diff --git a/docs/Tuning-Guide.md b/docs/Tuning-Guide.md
index ae54f0a..94a6ec6 100644
--- a/docs/Tuning-Guide.md
+++ b/docs/Tuning-Guide.md
@@ -393,7 +393,7 @@ declaration.
 ## Benchmarking models
 
 The capability matrix asks whether a model *can* hold the tool contract. The
-benchmark asks whether it *finishes real work*: four canned milestones, each an
+benchmark asks whether it *finishes real work*: eight canned milestones, each an
 approved plan over a small seed project, judged by a hidden check the model
 never sees. Success means the check passes, not that the model said it was
 done.
@@ -404,6 +404,17 @@ done.
 | `fix-bug` | find and fix the bug a failing test exposes, without touching the test |
 | `multi-file` | thread a new option through several files |
 | `long-output` | find one failure in test output far longer than `max_output_chars` |
+| `trace-bug` | follow a wrong total to its cause in a shared helper, and fix it there rather than at the symptom |
+| `refactor-rename` | rename a function across a ten-module package, sparing two similar names a blind replace would break |
+| `spec-parser` | implement a parser from an eight-rule written spec when the plan gives only three examples, with tests |
+| `long-milestone` | carry out eight dependent steps (model, storage, sorting, CLI) over 25+ turns |
+
+The first four were the original set. By September 2026 all seven newer local
+models passed all of them, so the last four were added to separate models that
+can hold the tool contract from models that can do a milestone's real work.
+Their plans stay explicit, as the planner is told to write them; what makes
+them harder is the work itself: tracing a cause, searching a codebase,
+reading a spec closely, and holding a plan over many turns.
 
 It runs the executor exactly as the agent does (the same client wiring,
 context budget and `tool_choice` handling, and your config's tuning knobs),
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/hidden/test_hidden.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/hidden/test_hidden.py
new file mode 100644
index 0000000..6ad4224
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/hidden/test_hidden.py
@@ -0,0 +1,186 @@
+import json
+import os
+import subprocess
+import sys
+from datetime import date
+from pathlib import Path
+
+import pytest
+from tracker import storage
+from tracker.core import add_task, complete, overdue, sort_tasks
+from tracker.formatting import format_task
+from tracker.models import Task
+
+TODAY = date(2026, 10, 5)
+
+
+# step 1
+def test_task_has_an_optional_due_date() -> None:
+    assert Task(1, "a").due is None
+    assert Task(1, "a", False, date(2026, 1, 2)).due == date(2026, 1, 2)
+
+
+# step 2
+def test_save_writes_version_2_with_iso_dates(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    storage.save(path, [Task(1, "a", due=date(2026, 10, 1)), Task(2, "b")])
+    data = json.loads(path.read_text())
+    assert data["version"] == 2
+    assert [t["due"] for t in data["tasks"]] == ["2026-10-01", None]
+
+
+def test_round_trip_keeps_dates(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    tasks = [Task(1, "a", due=date(2026, 10, 1)), Task(2, "b", done=True)]
+    storage.save(path, tasks)
+    assert storage.load(path) == tasks
+
+
+def test_version_1_files_still_load(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    path.write_text(
+        json.dumps({"version": 1, "tasks": [{"id": 4, "title": "old", "done": True}]})
+    )
+    assert storage.load(path) == [Task(4, "old", done=True, due=None)]
+
+
+@pytest.mark.parametrize("version", [0, 3, 99])
+def test_other_versions_are_refused(tmp_path: Path, version: int) -> None:
+    path = tmp_path / "t.json"
+    path.write_text(json.dumps({"version": version, "tasks": []}))
+    with pytest.raises(ValueError, match=f"unsupported tasks file version: {version}"):
+        storage.load(path)
+
+
+# step 3
+def test_add_task_takes_a_due_date() -> None:
+    tasks: list[Task] = []
+    assert add_task(tasks, "a", due=date(2026, 1, 1)).due == date(2026, 1, 1)
+    assert add_task(tasks, "b").due is None
+
+
+# step 4
+def test_sort_order() -> None:
+    tasks = [
+        Task(1, "undated"),
+        Task(2, "late", due=date(2026, 12, 1)),
+        Task(3, "early", due=date(2026, 9, 1)),
+        Task(4, "done dated", done=True, due=date(2026, 1, 1)),
+        Task(5, "done undated", done=True),
+        Task(6, "early twin", due=date(2026, 9, 1)),
+        Task(7, "undated twin"),
+    ]
+    assert [t.id for t in sort_tasks(tasks)] == [3, 6, 2, 1, 7, 4, 5]
+
+
+# step 5
+def test_overdue() -> None:
+    tasks = [
+        Task(1, "due today", due=TODAY),
+        Task(2, "yesterday", due=date(2026, 10, 4)),
+        Task(3, "done late", done=True, due=date(2026, 1, 1)),
+        Task(4, "undated"),
+        Task(5, "long ago", due=date(2026, 1, 1)),
+    ]
+    assert [t.id for t in overdue(tasks, TODAY)] == [5, 2]
+
+
+# step 6
+@pytest.mark.parametrize(
+    ("task", "line"),
+    [
+        (Task(3, "Buy milk"), "[ ] 3 Buy milk"),
+        (
+            Task(4, "Call Bob", done=True, due=date(2026, 10, 1)),
+            "[x] 4 Call Bob (due 2026-10-01)",
+        ),
+        (Task(5, "Pay rent", due=date(2026, 9, 1)), "[!] 5 Pay rent (due 2026-09-01)"),
+        (Task(6, "Soon", due=date(2026, 10, 9)), "[ ] 6 Soon (due 2026-10-09)"),
+        (Task(7, "Today", due=TODAY), "[ ] 7 Today (due 2026-10-05)"),
+    ],
+)
+def test_format(task: Task, line: str) -> None:
+    assert format_task(task, TODAY) == line
+
+
+# step 7, end to end through the real command line
+def run(tmp_path: Path, *args: str) -> subprocess.CompletedProcess[str]:
+    env = {
+        **os.environ,
+        "TRACKER_FILE": str(tmp_path / "t.json"),
+        "TRACKER_TODAY": "2026-10-05",
+    }
+    return subprocess.run(
+        [sys.executable, "-m", "tracker", *args],
+        capture_output=True,
+        text=True,
+        env=env,
+        cwd=Path.cwd(),
+        check=False,
+    )
+
+
+def test_command_line_flow(tmp_path: Path) -> None:
+    assert run(tmp_path, "add", "Buy milk").returncode == 0
+    assert run(tmp_path, "add", "Pay rent", "--due", "2026-10-01").returncode == 0
+    assert run(tmp_path, "add", "Dentist", "--due", "2026-11-20").returncode == 0
+    assert run(tmp_path, "done", "1").returncode == 0
+    listing = run(tmp_path, "list")
+    assert listing.stdout.splitlines() == [
+        "[!] 2 Pay rent (due 2026-10-01)",
+        "[ ] 3 Dentist (due 2026-11-20)",
+        "[x] 1 Buy milk",
+    ], listing.stderr
+    assert run(tmp_path, "list", "--overdue").stdout.splitlines() == [
+        "[!] 2 Pay rent (due 2026-10-01)"
+    ]
+
+
+def test_a_bad_due_date_is_a_usage_error(tmp_path: Path) -> None:
+    result = run(tmp_path, "add", "x", "--due", "2026-13-40")
+    assert result.returncode == 2
+    assert "invalid --due date: 2026-13-40" in result.stderr
+
+
+def test_today_defaults_to_the_real_date(tmp_path: Path) -> None:
+    env = {**os.environ, "TRACKER_FILE": str(tmp_path / "t.json")}
+    env.pop("TRACKER_TODAY", None)
+    subprocess.run(
+        [sys.executable, "-m", "tracker", "add", "Ancient", "--due", "2000-01-01"],
+        env=env,
+        cwd=Path.cwd(),
+        check=True,
+        capture_output=True,
+    )
+    listing = subprocess.run(
+        [sys.executable, "-m", "tracker", "list", "--overdue"],
+        env=env,
+        cwd=Path.cwd(),
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert listing.stdout.splitlines() == ["[!] 1 Ancient (due 2000-01-01)"]
+
+
+def test_existing_behaviour_is_kept() -> None:
+    tasks: list[Task] = []
+    add_task(tasks, "a")
+    add_task(tasks, "b")
+    complete(tasks, 1)
+    assert [t.id for t in sort_tasks(tasks)] == [2, 1]
+
+
+# step 8
+def test_the_projects_own_tests_pass_and_cover_due_dates() -> None:
+    result = subprocess.run(
+        [sys.executable, "-m", "pytest", "-q", "tests"],
+        capture_output=True,
+        text=True,
+        check=False,
+        cwd=Path.cwd(),
+    )
+    assert result.returncode == 0, result.stdout[-1500:]
+    written = "\n".join(p.read_text() for p in Path("tests").glob("test_*.py"))
+    for name in ("due", "overdue", "version"):
+        assert name in written, f"no test mentions {name!r}"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_core.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_core.py
new file mode 100644
index 0000000..225af24
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_core.py
@@ -0,0 +1,26 @@
+import pytest
+from tracker.core import add_task, complete, sort_tasks
+from tracker.models import Task
+
+
+def test_ids_count_up() -> None:
+    tasks: list[Task] = []
+    assert [add_task(tasks, t).id for t in ("a", "b", "c")] == [1, 2, 3]
+
+
+def test_a_title_is_required() -> None:
+    with pytest.raises(ValueError):
+        add_task([], "   ")
+
+
+def test_complete_and_sort() -> None:
+    tasks: list[Task] = []
+    for title in ("a", "b", "c"):
+        add_task(tasks, title)
+    complete(tasks, 1)
+    assert [t.id for t in sort_tasks(tasks)] == [2, 3, 1]
+
+
+def test_completing_an_unknown_task() -> None:
+    with pytest.raises(KeyError):
+        complete([], 7)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_storage_and_cli.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_storage_and_cli.py
new file mode 100644
index 0000000..62fe3d9
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tests/test_storage_and_cli.py
@@ -0,0 +1,25 @@
+from pathlib import Path
+
+from tracker import storage
+from tracker.cli import main
+from tracker.models import Task
+
+
+def test_round_trip(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    storage.save(path, [Task(1, "a"), Task(2, "b", done=True)])
+    assert storage.load(path) == [Task(1, "a"), Task(2, "b", done=True)]
+
+
+def test_no_file_is_no_tasks(tmp_path: Path) -> None:
+    assert storage.load(tmp_path / "missing.json") == []
+
+
+def test_cli_add_done_list(tmp_path: Path, capsys) -> None:
+    path = str(tmp_path / "t.json")
+    main(["--file", path, "add", "Buy milk"])
+    main(["--file", path, "add", "Call Bob"])
+    main(["--file", path, "done", "1"])
+    capsys.readouterr()
+    main(["--file", path, "list"])
+    assert capsys.readouterr().out.splitlines() == ["[ ] 2 Call Bob", "[x] 1 Buy milk"]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__init__.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__init__.py
new file mode 100644
index 0000000..8e7987e
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__init__.py
@@ -0,0 +1 @@
+"""A small command-line task tracker."""
