middleware changedcli-sdks-libraries/middleware

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11---

22title: SDK middleware

33url: https://platform.claude.com/docs/en/cli-sdks-libraries/middleware

4description: Intercept and modify requests and responses in the Anthropic SDKs.

4description: Intercept and modify the SDK's requests and responses.

55---

7The Anthropic SDKs provide a middleware (or interceptor) hook that lets you run code before a request is sent and after the response is received. Use middleware for cross-cutting concerns such as logging, custom retries, request annotation, and refusal fallback handling.

7The Claude SDK provides a middleware (or interceptor) hook that lets you run code before a request is sent and after the response is received. Use middleware for cross-cutting concerns such as logging, custom retries, request annotation, and refusal fallback handling.

99```mermaid

1010sequenceDiagram

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2828

2929## Registering middleware

3030

31Each middleware is a function that receives the outgoing request and a `next` callable. Call `next` to forward the request to the rest of the chain (or directly to the SDK core if this is the last middleware), and return its response. Anything before the `next` call runs on the way out; anything after runs on the way back.

31Each middleware is a function that receives the outgoing request and the next handler. Call `call_next(request)` (python; typescript: `next(request)`; csharp: `next(request, cancellationToken)`; go: `next(req)`; java: `nextClient.execute(request, requestOptions)`; php: `$next($request)`; ruby: `call_next.call(request)`) to forward the request to the rest of the chain (or directly to the SDK core if this is the last middleware), and return its response. Anything before that call runs on the way out; anything after runs on the way back.

3232

3333<CodeGroup exclude="shell">

3434 ```python Python

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171171

172172## Replacing the HTTP client

173173

174Each SDK also accepts a custom HTTP client (for proxy configuration, custom TLS, or connection pooling). Only one HTTP client is used per SDK client; setting it replaces the default. The custom HTTP client receives requests after all middleware has run.

174The SDK also accepts a custom HTTP client (for proxy configuration, custom TLS, or connection pooling). Only one HTTP client is used per SDK client; setting it replaces the default. The custom HTTP client receives requests after all middleware has run.

175175

176176## Built-in middleware

177177

178The SDKs ship a refusal-fallback middleware that automatically retries requests Claude Fable 5 declines on a fallback model. See [Detect and retry on a fallback model](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) for setup and per-language examples.

178The SDK ships a refusal-fallback middleware that automatically retries requests Claude Fable 5 declines on a fallback model. See [Detect and retry on a fallback model](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback) for setup and per-language examples.

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