@@ -9,11 +9,13 @@
 """
 import importlib.util
 import json
+import shutil
 import sys
 import time
 import urllib.request
 from pathlib import Path
 from types import ModuleType
+from typing import Any
 
 import pytest
 
@@ -97,13 +99,16 @@ def own_cache(monkeypatch: pytest.MonkeyPatch,
 
 
 def plant_corpus(monkeypatch: pytest.MonkeyPatch, corpora: ModuleType, source: str,
-                 records: list[dict[str, str]], date: str, unit: str) -> None:
+                 records: list[dict[str, str]], date: str, unit: str,
+                 detail: dict[str, Any] | None = None) -> None:
     """`records` fetched as `source`'s local corpus into the cache, with no
     network: through a Source carrying the real one's credit and nothing
-    else, so that none of the real source's renew or check applies."""
+    else, so that none of the real source's renew or check applies in
+    this process. A child process loads the corpus under the real
+    Source, whose check reads `detail`, the meta.json detail it pins."""
     monkeypatch.setitem(corpora.SOURCES, source, corpora.Source(
         corpora.SOURCES[source].credit, unit,
-        lambda: corpora.Fetched(list(records), date, {})))
+        lambda: corpora.Fetched(list(records), date, dict(detail or {}))))
     corpora.fetch(source)
 
 
@@ -125,3 +130,25 @@ def urlopen(request: urllib.request.Request, timeout: int) -> _Response:
         return _Response(body)
     monkeypatch.setattr(urllib.request, "urlopen", urlopen)
     return fetched
+
+
+#: Appended to a copied package's __init__.py, a change every reading
+#: shows: each name is parsed with its words reversed.
+_PLANTED = """
+
+_planted_parse = Parser.parse
+Parser.parse = lambda self, text: _planted_parse(self, " ".join(reversed(text.split())))
+"""
+
+
+def checkout_copy(at: Path, *, planted: bool = False) -> Path:
+    """A checkout at `at` holding a copy of this one's nameparser/
+    package, and `planted`, a change every reading shows: a base tree
+    for --base, which a child process reads, so that a check across a
+    planted copy proves the base was read from its own tree."""
+    shutil.copytree(_TOOLS.parents[1] / "nameparser", at / "nameparser",
+                    ignore=shutil.ignore_patterns("__pycache__"))
+    if planted:
+        with open(at / "nameparser" / "__init__.py", "a", encoding="utf-8") as f:
+            f.write(_PLANTED)
+    return at
@@ -8,6 +8,7 @@
 another fetch or another scorer, and a corpus damaged under the stream. The names are invented.
 """
 import hashlib
+import importlib
 import json
 import sys
 import unicodedata
@@ -17,7 +18,7 @@
 
 from nameparser import Parser
 
-from ._differential_fixtures import load_tool, own_cache, plant_corpus
+from ._differential_fixtures import checkout_copy, load_tool, own_cache, plant_corpus
 
 agreement = load_tool("local_agreement")
 corpora = agreement.local_corpora
@@ -96,7 +97,10 @@ def test_a_title_box_holding_an_unlisted_word_the_parse_reads_otherwise_is_set_a
 
 def _corpus(monkeypatch: pytest.MonkeyPatch, *records: dict[str, str]) -> None:
     """A PSC corpus of `records` fetched into the cache, no network behind it."""
-    plant_corpus(monkeypatch, corpora, _PSC, list(records), "2026-10-22", "filed names")
+    # The sample pinned as the real source pins it: a --base child
+    # loads the corpus under the real Source and its check.
+    plant_corpus(monkeypatch, corpora, _PSC, list(records), "2026-10-22", "filed names",
+                 {"sample": f"1 in {importlib.import_module('local_psc').PSC_SAMPLE}"})
 
