Fix InvalidOperationException in CoalesceWebSearchToolCallContent

[stephentoub]

Fix InvalidOperationException in CoalesceWebSearchToolCallContent

Fixes #7414

Problem

CoalesceWebSearchToolCallContent merged streaming updates by mutating the original content objects in-place. In particular, existing.Queries = webSearchCall.Queries made two objects share the same Queries list instance. This is a problem because ToChatResponse can be called multiple times on the same streaming updates: FunctionInvokingChatClient calls it internally to process function calls (line 666), and the caller (e.g. ChatClientAgent from the Agents Framework) calls it again on the same collected updates. On the second call, both content objects now have non-null Queries pointing to the same list, so the foreach-merge path tries to iterate and add to the same list simultaneously, throwing InvalidOperationException: Collection was modified; enumeration operation may not execute.

Fix

Create a new WebSearchToolCallContent with the merged data instead of mutating the originals. This matches the pattern used by all the other coalescing methods (Coalesce<T> for TextContent, CodeInterpreterToolCallContent, etc.), which create new objects via factory delegates. When existing and webSearchCall are the same object instance (a degenerate case), the duplicate is simply removed without creating a new object.

Tests

Added a regression test that mirrors the real-world scenario: streaming updates with an in-progress web search (null Queries), a done web search (populated Queries), and a function call, with ToChatResponse called twice on the same updates. Parameterized over sync/async and whether the first item has queries (exercising both the reference-assignment and list-combining branches).

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes an InvalidOperationException thrown when ToChatResponse is invoked multiple times over the same streaming updates containing WebSearchToolCallContent, by avoiding in-place mutation during coalescing.

Changes:

• Update CoalesceWebSearchToolCallContent to create a new merged WebSearchToolCallContent instead of mutating existing instances.
• Add a regression test that calls ToChatResponse twice on the same update sequence (sync/async), covering both “first update has queries” and “first update has null queries” paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs	Changes web-search coalescing to be non-mutating by replacing the coalesced item with a newly constructed merged instance.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs	Adds a regression test reproducing the multi-ToChatResponse scenario that previously caused the exception.