diff --git a/src/aggregate.py b/src/aggregate.py
index 035136e..3cccd46 100644
--- a/src/aggregate.py
+++ b/src/aggregate.py
@@ -214,6 +214,22 @@ def build_file_items(base_dir: Path, files: list[str | dict[str, Any]]) -> list[
         content = summarize.summarise_file(path, content)
       elif view_mode == "none":
        content = "(context excluded)"
+      elif view_mode == "custom":
+       if custom_slices:
+        lines = content.splitlines()
+        slices_text = []
+        for s in custom_slices:
+         start = s.get("start_line", 1)
+         end = s.get("end_line", len(lines))
+         tag = s.get("tag", "unnamed")
+         comment = s.get("comment", "")
+         s_idx = max(0, start - 1)
+         e_idx = min(len(lines), end)
+         chunk = "\n".join(lines[s_idx:e_idx])
+         slices_text.append(f"---\n[Slice: {tag}] ({comment})\nLines {start}-{end}:\n{chunk}")
+        content = "\n\n".join(slices_text)
+       else:
+        content = summarize.summarise_file(path, content)
     except FileNotFoundError:
      content = f"ERROR: file not found: {path}"
      mtime = 0.0
diff --git a/tests/test_context_composition_phase6.py b/tests/test_context_composition_phase6.py
index 839a2bc..9b5b759 100644
--- a/tests/test_context_composition_phase6.py
+++ b/tests/test_context_composition_phase6.py
@@ -124,3 +124,42 @@ def test_files_section_rendering(tmp_path):
  assert "```txt\n**Summary** content\n```" not in sections
  assert "**Summary** content" in sections
 
+def test_view_mode_custom(tmp_path):
+ base_dir = tmp_path / "project"
+ base_dir.mkdir()
+ test_file = base_dir / "test.py"
+ content = "line1\nline2\nline3\nline4\nline5\n"
+ test_file.write_text(content, encoding="utf-8")
+ 
+ slices = [
+  {"start_line": 1, "end_line": 2, "tag": "top", "comment": "header"},
+  {"start_line": 4, "end_line": 5, "tag": "bottom", "comment": "footer"}
+ ]
+ files = [{"path": "test.py", "view_mode": "custom", "custom_slices": slices}]
+ items = aggregate.build_file_items(base_dir, files)
+ 
+ assert len(items) == 1
+ assert items[0]["view_mode"] == "custom"
+ 
+ # Check formatting
+ expected_1 = "---\n[Slice: top] (header)\nLines 1-2:\nline1\nline2"
+ expected_2 = "---\n[Slice: bottom] (footer)\nLines 4-5:\nline4\nline5"
+ assert expected_1 in items[0]["content"]
+ assert expected_2 in items[0]["content"]
+ assert "line3" not in items[0]["content"]
+
+def test_view_mode_custom_empty_default_to_summary(tmp_path):
+ base_dir = tmp_path / "project"
+ base_dir.mkdir()
+ test_file = base_dir / "test.py"
+ test_file.write_text("def hello():\n print('world')\n", encoding="utf-8")
+ 
+ files = [{"path": "test.py", "view_mode": "custom", "custom_slices": []}]
+ items = aggregate.build_file_items(base_dir, files)
+ 
+ assert len(items) == 1
+ assert items[0]["view_mode"] == "custom"
+ # Should default to summary
+ assert "**Python**" in items[0]["content"]
+
+