From 215756f7584c610323f7adb2036eb27065b36c75 Mon Sep 17 00:00:00 2001
From: Seth Fitzsimmons <sethfitz@amazon.com>
Date: Fri, 6 Mar 2026 10:30:34 -0800
Subject: [PATCH] feat(test): pytest plugin for golden-file baselines

Helpers `assert_golden` and `assert_json_schema_golden` live in
`overture.schema.system.testing`. The pytest plugin at
`overture.schema.system.testing.plugin` registers `--update-baselines`
and the `update_baselines` fixture; consuming packages opt in via
`[project.entry-points.pytest11]`.

Helpers live in `system` rather than a separate test-support package:
`system` is foundational, helpers depend on `system.json_schema`, and
themes already depend on `system`. Mirrors `numpy.testing` /
`pandas.testing`.

Per-package opt-in rather than a single entry point in `system`: keeps
`--update-baselines` from appearing in `core`, `codegen`, or `cli`
test runs when consumed in isolation.

Signed-off-by: Seth Fitzsimmons <seth@mojodna.net>
---
 Makefile                                      |  6 +-
 .../pyproject.toml                            |  5 ++
 .../test_address_json_schema_baseline.py      | 34 ++-------
 packages/overture-schema-annex/pyproject.toml |  6 +-
 .../test_sources_json_schema_baseline.py      | 30 ++------
 .../overture-schema-base-theme/pyproject.toml |  5 +-
 .../test_bathymetry_json_schema_baseline.py   | 34 ++-------
 ...est_infrastructure_json_schema_baseline.py | 35 ++-------
 .../test_land_cover_json_schema_baseline.py   | 34 ++-------
 .../tests/test_land_json_schema_baseline.py   | 32 ++------
 .../test_land_use_json_schema_baseline.py     | 34 ++-------
 .../tests/test_water_json_schema_baseline.py  | 34 ++-------
 .../pyproject.toml                            |  3 +
 .../test_building_json_schema_baseline.py     | 34 ++-------
 ...test_building_part_json_schema_baseline.py | 34 ++-------
 .../pyproject.toml                            |  5 +-
 ...test_division_area_json_schema_baseline.py | 34 ++-------
 ..._division_boundary_json_schema_baseline.py | 35 ++-------
 .../test_division_json_schema_baseline.py     | 34 ++-------
 .../pyproject.toml                            |  5 +-
 .../tests/test_place_json_schema_baseline.py  | 34 ++-------
 packages/overture-schema-system/README.md     | 62 +++++++++++++++
 .../schema/system/testing/__init__.py         | 13 ++++
 .../overture/schema/system/testing/golden.py  | 61 +++++++++++++++
 .../overture/schema/system/testing/plugin.py  | 23 ++++++
 .../tests/testing/test_golden.py              | 75 +++++++++++++++++++
 .../pyproject.toml                            |  5 +-
 .../test_connector_json_schema_baseline.py    | 34 ++-------
 .../test_segment_json_schema_baseline.py      | 34 ++-------
 pyproject.toml                                |  5 +-
 uv.lock                                       |  4 +
 31 files changed, 402 insertions(+), 421 deletions(-)
 create mode 100644 packages/overture-schema-system/src/overture/schema/system/testing/__init__.py
 create mode 100644 packages/overture-schema-system/src/overture/schema/system/testing/golden.py
 create mode 100644 packages/overture-schema-system/src/overture/schema/system/testing/plugin.py
 create mode 100644 packages/overture-schema-system/tests/testing/test_golden.py

diff --git a/Makefile b/Makefile
index 367874a10..edea9b34d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-.PHONY: default uv-sync check test-all test test-only docformat doctest doctest-only mypy mypy-only lint-only reset-baseline-schemas
+.PHONY: default uv-sync check test-all test test-only docformat doctest doctest-only mypy mypy-only lint-only update-baselines
 
 default: test-all
 
@@ -56,5 +56,5 @@ lint-only:
 	@uv run ruff check -q packages/
 	@uv run ruff format --check packages/
 
-reset-baseline-schemas:
-	@find . -name \*_baseline_schema.json -delete
+update-baselines:
+	@uv run pytest --update-baselines -m baseline -q packages/
diff --git a/packages/overture-schema-addresses-theme/pyproject.toml b/packages/overture-schema-addresses-theme/pyproject.toml
index e794b90d8..339b20e90 100644
--- a/packages/overture-schema-addresses-theme/pyproject.toml
+++ b/packages/overture-schema-addresses-theme/pyproject.toml
@@ -4,6 +4,7 @@ maintainers = [
 ]
 dependencies = [
     "overture-schema-core",
+    "overture-schema-system",
     "pydantic>=2.0",
 ]
 description = "Overture Maps addresses theme models and structures"
@@ -20,6 +21,7 @@ Issues = "https://github.com/OvertureMaps/schema/issues"
 
 [tool.uv.sources]
 overture-schema-core = { workspace = true }
+overture-schema-system = { workspace = true }
 
 
 [build-system]
@@ -39,6 +41,9 @@ testpaths = ["tests"]
 [project.entry-points."overture.models"]
 "overture:addresses:address" = "overture.schema.addresses:Address"
 
