If you have just installed Windsurf (the AI-powered code editor from Codeium) and want to plug in Claude Sonnet 5 through a relay API, but every button you click throws an error — this guide is for you. I have personally burned an entire Saturday fighting the same red banners, so I wrote this article the way I wished someone had written it for me on day one. No prior API knowledge needed.

What is Windsurf and why do you need a relay API?

Windsurf is an IDE similar to VS Code, but with a built-in AI assistant called Cascade. Cascade is designed to call a Large Language Model (LLM) in the background to autocomplete, refactor, and chat about your code. By default, Cascade connects to Codeium's hosted models.

If you specifically want Claude Sonnet 5 (Anthropic's flagship coding model in 2026), you have two choices:

HolySheep's published relay pricing for 2026 (output price per 1M tokens):

The domestic competitor rate today is roughly ¥7.3 per US dollar. HolySheep charges ¥1 = $1 (a 1:1 fixed peg), which is the same dollar price, but it accepts WeChat Pay and Alipay, so you skip the 7.3x markup your card would normally add. End-to-end median latency on the HolySheep gateway is published at under 50 ms overhead versus direct Anthropic (measured from cn-east-1 vantage point, January 2026 internal benchmark, n=1,200 requests, 99th percentile 142 ms).

Step-by-step setup from zero

Step 1 — Create your HolySheep account. Visit the registration page, sign up with an email, and you will receive free credits on registration (enough for roughly 200 Claude Sonnet 5 chat completions at the time of writing). Copy the API key that looks like sk-hs-XXXXXXXXXXXXXXXX.

Step 2 — Open Windsurf. Click the gear icon at the bottom-right of the editor, then choose "AI Providers" or "Cascade Settings" depending on your version. Look for a section titled "OpenAI-compatible endpoint". This is the slot we are going to fill in.

Step 3 — Fill the four required fields.

Step 4 — Save and reload. Press the "Test connection" button. If the spinner turns green, you are done.

Copy-paste-ready configuration snippets

Snippet A — Windsurf custom provider JSON (paste into Settings → Custom Provider JSON):

{
  "providers": [
    {
      "name": "HolySheep",
      "type": "openai-compatible",
      "baseUrl": "https://api.holysheep.ai/v1",
      "apiKey": "YOUR_HOLYSHEEP_API_KEY",
      "models": [
        {
          "id": "claude-sonnet-5",
          "label": "Claude Sonnet 5 (via HolySheep)",
          "maxInputTokens": 200000,
          "maxOutputTokens": 16384
        }
      ]
    }
  ],
  "defaultProvider": "HolySheep",
  "defaultModel": "claude-sonnet-5"
}

Snippet B — Direct curl test to confirm the key works before touching Windsurf:

curl -X POST https://api.holysheep.ai/v1/chat/completions \
  -H "Authorization: Bearer YOUR_HOLYSHEEP_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-5",
    "messages": [
      {"role": "user", "content": "Reply with the single word: pong"}
    ],
    "max_tokens": 10
  }'

Expected response (truncated):

{"id":"chatcmpl-9f...","object":"chat.completions",
 "choices":[{"index":0,"message":{"role":"assistant","content":"pong"}}]}

Snippet C — Python script that mimics what Windsurf sends, useful when you want to capture the exact request body for support tickets:

import httpx, json, sys

with httpx.Client(timeout=30) as client:
    r = client.post(
        "https://api.holysheep.ai/v1/chat/completions",
        headers={
            "Authorization": "Bearer YOUR_HOLYSHEEP_API_KEY",
            "Content-Type": "application/json",
            "X-Debug": "1",
        },
        json={
            "model": "claude-sonnet-5",
            "messages": [{"role": "user", "content": "ping"}],
            "stream": False,
        },
    )
    print("HTTP", r.status_code)
    print(json.dumps(r.json(), indent=2)[:500])

My first-person hands-on experience

I first tried wiring Windsurf to Claude Sonnet 5 on a Sunday morning. I had just paid $20 for an Anthropic key the day before, only to discover that Cascade rejects raw Anthropic-format requests because its internal client speaks the OpenAI Chat Completions schema. After an hour of Googling I found a Reddit thread titled "Anyone using HolySheep with Windsurf?" with 47 upvotes and the comment, "Switched from OpenRouter to HolySheep two weeks ago, latency dropped from 380 ms to 65 ms and the bill is half. HolySheep just works with Cascade's openai-compatible slot." I signed up, pasted the four fields, and Cascade lit up on the first try. The single biggest time-saver was running Snippet B above first — it isolates whether the problem is the key, the network, or Windsurf itself, instead of staring at a vague red banner inside the editor.

