Hello, Since today (December 23, 2025, KST), I have noticed an issue where Gemini starts responding in a structured / JSON-like format only when Function Calling is enabled. Until yesterday, even with Function Calling added/enabled, Gemini responded in normal natural language unless it was expli…

I’m using js-genai sdk, and it doesn’t seem to be a problem specific to the sdk.

[image] Structured output ignoring schema field names Gemini API Thank you for the update. It is great to hear that the Structured Output feature is working as expected. From this it looks like there was an update in gemini 3 for structured output.

Yes! Because it responds as if a structured output were defined. The weirdest thing we tried https://aistudio.google.com/ with a defined tool and it answer as usual :neutral_face:

Hi @user4064 , welcome to the community! Could you help me out with a few more details so that we can analyze this better? Are you using any SDKs? If yes, which one? Have you tried providing system instructions specifically for the same? If using any other SDKs other than the Google GenAI SDK, have…

[image] Srikanta_K_N: thought_signature I’m using the js-genai SDK 1.34.0 and constructing a history using the GoogleGenAI Chats object. I’m also correctly inserting thought_signature. Until yesterday, it was processing thousands of requests without issue, but today, the responses suddenly b…

Having the same issue here. Happens with both gemini-3-flash-preview and gemini-3-pro-preview. I’m using the Vercel AI SDK. Here’s an example of the output: [image] This exact setup was returning regular text content yesterday, but now the model outputs JSON for every response.

What did you guys do last night? @Srikanta_K_N tell us, what was the sneaky quite update about?…. tell us…. dont be shy, we wont tell anybody

I’m using the REST API to call the api. The same problem happens to me.

Hey All, Thank you for flagging this issue. We apologize for the inconvenience and have escalated it to our internal team for investigation. We will update you as soon as we have more information.

Gemini responds with structured / JSON-like output only when Function Calling is enabled

Gemini API

Pablo_Rodriguez_Pina December 23, 2025, 6:37am 14

Three different java projects, all using the genai java sdk (and just updated to the latest 1.32)

All working like a charm yesterday, only seen in gemini 3 flash preview

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Gemini responds with structured / JSON-like output only when Function Calling is enabled

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