Problem

Hermes is completely broken when configured to use Azure AI Foundry's Anthropic-compatible endpoint. Multiple overlapping bugs cause every request to fail with HTTP 404 or HTTP 401.

Root Causes (5 bugs, fixed in order of discovery)

1. Azure AI Foundry keys misidentified as OAuth tokens

_is_oauth_token() in runtime_provider.py only excluded sk-ant-api prefix keys, treating Azure keys (which don't start with sk-ant- at all) as OAuth tokens → wrong auth header sent.

2. OAuth token priority chain overrides Azure key

resolve_anthropic_token() checks ~/.claude/.credentials.json at priority 3, before ANTHROPIC_API_KEY at priority 4. If the user has Claude Code installed, the OAuth token wins every time, regardless of what's in the environment.

3. _resolve_named_custom_runtime intercepts the anthropic provider

When requested="anthropic" + an Azure base_url is passed, the custom runtime resolver intercepts it and returns provider="custom" with no api_mode set → falls through to chat_completions path.

4. _try_refresh_anthropic_client_credentials overwrites Azure key mid-session

Called before every request. Finds the OAuth token in ~/.claude/.credentials.json, sees it differs from the Azure key, and swaps it in → correct Azure key gets silently replaced on message 2+.

5. api-version appended to base_url produces malformed URLs

Previous fix attempt added api-version=2025-04-15 directly to the base URL string. The Anthropic SDK then appended /v1/messages to that, producing:

.../anthropic?api-version=2025-04-15/v1/messages

instead of:

.../anthropic/v1/messages?api-version=2025-04-15

Azure returns 404 on the malformed path.

Fixes

Configuration

After this fix, Azure AI Foundry works with the following config.yaml:

model:
  provider: anthropic
  base_url: https://<resource>.services.ai.azure.com/anthropic
  api_key_env: AZURE_ANTHROPIC_KEY
  default: claude-sonnet-4-6

With AZURE_ANTHROPIC_KEY set to the Azure AI Foundry API key (no sk-ant- prefix).