diff --git a/dev/src/server/adk_api_server.ts b/dev/src/server/adk_api_server.ts
index a52ad63ab..701bb152a 100644
--- a/dev/src/server/adk_api_server.ts
+++ b/dev/src/server/adk_api_server.ts
@@ -863,11 +863,13 @@ export class AdkApiServer {
         });
         req.on('end', async () => {
           this.logger.info(`Received Reasoning Engine raw body: ${rawBody}`);
-          // eslint-disable-next-line @typescript-eslint/no-explicit-any
-          let body: any = {};
+          let body: object = {};
           if (rawBody) {
             try {
-              body = JSON.parse(rawBody);
+              const parsed: unknown = JSON.parse(rawBody);
+              if (typeof parsed === 'object' && parsed !== null) {
+                body = parsed;
+              }
             } catch (e) {
               this.logger.error(`Failed to parse raw body as JSON: ${e}`);
             }
diff --git a/dev/test/server/adk_api_server_test.ts b/dev/test/server/adk_api_server_test.ts
index d6db712d2..b79153a66 100644
--- a/dev/test/server/adk_api_server_test.ts
+++ b/dev/test/server/adk_api_server_test.ts
@@ -1152,6 +1152,34 @@ describe('AdkWebServer', () => {
       );
     });
 
+    it('should return 400 when the raw body is JSON null', async () => {
+      const res = await fetch(`${server.url}/api/reasoning_engine`, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json,application/json',
+        },
+        body: 'null',
+      });
+
+      expect(res.status).toBe(400);
+      const data = (await res.json()) as {error: string};
+      expect(data.error).toContain('appName is required');
+    });
+
+    it('should return 400 when the raw body is a JSON primitive', async () => {
+      const res = await fetch(`${server.url}/api/reasoning_engine`, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json,application/json',
+        },
+        body: '"hello"',
+      });
+
+      expect(res.status).toBe(400);
+      const data = (await res.json()) as {error: string};
+      expect(data.error).toContain('appName is required');
+    });
+
     it('should return 400 if appName is missing', async () => {
       try {
         await client.post('/api/reasoning_engine', {