diff --git a/README.md b/README.md
index 63c7b83c..ec4f6204 100644
--- a/README.md
+++ b/README.md
@@ -132,3 +132,12 @@ administrative truth. It also writes `results/local/scenarios.csv`, and the eval
 commands reuse that manifest by default instead of regenerating households from
 the current source dataset. Prediction CSVs also get a `.meta.json` sidecar so
 resumes only happen against the exact same manifest, model set, and program set.
+
+## License
+
+PolicyBench is free and unencumbered software released into the public domain
+under the
+[Unlicense](https://github.com/PolicyEngine/policybench/blob/main/LICENSE).
+Third-party files keep their own licenses and terms: the Quarto web libraries
+under `app/public/paper/web/site_libs/` and the captured Vals.ai leaderboard in
+`paper/external/`.
diff --git a/docs/myst.yml b/docs/myst.yml
index f4a37635..478529d4 100644
--- a/docs/myst.yml
+++ b/docs/myst.yml
@@ -11,7 +11,7 @@ project:
     - microsimulation
     - benchmarking
     - large language models
-  license: MIT
+  license: Unlicense
   toc:
     - file: index
     - file: paper
diff --git a/pyproject.toml b/pyproject.toml
index d40404bc..7de44741 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,7 +7,7 @@ name = "policybench"
 version = "2.0.0"
 description = "Benchmark: can AI models calculate tax/benefit outcomes without tools?"
 readme = "README.md"
-license = "MIT"
+license = "Unlicense"
 authors = [{name = "PolicyEngine", email = "hello@policyengine.org"}]
 requires-python = ">=3.11"
 dependencies = [
diff --git a/tests/test_license.py b/tests/test_license.py
new file mode 100644
index 00000000..56bdf657
--- /dev/null
+++ b/tests/test_license.py
@@ -0,0 +1,117 @@
+"""Every license declaration in the repository names the license in LICENSE.
+
+GitHub reads LICENSE, packaging reads pyproject.toml, the MyST docs site reads
+docs/myst.yml and readers read the README. From the first code commit (February
+2025) until October 2026, LICENSE held the Unlicense while pyproject.toml
+declared MIT (docs/myst.yml followed it in February 2026), so the code's license
+depended on where one looked.
+"""
+
+from __future__ import annotations
+
+import configparser
+import json
+import re
+import subprocess
+import tomllib
+from pathlib import Path
+
+import yaml
+
+ROOT = Path(__file__).resolve().parents[1]
+SPDX_ID = "Unlicense"
+LICENSE_URL = "https://github.com/PolicyEngine/policybench/blob/main/LICENSE"
+# File names whose formats carry a license field.
+MANIFESTS = {
+    ".zenodo.json",
+    "CITATION.cff",
+    "Cargo.toml",
+    "_quarto.yml",
+    "codemeta.json",
+    "myst.yml",
+    "package.json",
+    "pyproject.toml",
+    "setup.cfg",
+}
+# Vendored third-party code keeps its own license.
+THIRD_PARTY = ("app/public/paper/web/site_libs/",)
+
+
+def _tracked_manifests() -> list[str]:
+    listed = subprocess.run(
+        ["git", "-C", str(ROOT), "ls-files", "-z"],
+        capture_output=True,
+        text=True,
+    )
+    assert listed.returncode == 0, "the license check needs a git checkout. " + (
+        listed.stderr
+    )
+    return sorted(
+        path
+        for path in listed.stdout.split("\0")
+        if Path(path).name in MANIFESTS and not path.startswith(THIRD_PARTY)
+    )
+
+
+def _parse(path: Path):
+    text = path.read_text()
+    if path.suffix == ".toml":
+        return tomllib.loads(text)
+    if path.suffix == ".json":
+        return json.loads(text)
+    if path.suffix == ".cfg":
+        parser = configparser.ConfigParser(interpolation=None)
+        parser.read_string(text)
+        return {section: dict(parser[section]) for section in parser.sections()}
+    return yaml.safe_load(text)
+
+
+def _license_values(node) -> list[str]:
+    """Every value under a `license` key, at any depth. A mapping (MyST's
+    {content, code}, pyproject's legacy {text}) contributes each of its values."""
+    if isinstance(node, list):
+        return [value for item in node for value in _license_values(item)]
+    if not isinstance(node, dict):
+        return []
+    found = []
+    for key, value in node.items():
+        if str(key).lower() == "license":
+            values = value.values() if isinstance(value, dict) else [value]
+            found.extend(str(v) for v in values)
+        else:
+            found.extend(_license_values(value))
+    return found
+
+
+def test_license_file_is_the_unlicense():
+    text = (ROOT / "LICENSE").read_text()
+    assert text.startswith(
+        "This is free and unencumbered software released into the public domain."
+    )
+    assert text.rstrip().endswith(
+        "For more information, please refer to <https://unlicense.org>"
+    )
+
+
+def test_every_license_declaration_is_the_unlicense():
+    manifests = _tracked_manifests()
+    # The two files that disagreed must be found, or the check below is vacuous.
+    assert {"pyproject.toml", "docs/myst.yml"} <= set(manifests)
+    declared = {path: set(_license_values(_parse(ROOT / path))) for path in manifests}
+    assert declared["pyproject.toml"] == {SPDX_ID}
+    assert declared["docs/myst.yml"] == {SPDX_ID}
+    assert {path: ids for path, ids in declared.items() if ids - {SPDX_ID}} == {}
+    # MIT first appeared as a trove classifier as well as the license field.
+    project = _parse(ROOT / "pyproject.toml")["project"]
+    assert not [c for c in project.get("classifiers", []) if c.startswith("License")]
+
+
+def test_readme_names_only_the_unlicense():
+    readme = (ROOT / "README.md").read_text()
+    assert "\n## License\n" in readme
+    section = readme.split("\n## License\n", 1)[1].split("\n## ", 1)[0]
+    section = " ".join(section.split())
+    assert f"[Unlicense]({LICENSE_URL})" in section
+    assert not re.search(r"\b(MIT|Apache|BSD|L?GPL|MPL|CC[- ]?BY|CC0)\b", section)
+    badges = re.findall(r"shields\.io/badge/licen[cs]e-([^-/)\s]+)", readme, re.I)
+    assert [badge.lower() for badge in badges if badge.lower() != "unlicense"] == []