fix(lifecycle): reject claim self-contradiction

CHANGELOG.md

@@ -6,6 +6,13 @@ All notable changes to vouch are documented here. Format follows
 
 ## [Unreleased]
 
+### Fixed
+- `lifecycle.contradict()` no longer lets a claim contradict itself. calling
+  it with the same claim id on both sides previously wrote a self-loop
+  `contradicts` reference and flipped the claim to `contested` with no
+  actual counterparty; it now raises `LifecycleError`, mirroring the
+  existing guard in `supersede()`.
+
 ## [1.2.2] — 2026-07-07
 
 ### Packaging

src/vouch/lifecycle.py

@@ -76,6 +76,8 @@ def contradict(
     actor: str,
 ) -> tuple[Claim, Claim, Relation]:
     """Record that two claims contradict each other (symmetric)."""
+    if claim_a == claim_b:
+        raise LifecycleError("a claim cannot contradict itself")
     a = store.get_claim(claim_a)
     b = store.get_claim(claim_b)
     a.contradicts = sorted({*a.contradicts, b.id})

tests/test_storage.py

@@ -871,6 +871,16 @@ def test_contradict_marks_both_contested(store: KBStore) -> None:
     assert rel.relation == RelationType.CONTRADICTS
 
 
+def test_contradict_rejects_self_reference(store: KBStore) -> None:
+    src = store.put_source(b"e")
+    store.put_claim(Claim(id="a", text="x", evidence=[src.id]))
+    with pytest.raises(lifecycle.LifecycleError, match="cannot contradict itself"):
+        lifecycle.contradict(store, claim_a="a", claim_b="a", actor="u")
+    claim = store.get_claim("a")
+    assert claim.contradicts == []
+    assert claim.status != ClaimStatus.CONTESTED
+
+
 def test_archive_changes_status(store: KBStore) -> None:
     src = store.put_source(b"e")
     store.put_claim(Claim(id="c1", text="x", evidence=[src.id]))