diff --git a/pyproject.toml b/pyproject.toml
index 6bcf4482..a412b9ce 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,6 +33,8 @@ dev = [
   "ruff>=0.6.0",
   "black>=24.3.0",
   "mypy>=1.8.0",
+  # Validates the shared canonical `2.0` layout schema (drift guard with optiverse-web).
+  "jsonschema>=4.0",
 ]
 
 [project.urls]
diff --git a/schema/optiverse-2.0.schema.json b/schema/optiverse-2.0.schema.json
new file mode 100644
index 00000000..2141d31c
--- /dev/null
+++ b/schema/optiverse-2.0.schema.json
@@ -0,0 +1,147 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://optiverse.dev/schema/optiverse-2.0.schema.json",
+  "title": "Optiverse Layout 2.0",
+  "description": "The canonical Optiverse project layout format shared by the desktop app and the web project-zip interchange. This schema is the drift guard: both repos validate the same golden fixtures against it in CI so the two apps cannot silently diverge.",
+  "type": "object",
+  "required": ["version", "items"],
+  "properties": {
+    "version": { "const": "2.0" },
+    "items": { "type": "array", "items": { "$ref": "#/$defs/item" } },
+    "rulers": { "type": "array", "items": { "$ref": "#/$defs/ruler" } },
+    "texts": { "type": "array", "items": { "$ref": "#/$defs/text" } },
+    "angle_measures": {
+      "description": "Optiverse-web interchange EXTENSION (not emitted by the desktop app, which does not persist angle measurements). Carried so web angle annotations survive a web -> zip -> web round-trip; the desktop ignores this key.",
+      "type": "array",
+      "items": { "$ref": "#/$defs/angle_measure" }
+    },
+    "rectangles": { "type": "array" },
+    "path_measures": { "type": "array" },
+    "layer_state": { "$ref": "#/$defs/layer_state" }
+  },
+  "$defs": {
+    "item": {
+      "type": "object",
+      "required": ["_type"],
+      "properties": {
+        "_type": { "enum": ["component", "source"] },
+        "x_mm": { "type": "number" },
+        "y_mm": { "type": "number" },
+        "angle_deg": { "type": "number" },
+        "item_uuid": { "type": "string" },
+        "z_value": { "type": "number" }
+      },
+      "allOf": [
+        {
+          "if": { "properties": { "_type": { "const": "component" } } },
+          "then": {
+            "required": ["x_mm", "y_mm", "angle_deg", "image_path", "interfaces"],
+            "properties": {
+              "image_path": { "type": "string" },
+              "object_height_mm": { "type": "number" },
+              "interfaces": { "type": "array", "items": { "$ref": "#/$defs/interface" } }
+            }
+          }
+        },
+        {
+          "if": { "properties": { "_type": { "const": "source" } } },
+          "then": {
+            "required": ["x_mm", "y_mm", "angle_deg"],
+            "properties": {
+              "wavelength_nm": { "type": "number" },
+              "n_rays": { "type": "number" }
+            }
+          }
+        }
+      ]
+    },
+    "interface": {
+      "type": "object",
+      "required": ["element_type", "x1_mm", "y1_mm", "x2_mm", "y2_mm"],
+      "properties": {
+        "element_type": {
+          "enum": [
+            "beam_block",
+            "lens",
+            "mirror",
+            "beam_splitter",
+            "dichroic",
+            "polarizing_interface",
+            "polarizer",
+            "waveplate",
+            "pbs",
+            "detector",
+            "refractive",
+            "faraday"
+          ]
+        },
+        "x1_mm": { "type": "number" },
+        "y1_mm": { "type": "number" },
+        "x2_mm": { "type": "number" },
+        "y2_mm": { "type": "number" },
