diff --git a/src/openultrasast/cli.py b/src/openultrasast/cli.py
index b4cd5bd..5d3e46f 100644
--- a/src/openultrasast/cli.py
+++ b/src/openultrasast/cli.py
@@ -3,6 +3,7 @@ from __future__ import annotations
 import argparse
 import json
 import tempfile
+from collections.abc import Sequence
 from dataclasses import dataclass, replace
 from pathlib import Path
 from time import perf_counter
@@ -45,6 +46,7 @@ from .pair_gate import print_pair_metrics
 from .pairs import (
     DEFAULT_CATALOG,
     SLICE_NAMES,
+    PairCase,
     evaluate_catalog,
     load_pair_catalog,
     make_hunter_scan,
@@ -60,6 +62,7 @@ from .provenance import fingerprint
 from .provider.openrouter import OpenRouterEmbeddingClient, OpenRouterError
 from .rank import rank_targets, write_rankings
 from .regress import TRIGGERABLE, CandidateVerdict, run_regression, write_verdicts
+from .regress.candidate import hotspot_from_finding
 from .reports import scan_exit_code, write_manifest, write_markdown_report, write_sarif_report
 from .ruleset import DEFAULT_RULESET_DIR, load_ruleset
 from .run import ScanRun, create_scan_run
@@ -159,6 +162,10 @@ def main(argv: list[str] | None = None) -> int:
     pairs.add_argument("--hunter", action="store_true", help="also score the LLM tool hunter on overlay slices (needs a hunter model)")
     pairs.add_argument("--hunter-model", default=None, help="model id for --hunter (default: [models].hunter from openultrasast.toml)")
     pairs.add_argument("--pointers", action="store_true", help="allow network for non-vendored pointer pairs this run (nightly; CI never)")
+    pairs.add_argument(
+        "--loo", action="store_true", help="leave-one-out: score each pair against a store seeded from the other trusted pairs"
+    )
+    pairs.add_argument("--loo-out", type=Path, default=None, help="where to write loo.json (default: reports/loo.json)")
     pairs.add_argument("--json", action="store_true", help="print the pair scoreboard as JSON")
 
     subparsers.add_parser("mcp", help="run the narrow MCP server over stdio for OpenCode integration")
@@ -203,6 +210,8 @@ def main(argv: list[str] | None = None) -> int:
             hunter=args.hunter,
             hunter_model=args.hunter_model,
             pointers=args.pointers,
+            loo=args.loo,
+            loo_out=args.loo_out,
         )
     if args.command == "mechanisms":
         return _mechanisms_export(args.catalog, args.slice, args.store, json_out=args.json)
@@ -823,10 +832,14 @@ def _pairs(
     hunter: bool = False,
     hunter_model: str | None = None,
     pointers: bool = False,
+    loo: bool = False,
+    loo_out: Path | None = None,
 ) -> int:
     if not catalog.exists() or not catalog.is_file():
         raise SystemExit(f"pair catalog is not a file: {catalog}")
     cases = select_split(select_profile(select_slice(load_pair_catalog(catalog), slice_name), profile), split)
+    if loo:
+        return _pairs_loo(cases, loo_out or Path("reports") / "loo.json", json_out=json_out)
     scan = None
     if hunter:
         config_path = Path("openultrasast.toml")
@@ -877,16 +890,27 @@ def _search_variants(
 
 
 def _hotspot_from_variant(finding: StaticFinding) -> Hotspot:
-    return Hotspot(
-        path=finding.path,
-        function_name=finding.function_name,
-        score=0.0,
-        band="low",
-        signals={},
-        rationale="variant of a known mechanism (suspicion; sandbox may raise)",
-        test_hint=None,
-        inventory_finding_ids=(finding.finding_id,),
-    )
+    return hotspot_from_finding(finding, rationale="variant of a known mechanism (suspicion; sandbox may raise)")
+
+
+def _pairs_loo(cases: Sequence[PairCase], out: Path, *, json_out: bool) -> int:
+    """Req 4: the corpus's own detection rate; written as an artifact, never a gate."""
