AWS Bedrock Mantle support

OSS Product Help LangChain
1

A lot of Bedrocks newest models are only launching on Bedrock Mantle and not Bedrock runtime. GPT 5.5 and Gemma 4 for instance.

I can’t see anywhere in the docs on how to use Mantle with ChatBrdrockConverse or some equivalent. Is Bedrock Mantle not supported?

2

You’re right that there’s a gap here. Per the LangChain docs, ChatBedrockConverse targets the Bedrock Runtime Converse API, not Bedrock Mantle. Mantle isn’t documented or supported as a first-class integration in langchain-aws today.

Bedrock Mantle is a separate OpenAI-compatible inference layer (AWS announcement). Models like GPT 5.5 and Gemma 4 that launch only on Mantle won’t work through ChatBedrockConverse.

Workaround today: use ChatOpenAI with Mantle’s base URL, per the OpenAI-compatible endpoints docs:

from langchain_openai import ChatOpenAI

llm = ChatOpenAI(
    model="openai.gpt-oss-120b",  # use the Mantle model ID
    base_url="https://bedrock-mantle.us-east-1.api.aws/v1",
    api_key="...",  # Bedrock API key, or configure AWS SigV4 auth
)

This works for basic chat completions. Caveats:

  • ChatOpenAI only implements the standard OpenAI spec, Mantle-specific fields (e.g. reasoning) won’t be extracted
  • No built-in support for Mantle’s Responses API, Projects API, or other Mantle-only features
  • Auth may need extra setup for IAM/SigV4 vs. a Bedrock API key

Bottom line: Mantle isn’t officially supported yet. The OpenAI-compatible route is the practical option for now. A dedicated ChatBedrockMantle (or similar) in langchain-aws would be the right long-term fix, worth opening a feature request on langchain-ai/langchain-aws if you need first-class support.

4

@keenborder786 Models like GPT-5.5 aren’t supported by ChatOpenAI because AWS Bedrock Mantle doesn’t provide the Chat Completions API for them: GPT-5.5 - Amazon Bedrock .

ChatOpenAI works only with models that support the Chat Completions API, such as gpt-oss-120b and gpt-oss-20b.

5

You are right that GPT‑5.5 is a gap; on Mantle it’s Responses API only (.../openai/v1), so ChatOpenAI with default Chat Completions routing won’t reach it.

However, Gemma 4 is still different, though: Mantle exposes it on both Chat Completions and Responses (AWS blog). So ChatOpenAI(model="google.gemma-4-31b", base_url="https://bedrock-mantle.{region}.api.aws/openai/v1", api_key="<bedrock-key>") should work today via Chat Completions.

However, I would suggest you try the Responses API. You can experiment with:

ChatOpenAI(

model="openai.gpt-5.5",

use_responses_api=True,

base_url="https://bedrock-mantle.{region}.api.aws/openai/v1",

api_key="<bedrock-api-key>",

)

This is undocumented for Mantle and only covers Bearer/API‑key auth (IAM/SigV4 won’t work through ChatOpenAI). There’s no first‑class Mantle integration in langchain-aws yet, so a feature request on langchain-ai/langchain-aws is STILL the right long‑term path.