[Server] Carry trace context through a request (#452)

traceparent, tracestate and baggage reach handlers through RequestContext
and are echoed onto the notifications the request causes, so a span is not
broken by the response stream. Passed through verbatim: what they mean is
the tracing library's business, and no dependency is taken for it.
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Christopher Hertel 2026-08-19 00:20:32 +02:00 • committed by GitHub
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@ -5,6 +5,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
0.8.0
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* Carry W3C trace context through a request (SEP-414): `traceparent`, `tracestate` and `baggage` in a request's `_meta` are exposed to handlers as `RequestContext::getTraceContext()` and echoed onto the notifications that request causes, so a span stays joined across the response stream. Values pass through exactly as they arrived, and no OpenTelemetry dependency is added.
* Deliver notifications on a `subscriptions/listen` stream (SEP-2575), which previously acknowledged and then carried nothing for the rest of its life. New `Mcp\Server\Subscription\NotificationBusInterface` with two implementations — `InMemoryNotificationBus` for stdio and persistent runtimes, `Psr16NotificationBus` for PHP-FPM, where the worker holding the stream open and the worker publishing are different processes — set with `Builder::setNotificationBus()`. Registry changes are published automatically through a `PublishingEventDispatcher` that wraps whatever PSR-14 dispatcher was configured. `Builder::setSubscriptionLifetime()` replaces the hard-coded 30-second ceiling, where `0` means "until the client or the runtime ends it".
* Add `Mcp\Server\Wire\CachePolicy`, set with `Builder::setCachePolicy()`, to configure the SEP-2549 caching hints the 2026-07-28 lifecycle stamps on a cacheable result. The conservative `ttlMs: 0, cacheScope: private` stays the default, since `public` lets a shared proxy serve one caller's answer to another and only the operator can make that call. A `ReadResourceResult` may also carry its own `ttlMs`/`cacheScope`, which win over the policy.
* Add typed readers to `Mcp\Server\Stateless\InputContext`: `elicitResult()`, `samplingResult()` and `rootsResult()` deserialize a multi round-trip answer instead of handing back the raw array `response()` returns. A malformed answer reads as absent rather than throwing, so a handler asks again — which is what the spec says a server SHOULD do when the information it needs is still missing.