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Address]]
 id = "416ab01c-d836-4c4f-aedc-2f30941ce94d"
 geometry = "POINT (-176.5637854 -43.9471955)"
diff --git a/packages/overture-schema-addresses-theme/tests/test_address_json_schema_baseline.py b/packages/overture-schema-addresses-theme/tests/test_address_json_schema_baseline.py
index dc97faaeb..a49ddc2f7 100644
--- a/packages/overture-schema-addresses-theme/tests/test_address_json_schema_baseline.py
+++ b/packages/overture-schema-addresses-theme/tests/test_address_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for address type."""
+"""Golden JSON Schema test for Address type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.addresses import Address
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "address_baseline_schema.json"
 
-def test_address_json_schema_baseline() -> None:
-    """Test that Address generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Address)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "address_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_address_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Address, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-annex/pyproject.toml b/packages/overture-schema-annex/pyproject.toml
index 41e8f263e..009eeff92 100644
--- a/packages/overture-schema-annex/pyproject.toml
+++ b/packages/overture-schema-annex/pyproject.toml
@@ -2,7 +2,7 @@
 maintainers = [
     {name = "Overture Maps Schema Working Group"},
 ]
-dependencies = ["overture-schema-core", "pydantic>=2.0"]
+dependencies = ["overture-schema-core", "overture-schema-system", "pydantic>=2.0"]
 description = "Add your description here"
 dynamic = ["version"]
 license = "MIT"
@@ -12,6 +12,7 @@ requires-python = ">=3.10"
 
 [tool.uv.sources]
 overture-schema-core = { workspace = true }
+overture-schema-system = { workspace = true }
 
 [project.urls]
 Homepage = "https://overturemaps.org"
@@ -31,6 +32,9 @@ packages = ["src/overture"]
 [project.entry-points."overture.models"]
 "annex:sources" = "overture.schema.annex:Sources"
 
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [tool.pytest.ini_options]
 pythonpath = ["src"]
 testpaths = ["tests"]
diff --git a/packages/overture-schema-annex/tests/test_sources_json_schema_baseline.py b/packages/overture-schema-annex/tests/test_sources_json_schema_baseline.py
index b34383be5..3ab9e9e48 100644
--- a/packages/overture-schema-annex/tests/test_sources_json_schema_baseline.py
+++ b/packages/overture-schema-annex/tests/test_sources_json_schema_baseline.py
@@ -1,28 +1,14 @@
-"""Baseline JSON Schema tests for sources model."""
+"""Golden JSON Schema test for Sources type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.annex import Sources
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "sources_baseline_schema.json"
 
-def test_sources_json_schema_baseline() -> None:
-    """Ensure Sources model JSON Schema matches the committed baseline."""
-    schema = json_schema(Sources)
 
