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.NET: [Bug]: AGUI client crashes on non-JSON TOOL_CALL_RESULT content from handoff with JsonException ('T' is an invalid start of a value) #5587

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.NET: [Bug]: AGUI client crashes on non-JSON TOOL_CALL_RESULT content from handoff with JsonException ('T' is an invalid start of a value)#5587
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javiercn
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.NETUsage: [Issues, PRs], Target: .Netag-uiUsage: [Issues, PRs], Target: AG-UI protocol integrationbugUsage: [Issues], Target: all issues (Legacy, prefer issue type: bug)

Description

@statto1974
Toby Statham (statto1974)
opened on Apr 30, 2026

Description

I’m seeing a crash in AGUI streaming when a workflow handoff emits a TOOL_CALL_RESULT with plain text content.

Environment

  1. Microsoft Agent Framework (latest main as of 2026-04-30)
  2. AGUI client via AGUIChatClient
  3. Workflow built with handoff builder and exposed via AGUI endpoint

Repro

  1. Build workflow with handoff orchestration.
  2. Consume endpoint with AGUIChatClient.
  3. Call RunStreamingAsync.
  4. Observe AGUI SSE events:
  • RUN_STARTED
  • TOOL_CALL_START (handoff_to_1)
  • TOOL_CALL_ARGS
  • TOOL_CALL_END
  • TOOL_CALL_RESULT with content: Transferred.

Actual
Client throws:
System.Text.Json.JsonException: 'T' is an invalid start of a value
at ChatResponseUpdateAGUIExtensions.DeserializeResultIfAvailable(ToolCallResultEvent ...)

Expected
Non-JSON tool result content should not crash deserialization.
Either:

treat content as raw string result, or
deserialize as JsonElement only when content is valid JSON.
Root cause candidate
In ChatResponseUpdateAGUIExtensions:

SerializeResultContent returns raw string for FunctionResultContent strings.
DeserializeResultIfAvailable always attempts JsonSerializer.Deserialize(... JsonElement ...) for non-empty content.
This fails for plain text like Transferred.
Suggested fix
Fallback to raw string when JsonElement deserialization fails.

Code Sample



Error Messages / Stack Traces




Package Versions


Microsoft.Agents.AI.AGUI: 1.3.0-preview.260423.1


.NET Version


NET 10.0


Additional Context


No response
Activity
  1. Toby Statham (statto1974)
    added 
    bugUsage: [Issues], Target: all issues (Legacy, prefer issue type: bug)
     
    .NETUsage: [Issues, PRs], Target: .Net
     on Apr 30, 2026
  2. Toby Statham (statto1974)
    added the
    Bug
    issue type on Apr 30, 2026
  3. Evan Mattson (moonbox3)
    added 
    triageUsage: [Issues], Target: All issues that still need to be triaged
     on Apr 30, 2026
  4. Evan Mattson (moonbox3)
    assigned 
    Javier Calvarro Nelson (javiercn)
    on May 4, 2026
  5. Evan Mattson (moonbox3)
    added 
    ag-ui
     and removed 
    triageUsage: [Issues], Target: All issues that still need to be triaged
     on May 4, 2026
  6. awaescher commented on May 4, 2026 
    @awaescher
    Andreas Wäscher (awaescher)
    on May 4, 2026
    Seems to be related:
    

    This missing function exception ...
    

    "Error: Requested function \"ExecuteFormAction\" not found."

    

    ... gets hidden by this follow up exception:
    

    System.Text.Json.JsonReaderException: ''E' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.'

    
Image

    
Image
  7. max-meng1 commented on May 20, 2026 
    @max-meng1
    max-meng1
    on May 20, 2026
    <PackageReference Include="Microsoft.Agents.AI.Hosting.AGUI.AspNetCore" Version="1.0.0-preview.251125.1" />

message	{"threadId":"thread_0431523053504a11b04bbd6fd50ed360","runId":"run_9a5e473fdc8c4fdbb7b61e6056c07478","type":"RUN_STARTED"}
message	{"toolCallId":"call_oHFqbbIQTM5N6qEiy7WGsVnb","toolCallName":"GetRecordingFields","parentMessageId":"chatcmpl-DhOeiwRS1QsVvSGhHbvlfvX2KRIAI","type":"TOOL_CALL_START"}	
message	{"toolCallId":"call_oHFqbbIQTM5N6qEiy7WGsVnb","delta":"{}","type":"TOOL_CALL_ARGS"}	
message	{"toolCallId":"call_oHFqbbIQTM5N6qEiy7WGsVnb","type":"TOOL_CALL_END"}
message	{"threadId":"thread_0431523053504a11b04bbd6fd50ed360","runId":"run_9a5e473fdc8c4fdbb7b61e6056c07478","result":null,"type":"RUN_FINISHED"}

    

    /*Failed*/
<PackageReference Include="Microsoft.Agents.AI.Hosting.AGUI.AspNetCore" Version="1.6.1-preview.260514.1" />

message	{"threadId":"thread_d87185f9bab54954913eafdc0c5243d8","runId":"run_e27e114213174f638c11de78f1ec4ae8","type":"RUN_STARTED"}	
message	{"toolCallId":"call_rGsqLxDT6Hud5ophOVfmIirV","toolCallName":"GetRecordingFields","parentMessageId":"chatcmpl-DhOoT2W5d3zcxh3DEuTGr6rSQfgzU","type":"TOOL_CALL_START"}	
message	{"toolCallId":"call_rGsqLxDT6Hud5ophOVfmIirV","delta":"{}","type":"TOOL_CALL_ARGS"}	
message	{"toolCallId":"call_rGsqLxDT6Hud5ophOVfmIirV","type":"TOOL_CALL_END"}	
message	{"messageId":"ffa3714dd6e04ab899e3155258023886","toolCallId":"call_rGsqLxDT6Hud5ophOVfmIirV","content":"Success: Function completed.","role":"tool","type":"TOOL_CALL_RESULT"}	

    

    Same issue, there is an extra message TOOL_CALL_RESULT after TOOL_CALL_END, after upgrade the package.
  8. Evan Mattson (moonbox3)
    added 
    ui integration
     
    ag-uiUsage: [Issues, PRs], Target: AG-UI protocol integration
     and removed 
    ag-ui
     
    ui integration
     on Jun 17, 2026
  9. javiercn commented on Aug 18, 2026 
    @javiercn
    Javier Calvarro Nelson (javiercn)
    on Aug 18, 2026
    Contributor
    This issue is resolved by the current AGUI.Client implementation.
    

    The former converter serialized string tool results as plain text but then unconditionally attempted to deserialize that text as a JsonElement. Values such as "Transferred." or "Success: Function completed." therefore failed at their first non-JSON character.
    

    The current SDK passes ToolCallResultEvent.Content directly into FunctionResultContent:
    

    new FunctionResultContent(
    toolResult.ToolCallId,
    toolResult.Content);
    

    We exercised plain text, JSON-looking text, empty text, and null results. Plain-text results were returned as strings without exceptions in repeated runs. JSON-looking results are also preserved as strings; consumers that require a JSON DOM should parse them explicitly.
    

    Applications should use AGUI.Client instead of the removed Microsoft.Agents.AI.AGUI package. Since the failing conversion path has been replaced and the reported scenario now succeeds, this issue can be closed as resolved.
  10. Javier Calvarro Nelson (javiercn)
    closed this as completedon Aug 18, 2026
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