+    from .semantic.loo import evaluate_loo
+
+    result = evaluate_loo(select_vendored(cases))
+    payload = result.to_dict()
+    out.parent.mkdir(parents=True, exist_ok=True)
+    out.write_text(json.dumps(payload, indent=2, sort_keys=True) + "\n")
+    if json_out:
+        print(json.dumps(payload, indent=2, sort_keys=True))
+        return 0
+    for slice_name, metrics in result.per_slice.items():
+        print(
+            f"loo {slice_name}: pairs={metrics['pairs']} detected={metrics['detected']} silent={metrics['silent']} "
+            f"youden={metrics['youden']:+.3f} teaching={metrics['teaching']}"
+        )
+    print(f"artifact={out}")
+    return 0
 
 
 def _mechanisms_export(catalog: Path, slice_name: str, store_path: Path, *, json_out: bool) -> int:
diff --git a/src/openultrasast/semantic/loo.py b/src/openultrasast/semantic/loo.py
new file mode 100644
index 0000000..e0237d7
--- /dev/null
+++ b/src/openultrasast/semantic/loo.py
@@ -0,0 +1,203 @@
+"""Leave-one-out: the pair corpus's own detection rate (corpus-seeded-mechanisms, Req 4).
+
+For every scorable pair, a temporary mechanism store is seeded from every *other* trusted pair, both excerpts are
+materialized and searched for variants, and the pair is scored with the pair rules: detected when a hit lies inside the
+labeled function and names the labeled mechanism (or sink), silent when the fixed side yields no hit. Only ``seeded`` and
+``reviewed`` pairs teach; ``advisory`` and ``title`` pairs are scored as held-out targets but never seed. Reported, never
+gated.
+"""
+
+from __future__ import annotations
+
+import tempfile
+from collections.abc import Sequence
+from dataclasses import dataclass, field
+from pathlib import Path
+
+from ..pairs import GATING_TIERS, PairCase, _materialize, _targets
+from .facts import FactLoadError, SemanticFacts, load_facts
+from .functions import named_function_ranges, spans_named
+from .mechanisms import Mechanism, MechanismStore, append_from_pair, corpus_mechanisms
+from .seed import Lesson, _parse_language, pair_lessons
+from .variant_search import search_tree
+from .variants import Shape, trailing_name
+
+
+@dataclass(frozen=True)
+class LooOutcome:
+    pair: str
+    slice: str
+    provenance: str
+    mechanisms: tuple[str, ...]  # labeled mechanism ids of the held-out pair
+    detected: bool
+    silent: bool
+    found_by: tuple[tuple[str, tuple[str, ...]], ...]  # (mechanism record id, pairs that taught it) for detecting hits
+    hits_vuln: int
+    hits_fixed: int
+    teaches: bool  # the pair contributed at least one shape when others were held out
+    leaked_by: tuple[str, ...] = ()  # mechanism record ids that hit the fixed side
+
+    def to_dict(self) -> dict[str, object]:
+        return {
+            "pair": self.pair,
+            "slice": self.slice,
+            "provenance": self.provenance,
+            "mechanisms": list(self.mechanisms),
+            "detected": self.detected,
+            "silent": self.silent,
+            "found_by": [{"mechanism_id": mechanism_id, "pairs": list(pairs)} for mechanism_id, pairs in self.found_by],
+            "hits_vuln": self.hits_vuln,
+            "hits_fixed": self.hits_fixed,
+            "teaches": self.teaches,
+            "leaked_by": list(self.leaked_by),
+        }
+
+
+@dataclass(frozen=True)
+class LooResult:
+    outcomes: tuple[LooOutcome, ...]
