July 14th, 2026

New this release:

• 🎉 (server, @lauzadis) Support enum-typed members in server event stream unions. Previously, the server codegen rejected any model with an enum-trait shape reachable through an event stream, flagging it as an unsupported constraint. The enum trait is now excluded from that check, and the generated code is updated so it actually compiles and behaves correctly: event stream members skip the MaybeConstrained/builder unconstrained-type wrapping, and the generated unmarshaller calls build() on the parsed payload so a constraint violation surfaces as an unmarshalling error. Related: smithy-lang/smithy#1388.

• 🎉 (server, @lauzadis) Add a codegen.allowMissingUnionVariant configuration (boolean, default false). When true, a union JSON body whose object did not set any recognized variant (e.g. {} or {"unknownKey": ...}) parses to Ok(None) rather than returning a deserialization error. Opt in only for services that have shipped clients depending on the lenient behavior. Client codegen is unaffected.

• 🎉 (client, aws-sdk-rust#146) Add support for third-party libraries to self-identify in the SDK user agent via framework metadata, addressing the long-standing request to customize the user agent (aws-sdk-rust#146).

  A new public FrameworkMetadata type (re-exported as aws_config::FrameworkMetadata and on each client's config module) can be set on the client config builder, on SdkConfig, and via aws_config::ConfigLoader::framework_metadata:

  let config = aws_config::from_env()
      .framework_metadata(FrameworkMetadata::new("some-framework", Some("1.0"))?)
      .load()
      .await;

  Framework metadata is additive — multiple libraries (and the application) can each self-identify without clobbering one another. The name/version are validated against the same charset as AppName (rejecting, not sanitizing, invalid characters to prevent header injection). The UserAgentInterceptor de-duplicates entries on (name, version) preserving first-seen order, caps the total at 10 unique entries, and renders each as lib/{name}/{version} in the x-amz-user-agent header.

• 🐛 (all, smithy-rs#4435) Gate event-stream try_recv_initial to RPC protocols (awsJson, awsQuery, rpcv2Cbor) on both the client fluent builder and the server protocol generator. Previously, all event-stream operations performed an unconditional try_recv_initial, which can hang indefinitely on REST-bound operations whose streams take a long time to produce their first event. This change is a stopgap; the planned permanent fix tracked in #4435 will instead surface initial messages only when explicitly requested.

Contributors
Thank you for your contributions! ❤

• @lauzadis