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__main__.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__main__.py
new file mode 100644
index 0000000..60ffd92
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/__main__.py
@@ -0,0 +1,3 @@
+from tracker.cli import main
+
+raise SystemExit(main())
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/cli.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/cli.py
new file mode 100644
index 0000000..3236326
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/cli.py
@@ -0,0 +1,41 @@
+"""python -m tracker [--file PATH] {add,list,done} ...
+
+The file defaults to $TRACKER_FILE, or tasks.json in the current directory.
+"""
+
+import argparse
+import os
+from pathlib import Path
+
+from tracker import storage
+from tracker.core import add_task, complete, sort_tasks
+from tracker.formatting import format_task
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(prog="tracker")
+    parser.add_argument("--file", default=os.environ.get("TRACKER_FILE", "tasks.json"))
+    commands = parser.add_subparsers(dest="command", required=True)
+    add = commands.add_parser("add", help="add a task")
+    add.add_argument("title")
+    commands.add_parser("list", help="list tasks")
+    done = commands.add_parser("done", help="mark a task done")
+    done.add_argument("id", type=int)
+    args = parser.parse_args(argv)
+
+    path = Path(args.file)
+    tasks = storage.load(path)
+    if args.command == "add":
+        task = add_task(tasks, args.title)
+        storage.save(path, tasks)
+        print(f"added {task.id}")
+    elif args.command == "done":
+        try:
+            complete(tasks, args.id)
+        except KeyError as exc:
+            parser.error(str(exc.args[0]))
+        storage.save(path, tasks)
+    else:
+        for task in sort_tasks(tasks):
+            print(format_task(task))
+    return 0
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/core.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/core.py
new file mode 100644
index 0000000..fd26fce
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/core.py
@@ -0,0 +1,25 @@
+"""Operations on a list of tasks."""
+
+from tracker.models import Task
+
+
+def add_task(tasks: list[Task], title: str) -> Task:
+    title = title.strip()
+    if not title:
+        raise ValueError("a task needs a title")
+    task = Task(id=max((t.id for t in tasks), default=0) + 1, title=title)
+    tasks.append(task)
+    return task
+
+
+def complete(tasks: list[Task], task_id: int) -> Task:
+    for task in tasks:
+        if task.id == task_id:
+            task.done = True
+            return task
+    raise KeyError(f"no task {task_id}")
+
+
+def sort_tasks(tasks: list[Task]) -> list[Task]:
+    """Tasks not done first, then by id."""
+    return sorted(tasks, key=lambda t: (t.done, t.id))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/formatting.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/formatting.py
new file mode 100644
index 0000000..85be357
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/formatting.py
@@ -0,0 +1,8 @@
+"""One line per task."""
+
+from tracker.models import Task
+
+
+def format_task(task: Task) -> str:
+    mark = "x" if task.done else " "
+    return f"[{mark}] {task.id} {task.title}"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/models.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/models.py
new file mode 100644
index 0000000..6c3c8f6
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/models.py
@@ -0,0 +1,10 @@
+"""The task record."""
+
+from dataclasses import dataclass
+
+
+@dataclass
+class Task:
+    id: int
+    title: str
+    done: bool = False
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/storage.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/storage.py
new file mode 100644
index 0000000..c072ad1
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/seed/tracker/storage.py
@@ -0,0 +1,24 @@
+"""Tasks are kept in one JSON file."""
+
+import json
+from pathlib import Path
+
+from tracker.models import Task
+
+VERSION = 1
+
+
+def load(path: Path) -> list[Task]:
+    """Every task in the file; an empty list when there is no file yet."""
+    if not path.exists():
+        return []
+    data = json.loads(path.read_text())
+    return [Task(id=t["id"], title=t["title"], done=t["done"]) for t in data["tasks"]]
+
+
+def save(path: Path, tasks: list[Task]) -> None:
+    data = {
+        "version": VERSION,
+        "tasks": [{"id": t.id, "title": t.title, "done": t.done} for t in tasks],
+    }
+    path.write_text(json.dumps(data, indent=2))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tests/test_storage_and_cli.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tests/test_storage_and_cli.py
new file mode 100644
index 0000000..8587984
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tests/test_storage_and_cli.py
@@ -0,0 +1,76 @@
+import json
+from datetime import date
+from pathlib import Path
+
+import pytest
+from tracker import storage
+from tracker.cli import main
+from tracker.core import add_task, overdue, sort_tasks
+from tracker.formatting import format_task
+from tracker.models import Task
+
+
+def test_round_trip(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    tasks = [Task(1, "a"), Task(2, "b", done=True, due=date(2026, 10, 1))]
+    storage.save(path, tasks)
+    assert storage.load(path) == tasks
+    assert json.loads(path.read_text())["version"] == 2
+
+
+def test_no_file_is_no_tasks(tmp_path: Path) -> None:
+    assert storage.load(tmp_path / "missing.json") == []
+
+
+def test_version_1_files_load(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    path.write_text(
+        json.dumps({"version": 1, "tasks": [{"id": 1, "title": "a", "done": False}]})
+    )
+    assert storage.load(path) == [Task(1, "a")]
+
+
+def test_unknown_versions_are_refused(tmp_path: Path) -> None:
+    path = tmp_path / "t.json"
+    path.write_text(json.dumps({"version": 9, "tasks": []}))
+    with pytest.raises(ValueError, match="unsupported tasks file version: 9"):
+        storage.load(path)
+
+
+def test_due_dates_sort_and_overdue() -> None:
+    tasks: list[Task] = []
+    add_task(tasks, "undated")
+    add_task(tasks, "later", due=date(2026, 12, 1))
+    add_task(tasks, "sooner", due=date(2026, 9, 1))
+    assert [t.title for t in sort_tasks(tasks)] == ["sooner", "later", "undated"]
+    assert [t.title for t in overdue(tasks, date(2026, 10, 1))] == ["sooner"]
+
+
+def test_format_marks_overdue() -> None:
+    today = date(2026, 10, 5)
+    assert format_task(Task(5, "Pay rent", due=date(2026, 9, 1)), today) == (
+        "[!] 5 Pay rent (due 2026-09-01)"
+    )
+    assert format_task(Task(3, "Buy milk"), today) == "[ ] 3 Buy milk"
+
+
+def test_cli_add_done_list(tmp_path: Path, capsys, monkeypatch) -> None:
+    monkeypatch.setenv("TRACKER_TODAY", "2026-10-05")
+    path = str(tmp_path / "t.json")
+    main(["--file", path, "add", "Buy milk"])
+    main(["--file", path, "add", "Pay rent", "--due", "2026-10-01"])
+    main(["--file", path, "done", "1"])
+    capsys.readouterr()
+    main(["--file", path, "list"])
+    assert capsys.readouterr().out.splitlines() == [
+        "[!] 2 Pay rent (due 2026-10-01)",
+        "[x] 1 Buy milk",
+    ]
+    main(["--file", path, "list", "--overdue"])
+    assert capsys.readouterr().out.splitlines() == ["[!] 2 Pay rent (due 2026-10-01)"]
+
+
+def test_cli_rejects_a_bad_date(tmp_path: Path) -> None:
+    with pytest.raises(SystemExit) as caught:
+        main(["--file", str(tmp_path / "t.json"), "add", "x", "--due", "soon"])
+    assert caught.value.code == 2
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/cli.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/cli.py
new file mode 100644
index 0000000..616008f
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/cli.py
@@ -0,0 +1,57 @@
+"""python -m tracker [--file PATH] {add,list,done} ...
+
+The file defaults to $TRACKER_FILE, or tasks.json in the current directory.
+"""
+
+import argparse
+import os
+from datetime import date
+from pathlib import Path
+
+from tracker import storage
+from tracker.core import add_task, complete, overdue, sort_tasks
+from tracker.formatting import format_task
+
+
+def today() -> date:
+    value = os.environ.get("TRACKER_TODAY")
+    return date.fromisoformat(value) if value else date.today()  # noqa: DTZ011 -- the plan asks for the local date
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(prog="tracker")
+    parser.add_argument("--file", default=os.environ.get("TRACKER_FILE", "tasks.json"))
+    commands = parser.add_subparsers(dest="command", required=True)
+    add = commands.add_parser("add", help="add a task")
+    add.add_argument("title")
+    add.add_argument("--due", help="due date, YYYY-MM-DD")
+    listing = commands.add_parser("list", help="list tasks")
+    listing.add_argument("--overdue", action="store_true", help="only overdue tasks")
+    done = commands.add_parser("done", help="mark a task done")
+    done.add_argument("id", type=int)
+    args = parser.parse_args(argv)
+
+    path = Path(args.file)
+    tasks = storage.load(path)
+    now = today()
+    if args.command == "add":
+        due = None
+        if args.due is not None:
+            try:
+                due = date.fromisoformat(args.due)
+            except ValueError:
+                parser.error(f"invalid --due date: {args.due}")
+        task = add_task(tasks, args.title, due=due)
+        storage.save(path, tasks)
+        print(f"added {task.id}")
+    elif args.command == "done":
+        try:
+            complete(tasks, args.id)
+        except KeyError as exc:
+            parser.error(str(exc.args[0]))
+        storage.save(path, tasks)
+    else:
+        shown = overdue(tasks, now) if args.overdue else sort_tasks(tasks)
+        for task in shown:
+            print(format_task(task, now))
+    return 0
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/core.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/core.py
new file mode 100644
index 0000000..7392935
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/core.py
@@ -0,0 +1,34 @@
+"""Operations on a list of tasks."""
+
+from datetime import date
+
+from tracker.models import Task
+
+
+def add_task(tasks: list[Task], title: str, *, due: date | None = None) -> Task:
+    title = title.strip()
+    if not title:
+        raise ValueError("a task needs a title")
+    task = Task(id=max((t.id for t in tasks), default=0) + 1, title=title, due=due)
+    tasks.append(task)
+    return task
+
+
+def complete(tasks: list[Task], task_id: int) -> Task:
+    for task in tasks:
+        if task.id == task_id:
+            task.done = True
+            return task
+    raise KeyError(f"no task {task_id}")
+
+
+def sort_tasks(tasks: list[Task]) -> list[Task]:
+    """Not done first; then dated tasks, earliest first; then undated; then id."""
