diff --git a/chained_eclipse/__init__.py b/chained_eclipse/__init__.py
index 9684cec..5944444 100644
--- a/chained_eclipse/__init__.py
+++ b/chained_eclipse/__init__.py
@@ -1,6 +1,24 @@
-"""Chained solar-eclipse search package."""
+"""Chained solar-eclipse search package.
+
+Two distinct version identifiers live here on purpose:
+
+- ``__version__`` is the *package* version, derived from the installed
+  distribution metadata (single-sourced from ``pyproject.toml``). It moves
+  with releases.
+- ``MODEL_VERSION`` is the *model* version, defined in ``constants``. It is
+  bumped when computed outputs change for fixed inputs (see the CHANGELOG
+  policy) and is therefore allowed to differ from ``__version__``.
+"""
+
+from __future__ import annotations
+
+from importlib.metadata import PackageNotFoundError, version
 
 from .constants import MODEL_VERSION
 
-__all__ = ["MODEL_VERSION"]
+try:
+    __version__ = version("chained-eclipse")
+except PackageNotFoundError:  # source tree without an installed distribution
+    __version__ = "0+unknown"
 
+__all__ = ["MODEL_VERSION", "__version__"]
diff --git a/tests/test_versions.py b/tests/test_versions.py
new file mode 100644
index 0000000..613accd
--- /dev/null
+++ b/tests/test_versions.py
@@ -0,0 +1,46 @@
+"""Version single-sourcing checks.
+
+``pyproject.toml`` is the only place the package version is written by hand.
+``__version__`` must mirror the installed distribution, and ``CITATION.cff``
+must be bumped in lockstep with releases. ``MODEL_VERSION`` is deliberately
+independent (it tracks output-changing model revisions, not packaging), so it
+is only checked for shape, never for equality with the package version.
+"""
+
+from __future__ import annotations
+
+from importlib.metadata import PackageNotFoundError, version
+from pathlib import Path
+
+import pytest
+import yaml
+
+import chained_eclipse
+
+CITATION_FILE = Path(__file__).resolve().parents[1] / "CITATION.cff"
+
+
+def _installed_version() -> str:
+    try:
+        return version("chained-eclipse")
+    except PackageNotFoundError:  # pragma: no cover - CI always installs
+        pytest.skip("chained-eclipse is not installed")
+
+
+def test_dunder_version_matches_installed_distribution() -> None:
+    assert chained_eclipse.__version__ == _installed_version()
+
+
+def test_citation_cff_version_matches_installed_distribution() -> None:
+    citation = yaml.safe_load(CITATION_FILE.read_text(encoding="utf-8"))
+    assert str(citation["version"]) == _installed_version(), (
+        "CITATION.cff `version:` must be bumped in the same commit that changes "
+        "the package version in pyproject.toml"
+    )
+
+
+def test_model_version_is_present_and_separate() -> None:
+    assert isinstance(chained_eclipse.MODEL_VERSION, str)
+    assert chained_eclipse.MODEL_VERSION
+    # No equality assertion with __version__: the model version is allowed to
+    # differ from the package version by design.