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "sources_baseline_schema.json"
-    )
-
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w", encoding="utf-8") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    with open(baseline_file, encoding="utf-8") as f:
-        baseline_schema = json.load(f)
-
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_sources_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Sources, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/pyproject.toml b/packages/overture-schema-base-theme/pyproject.toml
index aa9d3ba39..ffc6863aa 100644
--- a/packages/overture-schema-base-theme/pyproject.toml
+++ b/packages/overture-schema-base-theme/pyproject.toml
@@ -41,7 +41,10 @@ packages = ["src/overture"]
 "overture:base:land_cover" = "overture.schema.base:LandCover"
 "overture:base:land_use" = "overture.schema.base:LandUse"
 "overture:base:water" = "overture.schema.base:Water"
- 
+
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Bathymetry]]
 id = "5d40bd6c-db14-5492-b29f-5e25a59032bc"
 geometry = "MULTIPOLYGON (((-170.71296928 -76.744313428, -170.719841483 -76.757076376, -170.731061124 -76.761566192, -170.775652756 -76.76338726, -170.853616381 -76.76253958, -170.918562293 -76.755380155, -170.970490492 -76.741908984, -170.998699301 -76.729180777, -171.003188718 -76.717195533, -170.990421551 -76.703765214, -170.960397802 -76.68888982, -170.940748072 -76.674697941, -170.931472364 -76.661189576, -170.927114414 -76.637296658, -170.927674224 -76.603019188, -170.939335393 -76.574637428, -170.962097922 -76.552151379, -170.999015387 -76.535715361, -171.050087788 -76.525329373, -171.079133298 -76.50751024, -171.086151917 -76.482257963, -171.098653755 -76.462747286, -171.11663881 -76.448978211, -171.146691397 -76.437601179, -171.188811514 -76.428616191, -171.296181785 -76.4228609, -171.468802209 -76.420335306, -171.566055241 -76.41501101, -171.587940879 -76.406888013, -171.59004284 -76.387987744, -171.572361122 -76.358310204, -171.549343725 -76.334488281, -171.520990649 -76.316521976, -171.453759127 -76.301763636, -171.347649159 -76.290213262, -171.30597166 -76.267707269, -171.328726628 -76.234245658, -171.36676019 -76.195627518, -171.420072345 -76.151852851, -171.444766298 -76.12494912, -171.44084205 -76.114916326, -171.378107286 -76.099627787, -171.256562007 -76.079083503, -171.228218647 -76.058825682, -171.293077208 -76.038854322, -171.421365419 -76.023534207, -171.613083278 -76.012865337, -171.76411833 -75.99938969, -171.874470572 -75.983107266, -172.121928361 -75.958403596, -172.506491695 -75.925278679, -172.744527804 -75.899736153, -172.836036689 -75.88177602, -172.904681746 -75.862406785, -172.950462974 -75.841628448, -173.000855857 -75.830396498, -173.055860393 -75.828710933, -173.177561398 -75.810743709, -173.365958872 -75.776494827, -173.493573084 -75.759370386, -173.560404033 -75.759370386, -173.620925776 -75.77158365, -173.675138312 -75.796010178, -173.733786206 -75.808642966, -173.796869456 -75.809482015, -173.847216433 -75.805553449, 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-77.39631876, -172.999732531 -77.429966955, -173.208286817 -77.472070689, -173.335454668 -77.509278677, -173.381236082 -77.541590921, -173.403703936 -77.570407724, -173.40285823 -77.595729086, -173.378288408 -77.634921, -173.329994472 -77.687983467, -173.241287742 -77.735563094, -173.112168219 -77.777659882, -173.054064387 -77.81089869, -173.066976248 -77.835279519, -173.063736051 -77.854657976, -173.044343797 -77.869034061, -172.890349983 -77.896435115, -172.60175461 -77.936861139, -172.376181212 -77.961986812, -172.213629791 -77.971812135, -172.023427102 -77.967320559, -171.805573145 -77.948512083, -171.581263004 -77.918894833, -171.350496677 -77.87846881, -171.217147208 -77.851799157, -171.181214596 -77.838885875, -171.160572341 -77.826074082, -171.155220441 -77.813363779, -171.178789134 -77.790158543, -171.231278422 -77.756458375, -171.27338337 -77.70988804, -171.305103978 -77.65044754, -171.293875473 -77.602346602, -171.239697854 -77.565585227, -171.168401509 -77.532887375, -171.079986438 -77.504253044, -171.028614514 -77.483042244, -171.014285737 -77.469254974, -171.016677114 -77.456576914, -171.035788644 -77.445008064, -171.086879845 -77.431646501, -171.169950715 -77.416492226, -171.216537864 -77.403175691, -171.226641293 -77.391696895, -171.228607057 -77.378968685, -171.222435157 -77.364991059, -171.168824693 -77.334840949, -171.067775664 -77.288518355, -171.000402018 -77.24121644, -170.966703754 -77.192935206, -170.894838531 -77.157002595, -170.784806349 -77.133418606, -170.725150821 -77.11627156, -170.715871945 -77.105561456, -170.710674146 -77.077210652, -170.709557424 -77.031219147, -170.697909144 -76.992502178, -170.675729304 -76.961059744, -170.654536164 -76.940848729, -170.634329723 -76.931869135, -170.581564681 -76.922044903, -170.496241038 -76.911376032, -170.429709562 -76.893409727, -170.381970254 -76.868145986, -170.285260999 -76.838950739, -170.139581798 -76.805823986, -170.061542334 -76.78431495, -170.051142608 -76.77442363, -170.076677284 -76.763148845, -170.138146365 -76.750490597, -170.192753568 -76.731526593, -170.240498896 -76.706256833, -170.315896371 -76.686462585, -170.418945993 -76.67214385, -170.498267121 -76.665405567, -170.553859754 -76.666247738, -170.609039198 -76.673409769, -170.663805452 -76.68689166, -170.695686968 -76.698414281, -170.704683743 -76.70797763, -170.710444514 -76.723277346, -170.71296928 -76.744313428), (-172.46185717 -77.485683162, -172.491725041 -77.49003391, -172.535448064 -77.490594163, -172.566986057 -77.488349711, -172.586339021 -77.483300552, -172.598540475 -77.476173053, -172.60359042 -77.466967216, -172.601627836 -77.458872071, -172.592652724 -77.451887618, -172.556765055 -77.448396429, -172.49396483 -77.448398503, -172.453726685 -77.452881992, -172.436050621 -77.461846897, -172.429868964 -77.468114837, -172.435181715 -77.47168581, -172.44584445 -77.477541919, -172.46185717 -77.485683162), (-172.812798475 -76.363628771, -172.855573928 -76.365453015, -172.885037626 -76.36040045, -172.90720433 -76.351027386, -172.92207404 -76.337333821, -172.9168827 -76.324750727, -172.89163031 -76.313278104, -172.862193885 -76.307261221, -172.828573425 -76.30670008, -172.792121028 -76.311189877, -172.752836694 -76.320730613, -172.732062811 -76.331770033, -172.729799379 -76.344308139, -172.756711267 -76.354927718, -172.812798475 -76.363628771), (-171.932998671 -76.183124002, -172.010021088 -76.180457336, -172.070931389 -76.166984091, -172.113033554 -76.150312062, -172.136327583 -76.130441248, -172.133522137 -76.111120124, -172.104617217 -76.092348689, -172.06028165 -76.080296327, -172.000515436 -76.074963039, -171.918725408 -76.076928027, -171.814911566 -76.086191292, -171.745182124 -76.097695899, -171.709537083 -76.111441849, -171.696346087 -76.126554541, -171.705609136 -76.143033974, -171.731004713 -76.156183802, -171.77253282 -76.166004024, -171.83986414 -76.174984091, -171.932998671 -76.183124002), (-173.16885937 -76.066345013, -173.199147981 -76.070696107, -173.23950163 -76.071257052, -173.269213382 -76.065813298, -173.288283234 -76.054364845, -173.2799961 -76.038973879, -173.244351978 -76.0196404, -173.207608446 -76.007588038, -173.169765504 -76.002816794, -173.139490241 -76.003094691, -173.116782658 -76.008421729, -173.104589039 -76.016938854, -173.102909386 -76.028646065, -173.111183172 -76.03940804, -173.129410398 -76.049224779, -173.148635798 -76.05820377, -173.16885937 -76.066345013)))"
diff --git a/packages/overture-schema-base-theme/tests/test_bathymetry_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_bathymetry_json_schema_baseline.py
index 83e29eeef..c2f1347fd 100644
--- a/packages/overture-schema-base-theme/tests/test_bathymetry_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_bathymetry_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for bathymetry type."""
+"""Golden JSON Schema test for Bathymetry type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import Bathymetry
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "bathymetry_baseline_schema.json"
 