+        "name": { "type": "string" },
+        "efl_mm": { "type": "number" },
+        "reflectivity": { "type": "number" },
+        "clear_aperture_mm": { "type": "number" },
+        "split_T": { "type": "number" },
+        "split_R": { "type": "number" },
+        "is_polarizing": { "type": "boolean" },
+        "pbs_transmission_axis_deg": { "type": "number" }
+      }
+    },
+    "ruler": {
+      "type": "object",
+      "required": ["type", "points"],
+      "properties": {
+        "type": { "const": "ruler" },
+        "points": {
+          "type": "array",
+          "items": { "type": "array", "items": { "type": "number" }, "minItems": 2 }
+        },
+        "item_uuid": { "type": "string" }
+      }
+    },
+    "angle_measure": {
+      "type": "object",
+      "required": ["type", "vertex", "point1", "point2"],
+      "properties": {
+        "type": { "const": "angle_measure" },
+        "vertex": { "type": "array", "items": { "type": "number" }, "minItems": 2 },
+        "point1": { "type": "array", "items": { "type": "number" }, "minItems": 2 },
+        "point2": { "type": "array", "items": { "type": "number" }, "minItems": 2 },
+        "item_uuid": { "type": "string" }
+      }
+    },
+    "text": {
+      "type": "object",
+      "required": ["type", "text", "x", "y"],
+      "properties": {
+        "type": { "const": "text" },
+        "text": { "type": "string" },
+        "x": { "type": "number" },
+        "y": { "type": "number" },
+        "point_size": { "type": "number" },
+        "item_uuid": { "type": "string" }
+      }
+    },
+    "layer_state": {
+      "type": "object",
+      "required": ["nodes"],
+      "properties": {
+        "version": { "type": "integer" },
+        "nodes": { "type": "array", "items": { "$ref": "#/$defs/node" } }
+      }
+    },
+    "node": {
+      "type": "object",
+      "required": ["type"],
+      "properties": {
+        "type": { "enum": ["group", "item"] },
+        "uuid": { "type": "string" },
+        "name": { "type": "string" },
+        "collapsed": { "type": "boolean" },
+        "children": { "type": "array", "items": { "$ref": "#/$defs/node" } }
+      }
+    }
+  }
+}
diff --git a/tests/fixtures/minimal_layout_2_0.json b/tests/fixtures/minimal_layout_2_0.json
new file mode 100644
index 00000000..d1c7f223
--- /dev/null
+++ b/tests/fixtures/minimal_layout_2_0.json
@@ -0,0 +1,128 @@
+{
+  "version": "2.0",
+  "items": [
+    {
+      "_type": "source",
+      "x_mm": -100.0,
+      "y_mm": 0.0,
+      "angle_deg": 0.0,
+      "size_mm": 5.0,
+      "n_rays": 9,
+      "ray_length_mm": 600.0,
+      "spread_deg": 0.0,
+      "color_hex": "#22cc55",
+      "wavelength_nm": 780.0,
+      "polarization_type": "horizontal",
+      "polarization_angle_deg": 0.0,
+      "source_type": "ray",
+      "beam_waist_mm": 0.5,
+      "item_uuid": "src-0001",
+      "locked": false,
+      "z_value": 0.0
+    },
+    {
+      "_type": "component",
+      "x_mm": 0.0,
+      "y_mm": 0.0,
+      "angle_deg": 45.0,
+      "object_height_mm": 25.4,
+      "mm_per_pixel": 0.05,
+      "name": "Beamsplitter 50/50",
+      "image_path": "@component/bs_5050/images/bs_5050.png",
+      "reference_line_mm": [-12.7, 0.0, 12.7, 0.0],
+      "interfaces": [
+        {
+          "x1_mm": -12.7,
+          "y1_mm": 0.0,
+          "x2_mm": 12.7,
+          "y2_mm": 0.0,
+          "element_type": "beam_splitter",
+          "name": "",
+          "split_T": 50.0,
+          "split_R": 50.0,
+          "is_polarizing": false
+        }
+      ],
+      "category": "beamsplitters",
+      "notes": "",
+      "item_uuid": "cmp-0001",
+      "locked": false,
