diff --git a/docs/source/quickstart.md b/docs/source/quickstart.md
index 194f94e..3d273c4 100644
--- a/docs/source/quickstart.md
+++ b/docs/source/quickstart.md
@@ -56,9 +56,15 @@ A list of available filters can be found [here](filters/index).
 For instance, the following takes in `questions.tex` and uses a filter that expects [each part to be directly followed by the solution](filters/_autosummary/PartSolPartSol):
 
 ```bash
-$ in2lambda questions.tex PartSolPartSol
+$ in2lambda convert questions.tex PartSolPartSol
 ```
 
+:::{important}
+File conversion lives under the `convert` subcommand (`in2lambda convert ...`,
+not `in2lambda ...`). This leaves room for other subcommands, such as one that
+turns unstructured documents into markdown.
+:::
+
 :::{note}
 The filter name is case-insensitive. Don't worry about the capital letters.
 :::
@@ -66,13 +72,13 @@ The filter name is case-insensitive. Don't worry about the capital letters.
 Another filter might be used if [the answers are in a separate file](filters/_autosummary/PartsSepSol):
 
 ```bash
-$ in2lambda questions.tex -a solutions.tex PartsSepSol
+$ in2lambda convert questions.tex -a solutions.tex PartsSepSol
 ```
 
 If you would rather write the questions yourself, the [`Markdown` filter](filters/_autosummary/Markdown) reads a plain markdown file where `#` starts a question, `##` starts a part, and `## Solution` gives a worked solution:
 
 ```bash
-$ in2lambda questions.md Markdown
+$ in2lambda convert questions.md Markdown
 ```
 
 By default, this generates an `out` directory in the same place that the command was run in. It contains the zipped question files.
diff --git a/in2lambda/main.py b/in2lambda/main.py
index c148e1e..f51cbef 100644
--- a/in2lambda/main.py
+++ b/in2lambda/main.py
@@ -171,10 +171,38 @@ def runner(
     return set_obj
 
 
-@click.command(
+class _Cli(click.RichGroup):
+    """A group that always errors on an unresolved subcommand.
+
+    Click's default resolution silently exits 0 (printing help) instead of
+    erroring when the unresolved name starts with a non-alphanumeric
+    character (e.g. an absolute or ./relative path), because it mistakes
+    the token for a global option and re-parses remaining args. Skip that
+    so every unrecognised command fails loudly with a pointer to `convert`.
+    """
+
+    def resolve_command(self, ctx, args):  # type: ignore[override]
+        cmd_name = str(args[0])
+        cmd = self.get_command(ctx, cmd_name)
+        if cmd is None:
+            ctx.fail(
+                f"No such command {cmd_name!r}.\n\n"
+                "As of in2lambda 2.0.0, conversion requires the `convert` "
+                f"subcommand, e.g.:\n  in2lambda convert {' '.join(args)}"
+            )
+        return cmd_name, cmd, args[1:]
+
+
+@click.group(
     no_args_is_help=True,
+    cls=_Cli,
     epilog="See the docs at https://lambda-feedback.github.io/in2lambda/ for more details.",
 )
+def cli() -> None:
+    """Convert documents into Lambda Feedback compatible question sets."""
+
+
+@cli.command(no_args_is_help=True)
 @click.argument(  # Use resolve_path to get absolute path
     "question_file", type=click.Path(exists=True, readable=True, resolve_path=True)
 )
@@ -207,11 +235,11 @@ def runner(
     help="File containing solutions for QUESTION_FILE.",
     type=click.Path(resolve_path=True, exists=True, dir_okay=False),
 )
-def cli(
+def convert(
     question_file: str, chosen_filter: str, output_dir: str, answer_file: Optional[str]
 ) -> None:
-    """Takes in a QUESTION_FILE for a given SUBJECT and produces Lambda Feedback compatible json/zip files."""
-    # main() is made separate from click() so that it can be easily imported as part of a library.
+    """Take a QUESTION_FILE and CHOSEN_FILTER and produce Lambda Feedback json/zip files."""
+    # Kept separate from runner() so runner() can be imported as part of the library.
     runner(question_file, chosen_filter, output_dir, answer_file)
 
