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The PHP SDK talks to Nexus through a PSR-18 ClientInterface and PSR-17 factories. It depends on the interfaces rather than on a concrete client, so it uses whichever HTTP stack your project already has - and does not install a second one alongside it.

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Discovery

Pass nothing and php-http/discovery finds the implementation your project has installed:
If there is none, create() raises an error naming what to install.

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Injecting your own

Pass your configured client to reuse its proxy settings, middleware and instrumentation for Nexus traffic too:
Each part falls back to discovery independently, so supplying only the client is fine. In Laravel this happens for you - the service provider takes the container’s PSR-18 binding when there is one.

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Any implementation works

Both are verified on every commit. One CI job installs the package into an empty project and fails if any HTTP stack comes with it; another runs the SDK end to end against Symfony HttpClient with Guzzle absent.

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What gets installed

Installing westyx/nexus brings 6 packages, about 1 MB:

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Status codes

PSR-18 clients return 4xx and 5xx responses rather than throwing, and the SDK maps every status to a typed exception itself.
The exceptions to catch around SDK calls are WestyxNexus\Exceptions\NexusException and its subclasses, not your HTTP client’s - the SDK never lets an implementation-specific error type reach your code. See Error handling.
The mapping is shared by every endpoint, so a condition reports identically wherever it occurs: a 429 on an A/B evaluation carries its Retry-After and its quarantine details exactly as one on a sync does.

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Redirects

A Nexus endpoint never redirects, and the SDK’s requests carry the API key, so the client must not follow a redirect to a server-chosen host. The SDK treats a 3xx it receives as an error.
Configure the client you pass (or the one discovery finds) to not follow redirects. Guzzle’s PSR-18 sendRequest() already does not; build Symfony HttpClient as new Psr18Client(HttpClient::create(['max_redirects' => 0])), since its PSR-18 client follows redirects by default and would replay the API key to the redirect target.
The SSE stream sets this itself (ext-curl, no CURLOPT_FOLLOWLOCATION).

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Response size

Every response is read with a hard byte limit enforced while reading - 10 MB for a sync, 1 MB for everything else. Exceeding it raises a size error naming the limit, rather than handing a truncated document to the JSON parser.

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Live updates are the exception

PSR-18’s sendRequest() returns a complete ResponseInterface, and whether the body is buffered before returning is left to the implementation. There is no standard way to request incremental delivery, so a buffering client would block forever on an endless event stream. connectStream() therefore uses ext-curl directly, which is also what gives it a real connection deadline and a liveness deadline. Without ext-curl it raises an actionable error and everything else, TTL polling included, works normally - losing live updates is a degradation, not a failure.