.NET: Fix AG-UI tool message ordering for multi-turn tool calls (#2699) #2700
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Pull request overview
This PR fixes a critical bug in AG-UI agents where multi-turn conversations with multiple tool calls would fail on the second turn with a 400 error from the OpenAI provider. The issue was caused by incorrect message ordering in the conversation history that violated the OpenAI tools protocol.
Key Changes
- Introduces a
FixToolMessageOrderingmethod that reorders AGUI messages before mapping to ChatMessages - Ensures each assistant message with tool calls is immediately followed by its corresponding tool result messages
- Handles edge cases including orphaned tool messages and tool calls without matching results
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
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dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
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dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs
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…pointRouteBuilderExtensions.cs Co-authored-by: Copilot <[email protected]>
Added XML documentation and changed FixToolMessageOrdering to internal for broader usage. Introduced extensive unit tests to validate message reordering behavior in various scenarios. Updated MapAGUIAgent to use FixToolMessageOrdering for consistent production logic. Added InternalsVisibleTo attribute for test assembly access and included a copyright notice in AssemblyInfo.cs.
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Motivation and Context
Why is this change required?
AG-UI agents fail on the second turn after executing multiple tool calls, because the message history sent back to the provider violates the OpenAI tools protocol. The model rejects the request with a 400 error.
What problem does it solve?
When AG-UI streams tool calls, it emits several assistant messages with ToolCalls, followed later by several tool messages with results. When replayed in this order, the provider complains that assistant messages containing tool_calls are not immediately followed by tool responses. This PR ensures the backend normalizes the ordering so that each assistant tool call is directly followed by its corresponding tool result.
What scenario does it contribute to?
Enables multi-turn conversations involving multiple tool calls, which is a core scenario for agents. Without this fix, any multi-tool-turn conversation breaks on the next user turn.
Linked issue:
Fixes #2699
Description
This change introduces a preprocessing step inside MapAGUI that reorders incoming AGUIMessage instances before they are mapped into ChatMessages.