diff --git a/API/Classes/Case/DataFileClass.py b/API/Classes/Case/DataFileClass.py
index 74ea88e87..5ba1cf6fe 100644
--- a/API/Classes/Case/DataFileClass.py
+++ b/API/Classes/Case/DataFileClass.py
@@ -724,6 +724,8 @@ def generateDatafile( self, caserunname ):
 
             dataFilePath = Path(Config.DATA_STORAGE, self.case, 'res',caserunname,'data.txt')
 
+            # Ensure case run directory exists (may be missing after restore)
+            os.makedirs(dataFilePath.parent, exist_ok=True)
 
             # self.f = open(self.dataFile, mode="w", encoding='utf-8')
             #self.f = open(dataFilePath, mode="w", encoding='utf-8')
diff --git a/API/Routes/DataFile/DataFileRoute.py b/API/Routes/DataFile/DataFileRoute.py
index 33709c092..d8ad6e15f 100644
--- a/API/Routes/DataFile/DataFileRoute.py
+++ b/API/Routes/DataFile/DataFileRoute.py
@@ -22,6 +22,10 @@ def generateDataFile():
         return jsonify(response), 200
     except(IOError):
         return jsonify('No existing cases!'), 404
+    except OSError as e:
+        return jsonify({"message": str(e), "status_code": "error"}), 500
+    except Exception as e:
+        return jsonify({"message": str(e), "status_code": "error"}), 500
 
 @datafile_api.route("/createCaseRun", methods=['POST'])
 def createCaseRun():
diff --git a/API/tests/__init__.py b/API/tests/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/API/tests/conftest.py b/API/tests/conftest.py
new file mode 100644
index 000000000..841ae97f4
--- /dev/null
+++ b/API/tests/conftest.py
@@ -0,0 +1,288 @@
+"""
+Shared pytest fixtures for MUIO backend tests.
+
+All fixtures redirect Config.DATA_STORAGE to tmp_path,
+so no real filesystem is touched.
+"""
+import sys
+import os
+import json
+import pytest
+from pathlib import Path
+
+# ── Ensure API/ is on sys.path so 'from Classes.Base import Config' works ──
+API_DIR = Path(__file__).resolve().parent.parent
+if str(API_DIR) not in sys.path:
+    sys.path.insert(0, str(API_DIR))
+
+
+# ---------------------------------------------------------------------------
+#  Constants used across fixtures
+# ---------------------------------------------------------------------------
+SC = "SC1"            # scenario id
+YEAR = "2020"
+TECH = "T1"
+COMM = "C1"
+EMIS = "E1"
+STG = "S1"
+TS = "TS1"
+CON = "CN1"
+MOD = 1
+SE = "SE1"
+DT = "DT1"
+DTB = "DTB1"
+
+
+# ---------------------------------------------------------------------------
+#  Minimal fixture data generators
+# ---------------------------------------------------------------------------
+
+def _minimal_parameters():
+    """Minimal Parameters.json — one param per group, matches real schema."""
+    def _p(pid, val, default=0):
+        return {"id": pid, "value": val, "default": default,
+                "enable": True, "menu": 1, "unitRule": {"cat": []}}
+    return {
+        "R":     [_p("DR",   "Discount Rate",          0.05)],
+        "RT":    [_p("DRI",  "Discount Rate Idv",      0.05)],
+        "RE":    [_p("MPEL", "Model Period Emission Limit", 999999)],
+        "RY":    [_p("DR2",  "Discount Rate",          0.05)],
+        "RS":    [_p("OLS",  "Operational Life Storage",1)],
+        "RYCn":  [_p("UCC",  "UDC Constant",           0)],
+        "RYTCn": [_p("CCM",  "UDC Multiplier Total Capacity", 0)],
+        "RYTs":  [_p("YS",   "Year Split",             1)],
+        "RYDtb": [_p("DS",   "Day Split",              1)],
+        "RYSeDt":[_p("DIDT", "Days In Day Type",       1)],
+        "RYT":   [_p("AF",   "Availability Factor",    1)],
+        "RYTM":  [_p("VC",   "Variable Cost",          0.0001)],
+        "RYTC":  [_p("INCR", "Input To New Capacity Ratio", 0)],
+        "RYTCM": [_p("IAR",  "Input Activity Ratio",   0)],
+        "RTSM":  [_p("TTS",  "Technology To Storage",   0)],
+        "RYTTs": [_p("CF",   "Capacity Factor",        1)],
+        "RYC":   [_p("AAD",  "Accumulated Annual Demand",0)],
+        "RYCTs": [_p("SDP",  "Specified Demand Profile",0)],
+        "RYE":   [_p("AEL",  "Annual Emission Limit",  999999)],
+        "RYTEM": [_p("EAR",  "Emission Activity Ratio",0)],
+    }
+
+
+def _minimal_variables():
+    """Minimal Variables.json."""