+    per_slice: dict[str, dict[str, float]]
+    per_profile: dict[str, dict[str, float]]
+    per_mechanism: dict[str, dict[str, float]]
+    skipped: tuple[tuple[str, str], ...] = ()
+    teaching_pairs: int = 0
+    degradations: tuple[dict[str, object], ...] = field(default=())
+
+    def to_dict(self) -> dict[str, object]:
+        return {
+            "per_slice": self.per_slice,
+            "per_profile": self.per_profile,
+            "per_mechanism": self.per_mechanism,
+            "outcomes": [outcome.to_dict() for outcome in self.outcomes],
+            "skipped": [{"pair": pair, "reason": reason} for pair, reason in self.skipped],
+            "teaching_pairs": self.teaching_pairs,
+            "degradations": list(self.degradations),
+        }
+
+
+def evaluate_loo(cases: Sequence[PairCase], *, facts: SemanticFacts | None = None) -> LooResult:
+    loaded = facts if facts is not None else _facts()
+    targets: list[PairCase] = []
+    skipped: list[tuple[str, str]] = []
+    for case in cases:
+        if case.known_limit:
+            skipped.append((case.name, f"known_limit:{case.known_limit}"))
+        elif not case.vuln_file.is_file() or not case.fixed_file.is_file():
+            skipped.append((case.name, "pointer_pair_not_cached" if not case.vendored else "excerpt_missing"))
+        else:
+            targets.append(case)
+    lessons = {case.name: pair_lessons(case, loaded) for case in targets if case.review_tier in GATING_TIERS}
+    outcomes = [_hold_out(case, targets, lessons, loaded) for case in targets]
+    return LooResult(
+        outcomes=tuple(outcomes),
+        per_slice=_group(outcomes, lambda item: (item.slice,)),
+        per_profile=_group(outcomes, lambda item: (item.provenance,)),
+        per_mechanism=_group(outcomes, lambda item: item.mechanisms),
+        skipped=tuple(skipped),
+        teaching_pairs=sum(1 for shapes in lessons.values() if shapes),
+    )
+
+
+def _hold_out(case: PairCase, targets: Sequence[PairCase], lessons: dict[str, list[Lesson]], facts: SemanticFacts) -> LooOutcome:
+    with tempfile.TemporaryDirectory(prefix="ousast-loo-") as scratch:
+        root = Path(scratch)
+        store = MechanismStore(root / "mechanisms.jsonl")
+        for teacher in targets:
+            if teacher.name == case.name:
+                continue
+            for lesson in lessons.get(teacher.name, ()):
+                append_from_pair(
+                    store,
+                    lesson.shape,
+                    summary=lesson.summary,
+                    cwe=lesson.cwe,
+                    pair=teacher.name,
+                    provenance=teacher.provenance,
+                    tier=teacher.review_tier,
+                )
+        scored = score_pair_with_store(case, store, facts, root=root)
+    return LooOutcome(**{**scored.__dict__, "teaches": bool(lessons.get(case.name))})
+
+
+def score_pair_with_store(case: PairCase, store: MechanismStore, facts: SemanticFacts, *, root: Path | None = None) -> LooOutcome:
+    """Search both sides of one pair with a fixed store and score it with the pair rules (shared by LOO and the lever)."""
+    labeled_mechanisms = tuple(sorted({row.mechanism for row in case.expected if row.mechanism}))
+    records = {record.id: record for record in corpus_mechanisms(store.load())}
+    with tempfile.TemporaryDirectory(prefix="ousast-loo-pair-") as scratch:
+        base = root if root is not None else Path(scratch)
+        vuln_root = _materialize(base / "vuln", case.vuln_file, case.relpath)
+        fix_root = _materialize(base / "fixed", case.fixed_file, case.relpath)
+        vuln_hits = search_tree(vuln_root, _targets(vuln_root), store, facts, max_mechanisms=len(records) or 1).hits
+        fix_hits = search_tree(fix_root, _targets(fix_root), store, facts, max_mechanisms=len(records) or 1).hits
+    text = case.vuln_file.read_text(errors="ignore")
+    ranges = named_function_ranges(case.relpath, text, _parse_language(case)) or ()
+    found: list[tuple[str, tuple[str, ...]]] = []
+    for hit in vuln_hits:
+        record = records.get(hit.mechanism_id)
+        if record is None or not _inside_label(case, hit.line, ranges):
+            continue
+        if _names_label(case, record, hit.sink_name):
+            found.append((record.id, tuple(record.pairs)))
+    return LooOutcome(
+        pair=case.name,
+        slice=case.slice,
+        provenance=case.provenance,
+        mechanisms=labeled_mechanisms,
+        detected=bool(found),
+        silent=not fix_hits,
+        found_by=tuple(found),
+        hits_vuln=len(vuln_hits),
+        hits_fixed=len(fix_hits),
+        teaches=False,
+        leaked_by=tuple(sorted({hit.mechanism_id for hit in fix_hits})),
+    )
+
+
+def _inside_label(case: PairCase, line: int, ranges: Sequence[tuple[str, int, int]]) -> bool:
+    functions = {row.function for row in case.expected if row.function}
+    if not functions:
+        return True
+    return any(start <= line <= end for function in functions for start, end in spans_named(ranges, function))
+
+
+def _names_label(case: PairCase, record: Mechanism, sink_name: str) -> bool:
+    """The hit names the pair's labeled mechanism, or its labeled sink when the row carries one."""
