diff --git a/.github/workflows/maintests.yml b/.github/workflows/maintests.yml
index e9b7070..3720b5e 100644
--- a/.github/workflows/maintests.yml
+++ b/.github/workflows/maintests.yml
@@ -23,9 +23,33 @@ jobs:
       - name: Run pre-commit
         uses: pre-commit/action@v3.0.0
 
+  no_extra_fields:
+    env:
+      AIWORKER_CACHE_HOME: ${{ github.workspace }}/.cache
+      HORDELIB_CI_ONGOING: "1"
+      TESTS_ONGOING: "1"
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: ["3.10", "3.11"]
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+      - name: Install any required packages
+        run: |
+          python -m pip install --upgrade pip
+          pip install --upgrade -r requirements.dev.txt
+      - name: Run no_extra_fields check # Enabled by HORDELIB_CI_ONGOING
+        run: tox -e tests
+
   build:
     env:
       AIWORKER_CACHE_HOME: ${{ github.workspace }}/.cache
+      TESTS_ONGOING: "1"
     runs-on: ubuntu-latest
     strategy:
       matrix:
diff --git a/.github/workflows/prtests.yml b/.github/workflows/prtests.yml
index b0b171d..57e3729 100644
--- a/.github/workflows/prtests.yml
+++ b/.github/workflows/prtests.yml
@@ -26,9 +26,34 @@ jobs:
       - name: Run pre-commit
         uses: pre-commit/action@v3.0.0
 
+  no_extra_fields:
+    env:
+      AIWORKER_CACHE_HOME: ${{ github.workspace }}/.cache
+      HORDELIB_CI_ONGOING: "1"
+      TESTS_ONGOING: "1"
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: ["3.10", "3.11"]
+
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+      - name: Install any required packages
+        run: |
+          python -m pip install --upgrade pip
+          pip install --upgrade -r requirements.dev.txt
+      - name: Run no_extra_fields check # Enabled by HORDELIB_CI_ONGOING
+        run: tox -e tests
   build:
     env:
       AIWORKER_CACHE_HOME: ${{ github.workspace }}/.cache
+      TESTS_ONGOING: "1"
     runs-on: ubuntu-latest
     strategy:
       matrix:
diff --git a/horde_model_reference/legacy/classes/raw_legacy_model_database_records.py b/horde_model_reference/legacy/classes/raw_legacy_model_database_records.py
index 10946aa..c2b15e1 100644
--- a/horde_model_reference/legacy/classes/raw_legacy_model_database_records.py
+++ b/horde_model_reference/legacy/classes/raw_legacy_model_database_records.py
@@ -42,7 +42,7 @@ class RawLegacy_StableDiffusion_ModelRecord(BaseModel):
     # This is a better representation of the legacy model reference than the one in `staging_model_database_records.py`
     # which is a hybrid representation of the legacy model reference and the new model reference format.
 
-    model_config = ConfigDict(extra="forbid")
+    model_config = ConfigDict(extra="allow")
 
     name: str
     baseline: str
diff --git a/horde_model_reference/legacy/classes/staging_model_database_records.py b/horde_model_reference/legacy/classes/staging_model_database_records.py
index d8d8b9b..3142411 100644
--- a/horde_model_reference/legacy/classes/staging_model_database_records.py
+++ b/horde_model_reference/legacy/classes/staging_model_database_records.py
@@ -41,7 +41,7 @@ def validate_model_file_has_sha256sum(self):
 class StagingLegacy_Config_DownloadRecord(BaseModel):
     """An entry in the `config` field of a `StagingLegacy_Generic_ModelRecord`."""
 
-    model_config = ConfigDict(extra="forbid")
+    model_config = ConfigDict(extra="allow")
 
     file_name: str
     file_path: str = ""
@@ -54,7 +54,7 @@ class StagingLegacy_Config_DownloadRecord(BaseModel):
 class StagingLegacy_Generic_ModelRecord(BaseModel):
     """This is a helper class, a hybrid representation of the legacy model reference and the new format."""
 
-    model_config = ConfigDict(extra="forbid")
+    model_config = ConfigDict(extra="allow")
 
     name: str
     type: str
@@ -100,7 +100,7 @@ class Legacy_StableDiffusion_ModelRecord(StagingLegacy_Generic_ModelRecord):
 class Legacy_Generic_ModelReference(BaseModel):
     """A helper class to convert the legacy model reference to the new model reference format."""
 