+    return sorted(
+        tasks,
+        key=lambda t: (t.done, t.due is None, t.due or date.max, t.id),
+    )
+
+
+def overdue(tasks: list[Task], today: date) -> list[Task]:
+    return [t for t in sort_tasks(tasks) if not t.done and t.due and t.due < today]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/formatting.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/formatting.py
new file mode 100644
index 0000000..a4fa732
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/formatting.py
@@ -0,0 +1,16 @@
+"""One line per task."""
+
+from datetime import date
+
+from tracker.models import Task
+
+
+def format_task(task: Task, today: date) -> str:
+    if task.done:
+        mark = "x"
+    elif task.due and task.due < today:
+        mark = "!"
+    else:
+        mark = " "
+    due = f" (due {task.due.isoformat()})" if task.due else ""
+    return f"[{mark}] {task.id} {task.title}{due}"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/models.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/models.py
new file mode 100644
index 0000000..da2e1f0
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/models.py
@@ -0,0 +1,12 @@
+"""The task record."""
+
+from dataclasses import dataclass
+from datetime import date
+
+
+@dataclass
+class Task:
+    id: int
+    title: str
+    done: bool = False
+    due: date | None = None
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/storage.py b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/storage.py
new file mode 100644
index 0000000..6ab222b
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/solution/tracker/storage.py
@@ -0,0 +1,44 @@
+"""Tasks are kept in one JSON file."""
+
+import json
+from datetime import date
+from pathlib import Path
+
+from tracker.models import Task
+
+VERSION = 2
+
+
+def load(path: Path) -> list[Task]:
+    """Every task in the file; an empty list when there is no file yet."""
+    if not path.exists():
+        return []
+    data = json.loads(path.read_text())
+    version = data.get("version")
+    if version not in (1, 2):
+        raise ValueError(f"unsupported tasks file version: {version}")
+    return [
+        Task(
+            id=t["id"],
+            title=t["title"],
+            done=t["done"],
+            due=date.fromisoformat(t["due"]) if t.get("due") else None,
+        )
+        for t in data["tasks"]
+    ]
+
+
+def save(path: Path, tasks: list[Task]) -> None:
+    data = {
+        "version": VERSION,
+        "tasks": [
+            {
+                "id": t.id,
+                "title": t.title,
+                "done": t.done,
+                "due": t.due.isoformat() if t.due else None,
+            }
+            for t in tasks
+        ],
+    }
+    path.write_text(json.dumps(data, indent=2))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/task.yaml b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/task.yaml
new file mode 100644
index 0000000..2b6cfee
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/long-milestone/task.yaml
@@ -0,0 +1,40 @@
+title: Carry out a long milestone whose steps build on each other
+check: python -m pytest -q .bench_check
+plan: |
+  Milestone 1: add due dates to the task tracker.
+
+  Run `python -m pytest -q` after each step; every test must still pass
+  before you start the next one. Use `datetime.date` for dates throughout.
+
+  1. `tracker/models.py`: add a field `due: date | None = None` to `Task`,
+     after `done`.
+  2. `tracker/storage.py`: `save` writes `"version": 2`, and each task's
+     `"due"` as an ISO string ("2026-10-01") or null. `load` reads version 1
+     files (their tasks have no "due"; use None) and version 2 files. Any
+     other version raises `ValueError("unsupported tasks file version: N")`,
+     with N the version found.
+  3. `tracker/core.py`: `add_task` takes a keyword argument
+     `due: date | None = None` and stores it on the new task.
+  4. `tracker/core.py`: change `sort_tasks` so that tasks not done come
+     before tasks done; within each of those two groups, tasks with a due
+     date come first, earliest due date first, then tasks without one; ties
+     are broken by id.
+  5. `tracker/core.py`: add `overdue(tasks: list[Task], today: date) -> list[Task]`:
+     tasks not done whose due date is before `today`, in `sort_tasks` order.
+  6. `tracker/formatting.py`: `format_task` takes a second argument
+     `today: date`. A task with a due date gets " (due 2026-10-01)" after its
+     title. An overdue task (as in step 5) shows `[!]` instead of `[ ]`.
+     Examples: "[ ] 3 Buy milk", "[x] 4 Call Bob (due 2026-10-01)",
+     "[!] 5 Pay rent (due 2026-09-01)".
+  7. `tracker/cli.py`: pass today's date to `format_task`. Today is the
+     `TRACKER_TODAY` environment variable (YYYY-MM-DD) when it is set, else
+     `date.today()`. `add` gains an option `--due YYYY-MM-DD`; an invalid date
+     must call `parser.error("invalid --due date: VALUE")`, which exits with
+     status 2. `list` gains a flag `--overdue` that lists only overdue tasks.
+  8. Add tests under `tests/` for each of steps 1-7, run `python -m pytest -q`
+     until everything passes, then check the whole flow by running:
+     `TRACKER_FILE=/tmp/tracker-check.json TRACKER_TODAY=2026-10-05 python -m tracker add "Pay rent" --due 2026-10-01`
+     then the same with `list --overdue`, which must print
+     `[!] 1 Pay rent (due 2026-10-01)`. Delete /tmp/tracker-check.json
+     before and after.
+  9. Call finish.
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/hidden/test_hidden.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/hidden/test_hidden.py
new file mode 100644
index 0000000..71a2b5e
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/hidden/test_hidden.py
@@ -0,0 +1,122 @@
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+ROOT = Path.cwd()
+OLD = re.compile(r"\bload_config\b")
+
+
+def project_files() -> list[Path]:
+    return [
+        p
+        for p in [*ROOT.rglob("*.py"), *ROOT.rglob("*.md")]
+        if ".bench_check" not in p.parts and "__pycache__" not in p.parts
+    ]
+
+
+def test_the_old_name_is_gone_everywhere() -> None:
+    left = [
+        f"{p.relative_to(ROOT)}:{n}: {line.strip()}"
+        for p in project_files()
+        for n, line in enumerate(p.read_text().splitlines(), 1)
+        if OLD.search(line)
+    ]
+    assert not left, "load_config still appears:\n" + "\n".join(left)
+
+
+def test_the_new_name_is_exported() -> None:
+    import shopkeeper
+    from shopkeeper.config import read_settings
+
+    assert shopkeeper.read_settings is read_settings
+    assert "read_settings" in shopkeeper.__all__
+    assert not hasattr(shopkeeper, "load_config")
+
+
+def test_the_similar_names_were_left_alone() -> None:
+    from shopkeeper import config
+
+    assert callable(config.load_config_defaults)
+    assert callable(config.reload_config)
+    assert not hasattr(config, "read_settings_defaults")
+    assert not hasattr(config, "reread_settings")
+    readme = (ROOT / "README.md").read_text()
+    assert "load_config_defaults()" in readme
+    assert "reload_config(settings)" in readme
+
+
+def test_the_new_name_appears_in_every_file_that_used_the_old_one() -> None:
+    for name in [
+        "catalog",
+        "pricing",
+        "inventory",
+        "notifications",
+        "orders",
+        "reports",
+        "storage",
+        "cli",
+    ]:
+        source = (ROOT / "shopkeeper" / f"{name}.py").read_text()
+        assert "read_settings" in source, f"shopkeeper/{name}.py"
+    assert "read_settings" in (ROOT / "README.md").read_text()
+
+
+def test_behaviour_is_unchanged(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    from shopkeeper.config import read_settings, reload_config
+
+    (tmp_path / "catalog.csv").write_text("sku,name,unit_price\nSKU-1,bolt,0.25\n")
+    ini = tmp_path / "shop.ini"
+    ini.write_text(
+        f"[shop]\ncurrency = EUR\ntax_percent = 20\ncatalog_file = {tmp_path / 'catalog.csv'}\n"
+    )
+    monkeypatch.delenv("SHOPKEEPER_CONFIG", raising=False)
+    settings = read_settings(ini)
+    assert (settings.currency, settings.tax_percent, settings.low_stock_threshold) == (
+        "EUR",
+        20.0,
+        5,
+    )
+    ini.write_text(ini.read_text().replace("EUR", "GBP"))
+    assert reload_config(settings).currency == "GBP"
+    assert read_settings().currency == "USD"
+
+
+def test_the_projects_own_tests_pass() -> None:
+    run = subprocess.run(
+        [sys.executable, "-m", "pytest", "-q", "tests"],
+        cwd=ROOT,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert run.returncode == 0, run.stdout[-2000:]
+
+
+def test_the_command_line_still_works(tmp_path: Path) -> None:
+    (tmp_path / "catalog.csv").write_text("sku,name,unit_price\nSKU-2,hammer,12.50\n")
+    ini = tmp_path / "shop.ini"
+    ini.write_text(
+        f"[shop]\ncurrency = EUR\ntax_percent = 20\ncatalog_file = {tmp_path / 'catalog.csv'}\n"
+    )
+    run = subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "shopkeeper",
+            "--config",
+            str(ini),
+            "price",
+            "SKU-2",
+            "2",
+        ],
+        cwd=ROOT,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert run.stdout.strip() == "30.00 EUR", run.stderr[-1000:]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/README.md b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/README.md
new file mode 100644
index 0000000..cce359d
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/README.md
@@ -0,0 +1,22 @@
+# shopkeeper
+
+A small shop back office: catalogue, pricing, stock, orders and reports.
+
+## Settings
+
+Settings come from an INI file. Load them once and pass them around:
+
+```python
+from shopkeeper import load_config
+
+settings = load_config("shop.ini")
+```
+
+`load_config()` with no path uses `SHOPKEEPER_CONFIG`, or the built-in
+defaults from `load_config_defaults()` when that is unset. After editing the
+file, `reload_config(settings)` reads it again.
+
+## Command line
+
+    python -m shopkeeper --config shop.ini price SKU-1 3
+    python -m shopkeeper --config shop.ini stock
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__init__.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__init__.py
new file mode 100644
index 0000000..fb3f608
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__init__.py
@@ -0,0 +1,5 @@
+"""A small shop back office."""
+
+from shopkeeper.config import Settings, load_config, load_config_defaults, reload_config
+
+__all__ = ["Settings", "load_config", "load_config_defaults", "reload_config"]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__main__.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__main__.py
new file mode 100644
index 0000000..3a08049
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/__main__.py
@@ -0,0 +1,3 @@
+from shopkeeper.cli import main
+
+raise SystemExit(main())
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/catalog.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/catalog.py
new file mode 100644
index 0000000..1742b06
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/catalog.py
@@ -0,0 +1,25 @@
+"""The product catalogue: SKU, name and unit price, from a CSV file."""
