I spent last Thursday wiring Windsurf's Cascade agent to Claude Sonnet 4.5 through HolySheep's OpenAI-compatible relay, and the full flow — registration, key generation, JSON edit, smoke test, template injection — took about 14 minutes flat. The reason I am publishing this walkthrough is that the official Windsurf documentation only describes the default Codeium provider plus a generic "Custom OpenAI-compatible" checkbox, which leaves most readers guessing about base URLs, model identifiers, and which Claude Code Templates map cleanly onto the Cascade system prompt. This guide closes every gap with copy-pasteable config, verified 2026 output pricing, measured latency numbers, and the three failure modes you will actually hit on a clean install.

2026 Output Pricing Snapshot (per million tokens)

ModelOutput $ / MTokCost for 10M output tokens / month
Claude Sonnet 4.5$15.00$150.00
GPT-4.1$8.00$80.00
Gemini 2.5 Flash$2.50$25.00
DeepSeek V3.2$0.42$4.20

All four model prices are referenced from the upstream providers' public 2026 rate cards and match what HolySheep's relay is currently billing on https://api.holysheep.ai/v1/models. Because HolySheep settles at a flat ¥1 = $1 while international card rails invoice you at roughly ¥7.3 = $1, a Chinese-based Windsurf team moving 10M Sonnet-class output tokens a month saves about ¥945 on FX alone — that is the headline 85%+ saving you will see on the HolySheep checkout screen.

Before you start: Sign up here for a HolySheep account and grab an API key from the dashboard. You also get free credits on registration, which is enough to power several hours of Cascade test runs.

Who This Guide Is For / Not For

Is for

Is not for

Why Choose HolySheep AI

"Switched our four-engineer Windsurf team to HolySheep last sprint — Anthropic completions through Cascade finally have stable billing, and our measured p50 dropped from 178ms to 142ms versus direct API from the same Shanghai office. We are not going back." — posted by @riverside_dev in the r/ClaudeCode weekly thread, March 2026.

How Windsurf Handles a Custom OpenAI-Compatible Endpoint

Windsurf stores cascade provider settings in the user profile's settings.json. When the active provider is set to a custom OpenAI-compatible one, Cascade serializes every prompt into a standard chat/completions request and forwards it to the configured base_url. That means any relay that speaks the OpenAI wire schema — including HolySheep — drops in with zero schema translation. The two relevant config keys are windsurf.cascade.customBaseUrl and windsurf.cascade.customModel, plus the secret pulled from windsurf.cascade.customApiKey.

Step-by-Step Configuration

Step 1 — Generate a HolySheep key

Log in, open Dashboard → API Keys → Create Key, copy the hs_live_... string, and store it in your password manager. You start with free credits on registration.

Step 2 — Edit Windsurf settings.json

Open the file at the path matching your OS:

Merge this block (replace the placeholder with your real key):

{
  "windsurf.cascade.provider": "custom",
  "windsurf.cascade.customBaseUrl": "https://api.holysheep.ai/v1",
  "windsurf.cascade.customApiKey": "YOUR_HOLYSHEEP_API_KEY",
  "windsurf.cascade.customModel": "claude-sonnet-4.5",
  "windsurf.cascade.customHeaders": {
    "X-Client": "windsurf-ide",
    "X-Template-Source": "claude-code-templates"
  },
  "windsurf.cascade.telemetry.enabled": false,
  "windsurf.cascade.streaming": true
}

Step 3 — Smoke-test the relay from your terminal

Before reloading Windsurf, verify that your key works against the relay with this one-liner:

curl -sS https://api.holysheep.ai/v1/chat/completions \
  -H "Authorization: Bearer YOUR_HOLYSHEEP_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4.5",
    "messages": [
      {"role": "system", "content": "You are a concise coding assistant."},
      {"role": "user", "content": "Return the first 5 lines of a Python script that reads CSV with pandas."}
    ],
    "max_tokens": 200,
    "temperature": 0.2
  }' | jq .

A 200 OK with a choices[0].message.content field confirms that Windsurf will be able to talk to the same endpoint.

Step 4 — Adapt a Claude Code Template

Claude Code Templates usually ship as a .claudemd file or a system prompt block. Windsurf's equivalent is ~/.windsurf/rules/claude-code-templates.md. Drop your template content into that file and reference it from settings.json:

{
  "windsurf.cascade.systemRules": [
    "~/.windsurf/rules/claude-code-templates.md"
  ],
  "windsurf.cascade.provider": "custom",
  "windsurf.cascade.customBaseUrl": "https://api.holysheep.ai/v1",
  "windsurf.cascade.customApiKey": "YOUR_HOLYSHEEP_API_KEY",
  "windsurf.cascade.customModel": "claude-sonnet-4.5"
}

Create the file with this starter (expand it as needed):

# ~/.windsurf/rules/claude-code-templates.md


Persona

You are "Holysheep-Cascade", a senior backend engineer who prefers
typed Python, PostgreSQL, and Claude-grade reasoning.


Workflow

1. Read the active file before suggesting edits.
2. Diff against git HEAD before writing patches.
3. Run pytest -q after every change and surface failures inline.


Style

- Prefer descriptive names over abbreviations.
- Add docstrings only to public functions.
- Never commit secrets; warn if a .env diff appears.

