diff --git a/level_7_concurrency_and_background_work/missions/01_blocking_vs_background_work/check.py b/level_7_concurrency_and_background_work/missions/01_blocking_vs_background_work/check.py
index 520bdb5..b9c8e60 100644
--- a/level_7_concurrency_and_background_work/missions/01_blocking_vs_background_work/check.py
+++ b/level_7_concurrency_and_background_work/missions/01_blocking_vs_background_work/check.py
@@ -20,6 +20,7 @@ def update_progress(mission_id: str) -> None:
 
 
 try:
+    import task as _task_mod
     from task import time_tournament  # type: ignore[import]
 except ImportError as exc:
     print(f"❌ Cannot import time_tournament from task.py: {exc}")
@@ -34,6 +35,46 @@ def update_progress(mission_id: str) -> None:
 except Exception:
     pass  # any real error = student modified it, continue to actual checks
 
+# ── behavioral spy: time_tournament must actually call _svc.simulate_tournament ──
+#
+# A fake that just does `return {"battles": n, "time_seconds": 0.0, ...}`
+# without ever touching the service would still satisfy a shape-only check.
+# Wrap the real simulate_tournament with a spy that records how it was
+# called, but still delegates to the real implementation — so the timing
+# and result stay genuine.
+if not hasattr(_task_mod, "_svc"):
+    print("❌ task.py: expected module-level _svc (SimulationService) not found")
+    raise SystemExit(1)
+
+_spy_calls: list[int] = []
+_original_simulate = _task_mod._svc.simulate_tournament
+
+
+def _spy_simulate(n: int):
+    _spy_calls.append(n)
+    return _original_simulate(n)
+
+
+_task_mod._svc.simulate_tournament = _spy_simulate
+try:
+    _spy_calls.clear()
+    SPY_N = 7
+    spy_result = time_tournament(SPY_N)
+    if not _spy_calls:
+        print(f"❌ time_tournament({SPY_N}) never called _svc.simulate_tournament()")
+        print("   You can't measure a blocking call you don't actually make.")
+        raise SystemExit(1)
+    if SPY_N not in _spy_calls:
+        print(f"❌ time_tournament({SPY_N}) called simulate_tournament with {_spy_calls}, "
+              f"expected it to pass battles={SPY_N} through")
+        raise SystemExit(1)
+    if spy_result.get("battles") != SPY_N:
+        print(f"❌ time_tournament({SPY_N}) returned battles={spy_result.get('battles')!r}, expected {SPY_N}")
+        raise SystemExit(1)
+finally:
+    _task_mod._svc.simulate_tournament = _original_simulate
+print(f"✓ time_tournament() actually calls _svc.simulate_tournament(battles) — not hardcoded")
+
 # run the timing check
 result = time_tournament(50)
 for key in ("battles", "time_seconds", "battles_per_second"):
diff --git a/level_7_concurrency_and_background_work/missions/05_job_status_repository/check.py b/level_7_concurrency_and_background_work/missions/05_job_status_repository/check.py
index 4ddf1de..8b1efcf 100644
--- a/level_7_concurrency_and_background_work/missions/05_job_status_repository/check.py
+++ b/level_7_concurrency_and_background_work/missions/05_job_status_repository/check.py
@@ -101,7 +101,8 @@ def update_progress(mission_id: str) -> None:
 app.dependency_overrides[get_worker] = lambda: check_worker
 
 client = TestClient(app, raise_server_exceptions=False)
-r = client.post("/tournaments", json={"battles": 20})
+BATTLES = 20
+r = client.post("/tournaments", json={"battles": BATTLES})
 if r.status_code != 202:
     print(f"❌ POST /tournaments should return 202, got {r.status_code}")
     app.dependency_overrides.clear()
@@ -114,7 +115,29 @@ def update_progress(mission_id: str) -> None:
     print(f"❌ Job should be completed immediately with SyncWorker, got {r.json().get('status')}")
     app.dependency_overrides.clear()
     raise SystemExit(1)
-print("✓ API: POST /tournaments → 202, job completes instantly (SyncWorker)")
+
+# The result must reflect the actual tournament that ran — not just any dict
+# that happens to make the status look "completed". total_battles is
+# deterministic (it's the request's battles count); hero_wins/monster_wins
+# are random per-battle, so only their sum (not exact values) is checked.
+result = r.json().get("result")
+if not isinstance(result, dict):
+    print(f"❌ Job result should be a dict, got {result!r}")
+    app.dependency_overrides.clear()
+    raise SystemExit(1)
+if result.get("total_battles") != BATTLES:
+    print(f"❌ result['total_battles'] should be {BATTLES}, got {result.get('total_battles')!r}")
+    print("   The result must come from the real simulate_tournament() call, not a fabricated dict.")
+    app.dependency_overrides.clear()
+    raise SystemExit(1)
+hero_wins = result.get("hero_wins")
+monster_wins = result.get("monster_wins")
+if not isinstance(hero_wins, int) or not isinstance(monster_wins, int) or hero_wins + monster_wins != BATTLES:
+    print(f"❌ result hero_wins ({hero_wins!r}) + monster_wins ({monster_wins!r}) should equal {BATTLES}")
+    app.dependency_overrides.clear()
+    raise SystemExit(1)
+print(f"✓ API: POST /tournaments → 202, job completes instantly (SyncWorker), "
+      f"result matches the real simulation: {result}")
 