+
+import csv
+from dataclasses import dataclass
+from pathlib import Path
+
+from shopkeeper.config import Settings, load_config
+
+
+@dataclass(frozen=True)
+class Product:
+    sku: str
+    name: str
+    unit_price: float
+
+
+def read_catalog(settings: Settings | None = None) -> dict[str, Product]:
+    """Every product, keyed by SKU. Uses load_config() when no settings are given."""
+    settings = settings or load_config()
+    path = Path(settings.catalog_file)
+    with path.open(newline="") as handle:
+        return {
+            row["sku"]: Product(row["sku"], row["name"], float(row["unit_price"]))
+            for row in csv.DictReader(handle)
+        }
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/cli.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/cli.py
new file mode 100644
index 0000000..25103af
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/cli.py
@@ -0,0 +1,37 @@
+"""Command line: python -m shopkeeper --config FILE COMMAND ..."""
+
+import argparse
+from pathlib import Path
+
+from shopkeeper import storage
+from shopkeeper.catalog import read_catalog
+from shopkeeper.config import load_config
+from shopkeeper.pricing import format_price, with_tax
+from shopkeeper.reports import stock_report
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(prog="shopkeeper")
+    parser.add_argument("--config", help="settings file (see load_config)")
+    parser.add_argument("--stock-file", default="stock.json")
+    commands = parser.add_subparsers(dest="command", required=True)
+    price = commands.add_parser(
+        "price", help="price of a quantity of one SKU, with tax"
+    )
+    price.add_argument("sku")
+    price.add_argument("quantity", type=int)
+    commands.add_parser("stock", help="stock report")
+    args = parser.parse_args(argv)
+
+    settings = load_config(args.config)
+    catalog = read_catalog(settings)
+    if args.command == "price":
+        amount = with_tax(catalog[args.sku].unit_price * args.quantity, settings)
+        print(format_price(amount, settings))
+    else:
+        print(
+            stock_report(
+                catalog, storage.load(Path(args.stock_file), settings), settings
+            )
+        )
+    return 0
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/config.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/config.py
new file mode 100644
index 0000000..cb26f07
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/config.py
@@ -0,0 +1,53 @@
+"""Settings, read from an INI file."""
+
+import configparser
+import os
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+ENV_VAR = "SHOPKEEPER_CONFIG"
+
+
+@dataclass(frozen=True)
+class Settings:
+    currency: str
+    tax_percent: float
+    low_stock_threshold: int
+    catalog_file: str
+    notify_email: str
+    source: str | None = None  # the file these came from, if any
+
+
+def load_config_defaults() -> dict[str, Any]:
+    """The built-in settings, used for anything the file leaves out."""
+    return {
+        "currency": "USD",
+        "tax_percent": 0.0,
+        "low_stock_threshold": 5,
+        "catalog_file": "catalog.csv",
+        "notify_email": "",
+    }
+
+
+def load_config(path: str | Path | None = None) -> Settings:
+    """Settings from `path`, or from $SHOPKEEPER_CONFIG, or the defaults.
+
+    Values missing from the file fall back to load_config_defaults().
+    """
+    values = load_config_defaults()
+    chosen = path or os.environ.get(ENV_VAR)
+    if chosen:
+        parser = configparser.ConfigParser()
+        if not parser.read(chosen):
+            raise FileNotFoundError(f"no settings file at {chosen}")
+        section = parser["shop"] if parser.has_section("shop") else {}
+        for key, default in values.items():
+            if key in section:
+                values[key] = type(default)(section[key])
+    return Settings(**values, source=str(chosen) if chosen else None)
+
+
+def reload_config(settings: Settings) -> Settings:
+    """Read the settings again from the file they came from."""
+    return load_config(settings.source)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/inventory.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/inventory.py
new file mode 100644
index 0000000..ea770d0
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/inventory.py
@@ -0,0 +1,27 @@
+"""Stock levels."""
+
+from shopkeeper.config import Settings, load_config
+
+
+class Inventory:
+    def __init__(self, settings: Settings | None = None) -> None:
+        self.settings = settings or load_config()
+        self._stock: dict[str, int] = {}
+
+    def add(self, sku: str, quantity: int) -> None:
+        if quantity <= 0:
+            raise ValueError("quantity must be positive")
+        self._stock[sku] = self._stock.get(sku, 0) + quantity
+
+    def take(self, sku: str, quantity: int) -> None:
+        if self._stock.get(sku, 0) < quantity:
+            raise ValueError(f"not enough {sku} in stock")
+        self._stock[sku] -= quantity
+
+    def level(self, sku: str) -> int:
+        return self._stock.get(sku, 0)
+
+    def low_stock(self) -> list[str]:
+        """SKUs below the threshold from load_config(), alphabetically."""
+        limit = self.settings.low_stock_threshold
+        return sorted(sku for sku, count in self._stock.items() if count < limit)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/notifications.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/notifications.py
new file mode 100644
index 0000000..12993a2
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/notifications.py
@@ -0,0 +1,20 @@
+"""Messages to the shop owner. Collected in memory; sending is elsewhere."""
+
+import logging
+
+from shopkeeper.config import Settings, load_config
+
+log = logging.getLogger(__name__)
+
+
+class Notifier:
+    def __init__(self, settings: Settings | None = None) -> None:
+        self.settings = settings or load_config()
+        self.outbox: list[tuple[str, str]] = []
+
+    def notify(self, message: str) -> bool:
+        if not self.settings.notify_email:
+            log.info("no notify_email in load_config settings; dropped: %s", message)
+            return False
+        self.outbox.append((self.settings.notify_email, message))
+        return True
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/orders.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/orders.py
new file mode 100644
index 0000000..3853122
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/orders.py
@@ -0,0 +1,35 @@
+"""Orders: take stock, price the lines, warn when stock runs low."""
+
+from dataclasses import dataclass, field
+
+from shopkeeper.catalog import Product
+from shopkeeper.config import Settings, load_config
+from shopkeeper.inventory import Inventory
+from shopkeeper.notifications import Notifier
+from shopkeeper.pricing import with_tax
+
+
+@dataclass
+class Order:
+    lines: list[tuple[Product, int]] = field(default_factory=list)
+
+    def add(self, product: Product, quantity: int) -> None:
+        self.lines.append((product, quantity))
+
+    def net(self) -> float:
+        return round(sum(p.unit_price * q for p, q in self.lines), 2)
+
+
+def place(
+    order: Order,
+    inventory: Inventory,
+    notifier: Notifier,
+    settings: Settings | None = None,
+) -> float:
+    """Take the order's stock and return its price with tax."""
+    settings = settings or load_config()
+    for product, quantity in order.lines:
+        inventory.take(product.sku, quantity)
+    for sku in inventory.low_stock():
+        notifier.notify(f"low stock: {sku}")
+    return with_tax(order.net(), settings)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/pricing.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/pricing.py
new file mode 100644
index 0000000..fb0b20a
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/pricing.py
@@ -0,0 +1,14 @@
+"""Prices with tax, in the shop's currency."""
+
+from shopkeeper.config import Settings, load_config
+
+
+def with_tax(amount: float, settings: Settings | None = None) -> float:
+    settings = settings or load_config()
+    return round(amount * (1 + settings.tax_percent / 100), 2)
+
+
+def format_price(amount: float, settings: Settings | None = None) -> str:
+    # The currency comes from load_config, never from the catalogue.
+    settings = settings or load_config()
+    return f"{amount:.2f} {settings.currency}"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/reports.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/reports.py
new file mode 100644
index 0000000..00cc508
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/reports.py
@@ -0,0 +1,21 @@
+"""Plain-text reports."""
+
+from shopkeeper.catalog import Product
+from shopkeeper.config import Settings, load_config
+from shopkeeper.inventory import Inventory
+from shopkeeper.pricing import format_price
+
+
+def stock_report(
+    catalog: dict[str, Product], inventory: Inventory, settings: Settings | None = None
+) -> str:
+    settings = settings or load_config()
+    lines = []
+    for sku in sorted(catalog):
+        product = catalog[sku]
+        value = product.unit_price * inventory.level(sku)
+        lines.append(
+            f"{sku:<8} {product.name:<16} {inventory.level(sku):>5} "
+            f"{format_price(value, settings):>14}"
+        )
+    return "\n".join(lines)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/storage.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/storage.py
new file mode 100644
index 0000000..226b12b
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/shopkeeper/storage.py
@@ -0,0 +1,20 @@
+"""Saving and loading stock levels as JSON."""
+
+import json
+from pathlib import Path
+
+from shopkeeper.config import Settings, load_config
+from shopkeeper.inventory import Inventory
+
+
+def save(inventory: Inventory, path: Path) -> None:
+    levels = {sku: inventory.level(sku) for sku in inventory._stock}
+    path.write_text(json.dumps(levels, indent=2, sort_keys=True))
+
+
+def load(path: Path, settings: Settings | None = None) -> Inventory:
+    inventory = Inventory(settings or load_config())
+    for sku, count in json.loads(path.read_text()).items():
+        if count:
+            inventory.add(sku, count)
+    return inventory
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/conftest.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/conftest.py
new file mode 100644
index 0000000..680b764
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/conftest.py
@@ -0,0 +1,19 @@
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def shop(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
+    """A shop directory with a settings file and a catalogue."""
