Incorrect Content-Type and Accept headers for bidirectional streaming operations in RPC v2 CBOR protocol

[vikramdara]

Describe the bug

When making API calls to bidirectional streaming operations using the RPC v2 CBOR protocol, the generated Rust client sets incorrect HTTP headers for Content-Type and Accept. Both headers are set to application/cbor instead of the expected vnd.amazon.eventstream for streaming operations.

Expected Behavior

According to the Smithy documentation, when an operation:

• Sends an EventStreamSender in the request, the Content-Type header should be vnd.amazon.eventstream
• Returns an EventReceiver in the response, the Accept header should be vnd.amazon.eventstream

Current Behavior

The generated client sets:

• Content-Type: application/cbor (should be vnd.amazon.eventstream)
• Accept: application/cbor (should be vnd.amazon.eventstream)

This causes the API call to hang indefinitely in the .await() block without returning a response.

Sample Request Headers

content-type: application/cbor 
smithy-protocol: rpc-v2-cbor 
accept: application/cbor 
x-amz-date: 20251124T171442Z 
authorization: **** 
x-amz-security-token: ****** 
amz-sdk-request: attempt=1; max=3 

Related Issue

Seems like this was an issue also on the server side, but has been fixed: #4322.

[landonxjames]

Do you have an example of an AWS service/operation that this is failing with that we can test a fix against?

[vikramdara]

Our smithy model we use is for an internal AWS team that isn't exposed to the public. However, the operation itself is pretty standard.

Here is a simplified, generic version of our model

main.smithy:

use aws.auth#sigv4
use smithy.protocols#rpcv2Cbor

@rpcv2Cbor
@sigv4(name: "generic-service")
service GenericStreamingService {
    version: "2018-05-10"
    operations: [
        BiDiOperation
    ]
}

bidi_operation.smithy:

@documentation("This operation starts a Bidirectional Stream .")
@http(code: 200, method: "POST", uri: "/v2/BiDiOperation")
operation BiDiOperation {
    input: BidiInput
    output: BidiOutput
    errors: [
        ....
    ]
}


structure BidiInput {
    @documentation("The input stream contains the input request events from client.")
    @required
    stream: BidiInputStream
}


@documentation("Client publishes an input request event through the input stream.")
@streaming
union BidiInputStream {
    update: UpdateHandler
    data: DataInserter
    rule: RuleInserter
    result: ResultFetcher
}


structure BidiOutput {
    @documentation("The output stream contains response events returning to client.")
    @required
    stream: BidiOutputStream
}

@documentation("Server publishes output events to client through the output stream.")
@streaming
union BidiOutputStream {
    notification: NotificationEvent

    result: ResultEvent

    data: DataEvent

    update: UpdateEvent
}