fix(vscode): deduplicate sync event delivery

[marius-kilocode]

What Problem This Solves

When multiple sessions stream concurrently, the shared VS Code connection receives two representations of several events: the normal live event and a compatibility sync envelope with the same event ID. Both representations currently reach every registered provider. This duplicates session resolution, event mapping, stream scheduling, and webview update work across the Kilo sidebar, editor tabs, and Agent Manager.

The duplication is especially costly during high-rate message.part.updated streaming. It increases extension-host and webview work even though the second representation carries no new information for those event types.

Why This Change Was Made

The connection now records the IDs of recently delivered live events and drops only a matching compatibility envelope that follows the live event. The filter is bounded to 1,024 IDs. It does not use time-based cleanup or scan the set on every event.

The filter intentionally preserves compatibility events that may contain information not present in the normal event path:

Event category	Behavior
message.updated, message.removed	Drop matching later sync envelope
message.part.updated, message.part.removed	Drop matching later sync envelope
session.created, session.deleted	Drop matching later sync envelope
session.updated	Preserve sync envelope for reconciliation sequence metadata
Replay-only sync events	Preserve

This is a VS Code client optimization only. It does not change the CLI server, database persistence, network delivery, or replay APIs.

User Impact

Users running multiple concurrent sessions should see less duplicate processing in the VS Code extension. Normal live updates, replay-only events, session reconciliation, and existing UI behavior remain available. The change applies to all VS Code clients using the shared connection service, including the sidebar, editor tabs, and Agent Manager.

Evidence

The same synthetic workload was run through the real SdkSSEAdapter path with 5,000,000 wire events, 11 sessions, and 8 downstream listeners.

Metric	Before	After	Change
Wire events received	5,000,000	5,000,000	No change
Events forwarded downstream	5,000,000	2,500,000	50% fewer
Downstream listener calls	40,000,000	20,000,000	50% fewer
CPU time	1,149 ms	1,113 ms	3.1% lower
Wall time	1,040 ms	993 ms	4.5% lower

The full SSE path is dominated by parsing and heartbeat timer work, so the end-to-end CPU reduction is smaller than the downstream reduction. The measurement proves that duplicate downstream dispatch is removed; it does not claim a reduction in CLI server CPU or total device CPU.

Validation	Result
Event-path unit tests	85 passed
VS Code typecheck	Passed
Formatting and diff checks	Passed
Replay-only sync behavior	Covered
session.updated reconciliation behavior	Covered

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• .changeset/calm-agent-manager-streams.md
• packages/kilo-vscode/src/services/cli-backend/connection-service.ts
• packages/kilo-vscode/src/services/cli-backend/connection-utils.ts
• packages/kilo-vscode/tests/unit/connection-utils.test.ts

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_{Reviewed by grok-4.6 · Input: 78.8K · Output: 13.3K · Cached: 588.4K}

_{Review guidance: REVIEW.md from base branch main}

[iscekic]

Approved, bot remarks below, feel free to ignore

[marius-kilocode]

Following up in 12417 @iscekic