.NET: fix parallel tool call rendering in AGUI translation layer

[Roger Barreto (rogerbarreto)]

Summary

Fix three interlocked bugs in the AGUI translation layer that prevent parallel tool calls from rendering correctly in AG-UI protocol clients.

Bugs fixed

#	Bug	Impact
1	Synthetic streamingMessageId leaks into ToolCallStartEvent.ParentMessageId	All parallel tool calls collapse into one assistant-message card in the FE
2	ToolCallResultEvent.MessageId shared across all results in a round trip	Only the first tool result renders; subsequent results overwrite it
3	AsChatMessages doesn't coalesce consecutive assistant-tool-call messages	Multi-turn replay produces consecutive assistant(tool_calls) without intervening tool messages - Azure OpenAI HTTP 400

Why PR #5800 didn't resolve this

PR #5800's ContainsToolResult branch is gated by IsNullOrWhiteSpace(chatResponse.MessageId). In the production path, MEAI's FunctionInvokingChatClient prefills MessageId (non-empty), so the branch is skipped entirely. Bug #1 (tool-call chunks carry FunctionCallContent, not FunctionResultContent) was never addressed by that fix.

Changes

ChatResponseUpdateAGUIExtensions.cs:

• FixThis repo is missing a LICENSE file #1: Replace streamingMessageId with textStreamingFallback scoped to text events only. Tool-call chunks pass through the raw provider MessageId (empty when omitted by OpenAI), so each parallel tool call gets a distinct identity in the FE.
• FixAdding Microsoft SECURITY.MD #2: Set ToolCallResultEvent.MessageId to result-{CallId} - deterministic, unique per result regardless of MEAI batching.
• Remove dead ContainsToolResult helper (superseded by fixAdding Microsoft SECURITY.MD #2).

AGUIChatMessageExtensions.cs:

• FixThis repo is missing important files #3: Buffer consecutive AGUIAssistantMessages with ToolCalls and flush as a single ChatMessage with multiple FunctionCallContent entries, restoring the assistant(tool_calls) -> tool, tool, ... alternation Azure OpenAI requires.

Reproduction

A standalone console repro app was built to independently validate all three bugs on current main before fixing. After fixing, the repro confirms all bugs resolved.

Tests

• 2 new streaming tests (ParallelToolCalls, ToolCallResults with shared MEAI MessageId)
• 3 new coalescing tests (consecutive, single, end-of-stream scenarios)
• Updated 2 PR .NET: fix: avoid AGUI tool result message id collisions #5800 test assertions for new contract (tool-call ParentMessageId reflects raw provider value)
• All 215 AGUI unit tests pass
• All 35 AspNetCore AGUI unit tests pass

Closes#6015

[Copilot]

Pull request overview

This PR fixes three related issues in the .NET AGUI translation layer that prevented parallel tool calls and their results from rendering correctly in AG-UI protocol clients, and that could also produce invalid chat history for Azure OpenAI when replaying tool-call sequences.

Changes:

• Scope text streaming fallback MessageId generation to text events only, so tool-call events use the raw provider MessageId and don’t collapse parallel tool calls under one parent.
• Make ToolCallResultEvent.MessageId unique and deterministic per tool call (result-{callId}) to prevent FE overwrites when MEAI batches multiple tool results under one shared MessageId.
• Coalesce consecutive assistant tool_calls messages into a single assistant message with multiple FunctionCallContent entries to preserve the required assistant(tool_calls) → tool → tool alternation for Azure OpenAI.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs	Fixes message-id fallback behavior for text vs tool-call surfaces; assigns deterministic per-result ids; removes dead helper.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/AGUIChatMessageExtensions.cs	Coalesces consecutive assistant tool-call messages into one OpenAI-compatible ChatMessage.
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs	Updates prior assertions and adds new streaming regressions for parallel tool calls and batched tool results.
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatMessageExtensionsTests.cs	Adds tests covering coalescing behavior (consecutive, single-message, end-of-stream).