diff --git a/app/src/confgen/diff.py b/app/src/confgen/diff.py
new file mode 100644
index 0000000..287df43
--- /dev/null
+++ b/app/src/confgen/diff.py
@@ -0,0 +1,157 @@
+"""Diff display for configuration changes using Rich."""
+
+
+class ConfigDiff:
+    """Diff viewer for configuration changes."""
+
+    def __init__(self):
+        pass
+
+    def show_diff(
+        self,
+        old_content: str,
+        new_content: str,
+        old_label: str = "Current",
+        new_label: str = "Generated",
+    ) -> None:
+        """Show a unified diff between two configurations."""
+        from rich.console import Console
+        from rich.panel import Panel
+        from rich.text import Text
+
+        console = Console()
+
+        if old_content == new_content:
+            console.print(Panel("[yellow]No changes detected.[/yellow]"))
+            return
+
+        diff_lines = self._generate_diff(old_content, new_content)
+
+        diff_text = Text()
+        for line in diff_lines:
+            if line.startswith("+") and not line.startswith("+++"):
+                diff_text.append(line + "\n", style="green")
+            elif line.startswith("-") and not line.startswith("---"):
+                diff_text.append(line + "\n", style="red")
+            elif line.startswith("@@"):
+                diff_text.append(line + "\n", style="dim")
+            else:
+                diff_text.append(line + "\n", style="white")
+
+        console.print(
+            Panel(
+                diff_text,
+                title=f"[bold]Diff: {old_label} → {new_label}[/bold]",
+                expand=False,
+            )
+        )
+
+    def _generate_diff(
+        self,
+        old_content: str,
+        new_content: str,
+        context: int = 3,
+        old_label: str = "Current",
+        new_label: str = "Generated",
+    ) -> list[str]:
+        """Generate unified diff lines."""
+        old_lines = old_content.splitlines(keepends=True)
+        new_lines = new_content.splitlines(keepends=True)
+
+        diff = []
+        diff.append(f"--- {old_label}")
+        diff.append(f"+++ {new_label}")
+
+        i, j = 0, 0
+        while i < len(old_lines) or j < len(new_lines):
+            if (
+                i < len(old_lines)
+                and j < len(new_lines)
+                and old_lines[i] == new_lines[j]
+            ):
+                diff.append("  " + old_lines[i].rstrip())
+                i += 1
+                j += 1
+            else:
+                start_i, start_j = i, j
+
+                while i < len(old_lines) and (
+                    j >= len(new_lines) or old_lines[i] != new_lines[j]
+                ):
+                    i += 1
+
+                while j < len(new_lines) and (
+                    i >= len(old_lines) or old_lines[i - 1] != new_lines[j]
+                ):
+                    j += 1
+
+                before = max(0, start_i - context)
+                after = min(len(old_lines), i + context)
+
+                if before < start_i:
+                    diff.append("...")
+
+                for k in range(before, start_i):
+                    diff.append("  " + old_lines[k].rstrip())
+
+                for k in range(start_i, i):
+                    diff.append("-" + old_lines[k].rstrip())
+
+                for k in range(start_j, j):
+                    diff.append("+" + new_lines[k].rstrip())
+
+                if after > i:
+                    diff.append("...")
+
+        return diff
+
+    def show_json_diff(
+        self,
+        old_data: dict,
+        new_data: dict,
+        old_label: str = "Current",
+        new_label: str = "Generated",
+    ) -> None:
+        """Show diff between two JSON configurations."""
+        import json
+
+        old_content = json.dumps(old_data, indent=2, sort_keys=True)
+        new_content = json.dumps(new_data, indent=2, sort_keys=True)
+
+        self.show_diff(old_content, new_content, old_label, new_label)
+
+    def show_yaml_diff(
+        self,
+        old_data: dict,
+        new_data: dict,
+        old_label: str = "Current",
+        new_label: str = "Generated",
+    ) -> None:
+        """Show diff between two YAML configurations."""
+        import yaml
+
+        old_content = yaml.dump(old_data, default_flow_style=False, sort_keys=True)
+        new_content = yaml.dump(new_data, default_flow_style=False, sort_keys=True)
+
+        self.show_diff(old_content, new_content, old_label, new_label)
+
+    def format_change_summary(
+        self,
+        old_content: str,
+        new_content: str,
+    ) -> dict[str, int]:
+        """Get a summary of changes."""
+        old_lines = set(old_content.splitlines())
+        new_lines = set(new_content.splitlines())
+
+        added = len(new_lines - old_lines)
+        removed = len(old_lines - new_lines)
+        unchanged = len(old_lines & new_lines)
+
+        return {
+            "added": added,
+            "removed": removed,
+            "unchanged": unchanged,
+            "total_old": len(old_lines),
+            "total_new": len(new_lines),
+        }