diff --git a/src/server/management/model-routes.ts b/src/server/management/model-routes.ts
index de218f205..2eb49fa6c 100644
--- a/src/server/management/model-routes.ts
+++ b/src/server/management/model-routes.ts
@@ -319,8 +319,9 @@ export async function handleModelRoutes(ctx: ManagementContext): Promise<Respons
   }
 
   if (url.pathname === "/api/custom-models" && req.method === "POST") {
-    let body: { provider?: unknown; modelId?: unknown; displayName?: unknown; contextWindow?: unknown; inputModalities?: unknown };
+    let body: unknown;
     try { body = await readManagementJsonBody(req); } catch (error) { rethrowManagementBodyTooLarge(error); return jsonResponse({ error: "invalid JSON body" }, 400); }
+    if (!isPlainRecord(body)) return jsonResponse({ error: "invalid JSON body" }, 400);
     const provider = typeof body.provider === "string" ? body.provider.trim() : "";
     const modelId = typeof body.modelId === "string" ? body.modelId.trim() : "";
     if (!provider || !modelId) return jsonResponse({ error: "provider and modelId are required" }, 400);
@@ -357,8 +358,9 @@ export async function handleModelRoutes(ctx: ManagementContext): Promise<Respons
   if (customPutMatch && req.method === "PUT") {
     let id: string;
     try { id = decodeURIComponent(customPutMatch[1]); } catch { return jsonResponse({ error: "invalid id encoding" }, 400); }
-    let body: { displayName?: unknown; contextWindow?: unknown; inputModalities?: unknown; modelId?: unknown };
+    let body: unknown;
     try { body = await readManagementJsonBody(req); } catch (error) { rethrowManagementBodyTooLarge(error); return jsonResponse({ error: "invalid JSON body" }, 400); }
+    if (!isPlainRecord(body)) return jsonResponse({ error: "invalid JSON body" }, 400);
     const list = config.customModels ?? [];
     const idx = list.findIndex(cm => cm.id === id);
     if (idx === -1) return jsonResponse({ error: "not found" }, 404);
diff --git a/tests/catalog-input-modality-enum.test.ts b/tests/catalog-input-modality-enum.test.ts
index e13e8762d..3b0afc13e 100644
--- a/tests/catalog-input-modality-enum.test.ts
+++ b/tests/catalog-input-modality-enum.test.ts
@@ -120,6 +120,20 @@ describe("custom-model API rejects out-of-enum input modalities", () => {
     expect(persistCalls).toBe(0);
   });
 
+  test("custom-model writes reject non-object JSON bodies", async () => {
+    for (const body of [null, [], "model"]) {
+      persistCalls = 0;
+      const posted = await callCustomModels("POST", body);
+      expect(posted?.status).toBe(400);
+      expect(await posted!.json()).toEqual({ error: "invalid JSON body" });
+
+      const put = await callCustomModels("PUT", body, "/api/custom-models/existing-uuid");
+      expect(put?.status).toBe(400);
+      expect(await put!.json()).toEqual({ error: "invalid JSON body" });
+      expect(persistCalls).toBe(0);
+    }
+  });
+
   test("an accepted modality set still passes", async () => {
     persistCalls = 0;
     const res = await callCustomModels("POST", {