diff --git a/corpus/CLAUDE.learned.md b/corpus/CLAUDE.learned.md
index 29611a5a..5258abcb 100644
--- a/corpus/CLAUDE.learned.md
+++ b/corpus/CLAUDE.learned.md
@@ -43,3 +43,4 @@ Engine-only install drops this file; reflect Accepted global rules land here.
 - Never pass a quoted command string through `sudo -i`, `su -`, or `su -l`. Those start a login shell that reads the command a second time, after the first shell already used up the quotes, so the string splits on spaces and its first word runs alone (`sudo -u demo -H -i bash -lc 'set -u; echo one'` fails with `set: -c: invalid option`; the same line without `-i` prints `one`). Copy the script to the host and run it by path, or drop `-i`.
 - The shell behind the Bash tool may be zsh, which does not split an unquoted variable into words: `for x in $LIST` runs once with the whole list as one item, exits 0, and reads as "all done". Run any loop over a list under bash (`bash <<'EOF' ... EOF`), count lookups that failed as unchecked rather than done, and spot-check one item before acting on the loop's summary. This is zsh's `SH_WORD_SPLIT` option being off by default (zsh manual, Options, https://zsh.sourceforge.io/Doc/Release/Options.html#index-SH_005fWORD_005fSPLIT).
 - Before asking me whether to run a script, read its whole body, not its header comment, usage text, or `--dry-run` output, and name in the question every process it kills, file it deletes, and service it restarts. A header states what the author meant; the body is what runs, so a consent question built from the header asks me to approve something other than what will happen. No known prior art.
+- When finishing a merge or rebase, reconcile three things before calling it resolved: what each side declared (its commit messages and PR description: the intent, and the tests it names), what each side's code actually does, and what the resolved result does. All three must agree: every declared intent is still visible in `git diff <base> -- <file>`, and every test either side named still passes on the result. A merge with no textual conflict can still break one side's declared behavior, so a clean merge is checked the same way. Never take one side's whole file (`git checkout --theirs` / `--ours <file>`, or copying the file from one branch) unless the other side's changes to that file are provably empty; taking a whole side silently reverts what the other branch added, including hunks that never conflicted (git-checkout(1), `--ours`/`--theirs`: https://git-scm.com/docs/git-checkout#Documentation/git-checkout.txt---ours). When a declaration and its code disagree, or the two sides' intents contradict, stop and flag it to me instead of choosing: run `why` on the conflicting lines (`git log -L` or `git log -S` plus each commit's PR body) and apply `principle-prove-it`, so the flag names the two commits or PRs, quotes the declared intent next to the code, and pastes the failing test output. Brun, Holmes, Ernst and Notkin, "Proactive Detection of Collaboration Conflicts" (ESEC/FSE 2011, https://doi.org/10.1145/2025113.2025139), across Git, Perl5 and Voldemort, found "that 33% of merges that were reported to contain no textual conflicts by the VCS in fact contained higher-order conflicts" (build or test failures), so a clean textual merge is not evidence the two intentions still hold.
diff --git a/tests/test_conflict_whole_side_rule.py b/tests/test_conflict_whole_side_rule.py
new file mode 100644
index 00000000..9cfdb71d
--- /dev/null
+++ b/tests/test_conflict_whole_side_rule.py
@@ -0,0 +1,169 @@
+#!/usr/bin/env python3
+"""The learned rule for finishing a merge or rebase.
+
+Two premise tests use real git. In the first, both branches change line 1 and
+only the current branch also changes line 5; `git checkout --theirs` resolves
+the conflict and silently drops the line-5 change, which never conflicted. In
+the second, git merges two branches with no conflict at all, yet the result
+fails the test one branch's commit declared. Both losses are what the rule's
+three-way reconciliation of declared intent, code, and result exists to catch.
