diff --git a/2021/README.md b/2021/README.md
index f246dcc..c6318ec 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -3,7 +3,7 @@
 <details>
   <summary>Setup</summary>
 
-  Using [asdf]:
+  Using [asdf](https://asdf-vm.com/#/):
 
   ```sh
   asdf plugin add erlang
@@ -19,4 +19,20 @@
 | 12  | 13  | 14  | 15  | 16  | 17  | 18  |
 | 19  | 20  | 21  | 22  | 23  | 24  | 25  |
 
-[asdf]: https://asdf-vm.com/#/
+---
+
+## Mix Tasks
+
+- `advent_of_code.gen.solution`
+- `advent_of_code.solve`
+- `advent_of_code.fetch_input`
+
+Run `mix help <task>` for details.
+
+## [`AdventOfCode.PuzzleSolver`](./lib/advent_of_code/puzzle_solver.ex)
+
+A behaviour for a solution to a puzzle. Must define a `solve/2` callback.
+
+## [`AdventOfCode.PuzzleCase`](./test/support/puzzle_case.ex)
+
+Case template defining an `assert_solution/2` helper.
diff --git a/2021/lib/mix/tasks/advent_of_code.gen.solution.ex b/2021/lib/mix/tasks/advent_of_code.gen.solution.ex
index f1d1165..d61cb68 100644
--- a/2021/lib/mix/tasks/advent_of_code.gen.solution.ex
+++ b/2021/lib/mix/tasks/advent_of_code.gen.solution.ex
@@ -33,6 +33,7 @@ defmodule Mix.Tasks.AdventOfCode.Gen.Solution do
         create_file(day_file, day_contents)
 
         for part <- 1..2 do
+          part = to_string(part)
           part_module = part_module(day, part)
 
           part_contents =