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…

We also stumbled across this issue using the OpenAI style endpoint for Gemini 3 models with tools. Since roughly a day, we get a Json string in the message content instead of just a normal string. Would like to have that resolved. For now the fallback is to go back to 2.5 flash instead of 3 flash.

Same happening for me with gemini 3 pro. But as well have infinite loop. It keeps returning same text when in a loop with AI SDK. And can not finish [529765060-5474e260-1b99-4bc2-a45d-2c31af0d6ea2]

The same is happening for me. Both Gemini 3 Pro and Flash behave the same way. I am using the Vercel AI SDK, I didn’t change anything in the code recently, and suddenly, Gemini 3 models started behaving strangely. ai@5.0.113 @ai-sdk/google@2.0.46

On Vercel AI SDK, this solved it for me for now.

I’m seeing this as well on gemini flash 2.5 and 3.0. Using the HTTP API. Regression seems to have started yesterday (Dec 22). All responses are coming back as JSON now (not using structured output, setting functionCallingConfig.mode to AUTO)

thanks for sharing same fix is working for me on genai, as long as i don’t set tool_config to AUTO it works again

I have tried @khbiotech ‘s approach, the issues is still going, i removed AUTO and sometimes it sends text parts that look like tool calls without the tool name and IT NEVER produces a text part if there is a tool call, there is no reason for that, a single turn can contain both text parts and tool…

Good news! Here is some workaround that works in the ai-sdk. You must apply a patch. Here is the GitHub issue discussion: [Bug] Gemini 3 Pro/Flash Preview models output internal JSON as text when tools are provided · Issue #11396 · vercel/ai · GitHub Here is an example commit you implement with up…

The provided work arounds (not setting AUTO) dont fix the problem, the model still tries to call tools using text parts (using latest java genai sdk)

Same problem here, with flash3 and gemini3 pro. I am in a refactoring process, so you can imagine that I thought the problem was with my code. I will change to Anthropic for now; I hope we get a fix on this.

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

Gemini API

Orca_flow December 24, 2025, 5:14pm 52

Yes I can help you with some code. Should I just put it here ?

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

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