diff --git a/.kiro/specs/corpus-seeded-mechanisms/design.md b/.kiro/specs/corpus-seeded-mechanisms/design.md
index 76cd1c0..85424ba 100644
--- a/.kiro/specs/corpus-seeded-mechanisms/design.md
+++ b/.kiro/specs/corpus-seeded-mechanisms/design.md
@@ -97,12 +97,12 @@ src/openultrasast/
 │   ├── seed.py         # export_mechanisms(cases, store) -> ExportReport (maintainer command `mechanisms export`)
 │   └── loo.py          # evaluate_loo(cases, *, store_factory) -> LooResult; payload
 ├── findings.py         # origin "variant" via tags/finding_id prefix "variant:" (no schema change)
-├── cli.py              # MAP: variant search after overlay; manifest counters; `pairs --loo`; `mechanisms export --slice`
+├── cli.py              # MAP: variant search after overlay; manifest counters; `pairs --loo`; `mechanisms export --slice`; `improve --mechanism-candidates/--mechanism-store/--no-mechanisms` (defaults: target calibration dir, then cwd)
 ├── improve/validator.py# MechanismEdit lever "mechanisms"
-├── improve/evolve.py   # proposer from loo misses/leaks; gate unchanged
+├── improve/evolve.py   # proposer from loo misses/leaks; mechanism edits decided by the per-profile clause over variant search (see §Lever)
 └── reports.py          # mechanism id, guard, provenance on findings
 tests/
-├── test_variants.py, test_loo.py, test_mechanism_export.py, test_improve.py (lever)
+├── test_variants.py, test_variant_search.py, test_variant_scan.py, test_mechanism_export.py, test_mechanism_seed.py, test_loo.py, test_mechanism_lever.py, test_mechanism_loop.py, test_mechanism_reports.py
 ```
 
 ### Modified Files
@@ -222,7 +222,9 @@ Only `seeded | reviewed` pairs seed; every trusted pair is held out once. `known
 
 ### Lever
 
-`MechanismEdit(lever="mechanisms", action="admit"|"retract", mechanism_id, source="loo"|"export", rationale)`. Validator: id must exist in the exporter's candidate set; shape guard in `GUARD_KINDS`; no free text fields beyond rationale. Proposer: admit candidates whose leave-one-out hit recovers a currently missed holdout pair in some profile; retract admitted records that leak on holdout fixed sides. Gate unchanged; revert restores `mechanisms.jsonl` byte for byte.
+`MechanismEdit(lever="mechanisms", action="admit"|"retract", mechanism_id, source="loo"|"export", rationale)`. Validator: id must exist in the exporter's candidate set; shape guard in `GUARD_KINDS`; no free text fields beyond rationale. Proposer: admit candidates whose leave-one-out hit recovers a currently missed holdout pair in some profile; retract admitted records that leak on holdout fixed sides. Application: admit appends the exporter record, retract appends a `retracted = true` tombstone (`MechanismStore.load` folds it away; the log stays append-only); `StoreSnapshot.restore` reverts byte for byte. A retraction may also name a record admitted in the scan store but no longer a candidate.
+
+Gate (implemented 2026-09-05, deviation from the first draft's "gate unchanged"): the rule gate and its per-profile clause are unchanged for rule edits; mechanism edits are decided by the *same* `profile_regressions` clause applied to `evaluate_mechanism_profiles` (holdout pairs scored per provenance profile by variant search with the store before and after, via `loo.score_pair_with_store`), because the overlay scorer cannot see what the store changes. Rejection reasons: `mechanism_leak:<id>` (an admission fires on a holdout fixed side, §Error Handling), `profile_regression:<profile>`, `mechanism_no_gain` (no profile gains a correct pair and nothing is retracted); every rejection restores the store byte for byte and the reverted key is blocked in later rounds regardless of rationale. The exporter writes `mechanism-candidates.jsonl`; only the lever writes the scan-time `mechanisms.jsonl`.