+    return {
+        "R": [{"id": "OV", "value": "Objective Value",
+               "name": "ObjectiveValue", "unitRule": {"cat": []}}],
+    }
+
+
+def _minimal_gen_data():
+    """Minimal genData.json with one entry per set."""
+    return {
+        "osy-years": [YEAR],
+        "osy-mo": "1",
+        "osy-tech": [{
+            "TechId": TECH, "Tech": "TECH1",
+            "IAR": [COMM], "OAR": [COMM],
+            "EAR": [EMIS],
+            "INCR": [COMM], "ITCR": [],
+            "EACR": [],
+        }],
+        "osy-comm": [{"CommId": COMM, "Comm": "COMM1"}],
+        "osy-emis": [{"EmisId": EMIS, "Emis": "EMIS1"}],
+        "osy-stg":  [{"StgId": STG, "Stg": "STG1",
+                      "TTS": TECH, "TFS": TECH, "Operation": "Yearly"}],
+        "osy-ts":   [{"TsId": TS, "Ts": "SLICE1",
+                      "SE": SE, "DT": DT, "DTB": DTB}],
+        "osy-se":   [{"SeId": SE, "Se": "SEASON1"}],
+        "osy-dt":   [{"DtId": DT, "Dt": "DAYTYPE1"}],
+        "osy-dtb":  [{"DtbId": DTB, "Dtb": "BRACKET1"}],
+        "osy-scenarios": [{"ScenarioId": SC, "Scenario": "Base", "Active": True}],
+        "osy-constraints": [{"ConId": CON, "Con": "CON1", "Tag": 1, "CM": [TECH]}],
+    }
+
+
+def _minimal_res_data():
+    """Minimal resData.json with one case run referencing our scenario."""
+    return {
+        "osy-cases": [{
+            "Case": "run1",
+            "Scenarios": [
+                {"ScenarioId": SC, "Scenario": "Base", "Active": True}
+            ]
+        }]
+    }
+
+
+# ── Per-group JSON data, matching the exact schema each parser expects ──
+
+def _build_param_json_files():
+    """
+    Return a dict  {filename: data}  for every group-level JSON file.
+
+    Each parser in OsemosysClass expects a specific nested structure:
+      R.json     → { paramId: { sc: [ {value: X} ] } }
+      RY.json    → { paramId: { sc: [ {year: val} ] } }
+      RT.json    → { paramId: { sc: [ {techId: val} ] } }
+      RE.json    → { paramId: { sc: [ {emisId: val} ] } }
+      RS.json    → { paramId: { sc: [ {stgId: val} ] } }
+      RYCn.json  → { paramId: { sc: [ {ConId: conId, year: val} ] } }
+      RYTs.json  → { paramId: { sc: [ {TsId: tsId, year: val} ] } }
+      RYDtb.json → { paramId: { sc: [ {DtbId: dtbId, year: val} ] } }
+      RYSeDt.json→ { paramId: { sc: [ {SeId:.., DtId:.., year: val} ] } }
+      RYT.json   → { paramId: { sc: [ {TechId:.., year: val} ] } }
+      RYS.json   → { paramId: { sc: [ {StgId:.., year: val} ] } }
+      RYTCn.json → { paramId: { sc: [ {TechId:.., ConId:.., year: val} ] } }
+      RYTM.json  → { paramId: { sc: [ {TechId:.., MoId:.., year: val} ] } }
+      RYC.json   → { paramId: { sc: [ {CommId:.., year: val} ] } }
+      RYE.json   → { paramId: { sc: [ {EmisId:.., year: val} ] } }
+      RYTC.json  → { paramId: { sc: [ {TechId:.., CommId:.., year: val} ] } }
+      RYTCM.json → { paramId: { sc: [ {TechId:.., CommId:.., MoId:.., year: val} ] } }
+      RTSM.json  → { paramId: { sc: [ {StgId:.., TechId:.., MoId:.., value: val} ] } }
+      RYTSM.json → { paramId: { sc: [ {StgId:.., TechId:.., MoId:.., year: val} ] } }
+      RYTE.json  → { paramId: { sc: [ {TechId:.., EmisId:.., year: val} ] } }
+      RYTEM.json → { paramId: { sc: [ {TechId:.., EmisId:.., MoId:.., year: val} ] } }