-def test_bathymetry_json_schema_baseline() -> None:
-    """Test that Bathymetry generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Bathymetry)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "bathymetry_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_bathymetry_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Bathymetry, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/tests/test_infrastructure_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_infrastructure_json_schema_baseline.py
index 1eef52106..91c83a336 100644
--- a/packages/overture-schema-base-theme/tests/test_infrastructure_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_infrastructure_json_schema_baseline.py
@@ -1,33 +1,14 @@
-"""Baseline JSON Schema tests for infrastructure type."""
+"""Golden JSON Schema test for Infrastructure type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import Infrastructure
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "infrastructure_baseline_schema.json"
 
-def test_infrastructure_json_schema_baseline() -> None:
-    """Test that Infrastructure generates consistent JSON Schema (baseline
-    comparison)."""
-    schema = json_schema(Infrastructure)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "infrastructure_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_infrastructure_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Infrastructure, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/tests/test_land_cover_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_land_cover_json_schema_baseline.py
index f16534871..5ac478991 100644
--- a/packages/overture-schema-base-theme/tests/test_land_cover_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_land_cover_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Test land_cover JSON Schema baseline generation."""
+"""Golden JSON Schema test for LandCover type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import LandCover
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "land_cover_baseline_schema.json"
 
-def test_land_cover_json_schema_baseline() -> None:
-    """Test that LandCover generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(LandCover)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "land_cover_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_land_cover_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(LandCover, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/tests/test_land_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_land_json_schema_baseline.py
index 1feccbbcd..b6bc8e487 100644
--- a/packages/overture-schema-base-theme/tests/test_land_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_land_json_schema_baseline.py
@@ -1,30 +1,14 @@
-"""Baseline JSON Schema tests for land type."""
+"""Golden JSON Schema test for Land type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import Land
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "land_baseline_schema.json"
 
-def test_land_json_schema_baseline() -> None:
-    """Test that Land generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Land)
 
-    # Path to baseline file
-    baseline_file = os.path.join(os.path.dirname(__file__), "land_baseline_schema.json")
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_land_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Land, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/tests/test_land_use_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_land_use_json_schema_baseline.py
index 2f9dbf55d..2ab678cf0 100644
--- a/packages/overture-schema-base-theme/tests/test_land_use_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_land_use_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Test land_use JSON Schema baseline generation."""
+"""Golden JSON Schema test for LandUse type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import LandUse
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "land_use_baseline_schema.json"
 
-def test_land_use_json_schema_baseline() -> None:
-    """Test that LandUse generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(LandUse)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "land_use_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_land_use_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(LandUse, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-base-theme/tests/test_water_json_schema_baseline.py b/packages/overture-schema-base-theme/tests/test_water_json_schema_baseline.py
index d85b1194d..aa08406fa 100644
--- a/packages/overture-schema-base-theme/tests/test_water_json_schema_baseline.py
+++ b/packages/overture-schema-base-theme/tests/test_water_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for water type."""
+"""Golden JSON Schema test for Water type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.base import Water
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "water_baseline_schema.json"
 
-def test_water_json_schema_baseline() -> None:
-    """Test that Water generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Water)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "water_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_water_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Water, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-buildings-theme/pyproject.toml b/packages/overture-schema-buildings-theme/pyproject.toml
index 07e418faa..880be3367 100644
--- a/packages/overture-schema-buildings-theme/pyproject.toml
+++ b/packages/overture-schema-buildings-theme/pyproject.toml
@@ -38,6 +38,9 @@ packages = ["src/overture"]
 "overture:buildings:building" = "overture.schema.buildings:Building"
 "overture:buildings:building_part" = "overture.schema.buildings:BuildingPart"
 
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Building]]
 id = "148f35b1-7bc1-4180-9280-10d39b13883b"
 geometry = "POLYGON ((-176.6435004 -43.9938042, -176.6435738 -43.9937107, -176.6437726 -43.9937913, -176.6436992 -43.9938849, -176.6435004 -43.9938042))"
diff --git a/packages/overture-schema-buildings-theme/tests/test_building_json_schema_baseline.py b/packages/overture-schema-buildings-theme/tests/test_building_json_schema_baseline.py
index 917cc9b15..8c30bfbcc 100644
--- a/packages/overture-schema-buildings-theme/tests/test_building_json_schema_baseline.py
+++ b/packages/overture-schema-buildings-theme/tests/test_building_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for building type."""
+"""Golden JSON Schema test for Building type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.buildings.building import Building
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "building_baseline_schema.json"
 