 
 ## Data Models
 
diff --git a/README.md b/README.md
index f9ae7de..24d27d0 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,8 @@ uv run ousast benchmark benchmarks/manifests/python-vulnerable.toml --mode quick
 uv run ousast pairs
 uv run ousast pairs --slice sast   # OWASP Benchmark Java/Python + Juliet
 uv run ousast pairs --slice vibe-py --pointers   # nightly: also harvest non-vendored pointer pairs into ~/.cache
+uv run ousast mechanisms export --slice vibe-py   # seed mechanism candidates from trusted pairs (offline); `ousast improve` admits them under the holdout gate
+uv run ousast pairs --slice vibe-py --loo        # leave-one-out recall of the corpus as a teacher
 uv run python -m openultrasast.pair_gate
 
 # Run the bounded self-improvement loop over the ruleset (benchmark-driven)
diff --git a/docs/examples.md b/docs/examples.md
index 5bb53c7..ef0d09a 100644
--- a/docs/examples.md
+++ b/docs/examples.md
@@ -37,6 +37,9 @@ uv run ousast pairs                              # local fixtures + vendored Git
 uv run ousast pairs --slice github --json        # honesty dashboard as JSON
 uv run ousast pairs --slice sast                 # OWASP Benchmark + Juliet Youden (TPR-FPR)
 uv run ousast pairs --slice vibe-py --pointers   # nightly: pointer pairs via the local cache (network)
+uv run ousast mechanisms export --slice vibe-py   # maintainer: seed mechanism candidates from trusted pairs (offline)
+uv run ousast improve bench.toml --pair-catalog benchmarks/pairs/catalog.toml   # the mechanisms lever admits candidates the holdout pairs prove; the scan then searches them
+uv run ousast pairs --slice vibe-py --loo        # leave-one-out: the corpus's own detection rate (reports/loo.json)
 uv run python -m openultrasast.pair_gate         # CI: local pairs must all pass
 ```
 
diff --git a/src/openultrasast/cli.py b/src/openultrasast/cli.py
index fdcbb7f..451a2f1 100644
--- a/src/openultrasast/cli.py
+++ b/src/openultrasast/cli.py
@@ -148,6 +148,19 @@ def main(argv: list[str] | None = None) -> int:
         "--profile-tolerance", type=float, default=0.0, help="allowed per-profile drop in pair_correct/Youden before rejecting"
     )
     improve.add_argument("--min-holdout-pairs", type=int, default=5, help="profiles with fewer holdout pairs are reported, not gated")
+    improve.add_argument(
+        "--mechanism-candidates",
+        type=Path,
+        default=None,
+        help="exporter candidates the mechanisms lever may admit (default: <target>/.openultrasast/calibration/mechanism-candidates.jsonl, then ./.openultrasast/calibration/mechanism-candidates.jsonl)",
+    )
+    improve.add_argument(
+        "--mechanism-store",
+        type=Path,
+        default=None,
+        help="scan-time mechanisms.jsonl the lever writes (default: <target>/.openultrasast/calibration/mechanisms.jsonl)",
+    )
+    improve.add_argument("--no-mechanisms", action="store_true", help="disable the mechanisms lever for this run")
 
     pairs = subparsers.add_parser(
         "pairs",
@@ -175,7 +188,10 @@ def main(argv: list[str] | None = None) -> int:
     export.add_argument("--slice", choices=SLICE_NAMES, default="all")
     export.add_argument("--catalog", type=Path, default=DEFAULT_CATALOG)
     export.add_argument(
-        "--store", type=Path, default=Path(CALIBRATION_DIR) / "mechanisms.jsonl", help="append-only mechanisms.jsonl the scan reads"
+        "--store",
+        type=Path,
+        default=Path(CALIBRATION_DIR) / "mechanism-candidates.jsonl",
+        help="append-only candidates file the improve lever admits from (the scan reads mechanisms.jsonl next to it; never written here)",
     )
     export.add_argument("--json", action="store_true")
 
@@ -199,6 +215,9 @@ def main(argv: list[str] | None = None) -> int:
             pair_catalog=None if args.no_pair_gate else args.pair_catalog,
             profile_tolerance=args.profile_tolerance,
             min_holdout_pairs=args.min_holdout_pairs,
+            mechanism_candidates=args.mechanism_candidates,
+            mechanism_store=args.mechanism_store,
+            mechanisms_enabled=not args.no_mechanisms,
         )
     if args.command == "pairs":
         return _pairs(
@@ -784,6 +803,9 @@ def _improve(
     pair_catalog: Path | None = None,
     profile_tolerance: float = 0.0,
     min_holdout_pairs: int = 5,
+    mechanism_candidates: Path | None = None,
+    mechanism_store: Path | None = None,
+    mechanisms_enabled: bool = True,
 ) -> int:
     if not manifest_path.exists() or not manifest_path.is_file():
         raise SystemExit(f"benchmark manifest is not a file: {manifest_path}")
@@ -794,6 +816,19 @@ def _improve(
         pair_cases = select_split(load_pair_catalog(pair_catalog), "holdout")
     manifest = load_benchmark_manifest(manifest_path)
     target = resolve_benchmark_source(manifest_path, manifest)
+    # corpus-seeded-mechanisms Req 5: the lever runs when a candidates file exists (exporter output) and the pair gate is on.