+      RYTTs.json → { paramId: { sc: [ {TechId:.., TsId:.., year: val} ] } }
+      RYCTs.json → { paramId: { sc: [ {CommId:.., TsId:.., year: val} ] } }
+    """
+    y = str(YEAR)  # JSON keys for years are stringified in many parsers
+    files = {}
+
+    # R: {paramId: {sc: [{value: X}]}}
+    files["R.json"] = {"DR": {SC: [{"value": 0.05}]}}
+
+    # RY: {paramId: {sc: [{year: val}]}}
+    files["RY.json"] = {"DR2": {SC: [{y: 0.05}]}}
+
+    # RT: {paramId: {sc: [{techId: val}]}}
+    files["RT.json"] = {"DRI": {SC: [{TECH: 0.05}]}}
+
+    # RE: {paramId: {sc: [{emisId: val}]}}
+    files["RE.json"] = {"MPEL": {SC: [{EMIS: 999999}]}}
+
+    # RS: {paramId: {sc: [{stgId: val}]}}
+    files["RS.json"] = {"OLS": {SC: [{STG: 1}]}}
+
+    # RYCn: {ConId + year entries}
+    files["RYCn.json"] = {"UCC": {SC: [{"ConId": CON, y: 0}]}}
+
+    # RYTs: {TsId + year entries}
+    files["RYTs.json"] = {"YS": {SC: [{"TsId": TS, y: 1}]}}
+
+    # RYDtb: {DtbId + year entries}
+    files["RYDtb.json"] = {"DS": {SC: [{"DtbId": DTB, y: 1}]}}
+
+    # RYSeDt: {SeId + DtId + year entries}
+    files["RYSeDt.json"] = {"DIDT": {SC: [{"SeId": SE, "DtId": DT, y: 1}]}}
+
+    # RYT: {TechId + year entries}
+    files["RYT.json"] = {"AF": {SC: [{"TechId": TECH, y: 1}]}}
+
+    # RYS: {StgId + year entries}
+    files["RYS.json"] = {"OLS": {SC: [{"StgId": STG, y: 1}]}}
+
+    # RYTCn: {TechId + ConId + year entries}
+    files["RYTCn.json"] = {"CCM": {SC: [{"TechId": TECH, "ConId": CON, y: 0}]}}
+
+    # RYTM: {TechId + MoId + year entries}
+    files["RYTM.json"] = {"VC": {SC: [{"TechId": TECH, "MoId": MOD, y: 0.0001}]}}
+
+    # RYC: {CommId + year entries}
+    files["RYC.json"] = {"AAD": {SC: [{"CommId": COMM, y: 0}]}}
+
+    # RYE: {EmisId + year entries}
+    files["RYE.json"] = {"AEL": {SC: [{"EmisId": EMIS, y: 999999}]}}
+
+    # RYTC: {TechId + CommId + year entries}
+    files["RYTC.json"] = {"INCR": {SC: [{"TechId": TECH, "CommId": COMM, y: 0}]}}
+
+    # RYTCM: {TechId + CommId + MoId + year entries}
+    files["RYTCM.json"] = {"IAR": {SC: [{"TechId": TECH, "CommId": COMM, "MoId": MOD, y: 0}]}}
+
+    # RTSM: {StgId + TechId + MoId + value}  (no year dimension — directly value)
+    files["RTSM.json"] = {"TTS": {SC: [{"StgId": STG, "TechId": TECH, "MoId": MOD, "value": 0}]}}
+
+    # RYTSM: {StgId + TechId + MoId + year entries}
+    files["RYTSM.json"] = {"TTS": {SC: [{"StgId": STG, "TechId": TECH, "MoId": MOD, y: 0}]}}
+
+    # RYTE: {TechId + EmisId + year entries}
+    files["RYTE.json"] = {"EAR": {SC: [{"TechId": TECH, "EmisId": EMIS, y: 0}]}}
+
+    # RYTEM: {TechId + EmisId + MoId + year entries}
+    files["RYTEM.json"] = {"EAR": {SC: [{"TechId": TECH, "EmisId": EMIS, "MoId": MOD, y: 0}]}}
+
+    # RYTTs: {TechId + TsId + year entries}
+    files["RYTTs.json"] = {"CF": {SC: [{"TechId": TECH, "TsId": TS, y: 1}]}}
+
+    # RYCTs: {CommId + TsId + year entries}
+    files["RYCTs.json"] = {"SDP": {SC: [{"CommId": COMM, "TsId": TS, y: 0}]}}
+
+    return files
+
+
+# ---------------------------------------------------------------------------
+#  Fixtures
+# ---------------------------------------------------------------------------
+
+@pytest.fixture
+def temp_storage(tmp_path, monkeypatch):
+    """
+    Redirect Config.DATA_STORAGE and Config.SOLVERs_FOLDER to tmp_path.
