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+# Microsoft.Agents.AI.Compaction — Namespace Design
+
+This document describes the types in the `Microsoft.Agents.AI.Compaction` namespace and how they relate to each other.
+
+The compaction system manages long-running chat histories by reducing token and message counts before they exceed model context limits.  Two integration points exist:
+
+- **Pre-write compaction** – `InMemoryChatHistoryProviderOptions.ChatReducer` runs the pipeline when messages are stored or retrieved.
+- **In-run compaction** – a `CompactingChatClient` inserted into the `IChatClient` pipeline runs compaction on every inference call.
+
+`ChatHistoryCompactionPipeline` implements `IChatReducer`, so it can be used at either integration point.
+
+## Class Diagram
+
+```mermaid
+classDiagram
+    direction TB
+
+    %% ── External contracts (Microsoft.Extensions.AI) ─────────────────────
+    class IChatReducer {
+        <<interface>>
+        +ReduceAsync(messages, cancellationToken) Task~IEnumerable~ChatMessage~~
+    }
+
+    class IChatClient {
+        <<interface>>
+    }
+
+    %% ── Core namespace interface ──────────────────────────────────────────
+    class IChatHistoryMetricsCalculator {
+        <<interface>>
+        +Calculate(messages) ChatHistoryMetric
+    }
+
+    %% ── Pipeline ──────────────────────────────────────────────────────────
+    class ChatHistoryCompactionPipeline {
+        +ReduceAsync(messages, cancellationToken) Task~IEnumerable~ChatMessage~~
+        +CompactAsync(messages, cancellationToken) CompactionPipelineResult
+        +Create(approach, size, chatClient)$ ChatHistoryCompactionPipeline
+    }
+
+    class PipelineApproach["ChatHistoryCompactionPipeline.Approach"] {
+        <<enum>>
+        Aggressive
+        Balanced
+        Gentle
+    }
+
+    class PipelineSize["ChatHistoryCompactionPipeline.Size"] {
+        <<enum>>
+        Compact
+        Adequate
+        Accomodating
+    }
+
+    %% ── Abstract base strategy ────────────────────────────────────────────
+    class ChatHistoryCompactionStrategy {
+        <<abstract>>
+        +Reducer IChatReducer
+        +Name string
+        #ShouldCompact(metrics) bool*
+        ~CompactAsync(history, calculator, ct) CompactionResult
+        #CurrentMetrics$ ChatHistoryMetric
+    }
+
+    %% ── Concrete strategies ───────────────────────────────────────────────
+    class ChatReducerCompactionStrategy {
+        +ChatReducerCompactionStrategy(reducer, condition)
+        #ShouldCompact(metrics) bool
+    }
+
+    class ToolResultCompactionStrategy {
+        +DefaultPreserveRecentGroups$ int
+        +ToolResultCompactionStrategy(maxTokens, preserveRecentGroups)
+        #ShouldCompact(metrics) bool
+    }
+
+    class SummarizationCompactionStrategy {
+        +DefaultSummarizationPrompt$ string
+        +SummarizationCompactionStrategy(chatClient, maxTokens, preserveRecentGroups, prompt)
+        #ShouldCompact(metrics) bool
+    }
+
+    class SlidingWindowCompactionStrategy {
+        +SlidingWindowCompactionStrategy(maxTurns)
+        #ShouldCompact(metrics) bool
+    }
+
+    class TruncationCompactionStrategy {
+        +TruncationCompactionStrategy(maxTokens, preserveRecentGroups)
+        #ShouldCompact(metrics) bool
+    }
+
+    %% ── Metrics & calculator ──────────────────────────────────────────────
+    class DefaultChatHistoryMetricsCalculator {
+        +Instance$ DefaultChatHistoryMetricsCalculator
+        +DefaultChatHistoryMetricsCalculator(charsPerToken)
+        +Calculate(messages) ChatHistoryMetric
+    }
+
+    class ChatHistoryMetric {
+        +TokenCount int
+        +ByteCount long
+        +MessageCount int
+        +ToolCallCount int
+        +UserTurnCount int
+        +Groups IReadOnlyList~ChatMessageGroup~
+    }
+
+    %% ── Message grouping ──────────────────────────────────────────────────
+    class ChatMessageGroup {
+        <<struct>>
+        +StartIndex int
+        +Count int
+        +Kind ChatMessageGroupKind
+    }
+
+    class ChatMessageGroupKind {
+        <<enum>>
+        System
+        UserTurn
+        AssistantToolGroup
+        AssistantPlain
+        ToolResult
+        Other
+    }
+
+    %% ── Result types ──────────────────────────────────────────────────────
+    class CompactionResult {
+        +StrategyName string
+        +Applied bool
+        +Before ChatHistoryMetric
+        +After ChatHistoryMetric
+        +Skipped(name, metrics)$ CompactionResult
+    }
+
+    class CompactionPipelineResult {
+        +Before ChatHistoryMetric
+        +After ChatHistoryMetric
+        +StrategyResults IReadOnlyList~CompactionResult~
+        +AnyApplied bool
+    }
+
+    %% ── Relationships ─────────────────────────────────────────────────────
+
+    %% Pipeline implements IChatReducer so it can plug into any reducer slot
+    ChatHistoryCompactionPipeline ..|> IChatReducer : implements
+
