context-diff

name	context-diff
description	Show what changed between sessions or git refs as a structured context summary. Invoke for: "context diff", "what changed since last session", "diff since main", "what's new since yesterday", "changes since checkpoint", "session diff", "summarize changes since", "what did I change", "context since last commit". Inspired by russbeye/claude-memory-bank /context-diff pattern.
argument-hint	git ref or timeframe (e.g. "main", "HEAD~5", "yesterday")
allowed-tools	Bash, Read, Grep

Skill: Context-Diff — Structured Change Summary

Role

Generate a human-readable, context-efficient summary of what changed between two git refs or since a specific timepoint. Inject this into the conversation so Claude understands what has shifted since last session without re-reading entire files.

When to invoke

• At session start after returning to a long-running project
• When picking up where another session left off
• After a merge or rebase to understand what's now in play
• Before a code review to understand the scope of changes

Instructions

1. Determine the comparison base:

   • If argument is a branch/ref: git diff <ref>...HEAD
   • If argument is "yesterday" or time-based: git log --since="1 day ago" --oneline
   • If no argument: compare to last commit (HEAD~1)

2. Run git diff --stat <base>...HEAD for file-level overview

3. For each changed file (up to 10), provide:

   • File path
   • What changed (1 line)
   • Why it matters (if determinable from commit messages)

4. Summarize at the top:

   • Files changed, insertions, deletions
   • Key themes (e.g., "mostly test additions + one routing fix")
   • Any breaking changes detected

5. List commits in scope with their subject lines

Output Format

## Context Diff: <base>...HEAD

### Summary
- 12 files changed, +340 / -45 lines
- Key themes: routing expansion, new skills, hook improvements
- Breaking changes: none detected

### Commits in scope
- abc123 feat: expand skill routing registry to 114 entries
- def456 fix: resolve duplicate trigger conflicts
- ghi789 feat: add PreCompact hook + SOUL/USER identity files

### Changed Files
| File | Change | Significance |
|------|--------|-------------|
| src/routing/skill_router.py | +200 lines | Routing registry expanded |
| .claude/hooks/session-start.sh | rewritten | Hot-memory injection added |
| .claude/SOUL.md | new | Agent identity file |

### Architectural Changes
[Any structural/interface changes that affect how things connect]