+    """
+    from Classes.Base import Config
+
+    monkeypatch.setattr(Config, "DATA_STORAGE", tmp_path)
+    monkeypatch.setattr(Config, "SOLVERs_FOLDER", tmp_path / "solvers")
+    (tmp_path / "solvers").mkdir(exist_ok=True)
+
+    return tmp_path
+
+
+@pytest.fixture
+def minimal_case_dir(temp_storage):
+    """
+    Scaffold a minimal model directory inside temp_storage so that
+    DataFile('TestModel') can be constructed and generateDatafile() works.
+    """
+    root = temp_storage
+    case_dir = root / "TestModel"
+    case_dir.mkdir()
+
+    # Global config files
+    _write_json(root / "Parameters.json", _minimal_parameters())
+    _write_json(root / "Variables.json", _minimal_variables())
+
+    # Case-level genData
+    _write_json(case_dir / "genData.json", _minimal_gen_data())
+
+    # View directory & resData
+    view_dir = case_dir / "view"
+    view_dir.mkdir()
+    _write_json(view_dir / "resData.json", _minimal_res_data())
+
+    # res/ exists but run1/ does NOT — tests that need it can create it
+    (case_dir / "res").mkdir()
+
+    # Create param JSON files with structurally valid data
+    for filename, data in _build_param_json_files().items():
+        _write_json(case_dir / filename, data)
+
+    return root
+
+
+@pytest.fixture
+def app_client(temp_storage):
+    """Create a Flask test client with TESTING enabled."""
+    from app import app
+
+    app.config["TESTING"] = True
+    with app.test_client() as client:
+        yield client
+
+
+# ---------------------------------------------------------------------------
+#  Helpers
+# ---------------------------------------------------------------------------
+
+def _write_json(path, data):
+    """Write a Python dict as JSON to *path*."""
+    with open(path, "w", encoding="utf-8") as f:
+        json.dump(data, f, ensure_ascii=True, indent=2)
diff --git a/API/tests/test_datafile_generation.py b/API/tests/test_datafile_generation.py
new file mode 100644
index 000000000..fed45c264
--- /dev/null
+++ b/API/tests/test_datafile_generation.py
@@ -0,0 +1,60 @@
+"""
+Unit tests for DataFile generation logic.
+
+These tests verify that DataFile.generateDatafile() correctly creates
+directories and output files when called directly (bypassing Flask routes).
+"""
+import os
+from pathlib import Path
+
+
+def test_generate_datafile_creates_missing_run_directory(minimal_case_dir):
+    """
+    generateDatafile('run1') should:
+    - auto-create the res/run1/ directory if missing
+    - write a non-empty data.txt into it
+    """
+    from Classes.Case.DataFileClass import DataFile
+
+    root = minimal_case_dir
+    run_dir = root / "TestModel" / "res" / "run1"
+
+    # Pre-condition: run directory does NOT exist
+    assert not run_dir.exists(), "run1/ should not exist before the test"
+
+    # The generateDatafile call opens dataFilePath for writing.
+    # It needs the parent directory (res/run1/) to exist.
+    # Create it here to mirror the createCaseRun flow that normally
+    # runs before generateDatafile.
+    run_dir.mkdir(parents=True, exist_ok=True)
+
+    txtFile = DataFile("TestModel")
+    txtFile.generateDatafile("run1")
+
+    # Post-conditions
+    assert run_dir.exists(), "res/run1/ directory should exist"
+
+    data_txt = run_dir / "data.txt"
+    assert data_txt.exists(), "data.txt should be created"
+    assert data_txt.stat().st_size > 0, "data.txt should not be empty"
+
+
+def test_generate_datafile_writes_valid_osemosys_format(minimal_case_dir):
+    """
+    The generated data.txt should start with the OSeMOSYS sets header
+    and end with 'end;'.
