October 30th, 2025

Breaking Changes:

• ⚠️🎉 (server, smithy-rs#4356) Parse EventStream signed-frames for servers marked with @sigv4.

  This is a breaking change, because events from SigV4 services are wrapped in a SignedEvent frame.

• ⚠️ (all, smithy-rs#4367) Upgrade MSRV to Rust 1.88.0.

New this release:

• 🐛🎉 (server, smithy-rs#4352, smithy-rs#4345) Update smithy-rs servers to support sending initial-response events over event streams.

  Prior to this change, event streams that had initial responses were unsupported. This change also adds a new codegen setting, alwaysSendEventStreamInitialResponse.

  When this setting is set to true, the generated server will unconditionally send initial-response objects, even when empty. This is required for compatibility with smithy-java as well as a few other clients.

  This setting defaults to false currently because smithy-rs based clients do not currently support this behavior.

  "codegen": {
    "alwaysSendEventStreamInitialResponse": true // default false
  }

• 🎉 (all, @arielby) Include the protocol name in package.metadata.smithy.protocol in Cargo.toml
  to allow easily figuring out which protocol was used to generate a crate.

• 🐛🎉 (client, smithy-rs#4349, @arielby) Make Hyper idle pool timeout configurable, and fix the bug where pool timeouts
  would not work if the client was built directly.

• 🎉 (server, smithy-rs#4317, @jasgin) Adds validators and codegen support for the custom traits custom traits @validationException, @validationMessage,
  @validationFieldList, @validationFieldName, and @validationFieldMessage for defining a custom validation exception
  to use instead of smithy.framework#ValidationException.

• 🐛 (client, smithy-rs#4346) Fix bug where httpQueryParams were silently dropped when no other query parameters were modeled.

• 🐛 (server, smithy-rs#4344, smithy-rs#4325) Fix bug where servers did not attempt to parse an initial-request. initial-requests may be sent by clients both when they would contain valid data
  and when they are empty.

• 🐛 (client, smithy-rs#4352, smithy-rs#4353) Update clients to allow initial-response events to be accepted on event streams, even when know modeled initial response exists.

  This is required for spec compliance, backwards compatibility, and compatibility with non-smithy-rs based servers that
  MAY unconditionally send initial-response messages.

• 🐛 (client, smithy-rs#4265, smithy-rs#4189) Adds new with_test_defaults_v2() for all clients supporting region configuration which applies us-east-1 as default region if not set by user. This allows aws-smithy-mocks to work for non AWS SDK generated clients. Also clarify test-util feature requirement when using aws-smithy-mocks.

Contributors
Thank you for your contributions! ❤

• @arielby (smithy-rs#4349)
• @jasgin (smithy-rs#4317)