[FEATURE]: Store per-message diffs separately and lazy-load them #16741

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Describe the enhancement you want to request

OpenCode currently stores per-message file diffs inline on user messages via message.summary.diffs, where each diff item includes full before and after file contents.

This has a major downside for long-lived coding sessions: message rows can become extremely large, especially when a turn touches generated files, large source maps, workflow files, or other bulky assets. Since session open/switch flows still load message rows from the session database, this can noticeably slow down the desktop app when switching between sessions.

I’d like to request a change to make per-message diffs lazy-loaded instead of embedding the full diff payload in MessageTable.data.

Proposed direction

• Keep lightweight per-message summary metadata inline on the message row (for example: title/body/files/additions/deletions)
• Move the full per-message diff payload (before / after content) into separate storage, ideally a dedicated table keyed by message_id
• Reuse or extend the existing diff API path so full per-message diffs are only fetched when the user actually opens turn-level review
• Leave session-level review diffs as a separate/lazy path as well

Why this would help

• Prevent very large message rows in SQLite
• Improve session switching/opening performance in the desktop app
• Reduce the cost of session prefetching
• Keep the review UX intact while moving heavy payloads off the hot path

Why I think this is worth doing

From local investigation, the main database growth came from oversized message rows containing summary.diffs[].after content. The hot path cost came from deserializing large message JSON blobs during normal session loading.

Minimal implementation shape

A low-risk version of this could be:

• add a message_diff table (or equivalent separate storage)
• stop persisting full summary.diffs inline on user messages
• keep only lightweight summary counts inline
• make the existing diff fetch path support messageID-scoped lazy loading
• update the turn review UI to fetch full per-message diffs only when needed

I’d be happy to help create a PR for this feature