diff --git a/src/web-server/routes/image-analysis-routes.ts b/src/web-server/routes/image-analysis-routes.ts
index 25ec7fc9..db7a44f3 100644
--- a/src/web-server/routes/image-analysis-routes.ts
+++ b/src/web-server/routes/image-analysis-routes.ts
@@ -404,6 +404,9 @@ router.put('/', async (req: Request, res: Response): Promise<void> => {
         if (!normalizedBackend || normalizedModel.length === 0) {
           return acc;
         }
+        if (!knownBackends.has(normalizedBackend)) {
+          throw new Error(`Unsupported provider backend "${backendId}".`);
+        }
         acc[normalizedBackend] = normalizedModel;
         return acc;
       },
@@ -470,6 +473,10 @@ router.put('/', async (req: Request, res: Response): Promise<void> => {
 
     res.json(await buildDashboardPayload());
   } catch (error) {
+    if (error instanceof Error && error.message.startsWith('Unsupported provider backend')) {
+      res.status(400).json({ error: error.message });
+      return;
+    }
     res.status(500).json({ error: (error as Error).message });
   }
 });
diff --git a/tests/unit/web-server/image-analysis-routes.test.ts b/tests/unit/web-server/image-analysis-routes.test.ts
index 27a6ef6f..20e6ca76 100644
--- a/tests/unit/web-server/image-analysis-routes.test.ts
+++ b/tests/unit/web-server/image-analysis-routes.test.ts
@@ -340,4 +340,31 @@ describe('image-analysis routes', () => {
       error: 'Profile mapping for "codexProfile" references an unknown backend.',
     });
   });
+
+  it('rejects unsupported provider backends and preserves existing configuration', async () => {
+    const putResponse = await fetch(`${baseUrl}/api/image-analysis`, {
+      method: 'PUT',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        providerModels: {
+          gemini: 'gemini-3-flash-preview',
+          evil: 'vision-model',
+        },
+        fallbackBackend: 'gemini',
+      }),
+    });
+
+    expect(putResponse.status).toBe(400);
+    expect(await putResponse.json()).toEqual({
+      error: 'Unsupported provider backend "evil".',
+    });
+
+    const getResponse = await fetch(`${baseUrl}/api/image-analysis`);
+    expect(getResponse.status).toBe(200);
+    const payload = await getResponse.json();
+    expect(payload.config.providerModels).toMatchObject({
+      gemini: 'gemini-3-flash-preview',
+      ghcp: 'claude-haiku-4.5',
+    });
+  });
 });