diff --git a/src/cleveragents/tui/permissions/models.py b/src/cleveragents/tui/permissions/models.py
index 8cbb3f905..a5eba5afe 100644
--- a/src/cleveragents/tui/permissions/models.py
+++ b/src/cleveragents/tui/permissions/models.py
@@ -139,21 +139,22 @@ class PermissionRequest(BaseModel):
         return left, right
 
     def auto_diff(self, *, context_lines: int = 3) -> str:
-        """Select unified or split diff based on available terminal width.
+        """Generate a context diff string for this file change.
 
-        Returns split (side-by-side) format for terminals >= 120 columns,
-        unified diff format otherwise.
+        Uses context diff format with *** and --- delimiters per spec §30139.
         """
-        import shutil
+        import difflib
 
-        terminal_width = shutil.get_terminal_size().columns
-        if terminal_width >= 120:
-            left, right = self.split_diff()
-            lines: list[str] = []
-            for l_line, r_line in zip(left, right, strict=True):
-                lines.append(f"{l_line:<40} | {r_line}")
-            return "\n".join(lines)
-        return self.unified_diff(context_lines=context_lines)
+        before_lines = (self.before_content or "").splitlines(keepends=True)
+        after_lines = (self.after_content or "").splitlines(keepends=True)
+        diff_lines = difflib.context_diff(
+            before_lines,
+            after_lines,
+            fromfile=f"a/{self.path}",
+            tofile=f"b/{self.path}",
+            n=context_lines,
+        )
+        return "".join(diff_lines)
 
     def render_diff(
         self,