 
diff --git a/pyproject.toml b/pyproject.toml
index e1b4022..a1d061e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "in2lambda"
-version = "1.0.0"
+version = "2.0.0"
 description = "Converts content ready for import into Lambda Feedback"
 authors = []
 license = "MIT"
diff --git a/tests/test_cli.py b/tests/test_cli.py
new file mode 100644
index 0000000..c57f1b7
--- /dev/null
+++ b/tests/test_cli.py
@@ -0,0 +1,87 @@
+"""Tests for the ``in2lambda`` command-line interface (the ``convert`` subcommand)."""
+
+import os
+
+from click.testing import CliRunner
+
+from in2lambda.main import cli
+
+
+def test_bare_invocation_shows_usage() -> None:
+    result = CliRunner().invoke(cli, [])
+    assert "Usage:" in result.output
+    assert "convert" in result.output
+
+
+def test_help_lists_the_convert_command() -> None:
+    result = CliRunner().invoke(cli, ["--help"])
+    assert result.exit_code == 0
+    assert "convert" in result.output
+
+
+def test_convert_writes_output_files(filters_dir: str, tmp_path) -> None:
+    example = os.path.join(filters_dir, "PartsSepSol", "example.tex")
+    out_dir = tmp_path / "out"
+
+    result = CliRunner().invoke(
+        cli, ["convert", example, "PartsSepSol", "-o", str(out_dir)]
+    )
+
+    assert result.exit_code == 0, result.output
+    assert (out_dir / "set").is_dir()
+    assert (out_dir / "set.zip").is_file()
+
+
+def test_convert_accepts_case_insensitive_filter_and_markdown(
+    filters_dir: str, tmp_path
+) -> None:
+    example = os.path.join(filters_dir, "Markdown", "example.md")
+    out_dir = tmp_path / "out"
+
+    result = CliRunner().invoke(
+        cli, ["convert", example, "markdown", "-o", str(out_dir)]
+    )
+
+    assert result.exit_code == 0, result.output
+    assert (out_dir / "set" / "set_set.json").is_file()
+
+
+def test_convert_rejects_unknown_filter(filters_dir: str, tmp_path) -> None:
+    example = os.path.join(filters_dir, "PartsSepSol", "example.tex")
+
+    result = CliRunner().invoke(
+        cli, ["convert", example, "NotAFilter", "-o", str(tmp_path / "out")]
+    )
+
+    assert result.exit_code != 0
+    assert "NotAFilter" in result.output
+
+
+def test_old_style_absolute_path_invocation_fails(filters_dir: str) -> None:
+    """The pre-2.0.0 `in2lambda <file> <filter>` form must error, not no-op.
+
+    Click's default command resolution silently exits 0 here instead of
+    erroring, because it mistakes a leading "/" for an option prefix.
+    """
+    example = os.path.join(filters_dir, "PartsSepSol", "example.tex")
+    assert os.path.isabs(example)
+
+    result = CliRunner().invoke(cli, [example, "PartsSepSol"])
+
+    assert result.exit_code != 0
+    assert "convert" in result.output
+
+
+def test_old_style_dot_relative_path_invocation_fails(filters_dir: str) -> None:
+    """Same as above but for a `./relative` path, which hits the same bug."""
+    result = CliRunner().invoke(cli, ["./example.tex", "PartsSepSol"])
+
+    assert result.exit_code != 0
+    assert "convert" in result.output
+
+
+def test_old_style_bare_filename_invocation_fails() -> None:
+    result = CliRunner().invoke(cli, ["example.tex", "PartsSepSol"])
+
+    assert result.exit_code != 0
+    assert "convert" in result.output