diff --git a/backend/tests/test_integration_v2.py b/backend/tests/test_integration_v2.py
index fd2fef2..4c47b4c 100644
--- a/backend/tests/test_integration_v2.py
+++ b/backend/tests/test_integration_v2.py
@@ -115,7 +115,12 @@ for _mod in [
     "passlib", "passlib.context",
 ]:
     if _mod not in sys.modules:
-        sys.modules[_mod] = ModuleType(_mod)
+        m = ModuleType(_mod)
+        if _mod == "boto3":
+            m.client = MagicMock()
+        elif _mod == "botocore.exceptions":
+            m.ClientError = Exception
+        sys.modules[_mod] = m
 
 # Now safe to import
 from app.models.enums import TestState, TestResult, TechniqueStatus
diff --git a/backend/tests/test_t106_workflow_service.py b/backend/tests/test_t106_workflow_service.py
index 9d32cd2..62ebdbe 100644
--- a/backend/tests/test_t106_workflow_service.py
+++ b/backend/tests/test_t106_workflow_service.py
@@ -1,139 +1,28 @@
 """Validation tests for T-106: Test Workflow Service.
 
-Uses mock objects to avoid needing a running database.
-The database module is stubbed before any app imports.
+Mocked DB sessions exercise the workflow service without PostgreSQL.
+Uses the same import stack as the rest of the suite (``tests.conftest``) so
+this module must **not** replace entries in ``sys.modules`` — doing so broke
+JWT verification in API tests by splitting :mod:`app.auth` (loaded early via
+conftest) from :mod:`app.routers.auth` / :mod:`app.dependencies.auth`.
 """
 
-import sys
-import os
 import uuid
 from unittest.mock import MagicMock, patch
-from types import ModuleType
-from datetime import datetime
-
-# ---------------------------------------------------------------------------
-# 0. Stub heavy dependencies BEFORE importing any app modules
-# ---------------------------------------------------------------------------
-
-# Ensure backend/ is on sys.path
-backend_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), ".."))
-if backend_dir not in sys.path:
-    sys.path.insert(0, backend_dir)
-
-# Stub pydantic_settings so config doesn't fail
-if "pydantic_settings" not in sys.modules:
-    pydantic_settings_mock = ModuleType("pydantic_settings")
-
-    class _BaseSettings:
-        def __init__(self, **kwargs):
-            pass
-
-        def __init_subclass__(cls, **kwargs):
-            super().__init_subclass__(**kwargs)
-
-    pydantic_settings_mock.BaseSettings = _BaseSettings
-    sys.modules["pydantic_settings"] = pydantic_settings_mock
-
-# Stub app.config
-config_mod = ModuleType("app.config")
-
-
-class _FakeSettings:
-    DATABASE_URL = "sqlite:///:memory:"
-    SECRET_KEY = "test"
-    ALGORITHM = "HS256"
-    ACCESS_TOKEN_EXPIRE_MINUTES = 60
-    REDIS_URL = "redis://localhost:6379/0"
-    MINIO_ENDPOINT = "localhost:9000"
-    MINIO_ACCESS_KEY = "test"
-    MINIO_SECRET_KEY = "test"
-    MINIO_BUCKET = "test"
-    MINIO_SECURE = False
-    MAX_RETEST_COUNT = 3
-    CORS_ORIGINS = "http://localhost:3000"
-    SCORING_WEIGHT_TESTS = 40
-    SCORING_WEIGHT_DETECTION_RULES = 20
-    SCORING_WEIGHT_D3FEND = 15
-    SCORING_WEIGHT_FRESHNESS = 15
-    SCORING_WEIGHT_PLATFORM_DIVERSITY = 10
-    REPORT_TEMPLATES_DIR = "app/templates/reports"
-    REPORT_OUTPUT_DIR = "/tmp/aegis_reports"
-    COMPANY_NAME = "Test Org"
-    COMPANY_LOGO_PATH = "app/templates/reports/assets/logo.png"
-    JIRA_ENABLED = False
-    JIRA_URL = ""
-    JIRA_USERNAME = ""
-    JIRA_API_TOKEN = ""
-    JIRA_IS_CLOUD = True
-    JIRA_DEFAULT_PROJECT = ""
-    JIRA_ISSUE_TYPE_TEST = "Task"
-    JIRA_ISSUE_TYPE_CAMPAIGN = "Epic"
-    TEMPO_ENABLED = False
-    TEMPO_API_TOKEN = ""
-    TEMPO_DEFAULT_WORK_TYPE = "Red Team"
-    NVD_API_KEY = ""
-    STALE_THRESHOLD_DAYS = 365
-
-
-config_mod.settings = _FakeSettings()
-sys.modules["app.config"] = config_mod
-
-# Stub app.database so no real engine is created
-db_mod = ModuleType("app.database")
-db_mod.Base = type("Base", (), {"metadata": MagicMock()})
-db_mod.get_db = MagicMock()
-sys.modules["app.database"] = db_mod
-
-# Stub taxii2client
-taxii_v20 = ModuleType("taxii2client.v20")
-taxii_v20.Server = MagicMock
-sys.modules["taxii2client"] = ModuleType("taxii2client")
-sys.modules["taxii2client.v20"] = taxii_v20
-
-# Stub jose
-jose_mod = ModuleType("jose")
-jose_mod.JWTError = Exception
-jose_mod.jwt = MagicMock()
-sys.modules["jose"] = jose_mod
-
-# Stub boto3
-boto3_mod = ModuleType("boto3")
-boto3_mod.client = MagicMock()
-sys.modules["boto3"] = boto3_mod
-sys.modules["botocore"] = ModuleType("botocore")
-sys.modules["botocore.exceptions"] = ModuleType("botocore.exceptions")
-sys.modules["botocore.exceptions"].ClientError = Exception
-
-# Stub apscheduler
-sys.modules["apscheduler"] = ModuleType("apscheduler")
-sys.modules["apscheduler.schedulers"] = ModuleType("apscheduler.schedulers")
-sys.modules["apscheduler.schedulers.background"] = ModuleType("apscheduler.schedulers.background")
-sys.modules["apscheduler.schedulers.background"].BackgroundScheduler = MagicMock
-sys.modules["apscheduler.triggers"] = ModuleType("apscheduler.triggers")
-sys.modules["apscheduler.triggers.cron"] = ModuleType("apscheduler.triggers.cron")
-sys.modules["apscheduler.triggers.cron"].CronTrigger = MagicMock
-
-# ---------------------------------------------------------------------------
-# Now we can safely import
-# ---------------------------------------------------------------------------
 
