diff --git a/pyproject.toml b/pyproject.toml
index ecafff2f..e61a01f4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,7 +15,7 @@ classifiers = [
     "Topic :: Software Development :: Code Generators",
 ]
 dependencies = [
-    "plain-parser==0.1.4",
+    "plain-parser==0.2.0",
 
     # Both transitive via plain-parser; pinned so an automatic mistletoe upgrade doesn't change spec hashes.
     "mistletoe==1.6.0",
diff --git a/tests/test_dependency_pins.py b/tests/test_dependency_pins.py
new file mode 100644
index 00000000..c3b4714b
--- /dev/null
+++ b/tests/test_dependency_pins.py
@@ -0,0 +1,52 @@
+"""The pins of requirements.txt and pyproject.toml must say the same thing.
+
+requirements.txt builds the development environment and the CI jobs. pyproject.toml is what the
+wheel carries, so it is what an installed client gets. A pin changed in one file and not the other
+passes every test here and still ships a client that cannot start.
+"""
+
+import os
+import re
+import tomllib
+
+REPO = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+
+
+def _name(requirement: str) -> str:
+    return re.split(r"[=<>@\[]", requirement, maxsplit=1)[0].strip().lower()
+
+
+def _requirements() -> dict[str, str]:
+    with open(os.path.join(REPO, "requirements.txt"), encoding="utf-8") as f:
+        lines = [line.strip() for line in f]
+    return {_name(line): line for line in lines if line and not line.startswith("#")}
+
+
+def _pyproject() -> tuple[dict[str, str], dict[str, str]]:
+    with open(os.path.join(REPO, "pyproject.toml"), "rb") as f:
+        pyproject = tomllib.load(f)["project"]
+    runtime = {_name(item): item for item in pyproject["dependencies"]}
+    dev = {_name(item): item for item in pyproject["optional-dependencies"]["dev"]}
+    return runtime, dev
+
+
+def test_every_pin_of_the_wheel_matches_requirements():
+    requirements = _requirements()
+    runtime, dev = _pyproject()
+
+    mismatched = {
+        name: (requirement, requirements.get(name))
+        for name, requirement in {**runtime, **dev}.items()
+        if requirements.get(name) != requirement
+    }
+
+    assert mismatched == {}, f"pyproject.toml and requirements.txt disagree: {mismatched}"
+
+
+def test_requirements_names_nothing_the_wheel_leaves_out():
+    requirements = _requirements()
+    runtime, dev = _pyproject()
+
+    missing = sorted(set(requirements) - set(runtime) - set(dev))
+
+    assert missing == [], f"requirements.txt pins what pyproject.toml does not declare: {missing}"