+    # Candidates: an explicit path, else the target's calibration dir, else the working directory's (where `mechanisms export`
+    # writes by default). The store always lives where a scan of this target reads it unless overridden.
+    calibration = target / CALIBRATION_DIR
+    candidates_path: Path | None = mechanism_candidates
+    if candidates_path is None:
+        for candidate in (calibration / "mechanism-candidates.jsonl", Path(CALIBRATION_DIR) / "mechanism-candidates.jsonl"):
+            if candidate.is_file():
+                candidates_path = candidate
+                break
+    if not mechanisms_enabled or not pair_cases or candidates_path is None or not candidates_path.is_file():
+        candidates_path = None
+    store_path = mechanism_store if mechanism_store is not None else calibration / "mechanisms.jsonl"
     policy = load_policy()
 
     # The loop writes its accepted ledger where `scan`/`benchmark` read it, so a
@@ -803,6 +838,11 @@ def _improve(
         if dry_run:
             ledger_path = Path(scratch) / "rule_policy.json"
             journal_path = Path(scratch) / "improve_journal.json"
+            if candidates_path is not None:  # a dry run must not mutate the scan-time store either
+                scratch_store = Path(scratch) / "mechanisms.jsonl"
+                if store_path.is_file():
+                    scratch_store.write_bytes(store_path.read_bytes())
+                store_path = scratch_store
         else:
             ledger_path = ledger or default_ledger
             journal_path = journal or ledger_path.with_name("improve_journal.json")
@@ -820,6 +860,8 @@ def _improve(
             pair_cases=pair_cases,
             profile_tolerance=profile_tolerance,
             min_holdout_pairs=min_holdout_pairs,
+            mechanism_candidates=candidates_path,
+            mechanism_store=store_path if candidates_path is not None else None,
         )
         _print_improve_outcomes(outcomes, manifest_path, target, ledger_path, dry_run=dry_run)
     return 0
@@ -951,7 +993,13 @@ def _print_improve_outcomes(outcomes: list[RoundOutcome], manifest_path: Path, t
     print(f"target={target}")
     accepted = [o for o in outcomes if o.accepted]
     for outcome in outcomes:
-        edits = ", ".join(f"{e.rule_id}:{e.from_status}->{e.to_status}" for e in outcome.edits) or "-"
+        edits = (
+            ", ".join(
+                [f"{e.rule_id}:{e.from_status}->{e.to_status}" for e in outcome.edits]
+                + [f"{m.action} {m.mechanism_id}" for m in outcome.mechanism_edits]
+            )
+            or "-"
+        )
         print(
             f"round {outcome.round}: {outcome.reason} | "
             f"recall {outcome.recall_before:.2%}->{outcome.recall_after:.2%} "
@@ -962,7 +1010,7 @@ def _print_improve_outcomes(outcomes: list[RoundOutcome], manifest_path: Path, t
             print(f"  profiles: regressed={outcome.profile_regressions or '-'} under_minimum={outcome.profiles_under_minimum or '-'}")
     print(f"rounds={len(outcomes)} accepted={len(accepted)}")
     if dry_run:
-        print("dry_run=true (no ledger written)")
+        print("dry_run=true (no ledger or mechanism store written)")
     elif accepted:
         print(f"ledger={ledger_path}")
     else:
diff --git a/src/openultrasast/improve/evolve.py b/src/openultrasast/improve/evolve.py
index 5fa8bea..2bc64f0 100644
--- a/src/openultrasast/improve/evolve.py
+++ b/src/openultrasast/improve/evolve.py
@@ -292,10 +292,13 @@ def run_round(
     try:
         for edit in edits:
             validator.validate(edit, ruleset_by_id, policy)
-        if mechanism_edits and candidates_store is not None:
+        if mechanism_edits and candidates_store is not None and mechanism_store is not None:
+            from ..semantic.mechanisms import MechanismStore
+
             by_id = {record.id: record for record in candidates_store.load()}
+            admitted_by_id = {record.id: record for record in MechanismStore(mechanism_store).load()}
             for mechanism_edit in mechanism_edits:
-                validator.validate_mechanism(mechanism_edit, by_id)
+                validator.validate_mechanism(mechanism_edit, by_id, admitted=admitted_by_id)
     except StrictValidationError as exc:
         _record(journal_path, round_index, edits, "rejected", before, before, str(exc), mechanism_edits=mechanism_edits)
         return _outcome(
@@ -416,6 +419,8 @@ def run_improvement(
     pair_cases: Sequence[object] | None = None,
     profile_tolerance: float = 0.0,
     min_holdout_pairs: int = 5,
+    mechanism_candidates: Path | None = None,
+    mechanism_store: Path | None = None,
 ) -> list[RoundOutcome]:
     """Run improvement rounds until convergence (no new proposals) or ``max_rounds``."""