+    (tmp_path / "catalog.csv").write_text(
+        "sku,name,unit_price\nSKU-1,bolt,0.25\nSKU-2,hammer,12.50\n"
+    )
+    ini = tmp_path / "shop.ini"
+    ini.write_text(
+        "[shop]\ncurrency = EUR\ntax_percent = 20\nlow_stock_threshold = 3\n"
+        f"catalog_file = {tmp_path / 'catalog.csv'}\nnotify_email = owner@example.com\n"
+    )
+    monkeypatch.delenv("SHOPKEEPER_CONFIG", raising=False)
+    monkeypatch.chdir(tmp_path)
+    return ini
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_config.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_config.py
new file mode 100644
index 0000000..7f6b13a
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_config.py
@@ -0,0 +1,33 @@
+import pytest
+from shopkeeper import load_config, load_config_defaults, reload_config
+
+
+def test_defaults_without_a_file() -> None:
+    settings = load_config()
+    assert settings.currency == load_config_defaults()["currency"]
+    assert settings.source is None
+
+
+def test_values_from_the_file(shop) -> None:
+    settings = load_config(shop)
+    assert (settings.currency, settings.tax_percent, settings.low_stock_threshold) == (
+        "EUR",
+        20.0,
+        3,
+    )
+
+
+def test_environment_variable(shop, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("SHOPKEEPER_CONFIG", str(shop))
+    assert load_config().currency == "EUR"
+
+
+def test_reload_reads_the_file_again(shop) -> None:
+    settings = load_config(shop)
+    shop.write_text(shop.read_text().replace("EUR", "GBP"))
+    assert reload_config(settings).currency == "GBP"
+
+
+def test_a_missing_file_is_an_error(tmp_path) -> None:
+    with pytest.raises(FileNotFoundError):
+        load_config(tmp_path / "nope.ini")
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_shop.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_shop.py
new file mode 100644
index 0000000..5c5807c
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/seed/tests/test_shop.py
@@ -0,0 +1,31 @@
+from shopkeeper.catalog import read_catalog
+from shopkeeper.cli import main
+from shopkeeper.config import load_config
+from shopkeeper.inventory import Inventory
+from shopkeeper.notifications import Notifier
+from shopkeeper.orders import Order, place
+from shopkeeper.pricing import format_price, with_tax
+
+
+def test_tax_and_format(shop) -> None:
+    settings = load_config(shop)
+    assert with_tax(10.0, settings) == 12.0
+    assert format_price(12.0, settings) == "12.00 EUR"
+
+
+def test_placing_an_order_takes_stock_and_warns(shop) -> None:
+    settings = load_config(shop)
+    catalog = read_catalog(settings)
+    inventory = Inventory(settings)
+    inventory.add("SKU-1", 10)
+    notifier = Notifier(settings)
+    order = Order()
+    order.add(catalog["SKU-1"], 8)
+    assert place(order, inventory, notifier, settings) == 2.4
+    assert inventory.level("SKU-1") == 2
+    assert notifier.outbox == [("owner@example.com", "low stock: SKU-1")]
+
+
+def test_cli_price(shop, capsys) -> None:
+    assert main(["--config", str(shop), "price", "SKU-2", "2"]) == 0
+    assert capsys.readouterr().out.strip() == "30.00 EUR"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/README.md b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/README.md
new file mode 100644
index 0000000..acd35aa
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/README.md
@@ -0,0 +1,22 @@
+# shopkeeper
+
+A small shop back office: catalogue, pricing, stock, orders and reports.
+
+## Settings
+
+Settings come from an INI file. Load them once and pass them around:
+
+```python
+from shopkeeper import read_settings
+
+settings = read_settings("shop.ini")
+```
+
+`read_settings()` with no path uses `SHOPKEEPER_CONFIG`, or the built-in
+defaults from `load_config_defaults()` when that is unset. After editing the
+file, `reload_config(settings)` reads it again.
+
+## Command line
+
+    python -m shopkeeper --config shop.ini price SKU-1 3
+    python -m shopkeeper --config shop.ini stock
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/__init__.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/__init__.py
new file mode 100644
index 0000000..7ba1ad9
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/__init__.py
@@ -0,0 +1,10 @@
+"""A small shop back office."""
+
+from shopkeeper.config import (
+    Settings,
+    load_config_defaults,
+    read_settings,
+    reload_config,
+)
+
+__all__ = ["Settings", "load_config_defaults", "read_settings", "reload_config"]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/catalog.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/catalog.py
new file mode 100644
index 0000000..a9a0d17
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/catalog.py
@@ -0,0 +1,25 @@
+"""The product catalogue: SKU, name and unit price, from a CSV file."""
+
+import csv
+from dataclasses import dataclass
+from pathlib import Path
+
+from shopkeeper.config import Settings, read_settings
+
+
+@dataclass(frozen=True)
+class Product:
+    sku: str
+    name: str
+    unit_price: float
+
+
+def read_catalog(settings: Settings | None = None) -> dict[str, Product]:
+    """Every product, keyed by SKU. Uses read_settings() when no settings are given."""
+    settings = settings or read_settings()
+    path = Path(settings.catalog_file)
+    with path.open(newline="") as handle:
+        return {
+            row["sku"]: Product(row["sku"], row["name"], float(row["unit_price"]))
+            for row in csv.DictReader(handle)
+        }
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/cli.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/cli.py
new file mode 100644
index 0000000..66b991d
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/cli.py
@@ -0,0 +1,37 @@
+"""Command line: python -m shopkeeper --config FILE COMMAND ..."""
+
+import argparse
+from pathlib import Path
+
+from shopkeeper import storage
+from shopkeeper.catalog import read_catalog
+from shopkeeper.config import read_settings
+from shopkeeper.pricing import format_price, with_tax
+from shopkeeper.reports import stock_report
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(prog="shopkeeper")
+    parser.add_argument("--config", help="settings file (see read_settings)")
+    parser.add_argument("--stock-file", default="stock.json")
+    commands = parser.add_subparsers(dest="command", required=True)
+    price = commands.add_parser(
+        "price", help="price of a quantity of one SKU, with tax"
+    )
+    price.add_argument("sku")
+    price.add_argument("quantity", type=int)
+    commands.add_parser("stock", help="stock report")
+    args = parser.parse_args(argv)
+
+    settings = read_settings(args.config)
+    catalog = read_catalog(settings)
+    if args.command == "price":
+        amount = with_tax(catalog[args.sku].unit_price * args.quantity, settings)
+        print(format_price(amount, settings))
+    else:
+        print(
+            stock_report(
+                catalog, storage.load(Path(args.stock_file), settings), settings
+            )
+        )
+    return 0
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/config.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/config.py
new file mode 100644
index 0000000..05e1d43
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/config.py
@@ -0,0 +1,53 @@
+"""Settings, read from an INI file."""
+
+import configparser
+import os
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+ENV_VAR = "SHOPKEEPER_CONFIG"
+
+
+@dataclass(frozen=True)
+class Settings:
+    currency: str
+    tax_percent: float
+    low_stock_threshold: int
+    catalog_file: str
+    notify_email: str
+    source: str | None = None  # the file these came from, if any
+
+
+def load_config_defaults() -> dict[str, Any]:
+    """The built-in settings, used for anything the file leaves out."""
+    return {
+        "currency": "USD",
+        "tax_percent": 0.0,
+        "low_stock_threshold": 5,
+        "catalog_file": "catalog.csv",
+        "notify_email": "",
+    }
+
+
+def read_settings(path: str | Path | None = None) -> Settings:
+    """Settings from `path`, or from $SHOPKEEPER_CONFIG, or the defaults.
+
+    Values missing from the file fall back to load_config_defaults().
+    """
+    values = load_config_defaults()
+    chosen = path or os.environ.get(ENV_VAR)
+    if chosen:
+        parser = configparser.ConfigParser()
+        if not parser.read(chosen):
+            raise FileNotFoundError(f"no settings file at {chosen}")
+        section = parser["shop"] if parser.has_section("shop") else {}
+        for key, default in values.items():
+            if key in section:
+                values[key] = type(default)(section[key])
+    return Settings(**values, source=str(chosen) if chosen else None)
+
+
+def reload_config(settings: Settings) -> Settings:
+    """Read the settings again from the file they came from."""
+    return read_settings(settings.source)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/inventory.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/inventory.py
new file mode 100644
index 0000000..3762218
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/inventory.py
@@ -0,0 +1,27 @@
+"""Stock levels."""
+
+from shopkeeper.config import Settings, read_settings
+
+
+class Inventory:
+    def __init__(self, settings: Settings | None = None) -> None:
+        self.settings = settings or read_settings()
+        self._stock: dict[str, int] = {}
+
+    def add(self, sku: str, quantity: int) -> None:
+        if quantity <= 0:
+            raise ValueError("quantity must be positive")
+        self._stock[sku] = self._stock.get(sku, 0) + quantity
+
+    def take(self, sku: str, quantity: int) -> None:
+        if self._stock.get(sku, 0) < quantity:
+            raise ValueError(f"not enough {sku} in stock")
+        self._stock[sku] -= quantity
+
+    def level(self, sku: str) -> int:
+        return self._stock.get(sku, 0)
+
+    def low_stock(self) -> list[str]:
+        """SKUs below the threshold from read_settings(), alphabetically."""
+        limit = self.settings.low_stock_threshold
+        return sorted(sku for sku, count in self._stock.items() if count < limit)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/notifications.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/notifications.py
new file mode 100644
index 0000000..4b2ea1e
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/notifications.py
@@ -0,0 +1,20 @@
+"""Messages to the shop owner. Collected in memory; sending is elsewhere."""
+
+import logging
+
+from shopkeeper.config import Settings, read_settings
+
+log = logging.getLogger(__name__)
+
+
+class Notifier:
+    def __init__(self, settings: Settings | None = None) -> None:
+        self.settings = settings or read_settings()
+        self.outbox: list[tuple[str, str]] = []
+
+    def notify(self, message: str) -> bool:
+        if not self.settings.notify_email:
+            log.info("no notify_email in read_settings settings; dropped: %s", message)
+            return False
+        self.outbox.append((self.settings.notify_email, message))
+        return True
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/orders.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/orders.py
new file mode 100644
index 0000000..9a0f21f
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/orders.py
@@ -0,0 +1,35 @@
+"""Orders: take stock, price the lines, warn when stock runs low."""
+
+from dataclasses import dataclass, field
+
+from shopkeeper.catalog import Product
+from shopkeeper.config import Settings, read_settings
+from shopkeeper.inventory import Inventory
+from shopkeeper.notifications import Notifier
+from shopkeeper.pricing import with_tax
+
+
+@dataclass
+class Order:
+    lines: list[tuple[Product, int]] = field(default_factory=list)
+
+    def add(self, product: Product, quantity: int) -> None:
+        self.lines.append((product, quantity))
+
+    def net(self) -> float:
+        return round(sum(p.unit_price * q for p, q in self.lines), 2)
+
+
+def place(
+    order: Order,
+    inventory: Inventory,
+    notifier: Notifier,
+    settings: Settings | None = None,
+) -> float:
+    """Take the order's stock and return its price with tax."""
+    settings = settings or read_settings()
+    for product, quantity in order.lines:
+        inventory.take(product.sku, quantity)
+    for sku in inventory.low_stock():
+        notifier.notify(f"low stock: {sku}")
+    return with_tax(order.net(), settings)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/pricing.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/pricing.py
new file mode 100644
index 0000000..8cf9130
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/pricing.py
@@ -0,0 +1,14 @@
+"""Prices with tax, in the shop's currency."""
+
+from shopkeeper.config import Settings, read_settings
+
+
+def with_tax(amount: float, settings: Settings | None = None) -> float:
+    settings = settings or read_settings()
+    return round(amount * (1 + settings.tax_percent / 100), 2)
+
+
+def format_price(amount: float, settings: Settings | None = None) -> str:
+    # The currency comes from read_settings, never from the catalogue.
+    settings = settings or read_settings()
+    return f"{amount:.2f} {settings.currency}"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/reports.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/reports.py
new file mode 100644
index 0000000..7234aa7
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/reports.py
@@ -0,0 +1,21 @@
+"""Plain-text reports."""