+    shape = Shape.from_dict(record.shape or {})
+    for row in case.expected:
+        if row.mechanism and row.mechanism == shape.mechanism:
+            return True
+        if row.sink and trailing_name(row.sink) == sink_name:
+            return True
+    return False
+
+
+def _group(outcomes: Sequence[LooOutcome], keys: object) -> dict[str, dict[str, float]]:
+    buckets: dict[str, list[LooOutcome]] = {}
+    for outcome in outcomes:
+        for key in keys(outcome):  # type: ignore[operator]
+            buckets.setdefault(str(key), []).append(outcome)
+    return {name: _metrics(items) for name, items in sorted(buckets.items())}
+
+
+def _metrics(outcomes: Sequence[LooOutcome]) -> dict[str, float]:
+    pairs = len(outcomes)
+    detected = sum(1 for item in outcomes if item.detected)
+    silent = sum(1 for item in outcomes if item.silent)
+    recall = detected / pairs if pairs else 0.0
+    silence = silent / pairs if pairs else 0.0
+    return {
+        "pairs": pairs,
+        "detected": detected,
+        "silent": silent,
+        "recall": recall,
+        "silence": silence,
+        "youden": recall + silence - 1.0 if pairs else 0.0,
+        "teaching": sum(1 for item in outcomes if item.teaches),
+    }
+
+
+def _facts() -> SemanticFacts:
+    try:
+        return load_facts()
+    except FactLoadError:
+        return SemanticFacts(version="", sources=(), sinks=(), sanitizers=())
diff --git a/tests/test_loo.py b/tests/test_loo.py
new file mode 100644
index 0000000..a6edaf2
--- /dev/null
+++ b/tests/test_loo.py
@@ -0,0 +1,107 @@
+"""corpus-seeded-mechanisms task 3.1: leave-one-out is the corpus's own detection rate (offline)."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from openultrasast.benchmark import ExpectedFinding
+from openultrasast.pairs import PairCase
+
+EXEC_A = (
+    "import sqlite3\n\n\ndef lookup(user):\n    c = sqlite3.connect('x').cursor()\n"
+    + "    c.execute(\"SELECT * FROM t WHERE u = '%s'\" % user)\n"
+)
+EXEC_A_FIX = (
+    "import sqlite3\n\n\ndef lookup(user):\n    c = sqlite3.connect('x').cursor()\n    c.execute('SELECT * FROM t WHERE u = ?', (user,))\n"
+)
+EXEC_B = (
+    "import sqlite3\n\n\ndef delete(name):\n    cur = sqlite3.connect('y').cursor()\n    cur.execute('DELETE FROM t WHERE n = ' + name)\n"
+)
+EXEC_B_FIX = (
+    "import sqlite3\n\n\ndef delete(name):\n    cur = sqlite3.connect('y').cursor()\n"
+    + "    cur.execute('DELETE FROM t WHERE n = ?', (name,))\n"
+)
+SYS_C = "import os\n\n\ndef ping(host):\n    os.system('ping ' + host)\n"
+SYS_C_FIX = "import os\nimport subprocess\n\n\ndef ping(host):\n    subprocess.run(['ping', host])\n"
+
+
+def _case(tmp_path: Path, name: str, vuln: str, fixed: str, *, function: str, cwe: str, provenance: str = "human") -> PairCase:
+    (tmp_path / f"{name}-v.py").write_text(vuln)
+    (tmp_path / f"{name}-f.py").write_text(fixed)
+    return PairCase(
+        name=name,
+        slice="toy",
+        language="python",
+        origin="test",
+        vuln_file=tmp_path / f"{name}-v.py",
+        fixed_file=tmp_path / f"{name}-f.py",
+        relpath="app.py",
+        expected=(
+            ExpectedFinding(