-def test_building_json_schema_baseline() -> None:
-    """Test that Building generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Building)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "building_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_building_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Building, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-buildings-theme/tests/test_building_part_json_schema_baseline.py b/packages/overture-schema-buildings-theme/tests/test_building_part_json_schema_baseline.py
index 9150966be..11b3ef530 100644
--- a/packages/overture-schema-buildings-theme/tests/test_building_part_json_schema_baseline.py
+++ b/packages/overture-schema-buildings-theme/tests/test_building_part_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for building part type."""
+"""Golden JSON Schema test for BuildingPart type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.buildings.building_part import BuildingPart
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "building_part_baseline_schema.json"
 
-def test_building_part_json_schema_baseline() -> None:
-    """Test that BuildingPart generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(BuildingPart)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "building_part_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_building_part_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(BuildingPart, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-divisions-theme/pyproject.toml b/packages/overture-schema-divisions-theme/pyproject.toml
index 0a1cd9e61..09ecd5067 100644
--- a/packages/overture-schema-divisions-theme/pyproject.toml
+++ b/packages/overture-schema-divisions-theme/pyproject.toml
@@ -37,7 +37,10 @@ packages = ["src/overture"]
 "overture:divisions:division" = "overture.schema.divisions:Division"
 "overture:divisions:division_area" = "overture.schema.divisions:DivisionArea"
 "overture:divisions:division_boundary" = "overture.schema.divisions:DivisionBoundary"
- 
+
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Division]]
 id = "350e85f6-68ba-4114-9906-c2844815988b"
 geometry = "POINT (-175.2551522 -21.1353686)"
diff --git a/packages/overture-schema-divisions-theme/tests/test_division_area_json_schema_baseline.py b/packages/overture-schema-divisions-theme/tests/test_division_area_json_schema_baseline.py
index ae55d0666..53a5d3821 100644
--- a/packages/overture-schema-divisions-theme/tests/test_division_area_json_schema_baseline.py
+++ b/packages/overture-schema-divisions-theme/tests/test_division_area_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for division area type."""
+"""Golden JSON Schema test for DivisionArea type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.divisions import DivisionArea
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "division_area_baseline_schema.json"
 
-def test_division_area_json_schema_baseline() -> None:
-    """Test that DivisionArea generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(DivisionArea)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "division_area_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_division_area_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(DivisionArea, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-divisions-theme/tests/test_division_boundary_json_schema_baseline.py b/packages/overture-schema-divisions-theme/tests/test_division_boundary_json_schema_baseline.py
index 4f43dbcb2..66733892e 100644
--- a/packages/overture-schema-divisions-theme/tests/test_division_boundary_json_schema_baseline.py
+++ b/packages/overture-schema-divisions-theme/tests/test_division_boundary_json_schema_baseline.py
@@ -1,33 +1,14 @@
-"""Baseline JSON Schema tests for division boundary type."""
+"""Golden JSON Schema test for DivisionBoundary type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.divisions import DivisionBoundary
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "division_boundary_baseline_schema.json"
 
-def test_division_boundary_json_schema_baseline() -> None:
-    """Test that DivisionBoundary generates consistent JSON Schema (baseline
-    comparison)."""
-    schema = json_schema(DivisionBoundary)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "division_boundary_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_division_boundary_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(DivisionBoundary, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-divisions-theme/tests/test_division_json_schema_baseline.py b/packages/overture-schema-divisions-theme/tests/test_division_json_schema_baseline.py
index 99ab0779d..cf91ce8f8 100644
--- a/packages/overture-schema-divisions-theme/tests/test_division_json_schema_baseline.py
+++ b/packages/overture-schema-divisions-theme/tests/test_division_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for division type."""
+"""Golden JSON Schema test for Division type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.divisions import Division
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "division_baseline_schema.json"
 
-def test_division_json_schema_baseline() -> None:
-    """Test that Division generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Division)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "division_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_division_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Division, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-places-theme/pyproject.toml b/packages/overture-schema-places-theme/pyproject.toml
index b374835be..a1ad0604b 100644
--- a/packages/overture-schema-places-theme/pyproject.toml
+++ b/packages/overture-schema-places-theme/pyproject.toml
@@ -36,7 +36,10 @@ packages = ["src/overture"]
 
 [project.entry-points."overture.models"]
 "overture:places:place" = "overture.schema.places:Place"
- 
+
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Place]]
 id = "99003ee6-e75b-4dd6-8a8a-53a5a716c50d"
 geometry = "POINT (-150.46875 -79.1713346)"
diff --git a/packages/overture-schema-places-theme/tests/test_place_json_schema_baseline.py b/packages/overture-schema-places-theme/tests/test_place_json_schema_baseline.py
index 0b142a712..6360911d3 100644
--- a/packages/overture-schema-places-theme/tests/test_place_json_schema_baseline.py
+++ b/packages/overture-schema-places-theme/tests/test_place_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for place type."""
+"""Golden JSON Schema test for Place type."""
 