     outcomes: list[RoundOutcome] = []
@@ -432,6 +437,8 @@ def run_improvement(
             pair_cases=pair_cases,
             profile_tolerance=profile_tolerance,
             min_holdout_pairs=min_holdout_pairs,
+            mechanism_candidates=mechanism_candidates,
+            mechanism_store=mechanism_store,
         )
         outcomes.append(outcome)
         if outcome.reason == "no_proposals":
@@ -507,6 +514,7 @@ def _outcome(
         accepted=accepted,
         reason=reason,
         edits=edits,
+        mechanism_edits=list(mechanism_edits or []),
         recall_before=before.recall,
         recall_after=after.recall,
         fp_before=before.fp_rate,
diff --git a/src/openultrasast/improve/journal.py b/src/openultrasast/improve/journal.py
index 5910282..e866190 100644
--- a/src/openultrasast/improve/journal.py
+++ b/src/openultrasast/improve/journal.py
@@ -31,7 +31,9 @@ def reverted_edit_keys(rounds: list[dict[str, object]]) -> set[str]:
             continue
         for edit in _edits(entry):
             key = edit.get("key")
-            if isinstance(key, str) and not edit.get("rationale"):
+            # Rule edits are blocked only without a new rationale; mechanism edits are blocked outright (the loop
+            # always writes a rationale for them, and a reverted store edit must not be re-proposed every round).
+            if isinstance(key, str) and (not edit.get("rationale") or edit.get("lever") == "mechanisms"):
                 keys.add(key)
     return keys
 
diff --git a/src/openultrasast/improve/validator.py b/src/openultrasast/improve/validator.py
index df09f09..807518e 100644
--- a/src/openultrasast/improve/validator.py
+++ b/src/openultrasast/improve/validator.py
@@ -118,13 +118,20 @@ class EvolveValidator:
         else:
             raise StrictValidationError(f"unknown edit type {type(edit).__name__}", kind="rule_change")
 
-    def validate_mechanism(self, edit: MechanismEdit, candidates: Mapping[str, object]) -> None:
-        """Req 5.2: only records the exporter derived from trusted pairs, with a closed guard and identifier-only shape text."""
+    def validate_mechanism(
+        self, edit: MechanismEdit, candidates: Mapping[str, object], admitted: Mapping[str, object] | None = None
+    ) -> None:
+        """Req 5.2: only records the exporter derived from trusted pairs, with a closed guard and identifier-only shape text.
+
+        A retraction may also name a record that is admitted in the scan store but no longer in the candidate set.
