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rt_check_unicode_steganography

Identify hidden Unicode characters like zero-width spaces and homoglyphs in text that can embed instructions visible to language models but invisible to human readers.

Instructions

Detect hidden Unicode characters in tool descriptions: zero-width spaces, zero-width joiners, word joiners, RTL/LTR override, BOM, invisible separators, homoglyph characters. These can hide instructions visible to LLM but invisible to humans.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "rt_check_unicode_steganography"
  }
}

Input Schema

Name	Required	Description
`env`	No	Additional environment variables for stdio
`url`	No	MCP server URL for HTTP/SSE transport (e.g. 'http://localhost:3000/mcp')
`args`	No	Command arguments for stdio (e.g. ['run', 'server.js'])
`command`	No	Server command for stdio transport (e.g. 'node', 'bun', 'npx')
`headers`	No	Custom HTTP headers (e.g. { 'Authorization': 'Bearer token' })
`timeout_ms`	No	Connection timeout in milliseconds (default: 30000)

Schema Changelog

Changes observed during successful MCP inspections.

Changed6 schema fields changedv1.1.1Jun 25, 2026

changedInput schema / properties / args / description

Previous value: -"Command arguments (e.g. ['run', 'server.js'])"New value: +"Command arguments for stdio (e.g. ['run', 'server.js'])"

changedInput schema / properties / command / description

Previous value: -"Server command to execute (e.g. 'node', 'bun', 'npx')"New value: +"Server command for stdio transport (e.g. 'node', 'bun', 'npx')"

changedInput schema / properties / env / description

Previous value: -"Additional environment variables"New value: +"Additional environment variables for stdio"

addedInput schema / properties / headers

Added value: +{
+  "additionalProperties": {
+    "type": "string"
+  },
+  "description": "Custom HTTP headers (e.g. { 'Authorization': 'Bearer token' })",
+  "type": "object"
+}

addedInput schema / properties / url

Added value: +{
+  "description": "MCP server URL for HTTP/SSE transport (e.g. 'http://localhost:3000/mcp')",
+  "type": "string"
+}

removedInput schema / required
```
Removed value: -[
-  "command"
-]
```

First observedv1.0.0Jun 24, 2026

TDQS

A3.6/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations are provided, so the description must carry the full burden. It describes what the tool detects and the rationale (hiding instructions), but fails to disclose whether the tool modifies anything, whether it requires read access, or what the output format is. This is adequate but not thorough.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is extremely concise: two sentences. The first sentence front-loads the core purpose and lists examples. The second sentence explains the security significance. Every word contributes value with no redundancy.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Despite no output schema, the description does not explain what the tool returns (e.g., list of findings, counts). It also fails to connect the input parameters (url, command, etc.) to the concept of 'tool descriptions' – it is unclear how the parameters relate to scanning. This leaves significant gaps given the tool's complexity.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% with all 6 parameters described in the input schema. The tool description adds no parameter-specific details, so it provides no additional value beyond the schema. A baseline score of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool detects hidden Unicode characters in tool descriptions and lists specific character types. This distinguishes it from sibling security checks like rt_check_ansi_injection or rt_check_prompt_injection, which focus on other injection vectors.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies use for inspecting tool descriptions, but does not explicitly state when to use this tool versus alternatives, nor does it provide when-not-to-use guidance. For example, it could clarify that it is intended for checking MCP server tool descriptions after server connection.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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