+"""
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+
+REPO_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+LEARNED = os.path.join(REPO_ROOT, "corpus", "CLAUDE.learned.md")
+sys.path.insert(0, os.path.join(REPO_ROOT, "scripts", "test"))
+
+from git_test_repo import init_repo  # noqa: E402
+
+BASE_LINES = ["one", "two", "three", "four", "five"]
+
+
+def git(root, *args, check=True):
+    return subprocess.run(
+        ["git", "-C", root, "-c", "user.name=t", "-c", "user.email=t@example.com", *args],
+        capture_output=True, text=True, check=check,
+    )
+
+
+def write_lines(root, lines):
+    with open(os.path.join(root, "notes.txt"), "w", encoding="utf-8") as handle:
+        handle.write("\n".join(lines) + "\n")
+
+
+def read_lines(root):
+    with open(os.path.join(root, "notes.txt"), encoding="utf-8") as handle:
+        return handle.read().splitlines()
+
+
+def working_style_section():
+    with open(LEARNED, encoding="utf-8") as handle:
+        text = handle.read()
+    start = text.index("# Working style")
+    end = text.find("\n# ", start + 1)
+    return text[start:] if end == -1 else text[start:end]
+
+
+class TestTakingTheirsDropsNonConflictingChanges(unittest.TestCase):
+    def test_checkout_theirs_loses_the_current_branch_line_five_change(self):
+        with tempfile.TemporaryDirectory() as root:
+            init_repo(root, "-b", "main")
+            write_lines(root, BASE_LINES)
+            git(root, "add", "notes.txt")
+            git(root, "commit", "-qm", "base")
+            git(root, "checkout", "-qb", "other")
+            write_lines(root, ["one-other"] + BASE_LINES[1:])
+            git(root, "commit", "-qam", "other edits line 1")
+            git(root, "checkout", "-q", "main")
+            write_lines(root, ["one-main", "two", "three", "four", "five-main"])
+            git(root, "commit", "-qam", "main edits lines 1 and 5")
+
+            merge = git(root, "merge", "other", check=False)
+            self.assertNotEqual(merge.returncode, 0, "the setup must produce a conflict")
+            git(root, "checkout", "--theirs", "notes.txt")
+
+            resolved = read_lines(root)
+            self.assertEqual(resolved[0], "one-other")
+            self.assertEqual(resolved[4], "five", "taking theirs reverted main's non-conflicting line 5")
+
+
+CALC_BASE = [
+    "def total(items):",
+    "    return sum(items)",
+    "",
+    "",
+    "",
+    "",
+    "",
+    "",
+    "",
+    "",
+]
+
+
+def write_file(root, name, lines):
+    with open(os.path.join(root, name), "w", encoding="utf-8") as handle:
+        handle.write("\n".join(lines) + "\n")
+
+
+class TestCleanMergeCanBreakDeclaredIntent(unittest.TestCase):
+    def test_textually_clean_merge_fails_the_test_one_side_declared(self):
+        with tempfile.TemporaryDirectory() as root:
+            init_repo(root, "-b", "main")
+            write_file(root, "calc.py", CALC_BASE)
+            git(root, "add", "calc.py")
+            git(root, "commit", "-qm", "base")
+
+            git(root, "checkout", "-qb", "skip-negatives")
+            write_file(root, "calc.py", ["def total(items):", "    return sum(i for i in items if i >= 0)"] + CALC_BASE[2:])
+            write_file(root, "test_calc.py", [
+                "import unittest",
+                "from calc import total",
+                "",
+                "",
+                "class T(unittest.TestCase):",
+                "    def test_negatives_are_ignored(self):",
+                "        self.assertEqual(total([2, -5]), 2)",
+            ])
+            git(root, "add", "calc.py", "test_calc.py")
+            git(root, "commit", "-qm", "total ignores negative items\n\nTest: test_calc.test_negatives_are_ignored")
+
+            git(root, "checkout", "-q", "main")
+            write_file(root, "calc.py", CALC_BASE[:-1] + ["total = lambda items: sum(items)"])
+            git(root, "commit", "-qam", "expose total as a lambda")
+
+            merge = git(root, "merge", "--no-edit", "skip-negatives", check=False)
+            self.assertEqual(merge.returncode, 0, merge.stdout + merge.stderr)
+
+            declared = git(root, "log", "-1", "--format=%B", "skip-negatives").stdout
+            self.assertIn("test_calc.test_negatives_are_ignored", declared)
+            run = subprocess.run(
+                [sys.executable, "-m", "unittest", "test_calc"],
+                cwd=root, capture_output=True, text=True,
+            )
+            self.assertNotEqual(run.returncode, 0, "the clean merge was expected to break the declared test")
+            self.assertIn("FAILED", run.stderr)
+
+
+class TestConflictReconcileRule(unittest.TestCase):
+    PREFIX = "- When finishing a merge or rebase"
+
+    def rule(self):
+        for line in working_style_section().splitlines():
+            if line.startswith(self.PREFIX):
+                return line
+        raise AssertionError("the conflict-resolution rule is missing")
+
+    def test_rule_forbids_taking_a_whole_side(self):
+        rule = self.rule()
+        self.assertIn("Never take one side's whole file", rule)
+        self.assertIn("`git checkout --theirs`", rule)
+
+    def test_rule_reconciles_declared_intent_code_and_result(self):
+        rule = self.rule()
+        self.assertIn("commit messages and PR description", rule)
+        self.assertIn("what each side's code actually does", rule)
+        self.assertIn("what the resolved result does", rule)
+        self.assertIn("`git diff <base> -- <file>`", rule)
+        self.assertIn("every test either side named still passes", rule)
+
+    def test_rule_checks_clean_merges_too(self):
+        self.assertIn("a clean merge is checked the same way", self.rule())
+
+    def test_rule_flags_mismatch_with_history_and_evidence(self):
+        rule = self.rule()
+        self.assertIn("stop and flag it to me instead of choosing", rule)
+        self.assertIn("run `why` on the conflicting lines", rule)
+        self.assertIn("`principle-prove-it`", rule)
+        self.assertIn("pastes the failing test output", rule)
+
+    def test_rule_names_its_source(self):
+        self.assertIn("https://doi.org/10.1145/2025113.2025139", self.rule())
+
+
+if __name__ == "__main__":
+    unittest.main()