+        """
         from ..semantic.variants import GUARD_KINDS, SOURCE_KINDS
 
         if edit.action not in MECHANISM_ACTIONS:
             raise StrictValidationError(f"mechanism edit action {edit.action!r} is not admit or retract", kind="mechanism_change")
         record = candidates.get(edit.mechanism_id)
+        if record is None and edit.action == "retract" and admitted is not None:
+            record = admitted.get(edit.mechanism_id)
         if record is None:
             raise StrictValidationError(
                 f"unknown mechanism {edit.mechanism_id!r}: not in the exporter candidate set", kind="mechanism_change"
diff --git a/src/openultrasast/semantic/loo.py b/src/openultrasast/semantic/loo.py
index e0237d7..cf1ffcd 100644
--- a/src/openultrasast/semantic/loo.py
+++ b/src/openultrasast/semantic/loo.py
@@ -36,6 +36,7 @@ class LooOutcome:
     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
+    degradations: tuple[dict[str, object], ...] = ()  # from variant search on either side
 
     def to_dict(self) -> dict[str, object]:
         return {
@@ -95,9 +96,22 @@ def evaluate_loo(cases: Sequence[PairCase], *, facts: SemanticFacts | None = Non
         per_mechanism=_group(outcomes, lambda item: item.mechanisms),
         skipped=tuple(skipped),
         teaching_pairs=sum(1 for shapes in lessons.values() if shapes),
+        degradations=_unique_degradations(outcomes),
     )
 
 
+def _unique_degradations(outcomes: Sequence[LooOutcome]) -> tuple[dict[str, object], ...]:
+    seen: set[str] = set()
+    unique: list[dict[str, object]] = []
+    for outcome in outcomes:
+        for item in outcome.degradations:
+            key = repr(sorted(item.items()))
+            if key not in seen:
+                seen.add(key)
+                unique.append(item)
+    return tuple(unique)
+
+
 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)
@@ -127,8 +141,9 @@ def score_pair_with_store(case: PairCase, store: MechanismStore, facts: Semantic
         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
+        vuln_search = search_tree(vuln_root, _targets(vuln_root), store, facts, max_mechanisms=len(records) or 1)
+        fix_search = search_tree(fix_root, _targets(fix_root), store, facts, max_mechanisms=len(records) or 1)
+        vuln_hits, fix_hits = vuln_search.hits, fix_search.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, ...]]] = []
@@ -150,6 +165,7 @@ def score_pair_with_store(case: PairCase, store: MechanismStore, facts: Semantic
         hits_fixed=len(fix_hits),
         teaches=False,
         leaked_by=tuple(sorted({hit.mechanism_id for hit in fix_hits})),
+        degradations=tuple(vuln_search.degradations) + tuple(fix_search.degradations),
     )
 
 
diff --git a/tests/test_loo.py b/tests/test_loo.py
index a6edaf2..fe6fe76 100644
--- a/tests/test_loo.py
+++ b/tests/test_loo.py
@@ -105,3 +105,54 @@ def test_cli_pairs_loo_writes_the_artifact_offline(tmp_path: Path, monkeypatch:
     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
+
+
+HELPER_SINK = "import os\n\n\ndef run(cmd):\n    os.system(cmd)\n\n\ndef ping(host):\n    return run('ping ' + host)\n"
+HELPER_SINK_FIX = (
+    "import subprocess\n\n\ndef run(cmd):\n    subprocess.run(cmd.split())\n\n\ndef ping(host):\n    return run('ping ' + host)\n"
+)
+
+
+def test_hits_outside_the_labeled_function_or_off_label_never_count(tmp_path: Path) -> None:
+    """Round-3 mutants: a hit must sit inside the labeled function and name the labeled mechanism or sink."""
