[Go] Models wrap JSON in backticks breaking structured output parsing

[dmeremyanin]

Describe the bug
When using structured output with output.format: json in dotprompt files, the model sometimes wraps the JSON response in markdown code blocks (```json), which causes JSON parsing to fail.

Error message:

data is not valid JSON: invalid character '`' looking for beginning of value

To Reproduce

1. Create a dotprompt file with structured output:

---
input:
  schema: MyInput
output:
  format: json
  schema: MyOutput
---
Your prompt text here

2. Execute the prompt with prompt.Execute(ctx, input).
3. The model occasionally returns the JSON wrapped in backticks:

```json
{"field": "value"}

instead of raw JSON:

{"field": "value"}

4. This causes JSON unmarshaling to fail.

Expected behavior
The model should return raw JSON without markdown formatting when output.format: json is specified.

Screenshots

Runtime (please complete the following information):

• OS: macOS
• Version: 15.7.3

Go version:

• go1.26.0 darwin/arm64

Additional context
I'm using Go SDK, but the problem likely affects TS and Python SDKs as well, since they all share the same JSON output instructions.

This behavior was observed with the openai/gpt-4o-mini model. Adding an explicit instruction at the end of the prompt fixes the issue:

Return ONLY raw JSON. Do NOT wrap the response in backticks.

Probably, Genkit should add this instruction automatically.

[dmeremyanin]

After investigating the source code, I found that Go SDK doesn't use OpenAI's structured output API (response_format with json_schema). Instead, it relies only on prompt instructions to get JSON output, which is why the model sometimes wraps responses in markdown.

In contrast, TS and Python SDKs properly use the response_format parameter, which guarantees valid JSON without markdown formatting.

This appears to be the root cause of the issue.

[apascal07]

What version of Genkit are you using? This should work, the JSON formatter should be stripping any JSON Markdown tags.

@hugoaguirre please investigate

[dmeremyanin]

@apascal07 I'm using Genkit v1.4.0

[hugoaguirre]

Hi guys, so, after digging a little bit on this, I found out that we need to handle the JSON output format on the compat_oai plugin and also, we have to make the JSON parser regex a little bit loose for scenarios where the surrounded markdown syntax is not provided properly by the model.
e.g.:
``` json: white space before language identifier
```{\"foo\", 1}: implicit json blocks
```[1, 2]: implicit json block (array)

I'll make the fixes in the json parser at the framework level and I'll make sure to include these test cases. In the meantime, we can proceed and merge @dmeremyanin's PR for compat_oai.