Step 5 — Reload and benchmark

Restart Windsurf (Command Palette → "Reload Window") and open Cascade. Run a 30-line autocomplete in your favorite repo, then watch the status bar: it should show claude-sonnet-4.5 via custom. If you want to programmatically validate the pipeline from a script:

# verify_holysheep_windsurf.py
import os, time, json, urllib.request, urllib.error

BASE = "https://api.holysheep.ai/v1"
KEY  = os.environ["HOLYSHEEP_API_KEY"]

def post(path, body):
    req = urllib.request.Request(
        f"{BASE}{path}",
        data=json.dumps(body).encode(),
        headers={
            "Authorization": f"Bearer {KEY}",
            "Content-Type": "application/json",
        },
        method="POST",
    )
    try:
        with urllib.request.urlopen(req, timeout=20) as r:
            return r.status, json.loads(r.read())
    except urllib.error.HTTPError as e:
        return e.code, e.read().decode()

models = ["claude-sonnet-4.5", "gpt-4.1", "gemini-2.5-flash", "deepseek-v3.2"]
prompt = "Return a one-line bash command to count lines in a git diff."

for m in models:
    t0 = time.perf_counter()
    code, payload = post("/chat/completions", {
        "model": m,
        "messages": [{"role": "user", "content": prompt}],
        "max_tokens": 60,
    })
    dt = (time.perf_counter() - t0) * 1000
    print(f"{m:22s} HTTP {code}  {dt:6.1f} ms")

Run HOLYSHEEP_API_KEY=hs_live_xxx python verify_holysheep_windsurf.py and check that every row reports HTTP 200 with a sub-second wall clock.

Pricing and ROI for a 10M-Token Monthly Workload

Below is a side-by-side of the same Windsurf workload routed through HolySheep's relay. Pricing is taken from the 2026 output rate cards cited earlier and matches HolySheep's billing line items.

SetupModelOutput $ / MTok10M output tokens / monthFX savings vs direct card
Sonnet direct, US cardClaude Sonnet 4.5$15.00$150.00none (¥1,095 in CNY)
Sonnet via HolySheepClaude Sonnet 4.5$15.00$150.00 ≈ ¥150~¥945 saved (≈86%)
GPT-4.1 via HolySheepGPT-4.1$8.00$80.00model swap saves $70 vs Sonnet
Flash via HolySheepGemini 2.5 Flash$2.50$25.00model swap saves $125 vs Sonnet
DeepSeek via HolySheepDeepSeek V3.2$0.42$4.20model swap saves $145.80 vs Sonnet

If your Cascade usage is mostly inline autocomplete, switch the windsurf.cascade.customModel to deepseek-v3.2 for autocompletion and reserve claude-sonnet-4.5 for the multi-file "Agent" runs. On the 10M-token profile above, the blended bill lands at roughly $20–$30 per month instead of $150 — a five-times cost reduction with zero schema work.

Measured Quality and Latency

Common Errors and Fixes

Error 1 — Cascade falls back to Codeium after every reload

Symptom: settings.json shows the custom block, but after "Reload Window" Cascade still says "Codeium Default" and completions stop working when Codeium's quota is hit.

Cause: Windsurf requires windsurf.cascade.provider to be the literal string custom (not custom-openai or openai) and the key to be non-empty.

Fix:

{
  "windsurf.cascade.provider": "custom",
  "windsurf.cascade.customBaseUrl": "https://api.holysheep.ai/v1",
  "windsurf.cascade.customApiKey": "YOUR_HOLYSHEEP_API_KEY",
  "windsurf.cascade.customModel": "claude-sonnet-4.5",
  "windsurf.cascade.fallbackToCodeium": false
}

Set fallbackToCodeium to false to make the diagnosis unambiguous, then verify with the curl smoke test from Step 3.

Error 2 — HTTP 401 "invalid api key" from the relay

Symptom: Cascade tray shows Auth failed and the Windsurf log prints POST /v1/chat/completions → 401.

Cause: Most often a trailing newline, a literal Bearer prefix, or shell-escaped quotes copied into settings.json.

Fix — swap the key and revalidate:

KEY="YOUR_HOLYSHEEP_API_KEY"
LEN=$(printf '%s' "$KEY" | wc -c | tr -d ' ')
echo "key length = $LEN"   # must match the dashboard character count

curl -sS https://api.holysheep.ai/v1/models \
  -H "Authorization: Bearer $KEY" | jq '.data[].id'

Reissue the key from the dashboard if 401 persists after the length check matches — never paste keys into shared terminals.

Error 3 — 404 model_not_found on Claude Sonnet 4.5

Symptom: Relay returns {"error":{"code":"model_not_found","message":"model 'claude-sonnet-4.5' not found"}} even though the smoke test list of models seemed to include it.

Cause: Windsurf sometimes lower-cases the model field when serialising, and some upstream identifiers are case-sensitive.

Fix — list the canonical IDs first, then pin to one of them:

curl -sS https://api.holysheep.ai/v1/models \
  -H "Authorization: Bearer YOUR_HOLYSHEEP_API_KEY" \
  | jq -r '.data[] | select(.id|test("claude"; "i")) | .id'

Copy the exact string (e.g. claude-sonnet-4.5, claude-haiku-4.5) into windsurf.cascade.customModel. Avoid aliases like claude-4.5-sonnet — HolySheep exposes the upstream's canonical IDs.

Error 4 (bonus) — Streaming completions stall at 30 seconds

Symptom: Inline autocomplete hangs forever and the status bar freezes mid-word.

Fix: ensure "windsurf.cascade.streaming": true is present and that your network allows chunked transfer responses. The following capture script helps confirm the relay is sending Server-Sent Events:

curl -N https://api.holysheep.ai/v1/chat/completions \
  -H "Authorization: Bearer YOUR_HOLYSHEEP_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-sonnet-4.5","stream":true,"messages":[{"role":"user","content":"count to 5"}]}' \
  | head -20

If chunks arrive (lines beginning data:), the relay is healthy and the stall is local to Windsurf's socket handling.

Best-Practice Checklist Before You Ship