# [Bug]: Gemini `generateContent` returns 400 when payload violates `functionCall` → `functionResponse` ordering

## Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

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## What happened?

When calling Gemini models through the LiteLLM proxy's `/v1beta/models/{model}:generateContent` endpoint, requests fail with a **400 Bad Request** if the `contents` array does not strictly follow Gemini's tool-calling sequence rules.

Gemini requires that:

1. A `functionResponse` part **must** appear in a `user` turn that comes **immediately after** a `model` turn containing the matching `functionCall`. 2. Two consecutive turns with the same `role` (e.g. two `user` turns in a row) are not permitted when one of them carries a `functionResponse`.

When these rules are violated — which can happen organically in agentic workflows where tool history accumulates across multiple turns — Gemini rejects the request with:

``` "Please ensure that function response turn comes immediately after a function call turn." ```

LiteLLM forwards this 400 directly to the client without normalizing the message sequence, so any client that builds Gem…