diff --git a/src/web/routes/case-routes.ts b/src/web/routes/case-routes.ts
index 66a073f0..8eb2ff87 100644
--- a/src/web/routes/case-routes.ts
+++ b/src/web/routes/case-routes.ts
@@ -60,6 +60,7 @@ import {
   isAdmin,
   isWorkingDirAllowed,
   ownerFor,
+  requireAdmin,
   resolveCasesDir,
   SETTINGS_PATH,
   validatePathWithinBase,
@@ -1665,7 +1666,13 @@ export function registerCaseRoutes(app: FastifyInstance, ctx: EventPort & Config
 
   // ========== Reorder Cases ==========
 
-  app.put('/api/cases/order', async (req): Promise<ApiResponse<{ order: string[] }>> => {
+  app.put('/api/cases/order', async (req, reply): Promise<ApiResponse<{ order: string[] }> | undefined> => {
+    // `caseOrder` is ONE list in the server-wide settings.json, read by every user's
+    // GET /api/cases, so a non-admin's reorder would replace everyone's order (and
+    // publish their case names to every user through GET /api/settings). Admin-only in
+    // multi-user mode, like PUT /api/settings; a per-user order would need per-user
+    // storage. No-op in single-user mode.
+    if (isMultiUserMode() && !requireAdmin(req, reply)) return;
     const { order } = parseBody(CaseOrderSchema, req.body, 'Invalid order data');
 
     try {
diff --git a/src/web/routes/ralph-routes.ts b/src/web/routes/ralph-routes.ts
index ae068b28..e8df0f74 100644
--- a/src/web/routes/ralph-routes.ts
+++ b/src/web/routes/ralph-routes.ts
@@ -16,6 +16,7 @@ import { SseEvent } from '../sse-events.js';
 import {
   autoConfigureRalph,
   getAuthUser,
+  isAdmin,
   ownerFor,
   resolveCasesDir,
   sessionCapacityMessage,
@@ -463,20 +464,25 @@ export function registerRalphRoutes(
       });
     }
 
-    // Save lastUsedCase
-    try {
-      let settings: Record<string, unknown> = {};
+    // Save lastUsedCase. It is a key of the server-wide settings.json, readable by every
+    // user through GET /api/settings, so a non-admin's case name is not written there in
+    // multi-user mode (writes are admin-only: PUT /api/settings). isAdmin is always true
+    // in single-user mode.
+    if (isAdmin(req)) {
       try {
-        settings = JSON.parse(await fs.readFile(SETTINGS_PATH, 'utf-8'));
+        let settings: Record<string, unknown> = {};
+        try {
+          settings = JSON.parse(await fs.readFile(SETTINGS_PATH, 'utf-8'));
+        } catch {
+          /* ignore */
+        }
+        settings.lastUsedCase = caseName;
+        const dir = dirname(SETTINGS_PATH);
+        if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
+        fs.writeFile(SETTINGS_PATH, JSON.stringify(settings, null, 2)).catch(() => {}); // Ignore - persisting lastUsedCase is non-critical
       } catch {
-        /* ignore */
+        /* non-critical */
       }
-      settings.lastUsedCase = caseName;
-      const dir = dirname(SETTINGS_PATH);
-      if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
-      fs.writeFile(SETTINGS_PATH, JSON.stringify(settings, null, 2)).catch(() => {}); // Ignore - persisting lastUsedCase is non-critical
-    } catch {
-      /* non-critical */
     }
 
     const sessionId = session.id;
diff --git a/src/web/routes/session-routes.ts b/src/web/routes/session-routes.ts
index 92529530..d0ebc07e 100644
--- a/src/web/routes/session-routes.ts
+++ b/src/web/routes/session-routes.ts
@@ -4477,28 +4477,33 @@ export function registerSessionRoutes(
         ).catch(() => {});
       }
 