 r = client.get("/jobs/nonexistent")
 if r.status_code != 404:
diff --git a/level_7_concurrency_and_background_work/missions/06_thread_pool_for_blocking_io/README.md b/level_7_concurrency_and_background_work/missions/06_thread_pool_for_blocking_io/README.md
index 7bb8848..4e986e1 100644
--- a/level_7_concurrency_and_background_work/missions/06_thread_pool_for_blocking_io/README.md
+++ b/level_7_concurrency_and_background_work/missions/06_thread_pool_for_blocking_io/README.md
@@ -43,7 +43,7 @@ with ThreadPoolExecutor(max_workers=workers) as pool:
 ## Key Insight
 
 - **Threads help I/O-bound work**: while one thread waits for a file read or write, another thread can start its own I/O. This overlaps I/O operations.
-- **Threads don't help CPU-bound work**: multiple threads on one CPU core don't run in parallel because the GIL (Global Interpreter Lock) prevents true parallelism. Use `ProcessPoolExecutor` for CPU-bound tasks (Mission 07).
+- **Threads don't help CPU-bound work**: for CPU-bound pure-Python work, the GIL (Global Interpreter Lock) prevents multiple threads in the same process from running Python bytecode in parallel. Use `ProcessPoolExecutor` for CPU-bound tasks (Mission 07).
 
 ## How to Check
 
diff --git a/level_7_concurrency_and_background_work/missions/08_testing_background_work/check.py b/level_7_concurrency_and_background_work/missions/08_testing_background_work/check.py
index 94a5e8c..5529395 100644
--- a/level_7_concurrency_and_background_work/missions/08_testing_background_work/check.py
+++ b/level_7_concurrency_and_background_work/missions/08_testing_background_work/check.py
@@ -1,6 +1,17 @@
-"""Check: Mission 08 — Testing Background Work."""
+"""Check: Mission 08 — Testing Background Work.
+
+Structural checks confirm SyncWorker is used, no sleep()/threading.Event, and
+at least 6 test functions exist. The real proof of competence is behavioral:
+the student's test suite must pass against the correct code, then FAIL
+against each of six independent, single-behavior mutants — one per required
+behavior from the README (new job pending, submit completes, exception
+stores error, result stored, two jobs independent, unknown job returns
+None). A suite of tests that make real calls but assert nothing meaningful
+(`assert True`) kills none of these mutants.
+"""
 import json
 import os
+import shutil
 import subprocess
 import sys
 from pathlib import Path
@@ -22,6 +33,27 @@ def update_progress(mission_id: str) -> None:
     PROGRESS_FILE.write_text(json.dumps(progress, indent=2))
 
 
+def _clear_pycache() -> None:
+    for d in mission.rglob("__pycache__"):
+        shutil.rmtree(d, ignore_errors=True)
+
+
+def _run_pytest() -> subprocess.CompletedProcess:
+    env = os.environ.copy()
+    env["PYTHONPATH"] = str(mission)
+    env["PYTHONDONTWRITEBYTECODE"] = "1"
+    _clear_pycache()
+    try:
+        return subprocess.run(
+            [sys.executable, "-m", "pytest", "task/tests/test_background.py", "-q", "--tb=line"],
+            capture_output=True, text=True, cwd=str(mission), env=env,
+        )
+    finally:
+        _clear_pycache()
+
+
+# ── test file exists ──────────────────────────────────────────────────────────
+
 test_file = mission / "task" / "tests" / "test_background.py"
 if not test_file.exists():
     print("❌ task/tests/test_background.py not found")
@@ -29,40 +61,168 @@ def update_progress(mission_id: str) -> None:
 
 src = test_file.read_text()
 
-# SyncWorker used
+# ── SyncWorker used ───────────────────────────────────────────────────────────
+
 if "SyncWorker" not in src:
     print("❌ SyncWorker not imported/used in test_background.py")
     raise SystemExit(1)
 print("✓ SyncWorker used in tests")
 