+    from openultrasast.semantic.loo import evaluate_loo
+
+    teacher = _case(tmp_path, "t", SYS_C, SYS_C_FIX, function="ping", cwe="CWE-78")
+    outside = _case(
+        tmp_path, "outside", HELPER_SINK, HELPER_SINK_FIX, function="ping", cwe="CWE-78"
+    )  # sink call lives in `run`, not `ping`
+    off_label = PairCase(
+        **{
+            **_case(tmp_path, "offlabel", SYS_C, SYS_C_FIX, function="ping", cwe="CWE-78").__dict__,
+            "expected": (
+                ExpectedFinding(
+                    cwe="CWE-78", vulnerability_class="x", path="app.py", evidence="", function="ping", mechanism="missing_auth_guard"
+                ),
+            ),
+        }
+    )
+    result = evaluate_loo([teacher, outside, off_label])
+    by_name = {outcome.pair: outcome for outcome in result.outcomes}
+    assert by_name["outside"].hits_vuln >= 1 and not by_name["outside"].detected
+    assert by_name["offlabel"].hits_vuln >= 1 and not by_name["offlabel"].detected
+
+
+def test_leave_one_out_forwards_search_degradations(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    from openultrasast.semantic.loo import evaluate_loo
+
+    monkeypatch.setenv("OPENULTRASAST_TREE_SITTER_PROBE", "0")
+    js = "function run(req) {\n  const cmd = req.query.cmd;\n  exec(cmd);\n}\n"
+    (tmp_path / "j-v.js").write_text(js)
+    (tmp_path / "j-f.js").write_text(js)
+    case = PairCase(
+        **{
+            **_toy(tmp_path)[0].__dict__,
+            "name": "j",
+            "language": "javascript",
+            "vuln_file": tmp_path / "j-v.js",
+            "fixed_file": tmp_path / "j-f.js",
+            "relpath": "app.js",
+        }
+    )
+    result = evaluate_loo([*_toy(tmp_path), case])
+    assert any(item["reason"] == "variants_language_unsupported" for item in result.degradations)
diff --git a/tests/test_mechanism_lever.py b/tests/test_mechanism_lever.py
index 9512f18..103ed7b 100644
--- a/tests/test_mechanism_lever.py
+++ b/tests/test_mechanism_lever.py
@@ -83,3 +83,30 @@ def test_apply_mechanism_edits_admits_into_the_scan_store_and_reverts_byte_for_b
     snapshot.restore()
     assert scan_store.read_bytes() == before  # byte-for-byte revert of the whole round
     assert b.id != a.id
+
+
+def test_retract_is_allowed_for_an_admitted_record_that_left_the_candidate_set(tmp_path: Path) -> None:
+    from openultrasast.improve.validator import EvolveValidator, MechanismEdit, StrictValidationError
+
+    scan = MechanismStore(tmp_path / "scan.jsonl")
+    stale = _record(scan, sink_name="popen")
+    validator = EvolveValidator()
+    with pytest.raises(StrictValidationError, match="unknown mechanism"):
+        validator.validate_mechanism(MechanismEdit(action="admit", mechanism_id=stale.id), {}, admitted={stale.id: stale})
+    validator.validate_mechanism(MechanismEdit(action="retract", mechanism_id=stale.id), {}, admitted={stale.id: stale})
+
+
+def test_export_defaults_to_a_candidates_file_and_never_writes_the_scan_store(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    import io
+    from contextlib import redirect_stdout
+
+    from openultrasast.cli import main
+
+    monkeypatch.chdir(tmp_path)
+    (tmp_path / "benchmarks").symlink_to(Path(__file__).resolve().parents[1] / "benchmarks")
+    buf = io.StringIO()
+    with redirect_stdout(buf):
+        assert main(["mechanisms", "export", "--slice", "vibe-py", "--json"]) == 0
+    payload = __import__("json").loads(buf.getvalue())
+    assert payload["store"].endswith("mechanism-candidates.jsonl")
+    assert not (tmp_path / ".openultrasast" / "calibration" / "mechanisms.jsonl").exists()
diff --git a/tests/test_mechanism_loop.py b/tests/test_mechanism_loop.py
index 3bddaae..56ef60e 100644
--- a/tests/test_mechanism_loop.py
+++ b/tests/test_mechanism_loop.py
@@ -157,5 +157,162 @@ def test_round_accepts_a_recovering_admission_and_rejects_a_leaking_one_byte_for
         mechanism_store=scan_store,
         scripted_mechanism_edits=[MechanismEdit(action="admit", mechanism_id=sys_record.id, source="export", rationale="scripted")],
     )
-    assert not forced.accepted and forced.reason.startswith("mechanism_leak") or forced.reason.startswith("profile_regression")