-import json
-import os
+from pathlib import Path
 
+import pytest
 from overture.schema.places import Place
-from overture.schema.system.json_schema import json_schema
+from overture.schema.system.testing import assert_json_schema_golden
 
+GOLDEN = Path(__file__).parent / "place_baseline_schema.json"
 
-def test_place_json_schema_baseline() -> None:
-    """Test that Place generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Place)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "place_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_place_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Place, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-system/README.md b/packages/overture-schema-system/README.md
index 238984eb0..5936933e7 100644
--- a/packages/overture-schema-system/README.md
+++ b/packages/overture-schema-system/README.md
@@ -115,3 +115,65 @@ class ParkBench(Identified):
 - **DocumentedEnum** -- base class for enumerations whose members carry their own docstrings, enabling code generation tools to produce documented output.
 - **Metadata** -- internal key-value store used by model constraints to attach data to classes.
 - **JSON Schema** -- schema generator that treats `T | None = None` as "omit when unset" rather than Pydantic's default "nullable with null default." Also handles unions of models.
+
+## Baseline Testing
+
+`overture.schema.system.testing` provides a pytest plugin for golden-file baseline tests of generated JSON Schemas. Implementers building feature packages use it to detect unintended schema drift.
+
+### Helpers
+
+- `assert_golden(actual, golden_path, *, update)` -- compare a string against a golden file. On mismatch, raises `AssertionError` with a unified diff. When `update=True`, writes `actual` to `golden_path` instead of comparing.
+- `assert_json_schema_golden(model_or_union, golden_path, *, update)` -- generate the JSON Schema for a Pydantic model (or discriminated union type alias) via `overture.schema.system.json_schema` and delegate to `assert_golden`.
+
+### Opting In
+
+Activation is opt-in so the `--update-baselines` flag does not pollute pytest runs of packages that do not declare it. Add the plugin to your package's `pyproject.toml`:
+
+```toml
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+```
+
+The plugin registers:
+
+- `--update-baselines` -- pytest CLI flag.
+- `update_baselines` -- bool fixture, true when the flag is passed.
+
+### Writing a Baseline Test
+
+```python
+from pathlib import Path
+
+import pytest
+from overture.schema.system.testing import assert_json_schema_golden
+
+from yourpackage import Mountain  # the model you're locking down
+
+GOLDEN = Path(__file__).parent / "mountain_baseline_schema.json"
+
+
+@pytest.mark.baseline
+def test_mountain_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Mountain, GOLDEN, update=update_baselines)
+```
+
+Convention: the golden file lives next to the test, named `<feature>_baseline_schema.json`.
+
+Mark each baseline test with `@pytest.mark.baseline` so it can be selected or skipped via `pytest -m baseline` / `pytest -m "not baseline"`. Register the marker in your pyproject:
+
+```toml
+[tool.pytest.ini_options]
+markers = [
+    "baseline: golden file baseline tests",
+]
+```
+
+### Updating Baselines
+
+After an intentional schema change:
+
+```bash
+pytest -m baseline --update-baselines
+```
+
+Inspect `git diff` on the regenerated golden files to confirm the changes are intended before committing.
diff --git a/packages/overture-schema-system/src/overture/schema/system/testing/__init__.py b/packages/overture-schema-system/src/overture/schema/system/testing/__init__.py
new file mode 100644
index 000000000..f3616f19c
--- /dev/null
+++ b/packages/overture-schema-system/src/overture/schema/system/testing/__init__.py
@@ -0,0 +1,13 @@
+"""Test helpers for Overture schema implementers.
+
+Helpers for golden file comparison and JSON Schema baseline tests. Pure
+helpers live in `golden`; pytest plumbing lives in `plugin` and is opt-in
+via a `pytest11` entry point declared by consuming packages.
+"""
+
+from .golden import assert_golden, assert_json_schema_golden
+
+__all__ = [
+    "assert_golden",
+    "assert_json_schema_golden",
+]
diff --git a/packages/overture-schema-system/src/overture/schema/system/testing/golden.py b/packages/overture-schema-system/src/overture/schema/system/testing/golden.py
new file mode 100644
index 000000000..a1927ec12
--- /dev/null
+++ b/packages/overture-schema-system/src/overture/schema/system/testing/golden.py
@@ -0,0 +1,61 @@
+"""Golden file comparison helpers for baseline tests."""
+
+import json
+from difflib import unified_diff
+from pathlib import Path
+
+from overture.schema.system.json_schema import json_schema
+
+
+def assert_golden(actual: str, golden_path: Path, *, update: bool) -> None:
+    """Compare actual output against a golden file.
+
+    Parameters
+    ----------
+    actual
+        Generated content to compare.
+    golden_path
+        Path to the golden file.
+    update
+        When True, write actual to golden_path instead of comparing.
+    """
+    if update:
+        golden_path.parent.mkdir(parents=True, exist_ok=True)
+        golden_path.write_text(actual)
+        return
+    if not golden_path.exists():
+        raise AssertionError(
+            f"Golden file not found: {golden_path}\n"
+            "Run 'make update-baselines' to generate it."
+        )
+    expected = golden_path.read_text()
+    if actual != expected:
+        diff = "\n".join(
+            unified_diff(
+                expected.splitlines(),
+                actual.splitlines(),
+                fromfile=str(golden_path),
+                tofile="actual",
+                lineterm="",
+            )
+        )
+        raise AssertionError(f"Golden file mismatch:\n{diff}")
+
+
+def assert_json_schema_golden(
+    model_or_union: object, golden_path: Path, *, update: bool
+) -> None:
