[quickbeard]

What

SessionSummary.summarize now computes the whole-session snapshot diff — computeDiff over all session messages (first step-start snapshot → last step-finish snapshot), reusing the already-computed turn diff for single-turn sessions — and publishes it via Session.Event.Diff together with real session.summary additions/deletions/files aggregates. GET /session/{id}/diff without a messageID (the hydrate path used by both the TUI and the web app) returns the computed session-wide diff instead of [].

This lights up three dormant surfaces at once, with zero client changes:

• the TUI sidebar's "Modified Files" section,
• the prompt-footer ctrl+b N todos · M files hint (feat(tui): show hidden-sidebar hint in prompt footer #69),
• the web app's session diff display and counters.

Upstream publishes a permanently empty session-level diff (summary.ts hardcodes diff: []; only session revert carries data). That was a deliberate perf fix — anomalyco/opencode#30127 removed automatic full-session diffs after a snapshot-heavy historical session recomputed a multi-megabyte diff every turn — so this is a fork-only divergence, documented in DIVERGENCES.md. We accept the recompute cost: the single-turn reuse covers the common case, and counts-only publishing (patch is optional in the schema) is the follow-up lever if large sessions bite.

Details:

• Single publish per summarize (no upfront empty publish → no sidebar flicker mid-computation); clearing paths (snapshots disabled, no messages, missing target) still publish zeros/empty.
• All returned/published paths go through a shared unquoteDiffs (git C-quoting), matching the existing per-message endpoint behavior.
• Legacy session_diff storage rows are still never read back, preserving the #26574 regression guard; that test's stale header comment updated.
• New no-mock test test/session/summary.test.ts (real git snapshots, modeled on revert-compact.test.ts): multi-turn session-wide diff + counters, single-turn reuse path, and the clearing path.

Known limitations (accepted)

• Session.Event.Diff now ships full patches per step-finish; share-next forwards it verbatim — larger SSE/share payloads on big sessions (upstream revert already ships full patches on this event).
• Multi-turn sessions pay a second diffFull per step-finish (serialized behind the snapshot lock); debounce is a follow-up if it ever shows in traces.
• Share full-sync still uses the Session.diff [] stub, so a freshly shared idle session shows an empty diff until its next activity.
• While a session sits reverted, a fresh hydrate recomputes over still-present messages rather than the revert-range diff from the last event; self-heals on the next prompt (upstream showed [] there).

Verification

bun typecheck + test/session/summary.test.ts, test/server/session-diff-missing-patch.test.ts, test/session/revert-compact.test.ts, test/snapshot/snapshot.test.ts, test/share/share-next.test.ts all green from packages/opencode.

Live TUI run (tmux, scratch git project, real model turns): agent edits 2 files → footer hint shows ctrl+b 2 modified files, sidebar lists both files with +1 -1; a later turn restoring one file to its original content drops it from the list (net diff, matches git status); TUI restart + -c reopen shows 1 modified file from the hydrate path, which previously always returned empty.

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