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ollama - 💡(How to fix) Fix Gemma 4 json output is fenced with markdown backticks [1 comments, 2 participants]

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ollama/ollama#15595•Fetched 2026-04-16 06:35:53
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CamJNCamJN
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CamJNCamJNrick-githubrick-github
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Code Example

> const response = await ollama.chat({
>     model: "gemma4:e4b",
>     stream: false,
>     think: false,
>     keep_alive: '24h',
>     options: {
>       temperature: 0.8,
>     },
>     format: JSON.stringify(z.toJSONSchema(InvoiceSchema)),
>     messages: [
>       { role: "system", content: t.my_message_string, },
>       { role: "user", content: t.parse_template_string.replace('{context_str}', context), },
>     ]
>   });
>
RAW_BUFFERClick to expand / collapse

Using ollama 0.20.7:

With the js library:

const response = await ollama.chat({
    model: "gemma4:e4b",
    stream: false,
    think: false,
    keep_alive: '24h',
    options: {
      temperature: 0.8,
    },
    format: JSON.stringify(z.toJSONSchema(InvoiceSchema)),
    messages: [
      { role: "system", content: t.my_message_string, },
      { role: "user", content: t.parse_template_string.replace('{context_str}', context), },
    ]
  });

The response.message.content is <code>"```json\n{\n "json details": "elided"\n}\n```"</code> which has erroneous <code>```json</code> and <code>```</code> markers.

Originally posted by @CamJN in #15315

This is also true of gemma4:31b which as an additional problem doesn't follow the response schema.

extent analysis

TL;DR

The issue can be fixed by properly handling the response format to remove erroneous JSON markers.

Guidance

  • Verify that the format option in the ollama.chat function is correctly set to expect a JSON response without additional markers.
  • Check the JSON.stringify(z.toJSONSchema(InvoiceSchema)) line to ensure it's not introducing the erroneous markers.
  • Consider parsing the response content to remove the unwanted markers before processing it further.
  • Test with a different model, like gemma4:31b, to see if the issue persists and if the response schema is being followed.

Example

const responseBody = response.message.content;
const cleanedResponse = responseBody.replace(/\\```json/g, '').replace(/\\```/g, '');

Notes

The provided code snippet and issue description suggest that the problem might be related to how the response is being formatted or parsed. However, without more information about the ollama library or the z.toJSONSchema function, it's difficult to provide a more specific solution.

Recommendation

Apply workaround: The best course of action seems to be to manually clean the response content to remove the erroneous markers, as shown in the example, until a more permanent fix can be found or the library is updated to handle this correctly.

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ollama - 💡(How to fix) Fix Gemma 4 json output is fenced with markdown backticks [1 comments, 2 participants]