ReadItemAsync and bulk execution throwing Bad Request (400) with special characters

Description When creating a CosmosClient and setting the option AllowBulkExecution to true, the client throws a Bad Request Exception with an empty Reason for specific ID / partitionKey values.

Example id value: NE3cDI8BljAatTkAptI++kPrmDCa+S8j6wBG4Kc7W/A=

To Reproduce

• Create a cosmos db with a container
• Set the id to be the partitionKey
• Create the cosmos client with the options for bulk execution to true
• Read the above id value

var key = "cosmosKey";
var endpoint = "cosmosEndpoint";
var dbName = "dbName";
var containerId = "containerId";

var options = new CosmosClientOptions()
{
    AllowBulkExecution = true
};

var client = new CosmosClient(endpoint, key, options);
var container = client.GetContainer(dbName, containerId);

var id = "NE3cDI8BljAatTkAptI++kPrmDCa+S8j6wBG4Kc7W/A=";
var pk = new PartitionKey(id);

try
{
    var item = await container.ReadItemAsync<dynamic>(id, pk);
}
catch (Exception ex)
{

}

Expected behavior Error 404 is thrown (Not Found).

Actual behavior Error 400 is thrown (Bad Request), with an empty Reason.

Environment summary SDK Version: 6.0.203 OS Version: windows Microsoft.Azure.Cosmos

• 3.29.0
• 3.29.0-preview
• 3.30.0
• 3.30.0-preview
• 3.30.1
• 3.30.1-preview

Additional context

• When I remove the slash from NE3cDI8BljAatTkAptI++kPrmDCa+S8j6wBG4Kc7W/A=, making it NE3cDI8BljAatTkAptI++kPrmDCa+S8j6wBG4Kc7WA= it does throw a 404 Not Found exception (expected).
• When I change AllowBulkExecution to false, it also throws a 404 Not Found exception (expected).

[silvioguiso]

Gotcha, that makes sense. Thank you for your input!

[ealsur]

The problem is that because of system design, Create operations send the Ids as part of the body. Read operations use the Id as part of the path/url, that is when the / character generates problems.

Ideally the service should reject these documents, but that behavior could be interpreted as a breaking change (some customers might be saving data and not using point reads, and it theory works).

But using these characters can lead to failures across different stacks (for example, trying to read those documents in the Portal might also fail).