from line 1120
11201120 <Step title="Re-send on a fallback model">
11211121 Send the same request with `model` set to a fallback model, such as Claude Opus 4.8. If the refused request sent `thinking: {"type": "between_tools"}`, change `thinking` first: only Claude Sonnet 5.5 accepts that value, so omit `thinking` or set a value the fallback model accepts. [Server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback) under the `2026-07-01` header makes this change for you when it falls back to Claude Sonnet 5. Another model can normally serve a request that Claude Fable 5.1 or Claude Fable 5 declines. How you handle the conversation history depends on whether you redeem a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit):
11221122
1123 * **Not redeeming a credit:** you can leave the earlier `thinking` and `redacted_thinking` blocks in place or strip them to save input tokens. The fallback model normally can't use them either way: only Claude Fable 5.1 and Claude Mythos 5.1 [read Claude Fable 5.1 blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model), and only those two models, Claude Fable 5, and Claude Mythos 5 read Claude Fable 5 blocks, so for any other fallback model the API drops them. The API also drops Claude Opus 5.5 blocks for every fallback model except Claude Fable 5.1 and Claude Mythos 5.1 (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). It drops Claude Sonnet 5.5 blocks too, for every fallback model except Claude Opus 5.5 on the Claude API and Google Cloud. A retry after a Claude Haiku 5.5 refusal is always in this case. Claude Opus 5.5 and Claude Sonnet 5.5 read Claude Haiku 5.5 blocks on the Claude API and Google Cloud, so leave those blocks in place when you fall back to either model there. For a fallback model that can't read them, the API drops them without billing them.
1123 * **Not redeeming a credit:** you can leave the earlier `thinking` and `redacted_thinking` blocks in place or strip them to save input tokens. The fallback model normally can't use them either way: only Claude Fable 5.1 and Claude Mythos 5.1 [read Claude Fable 5.1 blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model), and only those two models, Claude Fable 5, and Claude Mythos 5 read Claude Fable 5 blocks, so for any other fallback model the API drops them. The API also drops Claude Opus 5.5 blocks for every fallback model except Claude Fable 5.1 and Claude Mythos 5.1 (see [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). It drops Claude Sonnet 5.5 blocks too, for every fallback model except Claude Opus 5.5 on the Claude API and Claude Models on Google Cloud (Google managed). A retry after a Claude Haiku 5.5 refusal is always in this case. Claude Opus 5.5 and Claude Sonnet 5.5 read Claude Haiku 5.5 blocks on the Claude API and Claude Models on Google Cloud (Google managed), so leave those blocks in place when you fall back to either model there. For a fallback model that can't read them, the API drops them without billing them.
11241124 * **Redeeming a credit:** send the body unchanged, because redemption requires an exact match. The server handles the earlier model's thinking blocks on a redemption, so do not strip them (see [Fields that must match the refused request](https://platform.claude.com/docs/en/build-with-claude/fallback-credit#reference)).
11251125 </Step>
11261126
No line in this hunk matches that.