.NET: AGUI parallel tool calls render incorrectly — ParentMessageId and ToolCallResult MessageId collisions

[rogerbarreto]

Problem

Three interlocked bugs in the AGUI translation layer prevent parallel tool calls from rendering correctly in AG-UI protocol clients (@ag-ui/client, CopilotKit, etc.).

Bug 1: ParentMessageId collision on parallel tool calls

A single synthetic GUID (streamingMessageId) is reused as ParentMessageId on every ToolCallStartEvent across the entire SSE stream. OpenAI omits MessageId on FunctionCallContent chunks, so the fallback engages on every tool-call chunk — all parallel tool calls collapse into one assistant-message card in the frontend.

Bug 2: ToolCallResultEvent.MessageId collision

MEAI's FunctionInvokingChatClient packs all FunctionResultContent items from a single round trip into one ChatResponseUpdate with one synthetic MessageId. Reusing that ID on each emitted ToolCallResultEvent causes the frontend to overwrite results — only the first renders.

PR #5800 attempted to fix this with a ContainsToolResult branch, but it is gated by IsNullOrWhiteSpace(chatResponse.MessageId). Since MEAI prefills MessageId (non-empty), the branch is skipped entirely in the production path.

Bug 3: AsChatMessages doesn't coalesce consecutive assistant-tool-call messages

Once Bug 1 is fixed (empty ParentMessageId), the frontend produces a separate AGUIAssistantMessage per tool call. On multi-turn replay, these become consecutive assistant(tool_calls) messages without intervening tool messages — Azure OpenAI rejects with HTTP 400: "An assistant message with 'tool_calls' must be followed by tool messages responding to each 'tool_call_id'"

Why all three must be fixed together

• Fix only This repo is missing a LICENSE file #1 -> Bug This repo is missing important files #3 becomes fatal HTTP 400 on multi-turn
• Fix only Adding Microsoft SECURITY.MD #2 -> No visible effect (Bug This repo is missing a LICENSE file #1 still collapses everything)
• Fix only This repo is missing important files #3 -> No visible effect (Bug This repo is missing a LICENSE file #1 still funnels into one AGUIAssistantMessage)

Reproduction

Independently confirmed via console repro app feeding synthetic MEAI-shaped data through AsAGUIEventStreamAsync and AsChatMessages. All three bugs reproduce on current main (post-PR #5800).

Affected files

• dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
• dotnet/src/Microsoft.Agents.AI.AGUI/Shared/AGUIChatMessageExtensions.cs