diff --git a/tests/domain/scene-document-validation.test.ts b/tests/domain/scene-document-validation.test.ts
index 6eb5f517..6a7c3738 100644
--- a/tests/domain/scene-document-validation.test.ts
+++ b/tests/domain/scene-document-validation.test.ts
@@ -578,6 +578,7 @@ describe("validateSceneDocument", () => {
           interactionAngleDegrees: Number.NaN,
           allowLookInputTargetSwitch: "yes" as unknown as boolean,
           targetButtonCyclesActiveTarget: 1 as unknown as boolean,
+          invertMouseCameraHorizontal: "yes" as unknown as boolean,
           movementTemplate: {
             kind: "invalidTemplate",
             moveSpeed: 0,
diff --git a/tests/unit/entity-instances.test.ts b/tests/unit/entity-instances.test.ts
index eb7eea2b..473ba298 100644
--- a/tests/unit/entity-instances.test.ts
+++ b/tests/unit/entity-instances.test.ts
@@ -19,6 +19,7 @@ import {
   DEFAULT_PLAYER_START_CROUCH_SETTINGS,
   DEFAULT_PLAYER_START_EYE_HEIGHT,
   DEFAULT_PLAYER_START_ALLOW_LOOK_INPUT_TARGET_SWITCH,
+  DEFAULT_PLAYER_START_INVERT_MOUSE_CAMERA_HORIZONTAL,
   DEFAULT_PLAYER_START_INTERACTION_ANGLE_DEGREES,
   DEFAULT_PLAYER_START_INTERACTION_REACH_METERS,
   DEFAULT_PLAYER_START_JUMP_SETTINGS,
@@ -59,6 +60,8 @@ describe("entity registry defaults", () => {
         DEFAULT_PLAYER_START_ALLOW_LOOK_INPUT_TARGET_SWITCH,
       targetButtonCyclesActiveTarget:
         DEFAULT_PLAYER_START_TARGET_BUTTON_CYCLES_ACTIVE_TARGET,
+      invertMouseCameraHorizontal:
+        DEFAULT_PLAYER_START_INVERT_MOUSE_CAMERA_HORIZONTAL,
       movementTemplate: {
         kind: "default",
         moveSpeed: DEFAULT_PLAYER_START_MOVE_SPEED,
diff --git a/tests/unit/player-start-interaction-reach-serialization.test.ts b/tests/unit/player-start-interaction-reach-serialization.test.ts
index bcb0b5ba..96da400e 100644
--- a/tests/unit/player-start-interaction-reach-serialization.test.ts
+++ b/tests/unit/player-start-interaction-reach-serialization.test.ts
@@ -3,11 +3,13 @@ import { describe, expect, it } from "vitest";
 import { migrateSceneDocument } from "../../src/document/migrate-scene-document";
 import {
   createEmptySceneDocument,
+  PLAYER_START_TARGETING_SETTINGS_SCENE_DOCUMENT_VERSION,
   PLAYER_START_INTERACTION_REACH_SCENE_DOCUMENT_VERSION,
   PLAYER_START_INTERACT_BINDINGS_SCENE_DOCUMENT_VERSION
 } from "../../src/document/scene-document";
 import {
   DEFAULT_PLAYER_START_ALLOW_LOOK_INPUT_TARGET_SWITCH,
+  DEFAULT_PLAYER_START_INVERT_MOUSE_CAMERA_HORIZONTAL,
   DEFAULT_PLAYER_START_INTERACTION_ANGLE_DEGREES,
   DEFAULT_PLAYER_START_INTERACTION_REACH_METERS,
   DEFAULT_PLAYER_START_TARGET_BUTTON_CYCLES_ACTIVE_TARGET,
@@ -52,6 +54,7 @@ describe("Player Start interaction sector persistence", () => {
       interactionAngleDegrees: 42,
       allowLookInputTargetSwitch: false,
       targetButtonCyclesActiveTarget: true,
+      invertMouseCameraHorizontal: true,
       inputBindings: {
         keyboard: {
           clearTarget: "KeyQ"
@@ -76,6 +79,7 @@ describe("Player Start interaction sector persistence", () => {
       interactionAngleDegrees: 42,
       allowLookInputTargetSwitch: false,
       targetButtonCyclesActiveTarget: true,
+      invertMouseCameraHorizontal: true,
       inputBindings: {
         keyboard: {
           clearTarget: "KeyQ"
@@ -147,4 +151,29 @@ describe("Player Start interaction sector persistence", () => {
       }
     });
   });
+
+  it("migrates version 83 player starts to include the mouse inversion default", () => {
+    const playerStart = createPlayerStartEntity({
+      id: "entity-player-start-mouse-invert-legacy"
+    });
+    const legacyPlayerStart = {
+      ...playerStart
+    } as Record<string, unknown>;
+
+    delete legacyPlayerStart.invertMouseCameraHorizontal;
+
+    const migrated = migrateSceneDocument({
+      ...createEmptySceneDocument({ name: "Legacy Player Mouse Invert Scene" }),
+      version: PLAYER_START_TARGETING_SETTINGS_SCENE_DOCUMENT_VERSION,
+      entities: {
+        [playerStart.id]: legacyPlayerStart
+      }
+    });
+
+    expect(migrated.entities[playerStart.id]).toMatchObject({
+      kind: "playerStart",
+      invertMouseCameraHorizontal:
+        DEFAULT_PLAYER_START_INVERT_MOUSE_CAMERA_HORIZONTAL
+    });
+  });
 });