Add azure_anthropic backend for Anthropic models on Azure AI Foundry

[timothyjlaurent]

Summary

Add support for an azure_anthropic backend that enables using Anthropic models (Claude) hosted on Azure AI Foundry.

Motivation

Azure AI Foundry / Azure AI Model Catalog now offers Anthropic models as a managed service. Organizations using Azure often need to route API calls through their Azure endpoints for compliance, billing, and network policy reasons. The existing anthropic backend only targets api.anthropic.com, and the existing azure_openai backend only works with OpenAI models on Azure.

Proposed Implementation

The Anthropic Python SDK already supports a base_url parameter, so the implementation is straightforward — a thin wrapper around the existing AnthropicClient that reads Azure Foundry config.

Follows the ANTHROPIC_FOUNDRY_* env-var convention used by Claude Code:

Variable	Purpose
ANTHROPIC_FOUNDRY_API_KEY	API key
ANTHROPIC_FOUNDRY_RESOURCE	Azure resource name (derives base URL)
ANTHROPIC_FOUNDRY_BASE_URL	Explicit base URL (overrides resource)

Usage:

export ANTHROPIC_FOUNDRY_API_KEY="your-key"
export ANTHROPIC_FOUNDRY_RESOURCE="ml-platform-openai-stg-useast-2"

rlm ask . -q "Summarize this repo" --backend azure_anthropic --model claude-opus-4-6

Or via config:

backend: azure_anthropic
model: claude-opus-4-6
backend_kwargs:
  resource: ml-platform-openai-stg-useast-2