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Tool Call Guard
x402
Tool Call Guard is a paid API for AI agents from x402.agentindex.world, paid per call via x402, $0.001/call, status unknown (last checked 2026-10-01).
Evaluates a pending tool call against the user's original request and returns allow/ask/deny with a confidence probability to guide human-in-the-loop decisions.
Price / call$0.001
Success—
StatusUnknown
How this service works
Ask whether a tool call should run automatically, need human confirmation, or be denied, given the user's request - returns allow/ask/deny with a probability. Advisory only: like any LLM-based judge it is sensitive to prompt injection in the request or tool call, so the calling agent must keep the final decision, not delegate it outright. Try GET /guard/sample.
Output
Returns a decision string ('allow', 'ask', or 'deny') and an associated probability score indicating confidence in that verdict, helping an orchestrating agent decide whether to proceed automatically, pause for human confirmation, or block the tool call entirely.
Example use cases
Auto-approval gate for file writes
“I'm about to call `write_file` with path '/etc/hosts' and some content — before I do it automatically, check whether that should be allowed, flagged for my user to confirm, or outright denied given they asked me to 'update my hosts file to block ads'.”
Catching scope-creep in a shopping agent
“My agent is about to call `place_order` for $349 worth of items, but the user only asked to 'browse laptops under $300' — can you tell me if this tool call should run automatically, needs confirmation, or should be blocked?”
Human-in-the-loop for sensitive API calls
“Before my agent fires `delete_repo` with repo name 'my-prod-app', check if that action should be auto-allowed, require human sign-off, or be denied — the original user request was 'clean up my old test repositories'.”
Technical details
Endpoint
POSThttps://x402.agentindex.world/guard?utm_source=zero.xyzWhen to prefer
Use this endpoint when an AI agent needs a lightweight, pay-per-call advisory signal before executing a tool call that could have side effects, financial implications, or go beyond the user's stated intent. It is best suited for orchestration layers that want probabilistic allow/ask/deny verdicts without building a custom classifier. Prefer it over static allow-lists when tool calls are dynamic or user requests are open-ended. Remember that the verdict is advisory — the calling agent must make the final call.
Side effects
- Missing tool_call or user_request fields returns a validation error
- Prompt injection within tool arguments or user_request may skew the verdict — caller must retain final authority
- Low-confidence probability scores (near 0.5) indicate ambiguous cases requiring extra human scrutiny
- Malformed or deeply nested tool call objects may degrade classification accuracy
- Payment failure (insufficient USDC) returns HTTP 402 before any evaluation occurs
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| tool_call | object | The tool call an agent is about to make, e.g. {"name": ..., "arguments": {...}}. |
| user_request | string | The user's original request, in their own words. |
Output
Schema · Response
{
"type": "json",
"example": {
"decision": "deny",
"confidence": 0.89,
"probability": 0.92,
"x402_receipt": {
"upstream": "guardrail",
"latency_ms": 420,
"model_served": "jev",
"price_paid_usdc": 0
},
"probabilities": {
"ask": 0.08,
"deny": 0.92,
"allow": 0
}
}
}Tags
x402
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