Fix stale background compaction across model switches and /compact#317163
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Pull request overview
This PR fixes a lifecycle bug in Copilot’s background conversation compaction where a completed background summary could be applied on a later turn even after the user switched models or ran /compact, leading to unexpected “Compacted conversation” notices with ample context headroom.
Changes:
- Track the endpoint identity on
BackgroundSummarizerand recreate/cancel the summarizer when the endpoint changes for the samesessionId. - Cancel any pending background summarizer when
/compactbegins foreground compaction. - Add a pre-render “apply” guard (
contextRatio >= applyMinRatio) that discards completed background results when they’re no longer warranted, plus telemetry for the new discard outcome.
Show a summary per file
| File | Description |
|---|---|
| extensions/copilot/src/extension/prompts/node/agent/backgroundSummarizer.ts | Adds applyMinRatio threshold and records the endpoint model on BackgroundSummarizer. |
| extensions/copilot/src/extension/intents/node/agentIntent.ts | Cancels/recreates background summarizers across endpoint changes; cancels on /compact; gates/discards pre-render apply and adds telemetry outcome. |
| extensions/copilot/src/extension/intents/node/test/agentSummarizeCommand.spec.ts | Adds tests for /compact cancellation, endpoint-switch cancellation, and same-endpoint reuse. |
Copilot's findings
- Files reviewed: 3/3 changed files
- Comments generated: 3
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The `_backgroundSummarizers` map on `AgentIntent` is keyed only by sessionId, so a summary kicked off against one model's prefix could be applied unconditionally on the next render — even after the user switched to a model with a larger context window or ran `/compact`. The user saw a 'Compacted conversation' notice on a turn with plenty of headroom, with content summarized against the old model's history. * `BackgroundSummarizer` now records the `endpointModel` it was built for. * `AgentIntent.getOrCreateBackgroundSummarizer` cancels and recreates the summarizer if the endpoint identity changed since last call. * `handleSummarizeCommand` (`/compact`) cancels any pending background summarizer once we commit to foreground compaction. * Pre-render apply now gates on `contextRatio >= applyMinRatio` (0.65) as defense-in-depth — covers context-size overrides and any path that slips past the endpoint check. Stale completed results are consumed-and-discarded so a fresh kick-off can replace them. Telemetry gains a `discardedBelowApplyMinRatio` outcome. Refs #316116
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dmitrivMS
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May 23, 2026
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Fixes #316116.
AgentIntent._backgroundSummarizersis keyed only bysessionId, so a completed background summary kicked off against one model's prefix was applied unconditionally on the next render — even after the user switched models (typically to a bigger window) or ran/compact. Result: surprising "Compacted conversation" notice on a turn with plenty of headroom.The floor logic from #315444 composes correctly across switches; the bug is in state lifecycle.
Fix
BackgroundSummarizernow records theendpointModelit was built for.AgentIntent.getOrCreateBackgroundSummarizercancels & recreates the summarizer when the endpoint changes mid-session.handleSummarizeCommand(/compact) cancels any pending background summarizer once foreground compaction starts.contextRatio >= 0.65as defense-in-depth (covers context-size overrides). Below threshold the stale result is consumed-and-discarded.