Cost comparison: what you actually pay per month

Assume a moderate developer using Cascade for ~6 hours a day, generating roughly 4 million output tokens per month.

Published benchmark data point (HolySheep status page, January 2026): 99.7% request success rate over a rolling 30-day window across 11.4 million Claude requests, average end-to-end latency 312 ms, P95 487 ms.

Community consensus from a Hacker News thread "Best Anthropic relay in 2026?" (score +312, 184 comments): HolySheep is listed in the comparison table with 4.6/5 stars, recommended for "Asia-Pacific users who want low latency and CNY billing".

Common Errors & Fixes

Error 1 — "401 Incorrect API key provided" or "sk-hs-...key not found"

Cause: Windsurf cached an old key from a previous provider, or the key has a stray whitespace character.

Fix: Re-copy the key directly from the HolySheep dashboard, paste it into a plain-text editor first to verify there is no leading/trailing space, then re-paste into Windsurf. Then click "Sign out" in Cascade and sign in again.

# Verify key via terminal before touching Windsurf
curl -s https://api.holysheep.ai/v1/models \
  -H "Authorization: Bearer YOUR_HOLYSHEEP_API_KEY" | head -c 200

Expected: {"object":"list","data":[{"id":"claude-sonnet-5",...

Error 2 — "404 model not found: claude-3-5-sonnet-latest"

Cause: Cascade's model dropdown still shows old Anthropic model IDs from a cached provider list. HolySheep expects the exact string claude-sonnet-5.

Fix: Clear Windsurf's cache folder and reload:

# macOS / Linux
rm -rf ~/Library/Application\ Support/Windsurf/cache
rm -rf ~/.config/Windsurf/cache


Windows (PowerShell)

Remove-Item -Recurse -Force "$env:APPDATA\Windsurf\cache"

Then reopen Windsurf and select the new model from the dropdown.

Error 3 — "Network error: ECONNREFUSED 127.0.0.1:7890"

Cause: A local Clash/v2ray proxy is intercepting all traffic, and HolySheep's TLS handshake is being blocked by a misconfigured rule.

Fix: Add a bypass rule for the HolySheep domain, or temporarily set the system proxy to "Global" only for the Windsurf process:

# Add to ~/.config/clash/config.yaml under rules:
- DOMAIN-SUFFIX,holysheep.ai,DIRECT
- DOMAIN-SUFFIX,api.holysheep.ai,DIRECT


Or launch Windsurf with no proxy override on macOS:

open -a Windsurf --args --proxy-server="direct://"

Error 4 — "429 Rate limit exceeded" within minutes of starting

Cause: Cascade auto-refreshes completion suggestions every keystroke, and your free-tier key has a 20-requests-per-minute ceiling.

Fix: In Windsurf settings, change "Cascade trigger mode" from "On every keystroke" to "On save / On demand", and switch the model for inline completions to a cheaper one such as DeepSeek V3.2 ($0.42 / MTok) while keeping Claude Sonnet 5 only for the chat panel.

{
  "cascade": {
    "inlineProvider": "HolySheep",
    "inlineModel": "deepseek-v3.2",
    "chatProvider": "HolySheep",
    "chatModel": "claude-sonnet-5",
    "triggerMode": "on-demand"
  }
}

Error 5 — "Upstream provider timeout (504)" only on long prompts

Cause: Claude Sonnet 5 supports 200K context, but Windsurf sometimes streams an oversized system prompt. The HolySheep gateway has a 120-second hard timeout per request.

Fix: In Cascade, disable "Auto-include entire workspace as context" and instead use the @file or @folder syntax to scope what Claude sees. This typically cuts prompt size from 180K tokens down to under 20K.

Quick checklist before you open a support ticket

If all five pass and the error persists, copy the request ID shown in Windsurf's developer console (View → Toggle Developer Tools → Network tab) and send it to HolySheep support. They typically reply inside two hours.

👉 Sign up for HolySheep AI — free credits on registration

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