fix(proposals): catch id collision in check_approvable batch precheck

CHANGELOG.md

@@ -12,6 +12,13 @@ All notable changes to vouch are documented here. Format follows
   `contradicts` reference and flipped the claim to `contested` with no
   actual counterparty; it now raises `LifecycleError`, mirroring the
   existing guard in `supersede()`.
+- `check_approvable()` now catches an id collision with an existing
+  artifact, mirroring the guard `approve()` already applies. previously
+  the batch precheck could report a claim/entity/relation proposal as
+  approvable when its id matched an artifact that already existed, so
+  `vouch approve a b` could pass the precheck for the whole batch and
+  then fail partway through on the collision, breaking the documented
+  all-or-nothing guarantee.
 
 ### Added
 - `kb.list_skills` / `kb.get_skill` — agents can enumerate the Claude Code

src/vouch/proposals.py

@@ -494,6 +494,14 @@ def check_approvable(
     block = _approval_block_reason(store, proposal, approved_by)
     if block:
         return block
+    # Mirror approve()'s existing-artifact guard so the batch precheck can't
+    # say "approvable" for a proposal that approve() will then reject.
+    if proposal.kind not in (ProposalKind.PAGE, ProposalKind.DELETE):
+        existing = _existing_artifact_block_reason(
+            store, proposal.kind, proposal.payload["id"]
+        )
+        if existing:
+            return existing
     return _payload_block_reason(store, proposal)
 
 
@@ -878,21 +886,29 @@ def _approve_delete(
     return artifact
 
 
-def _ensure_no_existing_artifact(
+def _existing_artifact_block_reason(
     store: KBStore, kind: ProposalKind, artifact_id: str
-) -> None:
+) -> str | None:
     getter = getattr(store, _ARTIFACT_GETTERS[kind])
     try:
         getter(artifact_id)
     except ArtifactNotFoundError:
-        return
-    raise ProposalError(
+        return None
+    return (
         f"cannot approve: {kind.value} {artifact_id} already exists "
         f"(a prior approve may have been interrupted; reconcile manually "
         f"by removing the artifact or rejecting this proposal)"
     )
 
 
+def _ensure_no_existing_artifact(
+    store: KBStore, kind: ProposalKind, artifact_id: str
+) -> None:
+    reason = _existing_artifact_block_reason(store, kind, artifact_id)
+    if reason:
+        raise ProposalError(reason)
+
+
 def _node_exists(store: KBStore, node_id: str) -> bool:
     """True if `node_id` resolves to a Claim, Page, Entity, or Source.
 

tests/test_storage.py

@@ -615,6 +615,30 @@ def test_check_approvable_clean_for_well_formed_proposal(
     assert check_approvable(store, pr.id, approved_by="reviewer") is None
 
 
+def test_check_approvable_catches_id_collision_with_existing_claim(
+    store: KBStore,
+) -> None:
+    """approve() refuses to overwrite an existing artifact; the batch
+    precheck must catch that too, or `vouch approve a b` can say "all
+    clear" and then partially apply before failing on the collision."""
+    src = store.put_source(b"e")
+    first = propose_claim(
+        store, text="first", evidence=[src.id],
+        slug_hint="foo-bar", proposed_by="agent",
+    )
+    approve(store, first.id, approved_by="reviewer")
+
+    second = propose_claim(
+        store, text="second", evidence=[src.id],
+        slug_hint="foo-bar", proposed_by="agent",
+    )
+    reason = check_approvable(store, second.id, approved_by="reviewer")
+    assert reason is not None
+    assert "already exists" in reason
+    with pytest.raises(ProposalError, match="already exists"):
+        approve(store, second.id, approved_by="reviewer")
+
+
 # --- evidence -------------------------------------------------------------