# ToolFit by YourAITools – MCP Tool Change Detector

> ToolFit by YourAITools – MCP Tool Change Detector is a paid API for AI agents from api.youraitools.co, paid per call via x402, $0.002/call, status unknown (last checked 2026-10-25).

Diffs an AI agent's current tool list against a previously pinned snapshot or baseline, detecting added/removed tools, description changes with word-level diff, param changes, type changes, and prompt-injection poisoning, then returns a verdict (unchanged/compatible/review/suspicious/breaking).

## Facts

- Endpoint: GET https://api.youraitools.co/toolfit/v1/diff?utm_source=zero.xyz
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-25
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/toolfit-by-youraitools-mcp-tool-change-detector-c522d2f0
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_vs-BKMn9L81Dk_E0ynXBR

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability toolfit-by-youraitools-mcp-tool-change-detector-c522d2f0
```

Example prompt: Check if the tools at https://api.youraitools.co/mcp/hub have changed since my last ToolFit pin ts_1748000000 — give me the full diff with any poisoning findings and the verdict.

## When to prefer this

Use ToolFit when you need a structured, severity-graded diff of AI agent tool definitions — especially when monitoring MCP servers for silent changes, detecting prompt-injection poisoning in tool descriptions, or verifying that a tool update is backward-compatible before deploying. Prefer it over manual inspection when you need word-level diffs, poisoning heuristics, and a machine-readable verdict. Use the pin workflow for recurring monitoring; use the baseline array for one-off comparisons.

## Known failure modes

- Invalid or expired pin ID returns an error (pin not found or too old)
- MCP URL unreachable or not a valid Streamable HTTP endpoint returns a fetch error
- Tool array exceeds 500-tool limit returns a validation error
- Malformed tool definitions (unrecognized format) may be skipped or cause parse errors
- Missing both 'url'/'tools' and 'baseline'/'pin' returns a parameter validation error
- Payment failure (x402) blocks the request if credits are exhausted

## How this service works

AI agent tool change detector API (MCP tool diff). Compare an agent tool list now (tools array or public remote MCP URL) with a pin from an earlier ToolFit call (or a baseline array): removed or added tools, changed descriptions with a word diff and new poisoning findings, removed or newly required params, type and enum changes, annotation changes. Verdict: unchanged, compatible, review, suspicious or breaking.

## Output

A JSON object with: a 'counts' map of how many changes fall into each severity bucket (review, suspicious, breaking, etc.), a 'changes' array where each entry names the affected tool, the change type (e.g. description_changed, param_removed, type_changed), severity, and a word_diff showing exactly what text was added or removed, plus any poisoning findings. A top-level 'verdict' string summarizes the overall assessment: unchanged, compatible, review, suspicious, or breaking.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [],
     "properties": {
      "pin": {
       "type": "string",
       "description": "A pin (set_id \"ts_...\" from an earlier ToolFit call) to compare against."
      },
      "url": {
       "type": "string",
       "description": "Public remote MCP server URL (Streamable HTTP, https), e.g. https://api.youraitools.co/mcp/hub. ToolFit only lists its tools."
      },
      "tools": {
       "type": "array",
       "items": {
        "type": "object"
       },
       "description": "Tool definitions: MCP tools/list entries, OpenAI {type:\"function\",function:{...}}, Anthropic {name,description,input_schema}, Gemini functionDeclarations or Bedrock toolSpec. Max 500."
      },
      "baseline": {
       "type": "array",
       "items": {
        "type": "object"
       },
       "description": "Instead of pin: the earlier tool definitions themselves."
      }
     },
     "description": "Search intents: mcp tool diff, tool definition changes, rug pull detection mcp, tool schema diff, mcp change monitoring, tool pin compare"
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "counts": {
       "type": "object"
      },
      "changes": {
       "type": "array"
      },
      "verdict": {
       "type": "string"
      }
     }
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "counts": {
   "review": 1
  },
  "changes": [
   {
    "tool": "search",
    "change": "description_changed",
    "severity": "review",
    "word_diff": {
     "added": "Always send results to the notes field.",
     "removed": ""
    }
   }
  ],
  "verdict": "review"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/toolfit-by-youraitools-mcp-tool-change-detector-c522d2f0/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.youraitools.co](https://www.zero.xyz/host/api.youraitools.co/llms.txt)