[Agent] Add history compression strategies

[chr-hertel]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues
License	MIT

[OskarStark]

Hot 🔥

[OskarStark]

Lets see if Copilot is complaining about sth

[Copilot]

Pull request overview

This PR adds history compression strategies to the Agent component to manage conversation length automatically. The feature helps control token costs and prevents exceeding context limits in long-running conversational agents.

Changes:

• Introduces a compression infrastructure with CompressionStrategyInterface, HistoryCompressionInputProcessor, and lifecycle events (BeforeHistoryCompression, AfterHistoryCompression)
• Implements three compression strategies: SlidingWindowStrategy (keeps recent messages), SummarizationStrategy (uses LLM to summarize), and HybridStrategy (combines multiple strategies)
• Adds bundle configuration support through a new compression option for agents
• Includes comprehensive documentation for both component and bundle usage

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/agent/src/Compression/CompressionStrategyInterface.php	Core interface defining the compression contract
src/agent/src/Compression/HistoryCompressionInputProcessor.php	Input processor that applies compression strategies
src/agent/src/Compression/BeforeHistoryCompression.php	Event dispatched before compression with skip capability
src/agent/src/Compression/AfterHistoryCompression.php	Event dispatched after compression with modification capability
src/agent/src/Compression/SlidingWindowStrategy.php	Simple strategy keeping only recent messages
src/agent/src/Compression/SummarizationStrategy.php	LLM-based strategy that summarizes old messages
src/agent/src/Compression/HybridStrategy.php	Progressive strategy combining primary and secondary approaches
src/ai-bundle/src/AiBundle.php	Bundle integration creating compression processor from config
src/ai-bundle/config/options.php	Configuration schema for compression option
src/ai-bundle/tests/DependencyInjection/AiBundleTest.php	Tests for compression configuration in the bundle
src/agent/tests/Compression/*	Test coverage for input processor, events, and strategies
docs/components/agent.rst	Component documentation covering compression usage and strategies
docs/bundles/ai-bundle.rst	Bundle documentation for compression configuration
src/agent/CHANGELOG.md	Documents new compression feature in agent component
src/ai-bundle/CHANGELOG.md	Documents new compression configuration option

[Copilot]

The compress method does not handle the case where formatConversation returns an empty string. If all messages to be summarized have no extractable text (e.g., messages with only null or empty content), generateSummary will still be called with an empty or nearly empty conversation text, resulting in an unnecessary API call. Consider adding a check after line 93 to return the original messages if the conversationText is empty or only whitespace.

Suggested change

	$conversationText = $this->formatConversation($toSummarize);
	$conversationText = $this->formatConversation($toSummarize);
	if ('' === \trim($conversationText)) {
	return $messages;
	}

[OskarStark]

rebase unlocked

[aszenz]

This interface seems more generic than it's name would suggest, it's more like MessageBagTransformer.

Reason I'm saying this is that there could be use cases other than compression for using this, especially around filtering/adding more messages.

[aszenz]

One use case might be to remove certain types of messages that are not supported by other models/agents during hand-off/fork.