feat: Implement auto compression feature for chat context

[ethanyhou]

Demo:

Recording.2026-05-21.165012.mp4

The status context aligns with VSCode:

[jdneo]

Any consideration to expose a preference to disable auto compact?

I prefer to always enable it, enable it simply give users more value and advantages.

[Copilot]

Pull request overview

This PR adds an Auto Compress chat setting and end-to-end UI wiring for automatic conversation compression, including preference propagation to the language server and a “Compacting conversation...” status indicator in the chat view when compression notifications arrive.

Changes:

• Adds a new Auto Compress preference (default enabled) and propagates it to Copilot agent settings.
• Introduces LSP notification handling for $/copilot/compressionStarted / $/copilot/compressionCompleted, showing/hiding a spinner banner and refreshing context-size UI.
• Improves streaming UX by eagerly rendering trailing partial lines (without waiting for a newline) in chat turns.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/messages.properties	Adds localized strings for the Auto Compress preference UI.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/Messages.java	Exposes new preference strings via NLS.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/LanguageServerSettingManager.java	Sends autoCompress setting to the language server and reacts to preference changes.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/CopilotPreferenceInitializer.java	Defaults Auto Compress to enabled.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/preferences/ChatPreferencesPage.java	Adds the Auto Compress checkbox + description to the Chat preferences page.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/i18n/messages.properties	Adds “Compacting conversation...” user-facing string.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/i18n/Messages.java	Exposes the new compaction status string via NLS.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/CopilotTurnWidget.java	Adds compacting status UI (spinner + text) to the latest Copilot turn.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatView.java	Subscribes to compression events and updates turn UI + context size on completion; ensures cancel clears compaction UI.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatContentViewer.java	Adds a helper to safely fetch the latest Copilot turn widget.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/BaseTurnWidget.java	Eagerly renders trailing partial lines for better streaming visibility.
com.microsoft.copilot.eclipse.swtbot.test/test-plans/context-compress/context-compress.md	Adds a manual test plan for Auto Context Compression UX.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/CopilotAgentSettings.java	Adds autoCompress to agent settings serialization + equality/hash.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/CompressionStartedParams.java	Adds protocol type for compression-start notification.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/protocol/CompressionCompletedParams.java	Adds protocol type for compression-complete notification (with optional context info).
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/lsp/CopilotLanguageClient.java	Handles new compression notifications and forwards them via event broker.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/events/CopilotEventConstants.java	Defines new event topics for compression started/completed.
com.microsoft.copilot.eclipse.core/src/com/microsoft/copilot/eclipse/core/Constants.java	Adds the autoCompress preference key constant.

[iloveeclipse]

Any consideration to expose a preference to disable auto compact?

I prefer to always enable it, enable it simply give users more value and advantages.

We have errors reported by customers due the context "overflow" with Claude models, having this feature enabled by default would probably help to avoid the errors. What would be the point in not enabling it by default? Just the fear that the feature doesn't work properly or are there some use cases where it would harm if enabled?

If there is no use case for disabling the auto compression, I would hide the preference or just don't provide one at all.

[ethanyhou]

Any consideration to expose a preference to disable auto compact?

I prefer to always enable it, enable it simply give users more value and advantages.

@jdneo Removed as we aligned.

[ethanyhou]

Hi @iloveeclipse,

Thanks for the input. Agreed and it was removed. Compression will always be enabled.

[jdneo]

latestCopilotTurn is member, which can directly access, I don't see the benefit introduce getLatestCopilotTurn(), and this is why I propose to move logics into ChatContentViewer in #250 (comment)

[jdneo]

If using latestCopilotTurn directly, check dispose status here.

[jdneo]

I'm considering to rename notifyTurnEnd(). It's a little bit confused now since compression can happen in the middle of the turn. How about flushMessageBuffer()

[jdneo]

Let's make it simpler, hideCompactingStatusOnLatestCopilotTurn will always flush the buffer (which should have very tiny load because the buffer should be empty at this stage. If the buffer is not empty at this stage for some unknown reason, this can be a guard)

That way, boolean flushBufferedReply can be removed