diff --git a/server/catalog-server.mjs b/server/catalog-server.mjs
index 7d46b9c..7a6e1da 100644
--- a/server/catalog-server.mjs
+++ b/server/catalog-server.mjs
@@ -637,15 +637,26 @@ function promoteLegacyGridLodProfiles(settings) {
       selectionOutlineOpacityPercent: _selectionOutlineOpacityPercent,
       ...legacyFallback
     } = fallback;
+    const stepKm = settings[`gridLod${number}StepKm`] ?? fallback.stepKm;
+    const legacyRadiusKm = settings.gridRadiusKm ?? fallback.radiusKm;
+    // The flat v1 setting described one uniform grid and could legitimately
+    // be much larger than a fine LOD band can render. Migrate it as an upper
+    // bound per band so an existing application remains readable while the
+    // new five-band workload stays within its explicit safety limit.
+    const radiusKm = Math.min(
+      legacyRadiusKm,
+      fallback.radiusKm,
+      stepKm * MAX_LOCAL_GRID_INDEX,
+    );
     return {
       ...legacyFallback,
       maxHeightKm: settings[`gridLod${number}MaxHeightKm`] ?? fallback.maxHeightKm,
-      stepKm: settings[`gridLod${number}StepKm`] ?? fallback.stepKm,
+      stepKm,
       mode: settings[`gridLod${number}Mode`] ?? fallback.mode,
       heightMeters: settings.gridHeightMeters ?? fallback.heightMeters,
       max3dViewAngleDegrees: settings.gridMax3dViewAngleDegrees ?? fallback.max3dViewAngleDegrees,
       tileSizeKm: settings.gridTileSizeKm ?? fallback.tileSizeKm,
-      radiusKm: settings.gridRadiusKm ?? fallback.radiusKm,
+      radiusKm,
       lineDiameterMeters: settings.gridLineDiameterMeters ?? fallback.lineDiameterMeters,
       lineColor: settings.gridColor ?? fallback.lineColor,
       lineOpacity: settings.gridOpacity ?? fallback.lineOpacity,
@@ -658,7 +669,10 @@ function promoteLegacyGridLodProfiles(settings) {
       crossesWidthMeters: settings.gridCrossesWidthMeters ?? fallback.crossesWidthMeters,
       crossesColor: settings.gridCrossesColor ?? fallback.crossesColor,
       crossesOpacity: settings.gridCrossesOpacity ?? fallback.crossesOpacity,
-      graticuleLineWidthPx: settings.gridLineWidth ?? fallback.graticuleLineWidthPx,
+      graticuleLineWidthPx: Math.min(
+        3,
+        Math.max(1, settings.gridLineWidth ?? fallback.graticuleLineWidthPx),
+      ),
       graticuleColor: settings.gridColor ?? fallback.graticuleColor,
       graticuleOpacity: settings.gridOpacity ?? fallback.graticuleOpacity,
     };
diff --git a/server/map-grid-persistence.test.mjs b/server/map-grid-persistence.test.mjs
index 5ad8ffb..82ec0fd 100644
--- a/server/map-grid-persistence.test.mjs
+++ b/server/map-grid-persistence.test.mjs
@@ -98,6 +98,32 @@ test("flat layouts migrate mode-aware extensions while explicit invalid hierarch
   );
 });
 
+test("Robot2B legacy radius migrates per LOD without hiding the application", () => {
+  const legacyRobot2BSettings = {
+    gridHeightMeters: 500,
+    gridLod1MaxHeightKm: 10,
+    gridLod1StepKm: 1,
+    gridLod2MaxHeightKm: 50,
+    gridLod2StepKm: 5,
+    gridLod3StepKm: 25,
+    gridRadiusKm: 1_000,
+    gridLineWidth: 4,
+    gridColor: "#f5f5f5",
+    gridOpacity: 12,
+    gridDotsEnabled: true,
+    gridDotsSize: 7,
+    gridDotsColor: "#ffffff",
+    gridDotsOpacity: 58,
+  };
+  const migrated = structuredClone(serverGrid.validateGridLodProfiles(
+    serverGrid.promoteLegacyGridLodProfiles(legacyRobot2BSettings),
+  ));
+
+  assert.deepEqual(migrated.map((profile) => profile.radiusKm), [50, 1_000, 1_000, 1_000, 100]);
+  assert.ok(migrated.every((profile) => profile.radiusKm / profile.stepKm <= 512));
+  assert.ok(migrated.every((profile) => profile.graticuleLineWidthPx === 3));
+});
+
 test("Foundry MCP exposes every persisted sector-v2 profile field", () => {
   for (const key of extensionKeys) assert.match(mcpSource, new RegExp(`\\b${key}: \\{`));
 });