-    model_config = ConfigDict(extra="forbid")
+    model_config = ConfigDict(extra="allow")
 
     models: Mapping[str, StagingLegacy_Generic_ModelRecord]
 
diff --git a/horde_model_reference/legacy/validate_sd.py b/horde_model_reference/legacy/validate_sd.py
index a74185d..a480efb 100644
--- a/horde_model_reference/legacy/validate_sd.py
+++ b/horde_model_reference/legacy/validate_sd.py
@@ -9,7 +9,11 @@
 )
 
 
-def validate_legacy_stable_diffusion_db(sd_db: Path, write_to_path: Path | None = None) -> bool:
+def validate_legacy_stable_diffusion_db(
+    sd_db: Path,
+    write_to_path: Path | None = None,
+    fail_on_extra: bool = False,
+) -> bool:
     raw_json_sd_db: str
     with open(sd_db) as sd_db_file:
         raw_json_sd_db = sd_db_file.read()
@@ -42,8 +46,18 @@ def validate_legacy_stable_diffusion_db(sd_db: Path, write_to_path: Path | None
         },
         indent=4,
     )
+
     correct_json_layout += "\n"  # Add a newline to the end of the file, for consistency with formatters.
 
+    any_extra_fields = False
+    for key, record in parsed_db_records.items():
+        if record.model_extra:
+            logger.error(f"Extra fields found in {key}: {record.model_extra}")
+            any_extra_fields = True
+
+    if any_extra_fields and fail_on_extra:
+        raise ValueError("Extra fields found in stable diffusion model database.")
+
     if raw_json_sd_db != correct_json_layout:
         logger.error("Invalid stable diffusion model database.")
         if write_to_path:
diff --git a/legacy_stable_diffusion.schema.json b/legacy_stable_diffusion.schema.json
index ead3eab..0d82588 100644
--- a/legacy_stable_diffusion.schema.json
+++ b/legacy_stable_diffusion.schema.json
@@ -98,7 +98,7 @@
             "type": "object"
         },
         "RawLegacy_StableDiffusion_ModelRecord": {
-            "additionalProperties": false,
+            "additionalProperties": true,
             "description": "A model entry in the legacy model reference.",
             "properties": {
                 "name": {
diff --git a/tests/conftest.py b/tests/conftest.py
index d728bba..ba3924b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,3 +1,5 @@
+import os
+import sys
 from pathlib import Path
 
 import pytest
@@ -5,6 +7,15 @@
 
 from horde_model_reference.path_consts import LEGACY_REFERENCE_FOLDER_NAME
 
+os.environ["TESTS_ONGOING"] = "1"
+
+
+@pytest.fixture(scope="session")
+def env_var_checks() -> None:
+    """Check for required environment variables."""
+
+    assert "TESTS_ONGOING" in os.environ, "Environment variable 'TESTS_ONGOING' not set."
+
 
 @pytest.fixture(scope="session")
 def base_path_for_tests() -> Path:
@@ -30,6 +41,9 @@ def setup_logging(base_path_for_tests: Path):
                 "sink": base_path_for_tests.joinpath("test_log.txt"),
                 "format": "{time:YYYY-MM-DD HH:mm:ss} | {level} | {message}",
             },
+            # add sinks for stdout and stderr
+            {"sink": sys.stdout, "format": "{time:YYYY-MM-DD HH:mm:ss} | {level} | {message}"},
+            {"sink": sys.stderr, "format": "{time:YYYY-MM-DD HH:mm:ss} | {level} | {message}"},
         ],
     )
 
diff --git a/tests/test_scripts.py b/tests/test_scripts.py
index 851e4d8..d6cf08d 100644
--- a/tests/test_scripts.py
+++ b/tests/test_scripts.py
@@ -1,3 +1,4 @@
+import os
 from pathlib import Path
 
 from horde_model_reference.legacy.download_live_legacy_dbs import LegacyReferenceDownloadManager
@@ -13,9 +14,19 @@ def test_download_all_model_references(base_path_for_tests: Path):
 
 
 def test_validate_stable_diffusion_model_reference(legacy_folder_for_tests: Path):
-    assert validate_legacy_stable_diffusion_db(
-        sd_db=get_model_reference_file_path(
-            MODEL_REFERENCE_CATEGORY.stable_diffusion,
-            base_path=legacy_folder_for_tests,
-        ),
-    )
+    if os.environ.get("HORDELIB_CI_ONGOING"):
+        assert validate_legacy_stable_diffusion_db(
+            sd_db=get_model_reference_file_path(
+                MODEL_REFERENCE_CATEGORY.stable_diffusion,
+                base_path=legacy_folder_for_tests,
+            ),
+            fail_on_extra=True,
+        )
+    else:
+        assert validate_legacy_stable_diffusion_db(
+            sd_db=get_model_reference_file_path(
+                MODEL_REFERENCE_CATEGORY.stable_diffusion,
+                base_path=legacy_folder_for_tests,
+            ),
+            fail_on_extra=False,
+        )
diff --git a/tox.ini b/tox.ini
index b8fd760..c70577b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -23,6 +23,7 @@ commands = pre-commit run --all-files --show-diff-on-failure
 [testenv:tests]
 description = install pytest in a virtual environment and invoke it on the tests folder
 skip_install = false
+passenv = HORDELIB_CI_ONGOING
 deps =
     pytest>=7
     pytest-sugar