 
 _ALL = (_AGREES, _DOUBLE_SURNAME, _SUFFIX, _JOB_TITLE, _SHAPE_TITLE)
@@ -160,7 +164,7 @@ def test_a_record_of_another_fetch_is_refused(
         agreement.main([_PSC])
     path = _own_cache / "local" / _PSC / agreement.RECORD
     header = json.loads(path.read_text().splitlines()[0])
-    assert set(header) == {"raw_sha256", "scorer", "data_date", "tree"}
+    assert set(header) == {"raw_sha256", "scorer", "data_date", "tree", "entries"}
 
 
 def test_a_run_with_no_record_names_the_command_that_writes_one(
@@ -298,3 +302,141 @@ def test_a_tree_that_is_not_the_imported_package_is_refused(
     # The control: this checkout itself is the tree it imported.
     root = agreement.HERE.parents[1]
     assert agreement.main([_PSC, "--record", "--tree", str(root)]) == 0
+
+
+# -- --base ----------------------------------------------------------------------
+
+def _kept(cache: Path) -> list[Path]:
+    return sorted((cache / "local" / _PSC / agreement.KEPT).glob("*.jsonl"))
+
+
+def test_a_base_check_reports_a_change_planted_in_the_base_and_none_across_a_copy(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path,
+        capsys: pytest.CaptureFixture[str]) -> None:
+    # The harness must be able to report a difference: a base scored with
+    # the head's tree by mistake would show no move across a planted one.
+    _corpus(monkeypatch, *_ALL)
+    planted = checkout_copy(tmp_path / "planted", planted=True)
+    assert agreement.main([_PSC, "--base", str(planted)]) == 1
+    out = capsys.readouterr().out
+    assert f"base: {planted.resolve()}, nameparser/ " in out and "scored it now" in out
+    assert "progress: 3 records (" in out and "regression" not in out
+    # The control: an unplanted copy is the head's package byte for byte.
+    copy = checkout_copy(tmp_path / "copy")
+    assert agreement.main([_PSC, "--base", str(copy)]) == 0
+    out = capsys.readouterr().out
+    assert "matches the base" in out and "byte-identical" in out
+    assert len(_kept(_own_cache)) == 2
+
+
+def test_a_base_check_never_touches_the_shared_record(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path) -> None:
+    _corpus(monkeypatch, *_ALL)
+    shared = _own_cache / "local" / _PSC / agreement.RECORD
+    base = checkout_copy(tmp_path / "base", planted=True)
+    assert agreement.main([_PSC, "--base", str(base)]) == 1
+    assert not shared.exists()
+    assert agreement.main([_PSC, "--record"]) == 0
+    shared.write_text(shared.read_text().splitlines()[0] + "\n")  # would read as moves
+    before = (shared.stat().st_mtime_ns, shared.read_bytes())
+    assert agreement.main([_PSC, "--base", str(checkout_copy(tmp_path / "copy"))]) == 0
+    assert (shared.stat().st_mtime_ns, shared.read_bytes()) == before
+
+
+def test_a_kept_base_record_is_reused_and_a_damaged_one_scored_again(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path,
+        capsys: pytest.CaptureFixture[str]) -> None:
+    _corpus(monkeypatch, *_ALL)
+    base = checkout_copy(tmp_path / "base", planted=True)
+    assert agreement.main([_PSC, "--base", str(base)]) == 1
+    [kept] = _kept(_own_cache)
+    recorded = kept.read_bytes()
+    capsys.readouterr()
+    real = agreement._score_base
+
+    def unscored(*args: object) -> None:
+        raise AssertionError("scored the base again")
+    monkeypatch.setattr(agreement, "_score_base", unscored)
+    assert agreement.main([_PSC, "--base", str(base)]) == 1
+    assert "reused the kept record" in capsys.readouterr().out
+    # A record cut short on a line boundary still parses line by line;
+    # its header's count is what refuses it.
+    header = json.loads(recorded.decode().splitlines()[0])
+    uncounted = json.dumps({**header, "entries": None}) + "\n" + "".join(