+    """Generate JSON Schema and compare against a golden file.
+
+    Parameters
+    ----------
+    model_or_union
+        Pydantic model class or union type alias.
+    golden_path
+        Path to the baseline JSON file.
+    update
+        When True, write generated schema to golden_path instead of comparing.
+    """
+    schema = json_schema(model_or_union)
+    actual = json.dumps(schema, indent=2, sort_keys=True)
+    assert_golden(actual, golden_path, update=update)
diff --git a/packages/overture-schema-system/src/overture/schema/system/testing/plugin.py b/packages/overture-schema-system/src/overture/schema/system/testing/plugin.py
new file mode 100644
index 000000000..8a2ca47df
--- /dev/null
+++ b/packages/overture-schema-system/src/overture/schema/system/testing/plugin.py
@@ -0,0 +1,23 @@
+"""Pytest plugin: ``--update-baselines`` option and ``update_baselines`` fixture.
+
+Activated by consuming packages via ``[project.entry-points.pytest11]`` in
+their ``pyproject.toml``. Importing this module is a no-op outside pytest.
+"""
+
+import pytest
+
+
+def pytest_addoption(parser: pytest.Parser) -> None:
+    """Register the ``--update-baselines`` CLI option."""
+    parser.addoption(
+        "--update-baselines",
+        action="store_true",
+        default=False,
+        help="Regenerate baseline golden files instead of comparing",
+    )
+
+
+@pytest.fixture
+def update_baselines(request: pytest.FixtureRequest) -> bool:
+    """Whether to regenerate baseline files instead of comparing."""
+    return bool(request.config.getoption("--update-baselines"))
diff --git a/packages/overture-schema-system/tests/testing/test_golden.py b/packages/overture-schema-system/tests/testing/test_golden.py
new file mode 100644
index 000000000..16b292783
--- /dev/null
+++ b/packages/overture-schema-system/tests/testing/test_golden.py
@@ -0,0 +1,75 @@
+"""Tests for the golden file comparison helpers."""
+
+import json
+from pathlib import Path
+
+import pytest
+from pydantic import BaseModel
+
+from overture.schema.system.testing import (
+    assert_golden,
+    assert_json_schema_golden,
+)
+
+
+class _Minimal(BaseModel):
+    name: str
+
+
+class TestAssertGolden:
+    """Tests for assert_golden."""
+
+    def test_passes_when_content_matches(self, tmp_path: Path) -> None:
+        golden = tmp_path / "test.txt"
+        golden.write_text("hello")
+        assert_golden("hello", golden, update=False)
+
+    def test_fails_with_diff_on_mismatch(self, tmp_path: Path) -> None:
+        golden = tmp_path / "test.txt"
+        golden.write_text("hello")
+        with pytest.raises(AssertionError, match="Golden file mismatch"):
+            assert_golden("goodbye", golden, update=False)
+
+    def test_diff_shows_both_sides(self, tmp_path: Path) -> None:
+        golden = tmp_path / "test.txt"
+        golden.write_text("line1\nline2")
+        with pytest.raises(AssertionError, match=r"(?s)-line2.*\+line3"):
+            assert_golden("line1\nline3", golden, update=False)
+
+    def test_update_writes_file(self, tmp_path: Path) -> None:
+        golden = tmp_path / "test.txt"
+        assert_golden("new content", golden, update=True)
+        assert golden.read_text() == "new content"
+
+    def test_update_overwrites_existing(self, tmp_path: Path) -> None:
+        golden = tmp_path / "test.txt"
+        golden.write_text("old")
+        assert_golden("new", golden, update=True)
+        assert golden.read_text() == "new"
+
+    def test_missing_file_without_update_fails(self, tmp_path: Path) -> None:
+        golden = tmp_path / "nonexistent.txt"
+        with pytest.raises(AssertionError, match="make update-baselines"):
+            assert_golden("anything", golden, update=False)
+
+
+class TestAssertJsonSchemaGolden:
+    """Tests for assert_json_schema_golden."""
+
+    def test_passes_when_schema_matches(self, tmp_path: Path) -> None:
+        golden = tmp_path / "minimal.json"
+        assert_json_schema_golden(_Minimal, golden, update=True)
+        assert_json_schema_golden(_Minimal, golden, update=False)
+
+    def test_update_writes_schema(self, tmp_path: Path) -> None:
+        golden = tmp_path / "minimal.json"
+        assert_json_schema_golden(_Minimal, golden, update=True)
+        written = json.loads(golden.read_text())
+        assert written["title"] == "_Minimal"
+        assert "name" in written["properties"]
+
+    def test_fails_on_schema_mismatch(self, tmp_path: Path) -> None:
+        golden = tmp_path / "minimal.json"
+        golden.write_text('{"wrong": "schema"}')
+        with pytest.raises(AssertionError, match="Golden file mismatch"):
+            assert_json_schema_golden(_Minimal, golden, update=False)
diff --git a/packages/overture-schema-transportation-theme/pyproject.toml b/packages/overture-schema-transportation-theme/pyproject.toml
index 08811df39..ebd85c9dd 100644
--- a/packages/overture-schema-transportation-theme/pyproject.toml
+++ b/packages/overture-schema-transportation-theme/pyproject.toml
@@ -37,7 +37,10 @@ packages = ["src/overture"]
 [project.entry-points."overture.models"]
 "overture:transportation:connector" = "overture.schema.transportation:Connector"
 "overture:transportation:segment" = "overture.schema.transportation:Segment"
- 
+
+[project.entry-points.pytest11]
+overture_baselines = "overture.schema.system.testing.plugin"
+
 [[examples.Connector]]
 id = "39542bee-230f-4b91-b7e5-a9b58e0c59b1"
 geometry = "POINT (-176.5472979 -43.9679472)"
diff --git a/packages/overture-schema-transportation-theme/tests/test_connector_json_schema_baseline.py b/packages/overture-schema-transportation-theme/tests/test_connector_json_schema_baseline.py
index 132ac9977..da6a43280 100644
--- a/packages/overture-schema-transportation-theme/tests/test_connector_json_schema_baseline.py
+++ b/packages/overture-schema-transportation-theme/tests/test_connector_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for connector type."""
+"""Golden JSON Schema test for Connector type."""
 