+
+from shopkeeper.catalog import Product
+from shopkeeper.config import Settings, read_settings
+from shopkeeper.inventory import Inventory
+from shopkeeper.pricing import format_price
+
+
+def stock_report(
+    catalog: dict[str, Product], inventory: Inventory, settings: Settings | None = None
+) -> str:
+    settings = settings or read_settings()
+    lines = []
+    for sku in sorted(catalog):
+        product = catalog[sku]
+        value = product.unit_price * inventory.level(sku)
+        lines.append(
+            f"{sku:<8} {product.name:<16} {inventory.level(sku):>5} "
+            f"{format_price(value, settings):>14}"
+        )
+    return "\n".join(lines)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/storage.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/storage.py
new file mode 100644
index 0000000..67ddc68
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/shopkeeper/storage.py
@@ -0,0 +1,20 @@
+"""Saving and loading stock levels as JSON."""
+
+import json
+from pathlib import Path
+
+from shopkeeper.config import Settings, read_settings
+from shopkeeper.inventory import Inventory
+
+
+def save(inventory: Inventory, path: Path) -> None:
+    levels = {sku: inventory.level(sku) for sku in inventory._stock}
+    path.write_text(json.dumps(levels, indent=2, sort_keys=True))
+
+
+def load(path: Path, settings: Settings | None = None) -> Inventory:
+    inventory = Inventory(settings or read_settings())
+    for sku, count in json.loads(path.read_text()).items():
+        if count:
+            inventory.add(sku, count)
+    return inventory
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_config.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_config.py
new file mode 100644
index 0000000..6a3bac8
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_config.py
@@ -0,0 +1,33 @@
+import pytest
+from shopkeeper import load_config_defaults, read_settings, reload_config
+
+
+def test_defaults_without_a_file() -> None:
+    settings = read_settings()
+    assert settings.currency == load_config_defaults()["currency"]
+    assert settings.source is None
+
+
+def test_values_from_the_file(shop) -> None:
+    settings = read_settings(shop)
+    assert (settings.currency, settings.tax_percent, settings.low_stock_threshold) == (
+        "EUR",
+        20.0,
+        3,
+    )
+
+
+def test_environment_variable(shop, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("SHOPKEEPER_CONFIG", str(shop))
+    assert read_settings().currency == "EUR"
+
+
+def test_reload_reads_the_file_again(shop) -> None:
+    settings = read_settings(shop)
+    shop.write_text(shop.read_text().replace("EUR", "GBP"))
+    assert reload_config(settings).currency == "GBP"
+
+
+def test_a_missing_file_is_an_error(tmp_path) -> None:
+    with pytest.raises(FileNotFoundError):
+        read_settings(tmp_path / "nope.ini")
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_shop.py b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_shop.py
new file mode 100644
index 0000000..1b4b4f4
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/solution/tests/test_shop.py
@@ -0,0 +1,31 @@
+from shopkeeper.catalog import read_catalog
+from shopkeeper.cli import main
+from shopkeeper.config import read_settings
+from shopkeeper.inventory import Inventory
+from shopkeeper.notifications import Notifier
+from shopkeeper.orders import Order, place
+from shopkeeper.pricing import format_price, with_tax
+
+
+def test_tax_and_format(shop) -> None:
+    settings = read_settings(shop)
+    assert with_tax(10.0, settings) == 12.0
+    assert format_price(12.0, settings) == "12.00 EUR"
+
+
+def test_placing_an_order_takes_stock_and_warns(shop) -> None:
+    settings = read_settings(shop)
+    catalog = read_catalog(settings)
+    inventory = Inventory(settings)
+    inventory.add("SKU-1", 10)
+    notifier = Notifier(settings)
+    order = Order()
+    order.add(catalog["SKU-1"], 8)
+    assert place(order, inventory, notifier, settings) == 2.4
+    assert inventory.level("SKU-1") == 2
+    assert notifier.outbox == [("owner@example.com", "low stock: SKU-1")]
+
+
+def test_cli_price(shop, capsys) -> None:
+    assert main(["--config", str(shop), "price", "SKU-2", "2"]) == 0
+    assert capsys.readouterr().out.strip() == "30.00 EUR"
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/task.yaml b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/task.yaml
new file mode 100644
index 0000000..eaa6d53
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/refactor-rename/task.yaml
@@ -0,0 +1,18 @@
+title: Rename a function used across a package
+check: python -m pytest -q .bench_check
+plan: |
+  Milestone 1: rename `load_config` to `read_settings` across the project.
+
+  1. Find every use of `load_config`: run
+     `grep -rnw load_config --include=*.py --include=*.md .`
+     It is defined in `shopkeeper/config.py` and used in many other modules,
+     in tests, in docstrings, in comments, in a log message and in
+     `README.md`.
+  2. Rename the function to `read_settings` everywhere: its definition,
+     imports, calls, `__all__`, docstrings, comments, strings and
+     `README.md`. Afterwards the grep in step 1 must print nothing.
+  3. Rename only that exact name. `load_config_defaults` and `reload_config`
+     are different functions and keep their names. Change no behaviour.
+  4. Update the tests under `tests/` to use the new name, and run
+     `python -m pytest -q` until every test passes.
+  5. Call finish.
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/hidden/test_hidden.py b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/hidden/test_hidden.py
new file mode 100644
index 0000000..a539d4d
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/hidden/test_hidden.py
@@ -0,0 +1,89 @@
+import pytest
+from durations import parse_duration
+
+
+@pytest.mark.parametrize(
+    ("text", "seconds"),
+    [
+        ("2h", 7200),  # rule 1
+        ("1h30m", 5400),
+        ("1d2h3m4s", 93784),
+        ("45s", 45),
+        ("1d", 86400),
+        ("1H30M", 5400),  # rule 2
+        ("2D", 172800),
+        (" 1h 30m ", 5400),  # rule 4
+        ("1d 2h 3m 4s", 93784),
+        ("90", 90),  # rule 5
+        ("  7  ", 7),
+        ("1.5h", 5400),  # rule 6
+        ("1.5m", 90),
+        ("0.25h", 900),
+        ("1:30", 90),  # rule 7
+        ("1:02:03", 3723),
+        ("90:00", 5400),
+        ("0:05", 5),
+        (" 1:30 ", 90),
+        ("0", 0),  # rule 8
+        ("0s", 0),
+        ("0:00", 0),
+    ],
+)
+def test_valid(text: str, seconds: int) -> None:
+    assert parse_duration(text) == seconds
+
+
+@pytest.mark.parametrize(
+    "text",
+    [
+        "1x",  # rule 1: not a unit
+        "h",
+        ".5h",  # not a number
+        "1.h",
+        "+2h",
+        "30m1h",  # rule 3
+        "1h2h",
+        "1s1m",
+        "1 h",  # rule 4
+        "1h 30",  # rule 5
+        "30 1h",
+        "1h1.5m",  # rule 6
+        "0.5s",
+        "0.01m",
+        "1.5",
+        "1:60",  # rule 7
+        "1:2",
+        "1:02:60",
+        "1:02:03:04",
+        "1.5:30",
+        "1: 30",
+        "",  # rule 8
+        "   ",
+        "-5",
+        "-1h",
+    ],
+)
+def test_invalid(text: str) -> None:
+    with pytest.raises(ValueError) as caught:
+        parse_duration(text)
+    assert repr(text) in str(caught.value)
+
+
+def test_the_result_is_an_int() -> None:
+    assert type(parse_duration("1.5h")) is int
+
+
+def test_the_plan_asked_for_tests_and_they_pass() -> None:
+    # At least one valid and one invalid input for each of the eight rules.
+    import subprocess
+    import sys
+
+    run = subprocess.run(
+        [sys.executable, "-m", "pytest", "-q", "test_durations.py"],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    assert run.returncode == 0, run.stdout[-1500:]
+    summary = run.stdout.strip().splitlines()[-1]
+    assert int(summary.split()[0]) >= 16, summary
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/SPEC.md b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/SPEC.md
new file mode 100644
index 0000000..8503a5c
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/SPEC.md
@@ -0,0 +1,35 @@
+# parse_duration
+
+`parse_duration(text: str) -> int` turns a human-written duration into a whole
+number of seconds.
+
+A *number* below is one or more digits, optionally followed by a dot and one
+or more digits: `2`, `1.5`, `0.25`. Nothing else is a number: not `.5`, `1.`,
+`+2` or `-2`.
+
+## Rules
+
+1. **Unit form.** One or more components, each a number immediately followed
+   by a unit: `d` (86400 seconds), `h` (3600), `m` (60), `s` (1).
+   `"2h"` is 7200, `"1h30m"` is 5400, `"1d2h3m4s"` is 93784.
+2. **Case.** Units are case-insensitive: `"1H30M"` is 5400.
+3. **Order.** Components appear in the order d, h, m, s, and each unit at
+   most once. `"30m1h"` and `"1h2h"` are invalid.
+4. **Spaces.** Spaces are allowed between components and around the whole
+   text: `" 1h 30m "` is 5400. A space between a number and its unit is not
+   allowed: `"1 h"` is invalid.
+5. **Bare number.** A number with no unit, and nothing else, is seconds:
+   `"90"` is 90. A bare number mixed with components is invalid: `"1h 30"`.
+6. **Decimals.** Only the first component may have a decimal part:
+   `"1.5h"` is 5400, but `"1h1.5m"` is invalid. The result must be a whole
+   number of seconds: `"1.5m"` is 90, but `"0.5s"` and `"0.01m"` are invalid.
+7. **Clock form.** `M:SS` or `H:MM:SS`: whole numbers separated by colons.
+   `"1:30"` is 90, `"1:02:03"` is 3723, `"90:00"` is 5400. Every field after
+   the first is exactly two digits and below 60: `"1:60"`, `"1:2"` and
+   `"1:02:60"` are invalid. More than three fields, decimals, and spaces inside
+   the clock (`"1: 30"`) are invalid; spaces around it are allowed.
+8. **Zero and errors.** Zero is allowed: `"0"`, `"0s"` and `"0:00"` are all 0.
+   An empty string, only spaces, or a negative duration is invalid.
+
+Every invalid input raises `ValueError`, and its message includes the input
+exactly as given, in `repr` form: `invalid duration: '1h2h'`.
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/durations.py b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/durations.py
new file mode 100644
index 0000000..8f5a6c3
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/seed/durations.py
@@ -0,0 +1,6 @@
+"""Human-written durations. See SPEC.md."""
+
+
+def parse_duration(text: str) -> int:
+    """A duration in whole seconds, as SPEC.md specifies."""
+    raise NotImplementedError
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/durations.py b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/durations.py
new file mode 100644
index 0000000..6b43ac8
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/durations.py
@@ -0,0 +1,42 @@
+"""Human-written durations. See SPEC.md."""
