Summary

Adding a custom provider with OpenAI Chat wire + baseUrl `https://open.bigmodel.cn/api/paas/v4\` (GLM / 智谱) + api key → Test Connection shows `HTTP 404`, even though real generation via the same config would work.

Likely affects other Chinese OpenAI-compat endpoints that don't expose a GET `/models` list endpoint (e.g. some Baidu / Tencent / Moonshot / Aliyun paths).

Reproduction

1. Settings → API 服务 → Add custom endpoint
2. Protocol: OpenAI Chat
3. Name: GLM, baseUrl: `https://open.bigmodel.cn/api/paas/v4\`, api key: valid GLM key, default model: `glm-4.6v`
4. Click 测试连接 → `HTTP 404`

Root Cause

Connection test (`apps/desktop/src/main/connection-ipc.ts`) probes `GET ${baseUrl}/models` to verify reachability + auth. GLM's `/api/paas/v4` surface exposes `/chat/completions` but does not publish a standards-compliant `/models` JSON endpoint — it returns 404 / HTML.

Real generation POSTs to `/chat/completions` and works fine. So the 404 is a false negative on the probe, not a real outage. Users interpret the red banner as "broken" and stop here.

Impact

• Blocks onboarding for GLM users (a large Chinese user segment)
• Likely similar false negatives on other domestic OpenAI-compat endpoints
• Existing diagnostic hypothesis for 404 (`missingV1`) suggests "append /v1" — but GLM already uses `/v4`, so the hint is wrong and misleading

Expected Behavior

One of:

• (A) If `/models` returns 404, degrade-probe by firing a minimal `/chat/completions` call (short prompt, max_tokens=1) to verify the endpoint is alive
• (B) Show UI hint: "Some providers (GLM, 通义 via non-standard path, etc.) do not expose /models. You can skip the probe and save; test by sending a real prompt."
• (C) Provide a "skip probe" save button that commits the config regardless
• Do not fire the `missingV1` "append /v1" auto-fix hypothesis when baseUrl already ends in `/vN`

Suggested Fix

Prefer (A) as primary + (B) as fallback toast on degrade-probe also failing. Disable `missingV1` when `/v\d+$` regex matches baseUrl.

Related

• Connects to bug(desktop): generation fails with codesign:v1:generate 404 on Win11 relay setup #130 (generate-error → diagnose bridge) — test-connection degrade probe is the symmetric pre-save check
• Connects to bug: 第三方中转服务测试连接通过但生成返回 500 not implemented #158 (gateway compatibility detection)

cc @hqhq1025