diff --git a/pyproject.toml b/pyproject.toml
index 01b51b1..ff5752f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,6 +36,11 @@ pythonpath = ["src"]
 branch = true
 source = ["shard_wiki"]
 
+[tool.coverage.report]
+show_missing = true
+# Quality floor for `pytest --cov` / `coverage report` (not forced on a bare `pytest` run).
+fail_under = 90
+
 [tool.ruff]
 src = ["src", "tests"]
 target-version = "py311"
diff --git a/tests/test_error_paths.py b/tests/test_error_paths.py
new file mode 100644
index 0000000..afacfba
--- /dev/null
+++ b/tests/test_error_paths.py
@@ -0,0 +1,76 @@
+"""Error-path / contract tests across modules (keeps the suite honest about failure behaviour)."""
+
+from collections.abc import Iterable
+
+import pytest
+
+from shard_wiki import InformationSpace
+from shard_wiki.adapters import FolderAdapter, ShardAdapter, run_conformance
+from shard_wiki.engine import EngineKernel
+from shard_wiki.model import CapabilityProfile, Identity, Page, Placement
+from shard_wiki.provenance import ProvenanceEnvelope
+from shard_wiki.union import ResolutionKind, UnionGraph
+
+
+def _folder(tmp_path, name, files, writable=False):
+    root = tmp_path / name
+    root.mkdir(parents=True, exist_ok=True)
+    for rel, text in files.items():
+        (root / rel).write_text(text, encoding="utf-8")
+    return FolderAdapter(name, root, writable=writable)
+
+
+def test_resolution_single_on_red_link_raises():
+    u = UnionGraph("s")
+    res = u.resolve("ghost")
+    assert res.kind is ResolutionKind.RED_LINK
+    with pytest.raises(KeyError):
+        res.single()
+
+
+def test_apply_unknown_overlay_raises(tmp_path):
+    space = InformationSpace("t")
+    space.attach(_folder(tmp_path, "w", {"Home.md": "x"}, writable=True))
+    with pytest.raises(KeyError):
+        space.apply_overlay("does-not-exist")
+
+
+def test_apply_overlay_for_unattached_shard_raises(tmp_path):
+    space = InformationSpace("t")
+    space.attach(_folder(tmp_path, "w", {"Home.md": "x"}, writable=True))
+    # draft an overlay whose target shard is not attached -> apply can't find an adapter
+    ov = space.overlays.draft(Identity("ghost", "X"), "body", base_rev=None)
+    with pytest.raises(KeyError):
+        space.apply_overlay(ov.overlay_id)
+
+
+def test_kernel_delete_missing_raises():
+    with pytest.raises(KeyError):
+        EngineKernel("eng").delete("nope")
+
+
+def test_placement_str():
+    assert str(Placement("shardA", "sub/Page")) == "shardA/sub/Page"
+
+
+class _BrokenProfileAdapter(ShardAdapter):
+    """profile() raises — the conformance battery must report failure, not crash."""
+
+    @property
+    def shard_id(self) -> str:
+        return "broken"
+
+    def profile(self) -> CapabilityProfile:
+        raise RuntimeError("profile blew up")
+
+    def keys(self) -> Iterable[str]:
+        return []
+
+    def read(self, key: str) -> Page:
+        return Page(Identity("broken", key), "x", ProvenanceEnvelope(source_shard="broken"))
+
+
+def test_conformance_survives_a_broken_profile():
+    report = run_conformance(_BrokenProfileAdapter())
+    assert not report.ok
+    assert any(c.name == "profile-validates" and not c.ok for c in report.checks)