Why Does the Gemini Chat Model Return a String Instead of a JSON Object in Output Parsing?

Below is an explanation of the issue and some suggestions on how to address it:

What Is Happening

• Error Explanation:
  The error message

  [ { “code”: “invalid_type”, “expected”: “object”, “received”: “string”, “path”: [ “output” ], “message”: “Expected object, received string” } ]
  

  indicates that your output parser expected a JSON object for the key "output" but instead received a string.

• Different Behaviors:
  When using the OpenAI node, the agent produces a structured response (i.e. an object inside the "response" key), like:

  JSONCopied
  [
    { 
      "action": "parse",
      "response": { 
        "output":  {
          "capitulos": [
            { "numero": 1, "titulo": "El principito" },
            { "numero": 2, "titulo": "Índice" },
            ...
          ]
        }
      }
    }
  ]

  However, Gemini (with the auto-fixing Output Parser) returns:

  JSONCopied
  [
    { 
      "action": "parse",
      "text": "{\"output\":\"unknown\"}"
    }
  ]

  Note that here the value for the output is a string, not an object.

• Root Cause:
  Gemini generates unstructured text by default. This means that unless explicitly instructed, Gemini may return a plain text string that encapsulates JSON-like content rather than a directly parsed object. The auto-fixing node then wraps this text as a string, leading to the type mismatch.

How to Address the Issue

1. Explicitly Request Structured JSON Output:
   Modify the prompt or System Message for the Gemini node to include a clear schema or example. For example, instruct it to produce an object directly, similar to the output seen with the OpenAI node. This guides Gemini to output data as a JSON object rather than a string.

2. Use the “Require Specific Output Format” Option:
   With Gemini—as with other AI nodes in n8n—you can enable the option to enforce a specific output format. By doing so, n8n will prompt you to attach an appropriate output parser node (such as the Structured Output Parser). This parser will expect the output to be structured, pushing you to adjust the prompt accordingly.

3. Verify Your Output Parser Node Setup:
   Ensure that the auto-fixing or Structured Output Parser node attached to your Gemini node is correctly configured to parse JSON objects. If necessary, change or customize the parser instructions so that it converts the incoming text into an object.

Summary

The error arises because Gemini is returning a string (that contains what looks like JSON) instead of a JSON object. Adjusting your prompt to enforce a strict JSON schema for the agent’s output and using the dedicated output parsing options in n8n will help resolve the type mismatch error.

If you need further details on configuring output parsers or insights into the Gemini API’s structured output functionality, let me know!