feat(genapi): add json body invalid troubleshooting

Author: fpagny

pages/generative-apis/troubleshooting/fixing-common-issues.mdx

@@ -26,6 +26,19 @@ Below are common issues that you may encounter when using Generative APIs, their
   - `15k` tokens context window on `H100` Instances
   - `128k` tokens context window on `H100-2` Instances
 
+## 400: Bad Request - Invalid JSON body
+
+### Cause
+- You provided a JSON body that does not follow [standard JSON structure](https://datatracker.ietf.org/doc/html/rfc8259), therefore your request cannot be analyzed.
+
+### Solution
+- Verify the JSON body you send is valid:
+  - You can store your content in a file with `.json` extension (eg. named `file.json`), and open it with an IDE such as VSCode or Zed. Syntax errors should display if there are present.
+  - You can copy your content in a JSON formatter tool or linter available online, that will identify errors.
+  - Usually, most common errors include: 
+    - Missing or unecessary quotes `"`, `'` or commas `,`  on properties name and string values.
+    - Special characters that are not escaped, such as line break `\n` or backslash `\\`
+
 ## 403: Forbidden - Insufficient permissions to access the resource
 
 ### Cause
@@ -234,4 +247,4 @@ This snippet will output the model response, which is `4`.
 ### Debugging silent errors
 - For cases where no explicit error is returned:
   - Verify all fields in the API request are correctly named and formatted.
-  - Test the request with smaller and simpler inputs to isolate potential issues.
+  - Test the request with smaller and simpler inputs to isolate potential issues.