## Summary

GPT 5.5 on Amazon Bedrock runs on a new platform (Bedrock Mantle) that **only supports the Response API** (`/v1/responses`). It does NOT support the Chat Completions API (`/v1/chat/completions`).

Our enterprise customers use LiteLLM as a unified proxy and call all models via `/chat/completions`. Currently GPT 5.5 on Bedrock cannot be used through LiteLLM's standard chat interface.

## Request

LiteLLM should automatically convert Chat Completions requests to Response API format when the target model only supports Response API (e.g., GPT 5.5 on Bedrock Mantle).

## Current Behavior

- `litellm.completion(model="bedrock/gpt-5.5", messages=[...])` → fails or unsupported - GPT 5.5 on Bedrock only accepts Response API format via bedrock-mantle endpoint - Endpoint: `https://bedrock-mantle.{region}.api.aws/v1/responses` - Auth: API Key (not SigV4)

## Expected Behavior

- `litellm.completion(model="bedrock/gpt-5.5", messages=[...])` → works - LiteLLM detects GPT 5.5 uses Mantle/Response API - Auto-converts: messages[] → Response API input format - Auto-converts: Response API output → ChatCompletion response object - Transparent to caller — no code changes needed

## Key …