Auxiliary title generation 404 on custom Anthropic-mode providers (URL double-/v1)

Summary

When model.provider is a custom:<name> with api_mode: anthropic_messages and a base URL ending in /coding (e.g. Kimi Coding Plan), auxiliary tasks (title generation, compression, etc.) fail with HTTP 404 because the Anthropic SDK receives a base URL with /v1 already appended, then adds its own /v1/messages path — producing /coding/v1/v1/messages.

Reproduction

config.yaml:

model:
  default: kimi-for-coding
  provider: custom:kimi-coding-anthropic

custom_providers:
  - name: Kimi Coding Anthropic
    base_url: https://api.kimi.com/coding
    key_env: KIMI_API_KEY
    model: kimi-for-coding
    api_mode: anthropic_messages

Steps:

1. Start a new conversation via Telegram or CLI
2. First user message triggers title generation after the assistant responds
3. Observe: ⚠ Auxiliary title generation failed: HTTP 404: The requested resource was not found

Docker logs:

⚠️  API call failed (attempt 1/3): NotFoundError [HTTP 404]
   🔌 Provider: kimi-coding  Model: kimi-for-coding
   🌐 Endpoint: https://api.kimi.com/coding/v1
   📝 Error: HTTP 404: The requested resource was not found

Root Cause

In resolve_provider_client() (auxiliary_client.py), the anonymous custom provider branch with explicit_base_url:

# Line ~2118
custom_base = _to_openai_base_url(explicit_base_url).strip()
# explicit_base_url = "https://api.kimi.com/coding"
# custom_base       = "https://api.kimi.com/coding/v1"  ← rewritten for OpenAI SDK

# Line ~2143-2145
client = OpenAI(api_key=custom_key, base_url=_clean_base, **extra)
client = _wrap_if_needed(client, final_model, custom_base, custom_key)
#                                                ^^^^^^^^^^^
#   Passes /coding/v1 to _maybe_wrap_anthropic → build_anthropic_client

_maybe_wrap_anthropic() detects the Kimi endpoint and wraps the client in AnthropicAuxiliaryClient via build_anthropic_client(api_key, base_url="https://api.kimi.com/coding/v1").

The Anthropic SDK constructs the request URL as:

base_url.raw_path + "/v1/messages".lstrip("/")
= /coding/v1/ + v1/messages
= /coding/v1/v1/messages  ← 404

Correct behavior: build_anthropic_client should receive https://api.kimi.com/coding (raw URL), producing /coding/v1/messages.

Why main chat works

The main agent loop uses a separate code path (runtime_provider.py) that correctly constructs the Anthropic client with the raw base URL. Only the auxiliary client's anonymous-custom + explicit_base_url branch has this bug.

The named custom provider branch (line ~2188-2207) and _try_custom_endpoint() (line ~1410-1424) already handle anthropic_messages correctly by calling build_anthropic_client directly with the raw URL.

Suggested Fix

One-line change in resolve_provider_client(), anonymous custom branch:

# Before (buggy):
client = _wrap_if_needed(client, final_model, custom_base, custom_key)

# After (fixed):
# Pass the raw explicit_base_url, not the _to_openai_base_url()-rewritten version.
# AnthropicAuxiliaryClient needs the raw URL — the SDK appends /v1/messages.
client = _wrap_if_needed(client, final_model, explicit_base_url, custom_key)

Defensive hardening (optional, covers all call sites): _maybe_wrap_anthropic() could strip a trailing /v1 from base_url before passing to build_anthropic_client, since the Anthropic SDK always appends its own /v1 prefix:

# In _maybe_wrap_anthropic(), before build_anthropic_client():
anthropic_base = base_url.rstrip("/")
if anthropic_base.endswith("/v1"):
    anthropic_base = anthropic_base[:-3]  # strip trailing /v1
real_client = build_anthropic_client(api_key, anthropic_base)

Impact

• Affects: Any user with model.provider: custom:<name> where the custom provider uses api_mode: anthropic_messages (e.g. Kimi Coding Plan, third-party Anthropic-compatible gateways via anonymous custom config)
• Does NOT affect: Named custom providers (configured via custom_providers: list), OpenRouter, Anthropic, OpenAI, or any provider that doesn't go through the anonymous custom + explicit_base_url path
• Symptoms: Title generation fails silently (session gets NULL title); compression and other auxiliary tasks would also fail if they hit the same code path

Environment

• Hermes Agent v2026.4.30-ssh (Docker)
• anthropic SDK 0.96.0
• Python 3.13