[Agents] JsonReaderException: The input does not contain any JSON tokens during StreamingUpdate in Azure.AI.Agents.Persistent

[talleswp]

Description

I am encountering an intermittent but frequent System.Text.Json.JsonReaderException when using the Azure.AI.Agents.Persistent SDK (latest version). The error occurs when an Agent attempts to call a Microsoft Fabric tool.

The stack trace indicates that StreamingUpdate.FromEvent receives an empty or invalid payload, failing to parse it as JSON.

Regression Timeline & Comparison

We began noticing this behavior consistently starting on February 6th, 2026. Before this date, the integration was stable. Currently, the failure rate via SDK is extremely high.
Tests using Fabric Data Agent Directly works just fine. And Using ResponseApi also.

Reproduction Steps

1. Configure an Agent with a Microsoft Fabric tool connection.

2. Initialize a persistent thread and start a run with streaming enabled.

3. Iterate through the streaming updates using do while.

4. Observe the crash when the Agent triggers the Fabric tool.

Sample Code (Console App)

using Azure.AI.Agents.Persistent;
using Azure.Identity;

var url = "FOUNDRY_PROJECT_URL";
var agentId = "FOUNDRY_AGENT_ID";

var agentClient = new PersistentAgentsClient(url, new DefaultAzureCredential());

PersistentAgentThread thread = await agentClient.Threads.CreateThreadAsync();

await agentClient.Messages.CreateMessageAsync(thread.Id, MessageRole.User, content: "Give-me the client map locations using fabric");

var runTask = agentClient.Runs.CreateRunStreamingAsync(
    thread.Id, 
    agentId);
    
ThreadRun? currentRun;

do
{
    currentRun = null;
    await foreach (var update in runTask)
    {
        Console.WriteLine(update);
        switch (update)
        {
            case RunUpdate runUpdate:
                currentRun = runUpdate;
                break;
            case MessageContentUpdate messageContentUpdate:
                Console.WriteLine(messageContentUpdate.Text);
                break;
        }
    }
}while (currentRun?.Status != RunStatus.Cancelled ||
        currentRun?.Status != RunStatus.Cancelling ||
        currentRun?.Status != RunStatus.Completed ||
        currentRun?.Status != RunStatus.Failed);

Error Message / Stack Trace

Unhandled exception. System.Text.Json.JsonReaderException: The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.
   at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
   at System.Text.Json.Utf8JsonReader.Read()
   at System.Text.Json.JsonDocument.Parse(ReadOnlySpan`1 utf8JsonSpan, JsonReaderOptions readerOptions, MetadataDb& database, StackRowStack& stack)
   at Azure.AI.Agents.Persistent.StreamingUpdate.FromEvent(SseItem`1 sseItem)
   at Azure.AI.Agents.Persistent.AsyncStreamingUpdateCollection.AsyncStreamingUpdateEnumerator.MoveNextAsync()

Environment / Context

• SDK Version: Azure.AI.Agents.Persistent latest

• Tooling: Microsoft Fabric Tool.

• Service: Azure AI Foundry (Persistent Agents).

• Runtime: .NET 9 / .NET 10.