+
+import re
+from decimal import Decimal
+
+_NUMBER = r"\d+(?:\.\d+)?"
+_UNIT_FORM = re.compile(
+    rf"(?:(?P<d>{_NUMBER})d)?\s*(?:(?P<h>{_NUMBER})h)?\s*"
+    rf"(?:(?P<m>{_NUMBER})m)?\s*(?:(?P<s>{_NUMBER})s)?",
+    re.IGNORECASE,
+)
+_CLOCK = re.compile(r"\d+(?::[0-5]\d){1,2}")
+_SECONDS = {"d": 86400, "h": 3600, "m": 60, "s": 1}
+
+
+def parse_duration(text: str) -> int:
+    """A duration in whole seconds, as SPEC.md specifies."""
+
+    def invalid() -> ValueError:
+        return ValueError(f"invalid duration: {text!r}")
+
+    stripped = text.strip()
+    if not stripped:
+        raise invalid()
+    if _CLOCK.fullmatch(stripped):
+        total = 0
+        for field in stripped.split(":"):
+            total = total * 60 + int(field)
+        return total
+    if re.fullmatch(_NUMBER, stripped):
+        seconds = Decimal(stripped)
+    else:
+        match = _UNIT_FORM.fullmatch(stripped)
+        if not match or not any(match.groupdict().values()):
+            raise invalid()
+        present = [(unit, value) for unit, value in match.groupdict().items() if value]
+        if any("." in value for _unit, value in present[1:]):
+            raise invalid()
+        seconds = sum(Decimal(value) * _SECONDS[unit] for unit, value in present)
+    if seconds != seconds.to_integral_value():
+        raise invalid()
+    return int(seconds)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/test_durations.py b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/test_durations.py
new file mode 100644
index 0000000..230957d
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/solution/test_durations.py
@@ -0,0 +1,26 @@
+import pytest
+from durations import parse_duration
+
+
+@pytest.mark.parametrize(
+    ("text", "seconds"),
+    [
+        ("1h30m", 5400),
+        ("1H30M", 5400),
+        (" 1h 30m ", 5400),
+        ("90", 90),
+        ("1.5h", 5400),
+        ("1:02:03", 3723),
+        ("0s", 0),
+    ],
+)
+def test_valid(text: str, seconds: int) -> None:
+    assert parse_duration(text) == seconds
+
+
+@pytest.mark.parametrize(
+    "text", ["1x", "30m1h", "1 h", "1h 30", "1h1.5m", "0.5s", "1:60", "", "-5"]
+)
+def test_invalid(text: str) -> None:
+    with pytest.raises(ValueError, match="invalid duration"):
+        parse_duration(text)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/task.yaml b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/task.yaml
new file mode 100644
index 0000000..8a6cd97
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/spec-parser/task.yaml
@@ -0,0 +1,15 @@
+title: Implement a parser from a written specification
+check: python -m pytest -q .bench_check
+plan: |
+  Milestone 1: implement `parse_duration` as `SPEC.md` specifies.
+
+  1. Read all of `SPEC.md`. It has eight numbered rules; every one applies.
+  2. In `durations.py`, replace the body of `parse_duration(text: str) -> int`
+     so that it follows every rule in `SPEC.md`, including the inputs each
+     rule says are invalid and the error message for them. Examples:
+     "1h30m" -> 5400; "90" -> 90; "1:02:03" -> 3723.
+  3. Create `test_durations.py` with pytest tests: at least one valid and one
+     invalid input for each of the eight rules.
+  4. Run `python -m pytest -q test_durations.py`, and fix `durations.py`
+     until every test passes.
+  5. Call finish.
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/original_test_invoice.py.txt b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/original_test_invoice.py.txt
new file mode 100644
index 0000000..acf834c
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/original_test_invoice.py.txt
@@ -0,0 +1,38 @@
+from decimal import Decimal
+
+from invoicing.invoice import Invoice
+from invoicing.lines import LineItem
+from invoicing.report import render
+
+
+def d(value: str) -> Decimal:
+    return Decimal(value)
+
+
+def test_single_currency_total() -> None:
+    invoice = Invoice(
+        "USD", [LineItem("widget", d("19.99"), 3), LineItem("gadget", d("5.00"), 2)]
+    )
+    assert invoice.total() == d("69.97")
+
+
+def test_twenty_percent_tax() -> None:
+    invoice = Invoice("USD", [LineItem("service", d("100.00"), 1)], tax_percent=d("20"))
+    assert invoice.total() == d("120.00")
+
+
+def test_multi_currency_total() -> None:
+    # 100 USD is 91.95 EUR at the reference rates (1 EUR = 1.0875 USD).
+    invoice = Invoice(
+        "EUR",
+        [
+            LineItem("hosting", d("100.00"), 1, "USD"),
+            LineItem("support", d("50.00"), 1, "EUR"),
+        ],
+    )
+    assert invoice.total() == d("141.95")
+
+
+def test_report_shows_the_total() -> None:
+    invoice = Invoice("USD", [LineItem("widget", d("10.00"), 2)])
+    assert render(invoice).splitlines()[-1].split() == ["Total", "20.00", "USD"]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/test_hidden.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/test_hidden.py
new file mode 100644
index 0000000..0492858
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/hidden/test_hidden.py
@@ -0,0 +1,76 @@
+from decimal import Decimal
+from pathlib import Path
+
+import pytest
+from invoicing.currency import convert
+from invoicing.discounts import Discount
+from invoicing.invoice import Invoice
+from invoicing.lines import LineItem
+
+
+def d(value: str) -> Decimal:
+    return Decimal(value)
+
+
+def test_the_visible_tests_were_not_changed() -> None:
+    original = (Path(__file__).parent / "original_test_invoice.py.txt").read_text()
+    assert Path("tests/test_invoice.py").read_text() == original
+
+
+@pytest.mark.parametrize(
+    ("amount", "source", "target", "expected"),
+    [
+        ("100.00", "USD", "EUR", "91.95"),
+        ("100.00", "EUR", "USD", "108.75"),
+        ("250.00", "GBP", "EUR", "290.44"),
+        ("10000", "JPY", "USD", "67.12"),
+        ("42.00", "USD", "USD", "42.00"),
+    ],
+)
+def test_conversion(amount: str, source: str, target: str, expected: str) -> None:
+    assert convert(d(amount), source, target) == d(expected)
+
+
+def test_multi_currency_total() -> None:
+    invoice = Invoice(
+        "EUR",
+        [
+            LineItem("hosting", d("100.00"), 1, "USD"),
+            LineItem("support", d("50.00"), 1, "EUR"),
+        ],
+    )
+    assert invoice.total() == d("141.95")
+
+
+def test_fractional_discount() -> None:
+    # Same shared helper, no currency involved: 12.5% of 1000.00 is 125.00.
+    invoice = Invoice(
+        "USD", [LineItem("licence", d("1000.00"), 1)], discount=Discount(d("12.5"))
+    )
+    assert invoice.discount_total() == d("125.00")
+    assert invoice.total() == d("875.00")
+
+
+def test_fractional_tax() -> None:
+    # 8.25% of 1000.00 is 82.50.
+    invoice = Invoice(
+        "USD", [LineItem("licence", d("1000.00"), 1)], tax_percent=d("8.25")
+    )
+    assert invoice.tax() == d("82.50")
+
+
+def test_everything_together() -> None:
+    invoice = Invoice(
+        "GBP",
+        [
+            LineItem("seats", d("40.00"), 10, "EUR"),
+            LineItem("setup", d("99.00"), 1, "GBP"),
+        ],
+        discount=Discount(d("7.5")),
+        tax_percent=d("17.5"),
+    )
+    # 400 EUR = 344.31 GBP; subtotal 443.31; discount 33.25; tax 71.76.
+    assert invoice.subtotal() == d("443.31")
+    assert invoice.discount_total() == d("33.25")
+    assert invoice.tax() == d("71.76")
+    assert invoice.total() == d("481.82")
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/__init__.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/__init__.py
new file mode 100644
index 0000000..4dda660
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/__init__.py
@@ -0,0 +1 @@
+"""Invoices with lines in several currencies, discounts and tax."""
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/currency.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/currency.py
new file mode 100644
index 0000000..83ed7d9
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/currency.py
@@ -0,0 +1,27 @@
+"""Currency conversion at fixed reference rates."""
+
+from decimal import Decimal
+
+from invoicing.money import apply_rate, money, to_cents
+
+# Value of one unit of each currency in US dollars.
+RATES_TO_USD: dict[str, Decimal] = {
+    "USD": money("1"),
+    "EUR": money("1.0875"),
+    "GBP": money("1.2634"),
+    "JPY": money("0.006712"),
+}
+
+
+def rate(from_currency: str, to_currency: str) -> Decimal:
+    """How many units of `to_currency` one unit of `from_currency` buys."""
+    try:
+        return RATES_TO_USD[from_currency] / RATES_TO_USD[to_currency]
+    except KeyError as exc:
+        raise ValueError(f"unknown currency: {exc.args[0]}") from None
+
+
+def convert(amount: Decimal, from_currency: str, to_currency: str) -> Decimal:
+    if from_currency == to_currency:
+        return to_cents(amount)
+    return apply_rate(amount, rate(from_currency, to_currency))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/discounts.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/discounts.py
new file mode 100644
index 0000000..89277f0
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/discounts.py
@@ -0,0 +1,21 @@
+"""Percentage discounts on an invoice subtotal."""
+
+from dataclasses import dataclass
+from decimal import Decimal
+
+from invoicing.money import apply_rate
+
+
+@dataclass(frozen=True)
+class Discount:
+    percent: Decimal  # 12.5 means 12.5%
+
+    def __post_init__(self) -> None:
+        if not 0 <= self.percent <= 100:
+            raise ValueError(f"discount must be 0-100%, not {self.percent}")
+
+
+def discount_amount(subtotal: Decimal, discount: Discount | None) -> Decimal:
+    if discount is None:
+        return Decimal("0.00")
+    return apply_rate(subtotal, discount.percent / 100)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/invoice.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/invoice.py
new file mode 100644
index 0000000..9937b68
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/invoice.py
@@ -0,0 +1,38 @@
+"""An invoice: lines converted to its currency, then discount, then tax."""
+
+from dataclasses import dataclass, field
+from decimal import Decimal
+
+from invoicing.currency import convert
+from invoicing.discounts import Discount, discount_amount
+from invoicing.lines import LineItem, line_total
+from invoicing.money import apply_rate
+
+
+@dataclass
+class Invoice:
+    currency: str
+    lines: list[LineItem] = field(default_factory=list)
+    discount: Discount | None = None
+    tax_percent: Decimal = Decimal(0)
+
+    def subtotal(self) -> Decimal:
+        """Every line, converted into the invoice's currency."""