+        recorded.decode().splitlines(keepends=True)[1:])
+    for damage in ("not a record\n", "".join(recorded.decode().splitlines(keepends=True)[:-1]),
+                   uncounted):
+        kept.write_text(damage)
+        with pytest.raises(AssertionError, match="scored the base again"):
+            agreement.main([_PSC, "--base", str(base)])
+        assert "deleted the kept record" in capsys.readouterr().out
+        monkeypatch.setattr(agreement, "_score_base", real)
+        assert agreement.main([_PSC, "--base", str(base)]) == 1
+        assert kept.read_bytes() == recorded
+        assert not list(kept.parent.glob(".base-*"))  # the child's scratch is gone
+        monkeypatch.setattr(agreement, "_score_base", unscored)
+
+
+def test_a_base_that_changed_while_it_was_scored_keeps_no_record(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path) -> None:
+    _corpus(monkeypatch, *_ALL)
+    base = checkout_copy(tmp_path / "base", planted=True)
+    # The module base_entries imports by its bare name (local_agreement
+    # put its folder on sys.path), not a second load by path.
+    snapshot = importlib.import_module("snapshot")
+    answers = ["a" * 64, "b" * 64]
+    published: list[bool] = []
+
+    def digest(root: Path) -> str:
+        if not answers:
+            return "c" * 64
+        if len(answers) == 1:  # the check after the score: nothing kept yet
+            published.append(bool(_kept(_own_cache)))
+        return answers.pop(0)
+    monkeypatch.setattr(snapshot, "_source_digest", digest)
+    with pytest.raises(SystemExit, match="changed while it was scored"):
+        agreement.main([_PSC, "--base", str(base)])
+    # Written to the kept path before the check, a sibling could have
+    # reused it as the base it was keyed for in between.
+    assert published == [False]
+    assert not _kept(_own_cache)
+    assert not list((_own_cache / "local" / _PSC / agreement.KEPT).glob(".base-*"))
+
+
+def test_a_base_that_is_the_tree_read_or_no_checkout_is_refused(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path) -> None:
+    _corpus(monkeypatch, *_ALL)
+    with pytest.raises(SystemExit, match="is the tree being read"):
+        agreement.main([_PSC, "--base", str(agreement.HERE.parents[1])])
+    with pytest.raises(SystemExit, match="no nameparser package"):
+        agreement.main([_PSC, "--base", str(tmp_path)])
+    assert not _kept(_own_cache)
+
+
+def test_a_base_whose_import_fails_is_refused_with_the_childs_output(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path) -> None:
+    _corpus(monkeypatch, *_ALL)
+    base = checkout_copy(tmp_path / "base")
+    with open(base / "nameparser" / "__init__.py", "a", encoding="utf-8") as f:
+        f.write("\nraise ImportError('planted')\n")
+    with pytest.raises(SystemExit, match="(?s)scoring the base .* failed.*planted"):
+        agreement.main([_PSC, "--base", str(base)])
+    assert not _kept(_own_cache)
+
+
+def test_a_base_check_refuses_to_record(
+        monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]) -> None:
+    _corpus(monkeypatch, *_ALL)
+    for extra in (["--record"], ["--out", "x.jsonl"]):
+        with pytest.raises(SystemExit) as raised:
+            agreement.main([_PSC, "--base", ".", *extra])
+        assert raised.value.code == 2 and "--base compares" in capsys.readouterr().err
+
+
+def test_record_writes_where_out_says_and_leaves_the_shared_record(
+        monkeypatch: pytest.MonkeyPatch, _own_cache: Path, tmp_path: Path) -> None:
+    _corpus(monkeypatch, *_ALL)
+    out = tmp_path / "elsewhere.jsonl"
+    assert agreement.main([_PSC, "--record", "--out", str(out)]) == 0
+    assert agreement.read_record(out, corpora.load(_PSC))
+    assert not (_own_cache / "local" / _PSC / agreement.RECORD).exists()