+    assert not forced.accepted and (forced.reason.startswith("mechanism_leak") or forced.reason.startswith("profile_regression"))
     assert scan_store.read_bytes() == before  # byte-for-byte revert
+
+
+def test_no_gain_admission_is_rejected_and_reverted_mechanism_edits_are_not_reproposed(tmp_path: Path) -> None:
+    from openultrasast.improve.evolve import run_round
+    from openultrasast.improve.journal import reverted_edit_keys
+    from openultrasast.improve.validator import MechanismEdit
+
+    env = _round_env(tmp_path)
+    candidates = MechanismStore(tmp_path / "candidates.jsonl")
+    unrelated = append_from_pair(
+        candidates, _shape("popen"), summary="s", cwe="CWE-78", pair="teacher-c", provenance="human", tier="seeded"
+    )
+    holdout = [_pair(tmp_path, "x", EXEC_VULN, EXEC_FIX, function="lookup", cwe="CWE-89")]
+    scan_store = tmp_path / "scan" / "mechanisms.jsonl"
+    ledger = tmp_path / "rule_policy.json"
+    journal = tmp_path / "journal.json"
+    kwargs: dict[str, object] = {
+        "ledger_path": ledger,
+        "journal_path": journal,
+        "ruleset_dir": env["ruleset_dir"],
+        "pair_cases": holdout,
+        "min_holdout_pairs": 1,
+        "mechanism_candidates": candidates.log_path,
+        "mechanism_store": scan_store,
+    }
+    edit = MechanismEdit(action="admit", mechanism_id=unrelated.id, source="export", rationale="scripted: recovers nothing")
+    outcome = run_round(env["target"], env["manifest"], scripted_mechanism_edits=[edit], **kwargs)  # type: ignore[arg-type]
+    assert not outcome.accepted and outcome.reason == "mechanism_no_gain"
+    assert not scan_store.exists() or scan_store.read_bytes() == b""
+    assert edit.key() in reverted_edit_keys(__import__("json").loads(journal.read_text()))
+    # the same reverted edit, offered again with a rationale, is filtered before validation (novelty gate)
+    again = run_round(env["target"], env["manifest"], scripted_mechanism_edits=[edit], **kwargs)  # type: ignore[arg-type]
+    assert again.reason == "no_proposals" and again.mechanism_edits == []
+
+
+def _holdout_catalog(tmp_path: Path) -> Path:
+    (tmp_path / "x-v.py").write_text(EXEC_VULN)
+    (tmp_path / "x-f.py").write_text(EXEC_FIX)
+    catalog = tmp_path / "pairs.toml"
+    catalog.write_text(
+        '[[pair]]\nname = "x"\nslice = "github"\nlanguage = "python"\nvuln = "x-v.py"\nfixed = "x-f.py"\nrelpath = "app.py"\n'
+        'split = "holdout"\nreview_tier = "reviewed"\nreviewer = "t"\n\n'
+        '[[pair.expected]]\ncwe = "CWE-89"\nclass = "sql injection"\npath = "app.py"\nfunction = "lookup"\n'
+        'sink = "execute"\nmechanism = "source_reaches_sink"\n'
+    )
+    return catalog
+
+
+def test_ousast_improve_admits_candidates_into_the_scan_store_by_default(tmp_path: Path) -> None:
+    """Req 5: the lever is reachable from the operator command; export -> candidates -> improve -> mechanisms.jsonl the scan reads."""
+    from openultrasast.cli import main
+
+    env = _round_env(tmp_path)
+    repo = Path(str(env["target"]))
+    calibration = repo / ".openultrasast" / "calibration"
+    candidates = MechanismStore(calibration / "mechanism-candidates.jsonl")
+    exec_record = append_from_pair(
+        candidates, _shape("execute"), summary="s", cwe="CWE-89", pair="teacher-a", provenance="human", tier="seeded"
+    )
+    catalog = _holdout_catalog(tmp_path)
+    manifest = tmp_path / "bench.toml"
+    assert (
+        main(
+            ["improve", str(manifest), "--ruleset-dir", str(env["ruleset_dir"]), "--pair-catalog", str(catalog), "--min-holdout-pairs", "1"]
+        )
+        == 0
+    )
+    admitted = MechanismStore(calibration / "mechanisms.jsonl").load()
+    assert [r.id for r in admitted] == [exec_record.id]
+    journal = json.loads((calibration / "improve_journal.json").read_text())
+    assert any(e["lever"] == "mechanisms" and e["mechanism_id"] == exec_record.id for entry in journal for e in entry["edits"])
+
+
+def test_run_improvement_threads_mechanism_paths(tmp_path: Path) -> None:
+    from openultrasast.improve.evolve import run_improvement
+    from openultrasast.pairs import load_pair_catalog