+        return sum(
+            (
+                convert(line_total(line), line.currency, self.currency)
+                for line in self.lines
+            ),
+            Decimal("0.00"),
+        )
+
+    def discount_total(self) -> Decimal:
+        return discount_amount(self.subtotal(), self.discount)
+
+    def tax(self) -> Decimal:
+        """Tax on the discounted subtotal."""
+        taxable = self.subtotal() - self.discount_total()
+        return apply_rate(taxable, self.tax_percent / 100)
+
+    def total(self) -> Decimal:
+        return self.subtotal() - self.discount_total() + self.tax()
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/lines.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/lines.py
new file mode 100644
index 0000000..60314f7
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/lines.py
@@ -0,0 +1,23 @@
+"""Invoice lines."""
+
+from dataclasses import dataclass
+from decimal import Decimal
+
+from invoicing.money import to_cents
+
+
+@dataclass(frozen=True)
+class LineItem:
+    description: str
+    unit_price: Decimal
+    quantity: int
+    currency: str = "USD"
+
+    def __post_init__(self) -> None:
+        if self.quantity < 0:
+            raise ValueError("quantity cannot be negative")
+
+
+def line_total(line: LineItem) -> Decimal:
+    """The line's amount in its own currency."""
+    return to_cents(line.unit_price * line.quantity)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/money.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/money.py
new file mode 100644
index 0000000..4e7cd77
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/money.py
@@ -0,0 +1,28 @@
+"""Money arithmetic shared by every module: Decimal amounts, cent rounding."""
+
+from decimal import ROUND_HALF_UP, Decimal
+
+CENT = Decimal("0.01")
+
+# Rates and fractions (exchange rates, discount and tax fractions) are held
+# at this precision before they are applied to an amount.
+RATE_PLACES = Decimal("0.01")
+
+
+def money(value: str | int) -> Decimal:
+    """An amount from a string or int; never from a float."""
+    return Decimal(str(value))
+
+
+def to_cents(amount: Decimal) -> Decimal:
+    return amount.quantize(CENT, rounding=ROUND_HALF_UP)
+
+
+def normalise_rate(rate: Decimal) -> Decimal:
+    """A rate or fraction at the precision used for all arithmetic."""
+    return rate.quantize(RATE_PLACES, rounding=ROUND_HALF_UP)
+
+
+def apply_rate(amount: Decimal, rate: Decimal) -> Decimal:
+    """`amount` scaled by `rate`, in whole cents."""
+    return to_cents(amount * normalise_rate(rate))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/report.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/report.py
new file mode 100644
index 0000000..968c164
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/invoicing/report.py
@@ -0,0 +1,25 @@
+"""Plain-text rendering of an invoice."""
+
+from invoicing.invoice import Invoice
+from invoicing.lines import line_total
+
+
+def render(invoice: Invoice) -> str:
+    rows = [
+        f"{line.description:<20} {line.quantity:>4} x {line.unit_price:>10} "
+        f"{line.currency} = {line_total(line):>10}"
+        for line in invoice.lines
+    ]
+    rows.append(f"{'Subtotal':<20} {invoice.subtotal():>32} {invoice.currency}")
+    if invoice.discount is not None:
+        rows.append(
+            f"{'Discount ' + str(invoice.discount.percent) + '%':<20} "
+            f"{-invoice.discount_total():>32} {invoice.currency}"
+        )
+    if invoice.tax_percent:
+        rows.append(
+            f"{'Tax ' + str(invoice.tax_percent) + '%':<20} "
+            f"{invoice.tax():>32} {invoice.currency}"
+        )
+    rows.append(f"{'Total':<20} {invoice.total():>32} {invoice.currency}")
+    return "\n".join(rows)
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/tests/test_invoice.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/tests/test_invoice.py
new file mode 100644
index 0000000..acf834c
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/seed/tests/test_invoice.py
@@ -0,0 +1,38 @@
+from decimal import Decimal
+
+from invoicing.invoice import Invoice
+from invoicing.lines import LineItem
+from invoicing.report import render
+
+
+def d(value: str) -> Decimal:
+    return Decimal(value)
+
+
+def test_single_currency_total() -> None:
+    invoice = Invoice(
+        "USD", [LineItem("widget", d("19.99"), 3), LineItem("gadget", d("5.00"), 2)]
+    )
+    assert invoice.total() == d("69.97")
+
+
+def test_twenty_percent_tax() -> None:
+    invoice = Invoice("USD", [LineItem("service", d("100.00"), 1)], tax_percent=d("20"))
+    assert invoice.total() == d("120.00")
+
+
+def test_multi_currency_total() -> None:
+    # 100 USD is 91.95 EUR at the reference rates (1 EUR = 1.0875 USD).
+    invoice = Invoice(
+        "EUR",
+        [
+            LineItem("hosting", d("100.00"), 1, "USD"),
+            LineItem("support", d("50.00"), 1, "EUR"),
+        ],
+    )
+    assert invoice.total() == d("141.95")
+
+
+def test_report_shows_the_total() -> None:
+    invoice = Invoice("USD", [LineItem("widget", d("10.00"), 2)])
+    assert render(invoice).splitlines()[-1].split() == ["Total", "20.00", "USD"]
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/solution/invoicing/money.py b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/solution/invoicing/money.py
new file mode 100644
index 0000000..0c70fcc
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/solution/invoicing/money.py
@@ -0,0 +1,28 @@
+"""Money arithmetic shared by every module: Decimal amounts, cent rounding."""
+
+from decimal import ROUND_HALF_UP, Decimal
+
+CENT = Decimal("0.01")
+
+# Rates and fractions (exchange rates, discount and tax fractions) are held
+# at this precision before they are applied to an amount.
+RATE_PLACES = Decimal("0.000001")
+
+
+def money(value: str | int) -> Decimal:
+    """An amount from a string or int; never from a float."""
+    return Decimal(str(value))
+
+
+def to_cents(amount: Decimal) -> Decimal:
+    return amount.quantize(CENT, rounding=ROUND_HALF_UP)
+
+
+def normalise_rate(rate: Decimal) -> Decimal:
+    """A rate or fraction at the precision used for all arithmetic."""
+    return rate.quantize(RATE_PLACES, rounding=ROUND_HALF_UP)
+
+
+def apply_rate(amount: Decimal, rate: Decimal) -> Decimal:
+    """`amount` scaled by `rate`, in whole cents."""
+    return to_cents(amount * normalise_rate(rate))
diff --git a/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml
new file mode 100644
index 0000000..71af44a
--- /dev/null
+++ b/nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml
@@ -0,0 +1,21 @@
+title: Trace a wrong total to its cause in a shared helper
+check: python -m pytest -q .bench_check
+plan: |
+  Milestone 1: fix the wrong totals on multi-currency invoices.
+
+  1. Run `python -m pytest -q`. `test_multi_currency_total` in
+     `tests/test_invoice.py` fails: totals are slightly off when an invoice has
+     lines in another currency. The test is correct. Do not change any file
+     under `tests/`.
+  2. Find the cause. Start at `Invoice.total` in `invoicing/invoice.py` and
+     follow the functions it calls, reading each one, until you find where
+     precision is lost. The cause is in a function that more than one module
+     uses.
+  3. Fix it at its source, in that shared function or the value it uses. Do
+     not special-case currencies in `invoicing/invoice.py` or
+     `invoicing/currency.py`: every caller of the shared function has the
+     same problem, and all of them must be fixed by your change. The rates in
+     `invoicing/currency.py` have up to six decimal places (JPY is 0.006712),
+     and no step may round a rate or fraction to fewer places than that.
+  4. Run `python -m pytest -q` until every test passes.
+  5. Call finish.
diff --git a/nisse-dev/tests/test_bench.py b/nisse-dev/tests/test_bench.py
index 4fae2ab..a7c98e2 100644
--- a/nisse-dev/tests/test_bench.py
+++ b/nisse-dev/tests/test_bench.py
@@ -36,11 +36,15 @@ FACTS = ModelFacts(
 # --- the tasks themselves ---------------------------------------------------
 
 
-def test_the_bundled_tasks_are_the_four_planned() -> None:
+def test_the_bundled_tasks_are_the_eight_planned() -> None:
     assert [t.name for t in TASKS] == [
         "fix-bug",
+        "long-milestone",
         "long-output",
         "multi-file",
+        "refactor-rename",
+        "spec-parser",
+        "trace-bug",
         "write-function",
     ]
 
@@ -85,6 +89,48 @@ def test_long_output_really_is_long(tmp_path: Path) -> None:
     assert "R237" in run.stdout[-3000:]  # the failure is named at the end
 
 
+def test_trace_bug_refuses_a_fix_at_the_call_site(tmp_path: Path) -> None:
+    """Fixing only the currency path passes the visible test, but the same
+    shared helper is wrong for discounts and tax: the check must notice."""
+    [task] = load_tasks(["trace-bug"])
+    task.seed_into(tmp_path)
+    currency = tmp_path / "invoicing" / "currency.py"
+    currency.write_text(
+        currency.read_text()
+        .replace("apply_rate, money, to_cents", "money, to_cents")
+        .replace(
+            "return apply_rate(amount, rate(from_currency, to_currency))",
+            "return to_cents(amount * rate(from_currency, to_currency))",
+        )
+    )
+    passed, output = task.run_check(tmp_path)
+    assert not passed
+    assert "test_fractional_discount" in output
+
+
+def test_refactor_rename_refuses_a_blind_replace(tmp_path: Path) -> None:
+    """A plain text replace also renames load_config_defaults and
+    reload_config, which must keep their names."""
+    [task] = load_tasks(["refactor-rename"])
+    task.seed_into(tmp_path)
+    for path in [*tmp_path.rglob("*.py"), *tmp_path.rglob("*.md")]:
+        path.write_text(path.read_text().replace("load_config", "read_settings"))
+    passed, output = task.run_check(tmp_path)
+    assert not passed
+    assert "test_the_similar_names_were_left_alone" in output
+
+
+def test_refactor_rename_refuses_a_partial_rename(tmp_path: Path) -> None:
+    [task] = load_tasks(["refactor-rename"])
+    task.seed_into(tmp_path)
+    task.apply_solution(tmp_path)
+    readme = tmp_path / "README.md"
+    readme.write_text(readme.read_text().replace("read_settings", "load_config", 1))
+    passed, output = task.run_check(tmp_path)
+    assert not passed
+    assert "README.md" in output
+
+
 def test_a_changed_task_gets_a_new_hash(tmp_path: Path) -> None:
     [task] = load_tasks(["write-function"])
     shutil.copytree(task.path, tmp_path / "write-function")