[Irakli Gabisonia (gabisonia)]

When a compaction context provider receives no session, it currently applies its strategy and can remove input messages. Return the original messages and options without invoking the strategy, matching the .NET 1.22.0 provider.

This changes the previously documented behavior for direct provider calls and middleware calls with no session. Callers that need stateless compaction should use compaction.Compact. Normal Agent.Run calls continue to compact because the agent creates an implicit session.

Update the API documentation and add regression coverage for omitted/nil sessions, preservation of input messages and options, middleware passthrough, and normal agent execution with an implicit session. Existing source-attribution tests now provide a local session.

Validation on macOS arm64 with Go 1.26.1:

• Regression tests fail against the original provider and pass with this change.
• Executed equivalent cases against the published .NET 1.22.0 assembly, verifying source revision 0c9944cc9f577d51277ac7c55dbc388b60a577af. For six input messages, both implementations preserve all six without a session and retain two with an explicit session, an implicit agent session, or the standalone compaction API.
• go test ./...: 54 packages passed.
• go test -race -shuffle=on ./...: 54 packages passed, no races.
• golangci-lint run (2.15.0): zero issues.
• Formatting and git diff --check passed.

[Copilot]

🟢 Approval recommended

No active review issues are reported for the sessionless compaction passthrough change.

No open issues 🎉

What changed in this PR

This PR makes the compaction context provider pass through messages and options unchanged when no session is available. Stateless callers are directed to compaction.Compact, while normal agent runs retain compaction through implicit sessions.

Changes:

• Skip strategy execution when the session is omitted or nil.
• Document the session requirement and standalone compaction alternative.
• Add regression coverage for passthrough and implicit-session behavior, and update source-attribution tests to use local sessions.

File	Description
agent/​compaction/​provider.go	Return inputs unchanged without a session; update documentation and debug logging.
agent/​compaction/​compaction_test.go	Cover direct and middleware passthrough, implicit-session compaction, and session-backed source attribution.

_{Review effort: Balanced}

[github-actions]

API consistency review

Result	Scope
aligned	user-visible behavior

Changed Go contract

No exported API changes. compaction.NewContextProvider now returns messages and options unchanged (without invoking the strategy) when there is no session. Previously it compacted statelessly.

Upstream evidence reviewed

Implementation	Source	Contract checked
.NET	CompactionProvider.InvokingCoreAsync	session is null returns context.AIContext unchanged. Ad-hoc compaction is via the static CompactAsync. Services-managed sessions are skipped.

Assessment

Go now matches .NET: no session means passthrough, and stateless compaction stays available through compaction.Compact, the counterpart of CompactAsync. Service-managed session skipping is unchanged. I did not review a Python equivalent. The .NET source is the parity reference cited by the PR and is sufficient for this change. No public-api-change label is needed.

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