-      // Save lastUsedCase to settings for TUI/web sync
-      try {
-        const settingsFilePath = SETTINGS_PATH;
-        let settings: Record<string, unknown> = {};
+      // Save lastUsedCase to settings for TUI/web sync. It is a key of the server-wide
+      // settings.json, readable by every user through GET /api/settings, so a non-admin's
+      // case name is not written there in multi-user mode (writes are admin-only, like
+      // PUT /api/settings). isAdmin is always true in single-user mode.
+      if (isAdmin(req)) {
         try {
-          settings = JSON.parse(await fs.readFile(settingsFilePath, 'utf-8'));
+          const settingsFilePath = SETTINGS_PATH;
+          let settings: Record<string, unknown> = {};
+          try {
+            settings = JSON.parse(await fs.readFile(settingsFilePath, 'utf-8'));
+          } catch (err) {
+            if ((err as NodeJS.ErrnoException).code !== 'ENOENT') throw err;
+          }
+          settings.lastUsedCase = caseName;
+          const dir = dirname(settingsFilePath);
+          if (!existsSync(dir)) {
+            mkdirSync(dir, { recursive: true });
+          }
+          // Use async write to avoid blocking event loop
+          fs.writeFile(settingsFilePath, JSON.stringify(settings, null, 2)).catch((err) => {
+            // Non-critical but log for debugging
+            console.warn('[Server] Failed to save settings (lastUsedCase):', err);
+          });
         } catch (err) {
-          if ((err as NodeJS.ErrnoException).code !== 'ENOENT') throw err;
-        }
-        settings.lastUsedCase = caseName;
-        const dir = dirname(settingsFilePath);
-        if (!existsSync(dir)) {
-          mkdirSync(dir, { recursive: true });
-        }
-        // Use async write to avoid blocking event loop
-        fs.writeFile(settingsFilePath, JSON.stringify(settings, null, 2)).catch((err) => {
           // Non-critical but log for debugging
-          console.warn('[Server] Failed to save settings (lastUsedCase):', err);
-        });
-      } catch (err) {
-        // Non-critical but log for debugging
-        console.warn('[Server] Failed to prepare settings update:', err);
+          console.warn('[Server] Failed to prepare settings update:', err);
+        }
       }
 
       return {
diff --git a/src/web/routes/system-routes.ts b/src/web/routes/system-routes.ts
index 91f49b04..3b9b2ca8 100644
--- a/src/web/routes/system-routes.ts
+++ b/src/web/routes/system-routes.ts
@@ -869,7 +869,10 @@ export function registerSystemRoutes(
     return { config: ctx.store.getConfig() };
   });
 
-  app.put('/api/config', async (req) => {
+  app.put('/api/config', async (req, reply) => {
+    // Process-wide app config in state.json (poll interval, timeouts, the session cap,
+    // respawn defaults): admin-only in multi-user mode, like PUT /api/settings.
+    if (isMultiUserMode() && !requireAdmin(req, reply)) return;
     const configData = parseBody(ConfigUpdateSchema, req.body, 'Invalid config');
     ctx.store.setConfig(configData as Partial<ReturnType<typeof ctx.store.getConfig>>);
     return { config: ctx.store.getConfig() };
@@ -953,7 +956,13 @@ export function registerSystemRoutes(
     return readJsonConfig(SETTINGS_PATH, 'settings', {});
   });
 
-  app.put('/api/settings', async (req) => {
+  app.put('/api/settings', async (req, reply) => {
+    // Multi-user mode: settings.json is ONE file for the whole server (feature switches,
+    // the plan-usage collection switch, run mode, respawn presets, the tunnel), so a write
+    // is admin-only, like every other server-wide setting (docs/multi-user-plan.md,
+    // Settings row). Refused FIRST: nothing is parsed, written, toggled or started for a
+    // non-admin. GET above stays readable by everyone. No-op in single-user mode.
+    if (isMultiUserMode() && !requireAdmin(req, reply)) return;
     const settings = parseBody(SettingsUpdateSchema, req.body, 'Invalid settings') as Record<string, unknown>;
 
     // COD-55: enabling the Cloudflare tunnel publishes the whole app (full terminal
@@ -1092,7 +1101,10 @@ export function registerSystemRoutes(
     return { success: true, data: settings.modelConfig || {} };
   });
 
-  app.put('/api/execution/model-config', async (req) => {
+  app.put('/api/execution/model-config', async (req, reply) => {
+    // The `modelConfig` key of the server-wide settings.json (cron's and the plan
+    // orchestrator's default model): admin-only in multi-user mode, like PUT /api/settings.
+    if (isMultiUserMode() && !requireAdmin(req, reply)) return;
     const modelConfig = parseBody(ModelConfigUpdateSchema, req.body, 'Invalid model config') as Record<string, unknown>;
 
     try {