+
+    env = _round_env(tmp_path)
+    candidates = MechanismStore(tmp_path / "candidates.jsonl")
+    exec_record = append_from_pair(
+        candidates, _shape("execute"), summary="s", cwe="CWE-89", pair="teacher-a", provenance="human", tier="seeded"
+    )
+    cases = load_pair_catalog(_holdout_catalog(tmp_path))
+    scan_store = tmp_path / "scan" / "mechanisms.jsonl"
+    outcomes = run_improvement(
+        env["target"],
+        env["manifest"],
+        ledger_path=tmp_path / "ledger.json",
+        journal_path=tmp_path / "journal.json",  # type: ignore[arg-type]
+        ruleset_dir=env["ruleset_dir"],
+        pair_cases=cases,
+        min_holdout_pairs=1,  # type: ignore[arg-type]
+        mechanism_candidates=candidates.log_path,
+        mechanism_store=scan_store,
+        max_rounds=3,
+    )
+    assert any(o.accepted and o.mechanism_edits for o in outcomes)
+    assert [r.id for r in MechanismStore(scan_store).load()] == [exec_record.id]
+    assert outcomes[-1].reason == "no_proposals"  # converges once the candidate is admitted
+
+
+def _cli_env(tmp_path: Path) -> tuple[dict[str, object], Path, Path]:
+    env = _round_env(tmp_path)
+    calibration = Path(str(env["target"])) / ".openultrasast" / "calibration"
+    candidates = MechanismStore(calibration / "mechanism-candidates.jsonl")
+    append_from_pair(candidates, _shape("execute"), summary="s", cwe="CWE-89", pair="teacher-a", provenance="human", tier="seeded")
+    return env, calibration, _holdout_catalog(tmp_path)
+
+
+def test_dry_run_and_no_mechanisms_never_write_the_scan_store(tmp_path: Path, capsys) -> None:  # type: ignore[no-untyped-def]
+    from openultrasast.cli import main
+
+    env, calibration, catalog = _cli_env(tmp_path)
+    base = [
+        "improve",
+        str(tmp_path / "bench.toml"),
+        "--ruleset-dir",
+        str(env["ruleset_dir"]),
+        "--pair-catalog",
+        str(catalog),
+        "--min-holdout-pairs",
+        "1",
+    ]
+    assert main([*base, "--dry-run"]) == 0
+    assert not (calibration / "mechanisms.jsonl").exists()  # a dry run mutates nothing under the target
+    assert main([*base, "--no-mechanisms"]) == 0
+    assert not (calibration / "mechanisms.jsonl").exists()
+    (calibration / "mechanism-candidates.jsonl").unlink()
+    capsys.readouterr()  # the dry run legitimately reports its scratch admission; judge only the candidate-less round below
+    assert main(base) == 0
+    assert not (calibration / "mechanisms.jsonl").exists()  # no candidates file: the lever is inert, the round is a plain rule round
+    out = capsys.readouterr().out
+    assert "admit " not in out and "retract " not in out
+
+
+def test_documented_default_flow_connects_export_to_improve(tmp_path: Path, monkeypatch, capsys) -> None:  # type: ignore[no-untyped-def]
+    """README flow: `ousast mechanisms export` (cwd default) then `ousast improve --pair-catalog` admits into the target's store."""
+    from openultrasast.cli import main
+
+    env = _round_env(tmp_path)
+    catalog = _holdout_catalog(tmp_path)
+    monkeypatch.chdir(tmp_path)
+    # export a candidates file exactly as documented (cwd default), seeded from a tiny trusted catalog of our own
+    assert main(["mechanisms", "export", "--catalog", str(catalog), "--slice", "all"]) == 0
+    exported = tmp_path / ".openultrasast" / "calibration" / "mechanism-candidates.jsonl"
+    assert exported.is_file() and not (tmp_path / ".openultrasast" / "calibration" / "mechanisms.jsonl").exists()
+    # the target is a different directory (the benchmark fixture); improve must still find the cwd candidates by default
+    assert (
+        main(
+            ["improve", "bench.toml", "--ruleset-dir", str(env["ruleset_dir"]), "--pair-catalog", str(catalog), "--min-holdout-pairs", "1"]
+        )
+        == 0
+    )
+    target_store = Path(str(env["target"])) / ".openultrasast" / "calibration" / "mechanisms.jsonl"
+    assert MechanismStore(target_store).load()  # admitted where the scan of that target reads
+    out = capsys.readouterr().out
+    assert "admit corpus:" in out  # the accepted round names its mechanism edits