/diff-preview - Preview Before Apply

See changes before they happen. Safer editing with confirmation prompts.

When To Use

User says:

• "preview changes"
• "show me what will change"
• "diff before apply"
• "safe mode editing"
• Before any multi-file edit operation

How It Works

Before applying edits:

1. Run git diff to show pending changes
2. Present diff in readable format
3. Ask user to confirm: Apply these changes? [y/n]
4. Only apply if user confirms

For new files:

1. Show what content will be created
2. Show file path
3. Ask for confirmation

Usage Patterns

Before Multi-File Edits

# User: "Rename function across all files"

You do:
1. Run: git diff to check current state
2. Calculate changes: grep -r "oldFunction" --include="*.py"
3. Show preview:

📋 Proposed Changes:

src/auth.py:42

• def oldFunction(user):

• def newFunction(user):

src/api.py:15

• return oldFunction(data)

• return newFunction(data)

2 files will be modified

4. Ask: "Apply these changes? [y/n/diff]"
- y: Apply all changes
- n: Cancel
- diff: Show full git diff

Before Creating New Files

# User: "Create a new API endpoint"

You do:
1. Show what will be created:

📋 Will Create: src/api/endpoints/user.py

Content:

from fastapi import APIRouter

router = APIRouter()

@router.get("/users/{user_id}")
async def get_user(user_id: str):
    ...

File size: ~250 bytes

2. Ask: "Create this file? [y/n/edit]"

Integration with Edit Tool

# Before using Edit tool:
1. Read file to get current content
2. Show proposed edit in diff format:

📋 Edit Preview: src/main.py

@@ -15,7 +15,7 @@ - port = int(os.getenv("PORT", 8000)) + port = int(os.getenv("PORT", 3000))

3. Ask: "Apply this edit? [y/n]"
4. Only call Edit tool if user confirms

Diff Format

Compact Preview (default)

📋 Proposed Changes (3 files):

✓ src/config.py:12     - Change default port 8000→3000
✓ src/auth.py:45      - Add rate limiting
✓ src/utils/helpers.py - Extract common function

Lines added: 15 | Lines removed: 5 | Files changed: 3

Full Diff

📋 Full Diff:

src/config.py
@@ -12,7 +12,7 @@
     """Load configuration from environment."""
-    port = int(os.getenv("PORT", 8000))
+    port = int(os.getenv("PORT", 3000))
     debug = os.getenv("DEBUG", "false").lower() == "true"

src/auth.py
@@ -45,6 +45,10 @@
     def authenticate(self, token: str) -> User:
+        # Rate limiting
+        if self._is_rate_limited(token):
+            raise RateLimitError()
         return self._verify_token(token)

Apply these changes? [y/n/diff/all]

Confirmation Prompts

Safety Features

Automatic Diff Before Write

# In your workflow, before Write tool:
1. If file exists: Read it first
2. Compute diff between old and new
3. Show preview to user
4. Only Write if confirmed

Dry Run Mode

# User says "dry run" or "just show me"
/diff-preview --dry-run "rename oldFunction to newFunction"

# Shows changes without applying any

Rollback Prompt

# After applying changes:
"Changes applied! Undo with: git checkout -- <files>"
"Create checkpoint? [y/n]"  # Offer to create git commit

Integration with Other Skills

With refactorer

# refactorer uses diff-preview automatically
User: "refactor to extract common validation"

You do:
1. Analyze code and plan changes
2. Show diff-preview of proposed refactor
3. Wait for confirmation
4. Apply changes

With batch-processor

# batch-processor shows summary before applying
User: "add license header to all Python files"

You do:
1. Find all matching files: 15 files
2. Show summary: "Will add license to 15 Python files"
3. Show first file as example
4. Confirm: "Apply to all 15 files? [y/n/first]"

With implement-plan

# After implementing a plan step:
"Step 3/5 complete: Added authentication

Changes:
+ src/auth.py (new file, 120 lines)
+ src/middleware/auth.py (new file, 45 lines)
~ src/main.py (modified, 3 lines added)

Continue to step 4? [y/n/review]"

Quick Win from Research

Competitor: Cline has built-in diff preview Problem: Direct file edits feel risky Solution: Always show changes before applying Result: Users feel more in control, safer experimentation

Implementation Checklist

When using diff-preview:

• Show WHAT will change (file list)
• Show HOW it will change (diff)
• Show IMPACT (lines added/removed)
• Ask for CONFIRMATION
• Only apply if confirmed
• Offer rollback option

Keywords

diff, preview, changes, safe mode, confirm before apply, show diff, dry run, rollback