Overwing README

Guardrails for LLM output, as MCP tools.
Score any text for safety, quality and compliance. Get pass / fail / review verdicts with calibrated confidence in under 500 ms.

Overwing is an API that checks what your model said before it ships. This package exposes it to any MCP-capable agent: Claude Desktop, Claude Code, Cursor, Windsurf, VS Code, OpenAI's Agents SDK, and anything else that speaks the Model Context Protocol.

Real verdicts, not vibes. Every rule returns a typed answer, a probability, and a confidence. fail means a rule matched; review means it was unsure; pass means neither.
Prebuilt content-safety rule set: toxicity, personal data, self-harm, sexual content, severity. Or write your own rules in plain language.
Built for agents. Sign up, pay, evaluate, rotate keys, and cancel, all as JSON. No CAPTCHA, no browser required. See overwing.ai/llms.txt.

Try it without installing anything: paste text into the console at overwing.ai.

Install

You need an API key. Get one at overwing.ai/login, or let your agent sign itself up:

Terminal

curl -X POST https://overwing.ai/api/v1/signup \
  -H "Content-Type: application/json" \
  -d '{"email":"you@example.com","password":"at-least-12-chars"}'

Claude Code

Terminal

claude mcp add overwing -e OVERWING_API_KEY=ow_live_... -- npx -y overwing-mcp

Claude Desktop, Cursor, Windsurf, VS Code (any JSON-configured client)

config.json

{
  "mcpServers": {
    "overwing": {
      "command": "npx",
      "args": ["-y", "overwing-mcp"],
      "env": { "OVERWING_API_KEY": "ow_live_..." }
    }
  }
}

Set OVERWING_BASE_URL to point at a self-hosted deployment. Requires Node 20+.

Tools

Tool	What it does
`evaluate`	Score one text against a rule set. Returns the verdict, a `recommended_action` (block, redact, review, or allow), aggregate score, confidence, latency, and per-rule results with each rule's action. Takes an optional `context` object (recipient, channel, ownership) that context-aware rule sets such as `outbound-message` read.
`evaluate_batch`	Score up to 50 texts in one call, with a summary and per-item verdicts and recommended actions.
`list_rule_sets` · `get_rule_set` · `create_rule_set`	Browse the prebuilt set or define your own rules: yes/no questions, classifications, or scored scales.
`get_evaluation` · `list_evaluations`	Read stored results, filter by verdict or rule set, page with a cursor.
`atlas_lookup`	Overwing Atlas: say what a User-Agent string claims to be and whether the claim can be trusted (Web Bot Auth, spoofable string, or unattributable). 100 free a day.
`atlas_agents` · `atlas_summary`	Search the registry of 241 AI crawlers, fetchers and browser agents; get traffic shares, sector field-scan headlines, and the agent-spending summary.
`get_usage` · `whoami` · `list_plans`	Today's quota, the org behind the key, and the public plan catalog.

The overwing://guide resource returns the full plain-text API guide.

Example

Ask your agent:

Check this reply before I send it: "Reach me at dana@example.com or 555-0142 to sort out the refund."

It calls evaluate and gets back:

Code

Verdict: FAIL  score=0.82  confidence=0.97  251ms
  toxicity: pass (answer="safe", confidence=1)
  pii_detected: fail (answer=true, confidence=0.98)
  self_harm: pass (answer=false, confidence=1)
  sexual_content: pass (answer="none", confidence=1)
  severity: pass (answer=0.03, confidence=0.97)

How verdicts work

Each rule has a fail condition, an optional review threshold, and a weight.

fail: a rule's fail condition matched. Block it, redact it, or regenerate.
review: nothing failed, but a rule's confidence was below its threshold. Route to a person or a slower model.
pass: everything else.

Every rule also carries an action (block, redact, or review), and the response rolls them up into one recommended_action: block beats redact beats review beats allow. Branch on that field. Pass a context object (recipient, channel, owns_contact_info) with the outbound-message rule set and its rules read it, so a customer's own phone number in a reply to that customer is not flagged.

aggregate_score is 0 to 1 (pass = 1, review = 0.5, fail = 0 per rule, weighted). confidence is the minimum across rules.

Pricing

Free: 250 evaluations a day. Paid plans from $29/month. Every plan includes every endpoint, custom rule sets, webhooks, and the dashboard. Full details at overwing.ai/#pricing or GET https://overwing.ai/api/v1/plans.

Development

Terminal

npm install
npm run build
OVERWING_API_KEY=ow_live_... node dist/index.js

Install

Tools

Example

How verdicts work

Pricing

Links

Development