[Feature]: Expose compaction/compression hooks for cross-plugin compatibility (e.g., Engram memory persistence)

[Milor123]

Problem Summary

DCP performs context compression independently of OpenCode's native compaction system. While this is generally a good thing, it breaks the workflow for users who rely on other plugins that hook into experimental.session.compacting — specifically, memory/persistence plugins like Engram that need to save a checkpoint when the context is compressed.

Currently, DCP:

• Does not fire OpenCode's experimental.session.compacting hook
• Does not expose any event/hook that other plugins can listen to
• Injects system messages about pruning, but these are not a reliable API for other plugins

Impact

When DCP compresses the conversation, memory plugins have no reliable way to know that a compression occurred. This means:

• Session summaries are not persisted to long-term memory
• The model loses track of what was accomplished before compression
• After compression, the model may hallucinate old tasks or forget the current active task

Reproduction Steps

1. Install DCP and a memory plugin (e.g., Engram) that uses experimental.session.compacting to persist session checkpoints.
2. Have a long conversation that triggers DCP compression.
3. Observe that the memory plugin never fires — no checkpoint is saved.
4. After compression, the model loses context of prior work.

Expected Behavior

DCP should provide a mechanism for other plugins to be notified when compression occurs, so they can persist state or inject recovery context.

Possible Solutions

Option A: Fire OpenCode's experimental.session.compacting hook

When DCP compresses, trigger the same hook that OpenCode's native compaction uses. This would be the most compatible approach.

Option B: Expose a DCP-specific event/hook

Add a new hook like experimental.dcp.compressing or similar that plugins can register. Pass metadata like:

• beforeTokens, afterTokens
• summaryText
• compressedRange

Option C: Standardized system message format

If hooks are not possible, document a stable system message format that DCP injects when compressing, so other plugins can parse it reliably.

Additional Context

Protected tools mismatch

The Engram tools are named mem_save, mem_search, mem_update, mem_session_summary, etc. .

Environment

• OpenCode version: (1.17.9)
• DCP version: 2.x
• Model context: 256K tokens
• Memory plugin: Engram (custom plugin using experimental.session.compacting)

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Would love to hear if this is on the roadmap or if there's a recommended workaround. Happy to test any PR or beta version.