fix: resolve entire explain hanging on repos with many checkpoints

[dvydra]

Summary

entire explain (and any code path calling getBranchCheckpoints) hangs indefinitely on repos that have accumulated hundreds of checkpoints. This repo has 236 checkpoints on main and the command never returns — requires Ctrl-Z + kill -9.

Root cause: Two O(N) go-git operations that don't scale:

1. Full transcript parsing for list view prompts

collectCheckpoint called ReadLatestSessionContent + extractPromptsFromTranscript for every checkpoint — reading multi-MB JSONL transcript blobs from git and parsing them as JSON, just to extract the first user prompt for the list view display.

Introduced in: 0e7a05f1 (Feb 2, PR #130) — replaced the cheap ReadSessionPromptFromTree (reads tiny prompt.txt) with full transcript parsing.

Fix: New readPromptFromMetadataTree reads prompt.txt directly from the metadata branch tree. The metadata tree is fetched once and reused for all checkpoints — O(1) per checkpoint instead of O(transcript_size).

2. Full tree diff for temporary checkpoint filtering

convertTemporaryCheckpoint called hasCodeChanges which does parentTree.Diff(commitTree) via go-git for every temporary checkpoint. Each diff resolves hundreds of tree/blob objects through go-git's pure-Go SHA1 packfile reader (no ARM64 assembly), making each call very slow.

Fix: New hasAnyChanges compares tree hashes in O(1). For the list view, showing a few metadata-only checkpoints is acceptable — the full hasCodeChanges diff is preserved for contexts that need precise filtering.

Performance

Metric	Before	After
entire explain	Hangs indefinitely	344ms
Transcript reads	236 × multi-MB blobs	0 (reads prompt.txt)
Tree diffs	32 × full diffs	32 × hash comparisons

Test plan

• mise run fmt — clean
• mise run lint — 0 issues
• mise run test — all pass
• mise run test:integration — all pass
• Manual test: entire explain --no-pager completes in 344ms
• Verify prompt text still shows correctly in list view
• Verify entire explain --checkpoint <id> detail view still works (uses different code path)

@pfleidi — this was a gradual degradation (code introduced Feb 2, grew worse as checkpoints accumulated + large packfile created Feb 25). The fix is surgical — only changes the list view's data fetching strategy, doesn't touch the detail view or any other code paths.

🤖 Generated with Claude Code

[cursor]

PR Summary

Medium Risk
Changes checkpoint list-view data fetching and filtering to avoid expensive go-git transcript reads/diffs; behavior may shift slightly (e.g., prompts missing in edge cases or metadata-only temporary checkpoints appearing).

Overview
Fixes entire explain hanging on repos with many checkpoints by removing two expensive per-checkpoint operations in the list view.

Committed checkpoint prompts are now read by fetching the metadata branch tree once and pulling prompt.txt directly via readPromptFromMetadataTree (instead of loading/parsing full session transcripts). Temporary checkpoint filtering now uses new hasAnyChanges (tree-hash comparison) rather than the full-diff hasCodeChanges, trading perfect “code vs metadata” filtering for much faster list rendering.

^{Written by Cursor Bugbot for commit eff1e18. Configure here.}

[dvydra]

The second fix definitely fixed the problem. The first one might be a red herring

Full transcript parsing for list view prompts

Check if reverting that bit makes a difference to timing.

[Copilot]

Pull request overview

Improves the performance of entire explain list view by removing per-checkpoint expensive go-git operations (full transcript parsing and tree diffs) that don’t scale with hundreds of checkpoints.

Changes:

• Adds a fast path to read the session prompt preview directly from prompt.txt in the metadata-branch tree (avoids reading/parsing transcripts per checkpoint).
• Replaces diff-based temporary-checkpoint filtering with an O(1) tree-hash comparison to avoid repeated expensive go-git tree diffs.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.