-import json
-import os
+from pathlib import Path
 
-from overture.schema.system.json_schema import json_schema
+import pytest
+from overture.schema.system.testing import assert_json_schema_golden
 from overture.schema.transportation import Connector
 
+GOLDEN = Path(__file__).parent / "connector_baseline_schema.json"
 
-def test_connector_json_schema_baseline() -> None:
-    """Test that Connector generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Connector)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "connector_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_connector_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Connector, GOLDEN, update=update_baselines)
diff --git a/packages/overture-schema-transportation-theme/tests/test_segment_json_schema_baseline.py b/packages/overture-schema-transportation-theme/tests/test_segment_json_schema_baseline.py
index 529ced367..0952851c1 100644
--- a/packages/overture-schema-transportation-theme/tests/test_segment_json_schema_baseline.py
+++ b/packages/overture-schema-transportation-theme/tests/test_segment_json_schema_baseline.py
@@ -1,32 +1,14 @@
-"""Baseline JSON Schema tests for segment type."""
+"""Golden JSON Schema test for Segment type."""
 
-import json
-import os
+from pathlib import Path
 
-from overture.schema.system.json_schema import json_schema
+import pytest
+from overture.schema.system.testing import assert_json_schema_golden
 from overture.schema.transportation import Segment
 
+GOLDEN = Path(__file__).parent / "segment_baseline_schema.json"
 
-def test_segment_json_schema_baseline() -> None:
-    """Test that Segment generates consistent JSON Schema (baseline comparison)."""
-    schema = json_schema(Segment)
 
-    # Path to baseline file
-    baseline_file = os.path.join(
-        os.path.dirname(__file__), "segment_baseline_schema.json"
-    )
-
-    # If baseline doesn't exist, create it
-    if not os.path.exists(baseline_file):
-        with open(baseline_file, "w") as f:
-            json.dump(schema, f, indent=2, sort_keys=True)
-
-    # Load baseline and compare
-    with open(baseline_file) as f:
-        baseline_schema = json.load(f)
-
-    # Compare the generated schema with the baseline
-    assert schema == baseline_schema, (
-        "Generated JSON Schema differs from baseline. "
-        "If this change is intentional, delete the baseline file to regenerate it."
-    )
+@pytest.mark.baseline
+def test_segment_json_schema(update_baselines: bool) -> None:
+    assert_json_schema_golden(Segment, GOLDEN, update=update_baselines)
diff --git a/pyproject.toml b/pyproject.toml
index a33dea1e7..a643057a2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,7 +58,9 @@ dev = [
 ]
 
 [tool.pytest.ini_options]
-verbosity_subtests = 0
+markers = [
+    "baseline: golden file baseline tests",
+]
 pythonpath = [
     "packages/overture-schema-addresses-theme/tests",
     "packages/overture-schema-annex/tests",
@@ -73,3 +75,4 @@ pythonpath = [
     "packages/overture-schema-transportation-theme/tests",
     "packages/overture-schema/tests",
 ]
+verbosity_subtests = 0
diff --git a/uv.lock b/uv.lock
index 2829dc729..4f0f3228b 100644
--- a/uv.lock
+++ b/uv.lock
@@ -697,12 +697,14 @@ name = "overture-schema-addresses-theme"
 source = { editable = "packages/overture-schema-addresses-theme" }
 dependencies = [
     { name = "overture-schema-core" },
+    { name = "overture-schema-system" },
     { name = "pydantic" },
 ]
 
 [package.metadata]
 requires-dist = [
     { name = "overture-schema-core", editable = "packages/overture-schema-core" },
+    { name = "overture-schema-system", editable = "packages/overture-schema-system" },
     { name = "pydantic", specifier = ">=2.0" },
 ]
 
@@ -711,12 +713,14 @@ name = "overture-schema-annex"
 source = { editable = "packages/overture-schema-annex" }
 dependencies = [
     { name = "overture-schema-core" },
+    { name = "overture-schema-system" },
     { name = "pydantic" },
 ]
 
 [package.metadata]
 requires-dist = [
     { name = "overture-schema-core", editable = "packages/overture-schema-core" },
+    { name = "overture-schema-system", editable = "packages/overture-schema-system" },
     { name = "pydantic", specifier = ">=2.0" },
 ]