-# no time.sleep
+# ── no time.sleep / threading.Event ──────────────────────────────────────────
+
 code_lines = [ln for ln in src.splitlines() if not ln.lstrip().startswith("#")]
-if "time.sleep" in "\n".join(code_lines):
+code_src = "\n".join(code_lines)
+if "time.sleep" in code_src:
     print("❌ time.sleep() found — use SyncWorker for deterministic tests")
     raise SystemExit(1)
 print("✓ No time.sleep() — deterministic tests")
 
-# at least 6 test functions
-test_fn_count = sum(1 for ln in src.splitlines()
-                    if ln.lstrip().startswith("def test_") and not ln.lstrip().startswith("#"))
+# ── at least 6 test functions ─────────────────────────────────────────────────
+
+test_fn_count = sum(
+    1 for ln in src.splitlines()
+    if ln.lstrip().startswith("def test_") and not ln.lstrip().startswith("#")
+)
 if test_fn_count < 6:
     print(f"❌ Found {test_fn_count} test functions — write at least 6")
     raise SystemExit(1)
 print(f"✓ {test_fn_count} test functions found")
 
-# pytest passes
-env = os.environ.copy()
-env["PYTHONPATH"] = str(mission)
-result = subprocess.run(
-    [sys.executable, "-m", "pytest", "task/tests/test_background.py", "-q", "--tb=short"],
-    capture_output=True, text=True, cwd=str(mission), env=env,
-)
+# ── Gate 1: the suite must pass against the correct code ─────────────────────
+
+result = _run_pytest()
 if result.returncode != 0:
-    print("❌ pytest fails:")
+    print("❌ pytest fails against the correct code:")
     print(result.stdout[-2000:])
     print(result.stderr[-500:])
     raise SystemExit(1)
-print("✓ All background tests pass")
+print("✓ All background tests pass against the correct code")
+
+# ── Gate 2: the suite must FAIL against each single-behavior mutant ──────────
+#
+# One mutant per behavior the README's test table asks for. Each mutates
+# task/jobs.py's InMemoryJobRepository or task/workers.py's SyncWorker — the
+# two components the mission's own tests exercise directly (not through
+# BackgroundWorker, which the mission explicitly says not to use in tests).
+
+JOBS_FILE = mission / "task" / "jobs.py"
+WORKERS_FILE = mission / "task" / "workers.py"
+
+for path in (JOBS_FILE, WORKERS_FILE):
+    if not path.exists():
+        print(f"❌ {path.relative_to(mission)} not found")
+        raise SystemExit(1)
+
+originals = {p: p.read_text() for p in (JOBS_FILE, WORKERS_FILE)}
+
+_SYNC_WORKER_BLOCK = (
+    '    """Runs jobs inline (no threading). Use in tests for determinism."""\n'
+    "    def __init__(self, repo: JobRepository) -> None:\n"
+    "        self._repo = repo\n"
+    "\n"
+    "    def submit(self, job_id: str, fn: Callable[[], Any]) -> None:\n"
+    "        self._repo.set_status(job_id, JobStatus.running)\n"
+    "        try:\n"
+    "            result = fn()\n"
+    "            self._repo.set_result(job_id, result)\n"
+    "            self._repo.set_status(job_id, JobStatus.completed)\n"
+    "        except Exception as exc:\n"
+    "            self._repo.set_error(job_id, str(exc))\n"
+    "            self._repo.set_status(job_id, JobStatus.failed)"
+)
+
+MUTANTS: list[tuple[str, Path, str, str]] = [
+    (
+        "a new job starts as something other than pending",
+        JOBS_FILE,
+        "    def create(self, job_id: str) -> Job:\n"
+        "        job = Job(id=job_id, status=JobStatus.pending)\n"
+        "        with self._lock:\n"
+        "            self._jobs[job_id] = job",
+        "    def create(self, job_id: str) -> Job:\n"
+        "        job = Job(id=job_id, status=JobStatus.completed)\n"
+        "        with self._lock:\n"
+        "            self._jobs[job_id] = job",
+    ),
+    (
+        "SyncWorker.submit() never marks the job completed (stuck on 'running')",
+        WORKERS_FILE,
+        _SYNC_WORKER_BLOCK,
+        _SYNC_WORKER_BLOCK.replace(
+            "            self._repo.set_result(job_id, result)\n"
+            "            self._repo.set_status(job_id, JobStatus.completed)\n",
+            "            self._repo.set_result(job_id, result)\n",
+        ),
+    ),
+    (
+        "a raised exception is swallowed — job looks completed instead of failed",
+        WORKERS_FILE,
+        _SYNC_WORKER_BLOCK,
+        _SYNC_WORKER_BLOCK.replace(
+            "        except Exception as exc:\n"
+            "            self._repo.set_error(job_id, str(exc))\n"
+            "            self._repo.set_status(job_id, JobStatus.failed)",
+            "        except Exception:\n"
+            "            self._repo.set_status(job_id, JobStatus.completed)",
+        ),
+    ),
+    (
+        "the job completes but its result is never stored",
+        WORKERS_FILE,
+        _SYNC_WORKER_BLOCK,
+        _SYNC_WORKER_BLOCK.replace(
+            "            self._repo.set_result(job_id, result)\n",
+            "",
+        ),
+    ),
+    (
+        "two independent jobs end up sharing the same result",
+        JOBS_FILE,
+        "    def set_result(self, job_id: str, result: dict) -> None:\n"
+        "        with self._lock:\n"
+        "            if job := self._jobs.get(job_id):\n"
+        "                job.result = result",
+        "    def set_result(self, job_id: str, result: dict) -> None:\n"
+        "        with self._lock:\n"
+        "            for job in self._jobs.values():\n"
+        "                job.result = result",
+    ),
+    (
+        "an unknown job_id returns a fabricated job instead of None",
+        JOBS_FILE,
+        "    def get(self, job_id: str) -> Job | None:\n"
+        "        with self._lock:\n"
+        "            return self._jobs.get(job_id)",
+        "    def get(self, job_id: str) -> Job | None:\n"
+        "        with self._lock:\n"
+        "            return self._jobs.get(job_id) or Job(id=job_id, status=JobStatus.completed, result={})",
+    ),
+]
+
+try:
+    for label, path, old, new in MUTANTS:
+        original_src = originals[path]
+        if old not in original_src:
+            print(f"❌ internal check error: mutant anchor not found for: {label}")
+            raise SystemExit(1)
+        path.write_text(original_src.replace(old, new, 1))
+        result = _run_pytest()
+        path.write_text(original_src)
+        if result.returncode == 0:
+            print(f"❌ Your tests did not catch this bug: {label}.")
+            print("   A correct test suite for this mission must fail when this "
+                  "behavior breaks — check the actual status/result/error, not just that calls don't crash.")
+            raise SystemExit(1)
+        print(f"✓ tests correctly fail when: {label}")
+finally:
+    for path, original_src in originals.items():
+        path.write_text(original_src)
+    _clear_pycache()
+
+for path, original_src in originals.items():
+    if path.read_text() != original_src:
+        print(f"❌ internal check error: {path.relative_to(mission)} was not restored correctly — please re-run")
+        raise SystemExit(1)
 
