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API compatibility - 3 documented limits

Amazon Bedrock API compatibility

Amazon Bedrock documents an OpenAI-compatible API at https://bedrock-runtime.{region}.amazonaws.com/openai/v1 (chat/completions, responses and models (bedrock-mantle only)). Its Anthropic-compatible base URL, the value for ANTHROPIC_BASE_URL, is https://bedrock-runtime.{region}.amazonaws.com/anthropic, and it serves Claude models only. The docs list 3 limitations: 3 for the OpenAI-compatible layer.

Verified against Amazon Bedrock's own documentation. Not documented means the docs read that day do not say; it never means No.

Compare every provider in the API compatibility matrix.

OpenAI-compatible API

Supported
Base URL
https://bedrock-runtime.{region}.amazonaws.com/openai/v1
Endpoints
  • chat/completions
  • responses
  • models (bedrock-mantle only)
Auth
Authorization: Bearer <Amazon Bedrock API key>, or AWS SigV4 credentials
Regional and alternate URLs
  • Bedrock Runtimehttps://bedrock-runtime.{region}.amazonaws.com/openai/v1
  • Bedrock Mantlehttps://bedrock-mantle.{region}.api.aws/v1
Notes
bedrock-runtime is recommended for new applications; bedrock-mantle is a compatibility option when a model or capability is not on bedrock-runtime. On bedrock-mantle some models are served under /openai/v1 instead of /v1; check each model card. Per-model API support varies (for example GPT OSS supports Chat Completions but not Responses on bedrock-runtime). The base URL is per region.

From the docs

bedrock-runtime (recommended) | https://bedrock-runtime.{region}.amazonaws.com/openai/v1/chat/completions | AWS credentials (SigV4) or Amazon Bedrock API key ... bedrock-mantle (compatibility) | https://bedrock-mantle.{region}.api.aws/v1/chat/completions | Amazon Bedrock API key or AWS credentials
docs.aws.amazon.com/bedrock/latest/userguide/inference-chat-completions.html

Anthropic-compatible API

Supported

Claude models only. This endpoint gives native Messages API access to Anthropic's Claude models; it does not put other model families behind the Anthropic format.

ANTHROPIC_BASE_URL
https://bedrock-runtime.{region}.amazonaws.com/anthropic
Auth
x-api-key: <Amazon Bedrock API key> (docs use $AWS_BEARER_TOKEN_BEDROCK / BEDROCK_API_KEY), a short-term Bedrock bearer token, or AWS SigV4
Regional and alternate URLs
  • Bedrock Runtimehttps://bedrock-runtime.{region}.amazonaws.com/anthropic
  • Bedrock Mantlehttps://bedrock-mantle.{region}.api.aws/anthropic
Notes
This is native Anthropic Messages API access to Claude models on Bedrock, not a translation layer for other model families. bedrock-mantle requires the anthropic-version: 2023-06-01 header; count_tokens is on bedrock-mantle.

From the docs

The Anthropic Messages API provides native access to Claude models using the Anthropic request and response format. You can use the Messages API on both the bedrock-runtime and bedrock-mantle endpoints. For new applications, we recommend the bedrock-runtime endpoint.
docs.aws.amazon.com/bedrock/latest/userguide/inference-messages-api.md

Documented limitations

Parameters, content types and features Amazon Bedrock's docs say its compatibility layers ignore, reject or change, each with the line it comes from. A layer with nothing listed may still differ in ways the docs do not spell out.

OpenAI-compatible layer3

Anthropic-compatible layer0

No limitations listed in the docs read.

Example from Amazon Bedrock's docs

Copied verbatim from the provider's documentation, placeholders included.

Provider example
from openai import OpenAI
import os

client = OpenAI(
    base_url="https://bedrock-runtime.us-east-1.amazonaws.com/openai/v1",
    api_key=os.environ.get("OPENAI_API_KEY")
)

response = client.chat.completions.create(
    model="openai.gpt-oss-120b-1:0",
    messages=[{"role": "user", "content": "Hello"}]
)

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