+      "z_value": 1.0
+    },
+    {
+      "_type": "component",
+      "x_mm": 120.0,
+      "y_mm": 0.0,
+      "angle_deg": 90.0,
+      "object_height_mm": 25.4,
+      "mm_per_pixel": 0.05,
+      "name": "Mirror",
+      "image_path": "@component/flat_mirror/images/flat_mirror.png",
+      "reference_line_mm": [-12.7, 0.0, 12.7, 0.0],
+      "interfaces": [
+        {
+          "x1_mm": -12.7,
+          "y1_mm": 0.0,
+          "x2_mm": 12.7,
+          "y2_mm": 0.0,
+          "element_type": "mirror",
+          "name": "",
+          "reflectivity": 100.0
+        }
+      ],
+      "category": "mirrors",
+      "notes": "",
+      "item_uuid": "cmp-0002",
+      "locked": false,
+      "z_value": 2.0
+    }
+  ],
+  "rulers": [
+    {
+      "type": "ruler",
+      "points": [[-100.0, 0.0], [0.0, 0.0]],
+      "item_uuid": "rul-0001",
+      "z_value": 0.0
+    }
+  ],
+  "texts": [
+    {
+      "type": "text",
+      "text": "Input beam",
+      "x": -90.0,
+      "y": -10.0,
+      "color": "#0a0a28",
+      "point_size": 11.0,
+      "item_uuid": "txt-0001",
+      "z_value": 0.0
+    }
+  ],
+  "angle_measures": [
+    {
+      "type": "angle_measure",
+      "vertex": [0.0, 0.0],
+      "point1": [120.0, 0.0],
+      "point2": [0.0, 120.0],
+      "item_uuid": "ang-0001",
+      "z_value": 0.0
+    }
+  ],
+  "rectangles": [],
+  "path_measures": [],
+  "layer_state": {
+    "version": 1,
+    "nodes": [
+      {
+        "uuid": "grp-0001",
+        "type": "group",
+        "name": "Beam path",
+        "collapsed": false,
+        "children": [
+          { "uuid": "src-0001", "type": "item" },
+          { "uuid": "cmp-0001", "type": "item" }
+        ]
+      },
+      { "uuid": "cmp-0002", "type": "item" }
+    ]
+  }
+}
diff --git a/tests/test_layout_schema.py b/tests/test_layout_schema.py
new file mode 100644
index 00000000..a3f7b190
--- /dev/null
+++ b/tests/test_layout_schema.py
@@ -0,0 +1,47 @@
+"""Drift guard: the canonical `2.0` layout schema (shared with optiverse-web).
+
+The desktop app is the source of the `2.0` project file format. This test pins it
+against the same JSON Schema + golden fixture that optiverse-web validates in its
+own CI, so the two apps cannot silently diverge. Keep
+``schema/optiverse-2.0.schema.json`` and ``tests/fixtures/minimal_layout_2_0.json``
+byte-identical to the optiverse-web copies.
+"""
+
+from __future__ import annotations
+
+import importlib.util
+import json
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parents[1]
+_SCHEMA = _REPO / "schema" / "optiverse-2.0.schema.json"
+_FIXTURE = _REPO / "tests" / "fixtures" / "minimal_layout_2_0.json"
+_CONSTANTS = _REPO / "src" / "optiverse" / "core" / "constants.py"
+
+
+def _file_format_version() -> str:
+    """Read FILE_FORMAT_VERSION straight from the (import-free) constants module.
+
+    Loading by file path avoids dragging in the Qt-heavy package import chain, so
+    the drift guard runs in any environment that has the source tree.
+    """
+    spec = importlib.util.spec_from_file_location("optiverse_constants", _CONSTANTS)
+    assert spec and spec.loader
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module.FILE_FORMAT_VERSION
+
+
+def test_file_format_version_matches_schema() -> None:
+    schema = json.loads(_SCHEMA.read_text())
+    assert schema["properties"]["version"]["const"] == _file_format_version()
+
+
+def test_golden_fixture_matches_schema() -> None:
+    jsonschema = pytest.importorskip("jsonschema")
+    jsonschema.validate(
+        instance=json.loads(_FIXTURE.read_text()),
+        schema=json.loads(_SCHEMA.read_text()),
+    )