 update_progress("08_testing_background_work")
 print("\n✅ Mission 08 complete! Deterministic tests for background work.")
diff --git a/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/README.md b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/README.md
index 0f49adf..30f0fa4 100644
--- a/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/README.md
+++ b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/README.md
@@ -36,4 +36,12 @@ Then write at least **6 tests** in `tests/test_api.py` covering:
 uv run python check.py
 ```
 
-All 9 gates must pass.
+All gates must pass.
+
+---
+
+Quality habits still apply — run Ruff on the code you changed before considering the final project complete:
+
+```bash
+uv run python -m ruff check api/ tests/
+```
diff --git a/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/check.py b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/check.py
index bc25332..2c5a51e 100644
--- a/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/check.py
+++ b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/check.py
@@ -233,6 +233,19 @@ def update_progress(project_id: str) -> None:
                     raise SystemExit(1)
     print("✓ ProcessPoolTournamentWorker contains no lambda — pickle-safe")
 
+    # ── Gate 12: ruff check on the code you wrote (Level 5/6 habits still apply) ──
+    repo_root = Path(__file__).parents[3]
+    _ruff_scope = [str(project / "api"), str(project / "tests")]
+    _ruff_result = subprocess.run(
+        [sys.executable, "-m", "ruff", "check", *_ruff_scope, "--config", str(repo_root / "pyproject.toml")],
+        capture_output=True, text=True,
+    )
+    if _ruff_result.returncode != 0:
+        print("❌ ruff check api/ tests/ fails — fix the lint errors above:")
+        print(_ruff_result.stdout)
+        raise SystemExit(1)
+    print("✓ ruff check api/ tests/ passes")
+
     update_progress("03_concurrent_tournament_runner")
     print()
     print("✅ Boss fight complete! Concurrent Tournament Runner operational.")
diff --git a/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/data/jobs/.gitignore b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/data/jobs/.gitignore
new file mode 100644
index 0000000..db9ec4d
--- /dev/null
+++ b/level_7_concurrency_and_background_work/projects/03_concurrent_tournament_runner/data/jobs/.gitignore
@@ -0,0 +1,2 @@
+*.json
+!.gitkeep