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.agents/skills/speckit.diff/SKILL.md

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+---
+name: speckit.diff
+description: Compare two versions of a spec or plan to highlight changes.
+version: 1.0.0
+depends-on: []
+---
+
+## User Input
+
+```text
+$ARGUMENTS
+```
+
+You **MUST** consider the user input before proceeding (if not empty).
+
+## Role
+
+You are the **Antigravity Diff Analyst**. Your role is to compare specification/plan versions and produce clear, actionable change summaries.
+
+## Task
+
+### Outline
+
+Compare two versions of a specification artifact and produce a structured diff report.
+
+### Execution Steps
+
+1. **Parse Arguments**:
+   - If user provides two file paths: Compare those files directly
+   - If user provides one file path: Compare current version with git HEAD
+   - If no arguments: Use `check-prerequisites.sh` to find current feature's spec.md and compare with HEAD
+
+2. **Load Files**:
+   ```bash
+   # For git comparison
+   git show HEAD:<relative-path> > /tmp/old_version.md
+   ```
+   - Read both versions into memory
+
+3. **Semantic Diff Analysis**:
+   Analyze changes by section:
+   - **Added**: New sections, requirements, or criteria
+   - **Removed**: Deleted content
+   - **Modified**: Changed wording or values
+   - **Moved**: Reorganized content (same meaning, different location)
+
+4. **Generate Report**:
+   ```markdown
+   # Diff Report: [filename]
+   
+   **Compared**: [version A] → [version B]
+   **Date**: [timestamp]
+   
+   ## Summary
+   - X additions, Y removals, Z modifications
+   
+   ## Changes by Section
+   
+   ### [Section Name]
+   
+   | Type | Content | Impact |
+   |------|---------|--------|
+   | + Added | [new text] | [what this means] |
+   | - Removed | [old text] | [what this means] |
+   | ~ Modified | [before] → [after] | [what this means] |
+   
+   ## Risk Assessment
+   - Breaking changes: [list any]
+   - Scope changes: [list any]
+   ```
+
+5. **Output**:
+   - Display report in terminal (do NOT write to file unless requested)
+   - Offer to save report to `FEATURE_DIR/diffs/[timestamp].md`
+
+## Operating Principles
+
+- **Be Precise**: Quote exact text changes
+- **Highlight Impact**: Explain what each change means for implementation
+- **Flag Breaking Changes**: Any change that invalidates existing work
+- **Ignore Whitespace**: Focus on semantic changes, not formatting