diff --git a/CHANGELOG.md b/CHANGELOG.md
index 87ebf5f0..1c6d3a8a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ it releases that version ([RELEASING.md](https://github.com/energy-models/mathsp
 
 ## Upcoming version
 
+- feat(cli): `canonical --check` fails a model file that is not in the canonical form, and `--write` rewrites it ([#718](https://github.com/energy-models/mathspec/pull/718))
 - feat(language): two files that mean the same model write one text ([#530](https://github.com/energy-models/mathspec/pull/530))
 
 ## 0.1.0 (2026-09-25)
diff --git a/docs/howto/compare.md b/docs/howto/compare.md
index aecf855f..cc2a5a98 100644
--- a/docs/howto/compare.md
+++ b/docs/howto/compare.md
@@ -50,7 +50,30 @@ lists what the form sorts and what it keeps.
    Match only model files in `.gitattributes`. Git runs the command on every
    file the pattern matches, and a YAML file that is not a model fails to load.
 
-4. **Compare from Python** where the comparison is one step of a longer
+4. **Keep models in the canonical form in CI.** Then the files themselves
+   diff the way the form does, with no git setup. `--write` rewrites a file in
+   the form. The form holds no YAML comments, so `--write` drops them:
+
+   ```bash
+   python -m mathspec canonical --write model.yaml
+   ```
+
+   `--check` writes nothing. It exits with status 1 if the file is not in the
+   form, and names the rewrite:
+
+   ```text
+   model.yaml is not in the canonical form. Run `python -m mathspec canonical --write model.yaml` to rewrite it.
+   ```
+
+   Check every model, and fail the job if one of them fails:
+
+   ```bash
+   status=0
+   for model in models/*.yaml; do python -m mathspec canonical --check "$model" || status=1; done
+   exit $status
+   ```
+
+5. **Compare from Python** where the comparison is one step of a longer
    script. [`to_yaml`](../reference/api.md#mathspec.Spec.to_yaml) writes the
    same text with `canonical=True`:
 
diff --git a/docs/reference/reading.md b/docs/reference/reading.md
index 91eff525..8489eeea 100644
--- a/docs/reference/reading.md
+++ b/docs/reference/reading.md
@@ -271,6 +271,8 @@ The normal form loads to the same model. It does not load to a `Spec` equal to
 the original: a reprinted expression is a different string. Writing the form out
 again gives the same text, which is what the line above says.
 
-`python -m mathspec canonical model.yaml` writes it from a shell.
+`python -m mathspec canonical model.yaml` writes it from a shell. `--write`
+rewrites the file in the form, and `--check` exits with status 1 if the file is
+not in the form. The form holds no YAML comments, so `--write` drops them.
 [Compare two models](../howto/compare.md) shows how to diff two files in this
 form.
diff --git a/src/mathspec/__main__.py b/src/mathspec/__main__.py
index 34564248..53a58baf 100644
--- a/src/mathspec/__main__.py
+++ b/src/mathspec/__main__.py
@@ -5,7 +5,8 @@
 """``python -m mathspec <verb> model.yaml`` — the shell front.
 
 ``check`` loads the file and prints the language's advice, ``canonical``
-writes the model in the form two files that mean the same thing share, and one
+writes the model in the form two files that mean the same thing share, or with
+``--check`` asks whether the file is already in that form, and one
 further verb per typeset format, read off :data:`mathspec.typesetting.FORMATS`.
 Every verb reads the file as written, and nothing here writes a formulation out
 unasked. The typeset verbs take ``--expand``, because a shell cannot compose
@@ -36,7 +37,14 @@ def parser() -> argparse.ArgumentParser:
 
     canonical = verbs.add_parser('canonical', help='write the model in the form two files that mean the same share')
     canonical.add_argument('model', help='path to a mathspec YAML model')
-    canonical.add_argument('-o', '--out', help='write here instead of stdout')
+    target = canonical.add_mutually_exclusive_group()
+    target.add_argument('-o', '--out', help='write here instead of stdout')
+    target.add_argument(
+        '--write', action='store_true', help='rewrite the model file in the form, dropping its comments'
+    )
+    target.add_argument(
+        '--check', action='store_true', help='exit 1 if the model file is not in the form, writing nothing'
+    )
 
     for name in FORMATS:
         verb = verbs.add_parser(name, help=f'render a model as {name}')
@@ -57,10 +65,26 @@ def parser() -> argparse.ArgumentParser:
     return front
 
 
+def _checked(model: str, text: str) -> int:
+    """Exit status 0 if *model* already holds *text*, the form, and 1 with the rewrite on stderr if not.
+
+    A CI job runs this, so it compares bytes and writes nothing: the job fails,
+    and the author runs the rewrite the message names.
+    """
+    if Path(model).read_text(encoding='utf-8') == text:
+        return 0
+    sys.stderr.write(
+        f'{model} is not in the canonical form. Run `python -m mathspec canonical --write {model}` to rewrite it.\n'
+    )
+    return 1
+
+
 def main(argv: list[str] | None = None) -> int:
     """Run one verb; a refused file is its message on stderr and exit status 1.
 
