fix(custom-provider): preserve base URL for anthropic_messages api_mode

[terlanks]

Problem

When a custom provider is configured with api_mode: anthropic_messages, the code in resolve_provider_client() unconditionally rewrites the base URL via _to_openai_base_url(), converting e.g. http://host/anthropic → http://host/v1.

The Anthropic SDK then appends /v1/messages to the base URL, resulting in a request to /v1/v1/messages → 404.

Root Cause

The generic custom provider branch (~line 3271) lacks the api_mode guard that the named custom-provider branch (~line 3380) already has:

# named-custom branch (correct)
if entry_api_mode == "anthropic_messages":
    # keeps custom_base as-is
else:
    openai_base = _to_openai_base_url(custom_base)

The generic branch was missing this check entirely.

Fix

Apply the same guard to the generic custom branch: when api_mode == "anthropic_messages", keep the original base URL (only strip trailing slash). For all other api_modes, the existing _to_openai_base_url() rewrite is preserved.

Testing

Reproduced and verified with a local Anthropic-compatible proxy (base_url: http://localhost:6655/anthropic, api_mode: anthropic_messages). Before the fix: 404 on every call. After: requests route correctly to /anthropic/v1/messages.

[alt-glitch]

Duplicate of #29624 — same fix location in auxiliary_client.py generic custom provider branch: skip _to_openai_base_url() when api_mode == "anthropic_messages". PR #29624 is a broader fix (builds full Anthropic client); this PR is a minimal guard. Same bug class as #17084 (closed), #17467 (merged).

[teknium1]

Thanks for isolating the generic custom-provider path. The premise still holds on current main: agent/auxiliary_client.py:4636 rewrites an explicit custom URL before agent/auxiliary_client.py:4693 passes that rewritten value into the Anthropic wrapper. The named-provider and fallback-custom paths already preserve the Anthropic route (agent/auxiliary_client.py:2403-2419, agent/auxiliary_client.py:4756-4804).

Problems

• The diff has no regression test. Existing tests cover _try_custom_endpoint() (tests/agent/test_auxiliary_client_anthropic_custom.py:35) and named custom providers (tests/agent/test_auxiliary_named_custom_providers.py:341), not resolve_provider_client("custom", explicit_base_url=..., api_mode="anthropic_messages").

Suggested changes

• Add a focused test for that explicit-base generic branch, asserting it returns an AnthropicAuxiliaryClient whose base URL remains the supplied /anthropic URL, plus a non-Anthropic control assertion that preserves the existing OpenAI rewrite.

The target has moved to agent/auxiliary_client.py:4636, so salvage requires a small manual transplant rather than a clean cherry-pick. This is an automated hermes-sweeper review.

[teknium1]

This bug is now fixed on main via #85466 (salvage of #64891). Credit correction: your PR was submitted May 22 — the second-earliest fix for this branch (after #29624 on May 21), months before the version that merged, and your diff is nearly line-identical to what landed for the wrapper path. The pre-merge duplicate sweep missed it, which is our miss.

The merged variant additionally kept the /v1 rewrite for the plain-OpenAI fallback client and carried a regression test file, so it was taken as the base. Closing as resolved-on-main with early-submitter credit to you. Thanks, and sorry for the wait.