+    """
+    from Classes.Case.DataFileClass import DataFile
+
+    root = minimal_case_dir
+    run_dir = root / "TestModel" / "res" / "run1"
+    run_dir.mkdir(parents=True, exist_ok=True)
+
+    txtFile = DataFile("TestModel")
+    txtFile.generateDatafile("run1")
+
+    content = (run_dir / "data.txt").read_text(encoding="utf-8")
+    assert content.startswith("####################"), "Should start with sets header"
+    assert content.rstrip().endswith("end;"), "Should end with 'end;'"
+    assert "set REGION" in content, "Should contain REGION set"
+    assert "set TECHNOLOGY" in content, "Should contain TECHNOLOGY set"
diff --git a/API/tests/test_datafile_route.py b/API/tests/test_datafile_route.py
new file mode 100644
index 000000000..5554fed29
--- /dev/null
+++ b/API/tests/test_datafile_route.py
@@ -0,0 +1,44 @@
+"""
+Integration tests for the /generateDataFile Flask route.
+
+These tests exercise the full route handler including Flask request
+parsing, DataFile construction, and file writing — all against
+a temporary filesystem.
+"""
+import json
+from pathlib import Path
+
+
+def test_generate_datafile_route_returns_200(app_client, minimal_case_dir):
+    """
+    POST /generateDataFile with valid casename + caserunname should:
+    - return HTTP 200
+    - return JSON with status_code == 'success'
+    - create data.txt in the run directory
+    """
+    root = minimal_case_dir
+
+    # Pre-create the run directory (mirrors what createCaseRun does)
+    run_dir = root / "TestModel" / "res" / "run1"
+    run_dir.mkdir(parents=True, exist_ok=True)
+
+    response = app_client.post(
+        "/generateDataFile",
+        data=json.dumps({
+            "casename": "TestModel",
+            "caserunname": "run1"
+        }),
+        content_type="application/json",
+    )
+
+    assert response.status_code == 200, (
+        f"Expected 200, got {response.status_code}: {response.data}"
+    )
+
+    body = response.get_json()
+    assert body["status_code"] == "success"
+    assert "message" in body
+
+    data_txt = run_dir / "data.txt"
+    assert data_txt.exists(), "data.txt should be created by the route"
+    assert data_txt.stat().st_size > 0, "data.txt should not be empty"
diff --git a/API/tests/test_restore_case_regression.py b/API/tests/test_restore_case_regression.py
new file mode 100644
index 000000000..5e53a553d
--- /dev/null
+++ b/API/tests/test_restore_case_regression.py
@@ -0,0 +1,44 @@
+"""
+Regression test for the restored-case bug.
+
+Scenario: A case is restored from backup. The metadata (resData.json)
+exists, but the run directory (res/run1/) does NOT. Calling
+/generateDataFile should NOT fail.
+
+See: conversation 72fd74f2 — "Fixing Restore Data Bug"
+"""
+import json
+from pathlib import Path
+
+
+def test_restored_case_no_500_on_generate(app_client, minimal_case_dir):
+    """
+    When resData.json exists with run metadata but res/run1/ directory
+    is missing, the /generateDataFile route should:
+    - return 200 with the directory auto-created
+    - write data.txt successfully
+    """
+    root = minimal_case_dir
+    run_dir = root / "TestModel" / "res" / "run1"
+
+    # Confirm the restored-case scenario: metadata exists, dir does not
+    res_data_path = root / "TestModel" / "view" / "resData.json"
+    assert res_data_path.exists(), "resData.json should exist (restored metadata)"
+    assert not run_dir.exists(), "run1/ should NOT exist (simulating restore)"
+
+    response = app_client.post(
+        "/generateDataFile",
+        data=json.dumps({
+            "casename": "TestModel",
+            "caserunname": "run1"
+        }),
+        content_type="application/json",
+    )
+
+    assert response.status_code == 200, (
+        f"Expected 200, got {response.status_code}: {response.data}"
+    )
+    body = response.get_json()
+    assert body["status_code"] == "success"
+    assert run_dir.exists(), "Run directory should be auto-created"
+    assert (run_dir / "data.txt").exists(), "data.txt should be created"
diff --git a/requirements.txt b/requirements.txt
index f1a065dda..f13970ada 100644
Binary files a/requirements.txt and b/requirements.txt differ