-    Advice is not a refusal: ``check`` prints it and exits 0.
+    Advice is not a refusal: ``check`` prints it and exits 0. A file that
+    ``canonical --check`` finds out of the form exits 1, with the rewrite on
+    stderr.
     """
     args = parser().parse_args(argv)
     if args.verb == 'check':
@@ -77,6 +101,11 @@ def main(argv: list[str] | None = None) -> int:
         except MathSpecError as e:
             sys.stderr.write(f'{e}\n')
             return 1
+        if args.check:
+            return _checked(args.model, text)
+        if args.write:
+            Path(args.model).write_text(text, encoding='utf-8')
+            return 0
     else:
         model = to_spec(args.model).expand() if args.expand else args.model
         text = typeset(
diff --git a/tests/test_canonical.py b/tests/test_canonical.py
index 4a2901b6..e156409d 100644
--- a/tests/test_canonical.py
+++ b/tests/test_canonical.py
@@ -259,6 +259,62 @@ def test_the_shell_refuses_a_model_the_language_refuses(tmp_path, capsys):
     assert capsys.readouterr().err, 'and its message on stderr'
 
 
+def test_the_check_passes_a_file_in_the_form_and_writes_nothing(tmp_path, capsys):
+    model = tmp_path / 'model.yaml'
+    model.write_text(ms.to_spec(DISPATCH_MODEL).to_yaml(canonical=True))
+    assert main(['canonical', '--check', str(model)]) == 0, 'a file in the form passes'
+    assert capsys.readouterr() == ('', ''), 'a check that passes prints nothing on either stream'
+
+
+def test_the_check_fails_a_file_out_of_the_form_and_names_the_rewrite(tmp_path, capsys):
+    """What a CI job runs: the model loads, so the language accepts it, and the
+    job still fails, because the file is not the text the form writes."""
+    model = tmp_path / 'model.yaml'
+    written = ms.to_spec(DISPATCH_MODEL).to_yaml()
+    model.write_text(written)
+    assert main(['canonical', '--check', str(model)]) == 1, 'a file out of the form is exit status 1'
+    assert capsys.readouterr().err == (
+        f'{model} is not in the canonical form. Run `python -m mathspec canonical --write {model}` to rewrite it.\n'
+    )
+    assert model.read_text() == written, 'the check leaves the file as it found it'
+
+
+def test_the_rewrite_leaves_a_file_the_check_passes(tmp_path, capsys):
+    model = tmp_path / 'model.yaml'
+    model.write_text(ms.to_spec(DISPATCH_MODEL).to_yaml())
+    assert main(['canonical', '--write', str(model)]) == 0, 'the rewrite of a model the language accepts exits 0'
+    assert model.read_text() == ms.to_spec(DISPATCH_MODEL).to_yaml(canonical=True), 'the file now holds the form'
+    assert capsys.readouterr().out == '', 'the form goes into the file, not to stdout'
+    assert main(['canonical', '--check', str(model)]) == 0, 'and the check passes the file it wrote'
+
+
+@pytest.mark.parametrize('flag', ['--check', '--write'])
+def test_a_refused_file_is_neither_checked_nor_rewritten(tmp_path, capsys, flag):
+    model = tmp_path / 'broken.yaml'
+    written = 'constraints: {k: {dims: [], expression: "p >= "}}\n'
+    model.write_text(written)
+    assert main(['canonical', flag, str(model)]) == 1, 'a refused file is exit status 1'
+    assert capsys.readouterr().err, 'and its message on stderr'
+    assert model.read_text() == written, 'the file is left as the author wrote it'
+
+
+@pytest.mark.parametrize(
+    'flags',
+    [
+        pytest.param(['--check', '--write'], id='check-and-write'),
+        pytest.param(['--check', '-o', 'out.yaml'], id='check-and-out'),
+        pytest.param(['--write', '-o', 'out.yaml'], id='write-and-out'),
+    ],
+)
+def test_the_form_goes_to_one_place(tmp_path, flags):
+    """Each flag names where the form goes, so two of them name two places."""
+    model = tmp_path / 'model.yaml'
+    model.write_text(ms.to_spec(DISPATCH_MODEL).to_yaml())
+    with pytest.raises(SystemExit) as refused:
+        main(['canonical', *flags, str(model)])
+    assert refused.value.code == 2, 'argparse refuses the pair before the model is read'
+
+
 # ---------------------------------------------------------------------------
 # What the form promises, over trees nobody wrote by hand
 # ---------------------------------------------------------------------------