+                cwe=cwe, vulnerability_class="x", path="app.py", evidence="", function=function, mechanism="source_reaches_sink"
+            ),
+        ),
+        min_recall=1.0,
+        fix_policy="silent",
+        provenance=provenance,
+        review_tier="seeded",
+    )
+
+
+def _toy(tmp_path: Path) -> list[PairCase]:
+    return [
+        _case(tmp_path, "a", EXEC_A, EXEC_A_FIX, function="lookup", cwe="CWE-89"),
+        _case(tmp_path, "b", EXEC_B, EXEC_B_FIX, function="delete", cwe="CWE-89", provenance="agent"),
+        _case(tmp_path, "c", SYS_C, SYS_C_FIX, function="ping", cwe="CWE-78"),
+    ]
+
+
+def test_leave_one_out_reports_which_pair_taught_which(tmp_path: Path) -> None:
+    from openultrasast.semantic.loo import evaluate_loo
+
+    result = evaluate_loo(_toy(tmp_path))
+    by_name = {outcome.pair: outcome for outcome in result.outcomes}
+    assert set(by_name) == {"a", "b", "c"}
+    assert by_name["a"].detected and by_name["a"].silent and by_name["a"].found_by  # taught by b's execute shape
+    assert by_name["b"].detected and by_name["b"].silent and any("a" in pairs for _, pairs in by_name["b"].found_by)
+    assert not by_name["c"].detected and by_name["c"].found_by == ()  # nobody else teaches os.system: c only tests
+    slice_metrics = result.per_slice["toy"]
+    assert slice_metrics["pairs"] == 3 and slice_metrics["detected"] == 2 and slice_metrics["silent"] == 3
+    assert abs(slice_metrics["recall"] - 2 / 3) < 1e-9 and slice_metrics["silence"] == 1.0 and abs(slice_metrics["youden"] - 2 / 3) < 1e-9
+    assert result.per_profile["agent"]["pairs"] == 1 and result.per_profile["human"]["pairs"] == 2
+    assert "source_reaches_sink" in result.per_mechanism
+    payload = result.to_dict()
+    assert set(payload) >= {"per_slice", "per_profile", "per_mechanism", "outcomes"}
+    json.dumps(payload)  # artifact-ready
+
+
+def test_leave_one_out_seeds_only_trusted_pairs_and_skips_known_limit(tmp_path: Path) -> None:
+    from openultrasast.semantic.loo import evaluate_loo
+
+    cases = _toy(tmp_path)
+    cases[1] = PairCase(**{**cases[1].__dict__, "review_tier": "advisory"})  # b may be tested, never teaches
+    cases[2] = PairCase(**{**cases[2].__dict__, "known_limit": "engine"})
+    result = evaluate_loo(cases)
+    by_name = {outcome.pair: outcome for outcome in result.outcomes}
+    assert set(by_name) == {"a", "b"}  # known_limit excluded entirely
+    assert not by_name["a"].detected  # b is advisory: it cannot teach a
+    assert by_name["b"].detected  # a (seeded) teaches b, which is scored as a held-out target
+    assert result.skipped == (("c", "known_limit:engine"),)
+
+
+def test_cli_pairs_loo_writes_the_artifact_offline(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    import io
+    from contextlib import redirect_stdout
+
+    from openultrasast.cli import main
+
+    monkeypatch.chdir(Path.cwd())
+    buf = io.StringIO()
+    with redirect_stdout(buf):
+        assert main(["pairs", "--slice", "vibe-py", "--loo", "--json", "--loo-out", str(tmp_path / "loo.json")]) == 0
+    payload = json.loads(buf.getvalue())
+    assert payload["per_slice"]["vibe-py"]["pairs"] == 35 and "found_by" in payload["outcomes"][0]
+    assert json.loads((tmp_path / "loo.json").read_text()) == payload