+from app.domain.exceptions import InvalidTransitionError
 from app.models.enums import TestState
 from app.services.test_workflow_service import (
-    VALID_TRANSITIONS,
     can_transition,
-    transition_state,
-    start_execution,
-    submit_red_evidence,
-    submit_blue_evidence,
-    validate_as_red_lead,
-    validate_as_blue_lead,
-    check_dual_validation,
     reopen_test,
+    start_execution,
+    submit_blue_evidence,
+    submit_red_evidence,
+    transition_state,
+    validate_as_blue_lead,
+    validate_as_red_lead,
 )
 
-# We need the domain exceptions for assertions
-from app.domain.exceptions import InvalidTransitionError
-
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
diff --git a/backend/tests/test_tests.py b/backend/tests/test_tests.py
index 84ad8c5..bd669c8 100644
--- a/backend/tests/test_tests.py
+++ b/backend/tests/test_tests.py
@@ -75,3 +75,45 @@ def test_get_test_by_id(client, auth_headers, technique):
     response = client.get(f"/api/v1/tests/{test_id}", headers=auth_headers)
     assert response.status_code == 200
     assert response.json()["id"] == test_id
+
+
+def test_start_execution_twice_returns_invalid_transition(
+    client, auth_headers, technique, red_tech_user
+):
+    """Invalid workflow transition surfaces domain error JSON (FASE 0.4).
+
+    HttpOnly login cookies take precedence over the Authorization header.
+    Clear cookies before each phase so Bearer tokens match the intended user.
+    """
+    client.cookies.clear()
+    create_response = client.post(
+        "/api/v1/tests",
+        json={"technique_id": technique["id"], "name": "Workflow dup start"},
+        headers=auth_headers,
+    )
+    assert create_response.status_code == 201
+    test_id = create_response.json()["id"]
+
+    rl = client.post(
+        "/api/v1/auth/login",
+        data={"username": "redtech", "password": "redtech123"},
+    )
+    assert rl.status_code == 200
+    red_headers = {"Authorization": f"Bearer {rl.json()['access_token']}"}
+    client.cookies.clear()
+
+    first = client.post(
+        f"/api/v1/tests/{test_id}/start-execution",
+        headers=red_headers,
+    )
+    assert first.status_code == 200
+
+    client.cookies.clear()
+    second = client.post(
+        f"/api/v1/tests/{test_id}/start-execution",
+        headers=red_headers,
+    )
+    assert second.status_code == 400
+    body = second.json()
+    assert body.get("code") == "INVALID_TRANSITION"
+    assert "detail" in body