+    %% Pipeline owns an ordered set of strategies
+    ChatHistoryCompactionPipeline "1" *-- "1..*" ChatHistoryCompactionStrategy : executes in order
+
+    %% Pipeline uses a metrics calculator
+    ChatHistoryCompactionPipeline --> IChatHistoryMetricsCalculator : uses
+
+    %% Pipeline produces an aggregate result
+    ChatHistoryCompactionPipeline --> CompactionPipelineResult : returns
+
+    %% Factory enums are nested inside the pipeline
+    ChatHistoryCompactionPipeline --> PipelineApproach : factory param
+    ChatHistoryCompactionPipeline --> PipelineSize : factory param
+
+    %% Each strategy wraps an IChatReducer for the actual reduction work
+    ChatHistoryCompactionStrategy --> IChatReducer : wraps
+
+    %% Strategies evaluate metrics to decide whether to compact
+    ChatHistoryCompactionStrategy --> ChatHistoryMetric : evaluates
+
+    %% Strategy execution produces a per-strategy result
+    ChatHistoryCompactionStrategy --> CompactionResult : returns
+
+    %% Concrete strategy inheritance
+    ChatReducerCompactionStrategy --|> ChatHistoryCompactionStrategy
+    ToolResultCompactionStrategy --|> ChatHistoryCompactionStrategy
+    SummarizationCompactionStrategy --|> ChatHistoryCompactionStrategy
+    SlidingWindowCompactionStrategy --|> ChatHistoryCompactionStrategy
+    TruncationCompactionStrategy --|> ChatHistoryCompactionStrategy
+
+    %% SummarizationCompactionStrategy calls an LLM to generate summaries
+    SummarizationCompactionStrategy --> IChatClient : calls for summarization
+
+    %% Default calculator implements the interface
+    DefaultChatHistoryMetricsCalculator ..|> IChatHistoryMetricsCalculator : implements
+
+    %% Calculator produces metrics
+    DefaultChatHistoryMetricsCalculator --> ChatHistoryMetric : produces
+
+    %% Metric contains groups
+    ChatHistoryMetric "1" *-- "0..*" ChatMessageGroup : contains
+
+    %% Group is classified by kind
+    ChatMessageGroup --> ChatMessageGroupKind : typed by
+
+    %% Pipeline result aggregates per-strategy results
+    CompactionPipelineResult "1" *-- "1..*" CompactionResult : aggregates
+
+    %% Each result references before/after metrics
+    CompactionResult --> ChatHistoryMetric : before / after
+```
+
+## Type Overview
+
+### Pipeline
+
+| Type | Role |
+|------|------|
+| `ChatHistoryCompactionPipeline` | Orchestrates an ordered chain of strategies; implements `IChatReducer` for drop-in use |
+| `ChatHistoryCompactionPipeline.Approach` | Factory enum — `Aggressive`, `Balanced`, `Gentle` |
+| `ChatHistoryCompactionPipeline.Size` | Factory enum — `Compact`, `Adequate`, `Accomodating` |
+
+### Strategies
+
+| Type | Trigger | Effect |
+|------|---------|--------|
+| `ToolResultCompactionStrategy` | Token count > threshold **and** tool calls present | Collapses old `AssistantToolGroup` entries into `[Tool calls: …]` summaries (gentlest) |
+| `SummarizationCompactionStrategy` | Token count > threshold | Sends older messages to an LLM and replaces them with a single summary |
+| `SlidingWindowCompactionStrategy` | User-turn count > threshold | Keeps the N most-recent user turns and drops the rest |
+| `TruncationCompactionStrategy` | Token count > threshold | Drops oldest non-system groups until within budget (most aggressive) |
+| `ChatReducerCompactionStrategy` | Custom `Func<ChatHistoryMetric, bool>` | Delegates to any `IChatReducer` with caller-supplied trigger logic |
+
+### Data Models
+
+| Type | Description |
+|------|-------------|
+| `ChatHistoryMetric` | Immutable snapshot of a conversation: token count, byte count, message count, tool-call count, user-turn count, and a `Groups` index |
+| `ChatMessageGroup` | Value type identifying a contiguous, atomic slice of the message list (`StartIndex`, `Count`, `Kind`) |
+| `ChatMessageGroupKind` | `System` · `UserTurn` · `AssistantToolGroup` · `AssistantPlain` · `ToolResult` · `Other` |
+| `IChatHistoryMetricsCalculator` | Computes a `ChatHistoryMetric` from a message list |
+| `DefaultChatHistoryMetricsCalculator` | Default implementation using JSON-length heuristics for token estimation |
+
+### Results
+
+| Type | Description |
+|------|-------------|
+| `CompactionResult` | Outcome of a single strategy: `StrategyName`, `Applied`, `Before`/`After` metrics |
+| `CompactionPipelineResult` | Aggregate outcome of the full pipeline: overall `Before`/`After` metrics and per-strategy `StrategyResults` |
+
+## Integration Points
+
+```mermaid
+flowchart LR
+    subgraph InMemoryChatHistoryProvider
+        A[ChatReducer\nInMemoryChatHistoryProviderOptions]
+    end
+
+    subgraph IChatClient pipeline
+        B[CompactingChatClient\ninternal]
+    end
+
+    P[ChatHistoryCompactionPipeline\nimplements IChatReducer]
+
+    A -->|plug in| P
+    B -->|plug in| P
+```
+
+`ChatHistoryCompactionPipeline` can be wired in at either location, or both, to apply compaction before messages are stored/retrieved **and** on every inference call.