Honor an explicit MinimumPreserved=0 in compaction strategies instead of collapsing to the default

[PratikDhanave]

What

The four compaction strategies (TruncationStrategy, SlidingWindowStrategy, SummarizationStrategy, ToolResultStrategy) computed their minimum-preserved floor with:

minimumPreserved := cmp.Or(max(strategy.MinimumPreserved*, 0), default)

Because max(0, 0) == 0 and cmp.Or(0, default) == default, this folds three distinct inputs into the type default (truncation 32, sliding window 1, summarization 8, tool result 16):

• unset (the intended default case) — correct
• an explicit 0 — silently overridden by the default
• a negative value — silently overridden by the default

So a caller who set MinimumPreservedGroups: 0 to preserve nothing got the default instead, and the aggressive preserve-none configuration was inexpressible. Only positive values worked.

This changes each MinimumPreserved* field to *int: a nil config uses the default, while a non-nil value is honored as-is with a negative clamped to 0:

minimumPreserved := default
if strategy.MinimumPreserved* != nil {
    minimumPreserved = max(*strategy.MinimumPreserved*, 0)
}

Why

The .NET port treats this as a nullable int and applies EnsureNonNegative(value) => Math.Max(0, value), so an explicit 0 disables the floor and negatives clamp to 0. The Go port silently discarded the value instead, leaving no equivalent for the preserve-none case. This restores cross-SDK parity.

Tests

• Added TestTruncationStrategy_ExplicitZeroPreservesNone: an index with 5 non-system groups, nil Trigger/Target, and MinimumPreservedGroups: ptr(0) now compacts and excludes all removable non-system groups rather than preserving the default 32.
• Added TestSlidingWindowStrategy_NegativeMinimumClampsToZero: a negative floor clamps to 0 (excluding all turns) instead of using the default 1.
• Updated existing call sites (contextwindow.go, the step18 example) and existing tests to the pointer field type.

go build ./..., go vet ./agent/compaction/..., and go test ./agent/compaction/... all pass.

[Copilot]

Pull request overview

Updates the compaction strategies’ “minimum preserved” floors so callers can explicitly set 0 (preserve none) and have it honored, while keeping defaults when the value is unset. This aligns the Go SDK behavior with the .NET port by making the minimum-preserved fields nullable and clamping negative values to 0.

Changes:

• Changed MinimumPreservedGroups / MinimumPreservedTurns fields from int to *int across compaction strategies to distinguish “unset” vs “explicit 0”.
• Updated compaction logic to use defaults when the pointer is nil, otherwise clamp the provided value to >= 0.
• Updated examples and tests; added new tests covering explicit 0 and negative clamping behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
examples/02-agents/agents/step18_compaction_pipeline/main.go	Updates example strategy config to use pointer-valued minimum-preserved fields and adds a local ptr helper.
agent/compaction/truncation.go	Makes MinimumPreservedGroups nullable and honors explicit 0 / clamps negatives.
agent/compaction/toolresult.go	Makes MinimumPreservedGroups nullable and honors explicit 0 / clamps negatives.
agent/compaction/summarization.go	Makes MinimumPreservedGroups nullable and honors explicit 0 / clamps negatives while keeping existing prompt/default behavior.
agent/compaction/slidingwindow.go	Makes MinimumPreservedTurns nullable and honors explicit 0 / clamps negatives.
agent/compaction/index_test.go	Updates tests to use pointer-valued MinimumPreservedGroups.
agent/compaction/contextwindow.go	Updates internal strategy wiring to provide pointer-valued minimum-preserved config.
agent/compaction/compaction_test.go	Updates tests, adds new coverage for explicit 0 and negative clamping, and adds a ptr helper for tests.

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[github-actions]

Cross-SDK Parity Review ✅

Verdict: parity-approved — this change restores alignment with the upstream .NET implementation.

What changed

All four compaction strategy structs (TruncationStrategy, SlidingWindowStrategy, SummarizationStrategy, ToolResultStrategy) change their MinimumPreserved* field type from int to *int, so an explicit 0 can be distinguished from the zero value and honored as "preserve nothing".

Upstream .NET parity

The canonical .NET equivalents live in dotnet/src/Microsoft.Agents.AI/Compaction/:

• TruncationCompactionStrategy — constructor: int minimumPreservedGroups = DefaultMinimumPreserved, then EnsureNonNegative(value) (= Math.Max(0, value)). An explicit 0 is stored as-is; negative values clamp to 0.
• SlidingWindowCompactionStrategy — same pattern with minimumPreservedTurns = DefaultMinimumPreserved.

The old Go behavior (cmp.Or(max(v, 0), default)) diverged from this: because max(0, 0) == 0 and cmp.Or(0, default) == default, an explicit 0 was silently promoted to the type default (32 / 1 / 8 / 16 depending on strategy). The .NET EnsureNonNegative approach preserves 0 — matching the intent of this PR.

Go idiom alignment

Using *int (nil = unset → use default; non-nil → honor the caller) is the idiomatic Go equivalent of .NET's constructor default-parameter pattern. The semantic contract is identical.

No new divergence introduced

No upstream Python compaction implementation was found; the .NET reference is the primary source of truth for this subsystem and the PR is consistent with it.

Labels

• public-api-change — already present and correct; struct field types are part of the exported API surface.
• parity-approved — added by this review.

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