.NET: Introduce ToolResultReductionStrategy

[arjendev]

Motivation and Context

When using tools that return very large results — such as the M365 Retrieval API returning hundreds of thousands of tokens with relevance scores — there is currently no way for a framework user to selectively reduce individual tool results while preserving the tool-call message structure. The existing ToolResultCompactionStrategy collapses entire tool-call groups into YAML summaries, which loses the assistant/tool message pairing needed for multi-turn tool interactions.

This PR proposes a new ToolResultReductionStrategy that allows users to register per-tool Func<string, string> reducers — enabling them to deserialize, filter, sort, and re-serialize tool results within their own logic, while the framework handles the plumbing. I've submitted this as a PR (rather than an issue) to best convey the intent and proposed implementation for discussion.

Description

New classes:

• ToolResultStrategyBase — Abstract base class extracted from the shared logic between compaction and reduction. Implements a sealed CompactCoreAsync with a template-method pattern, delegating to two hooks:

  • IsToolGroupEligible(group) — virtual, defaults to true
  • TransformToolGroup(group) — abstract, returns the transformed messages

• ToolResultReductionStrategy — Applies per-tool Func<string, string> reducers to FunctionResultContent while preserving the original message structure (assistant tool-call + tool result messages). Defaults minimumPreservedGroups to 0 so reducers apply even to the current turn. Only tool-call groups containing at least one registered tool name are eligible; unregistered tools within a group are preserved as-is.

Refactored:

• ToolResultCompactionStrategy — Now extends ToolResultStrategyBase instead of CompactionStrategy directly, removing ~110 lines of duplicated loop/protect/eligibility logic. Behavior is unchanged.

Tests:

• 13 new tests in ToolResultReductionStrategyTests covering: trigger/target, reducer application, unregistered tool preservation, message structure preservation, current-turn reduction, minimum preserved groups, eligibility filtering, system message preservation, pre-excluded groups, no tool groups, pipeline composition, and a real-world RAG scenario (JSON chunks with relevance scores filtered within a character budget).

Key design decisions:

	Compaction	Reduction
Output	YAML summary (single message)	Structure-preserving (rebuilt tool-call group)
minimumPreservedGroups default	16	0
Group kind	Summary	ToolCall
Customization	ToolCallFormatter delegate	Per-tool Func<string, string> reducers

Both strategies compose naturally in a PipelineCompactionStrategy — reduction first (shrink results), then compaction (collapse older groups).

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Not a breaking change. No existing APIs are modified; ToolResultCompactionStrategy behavior is identical after the base-class refactor.

[Copilot]

Pull request overview

Introduces a new .NET compaction strategy to reduce individual tool results (per-tool reducers) while preserving assistant tool-call + tool-result message structure, and refactors shared tool-group processing into a new base class used by both reduction and existing compaction.

Changes:

• Added ToolResultReductionStrategy to apply per-tool Func<string,string> reducers to FunctionResultContent while keeping tool-call group structure.
• Added ToolResultStrategyBase to centralize the shared protected-group / eligibility / exclude+insert loop for tool-call-group strategies.
• Refactored ToolResultCompactionStrategy to inherit from ToolResultStrategyBase and added unit tests for reduction behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Compaction/ToolResultStrategyBase.cs	New shared base class implementing the common “exclude + insert replacement group” loop for tool-call groups.
dotnet/src/Microsoft.Agents.AI/Compaction/ToolResultReductionStrategy.cs	New strategy that reduces per-tool results while attempting to preserve tool-call message structure.
dotnet/src/Microsoft.Agents.AI/Compaction/ToolResultCompactionStrategy.cs	Refactor to use the new base class; core behavior moved into TransformToolGroup.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ToolResultReductionStrategyTests.cs	New unit tests validating reducer application, preservation semantics, and pipeline composition.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

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