API Ninjas

detectFaces

Read-only

Runs computer-vision face detection on an image supplied in the POST request body and returns the faces found. Takes no query parameters.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "image": "<image>"
    },
    "name": "detectFaces"
  }
}

Input Schema

Name	Required	Description	Default
`image`	Yes	Image file to be analyzed

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 9, 2026

TDQS

B3.2/5.0

Behavior3/5

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

Annotations declare readOnlyHint=true, so the safe read-only nature is already covered. The description adds useful context that input arrives via the POST body and that no query parameters are accepted. However, it says nothing about output shape (bounding boxes, count), limits, or accepted image formats.

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

Conciseness4/5

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

Two short sentences, front-loaded with the action and scope. Efficient with no filler, though the second sentence ('Takes no query parameters') is largely redundant against a single-property schema.

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

Completeness3/5

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

For a tool with no output schema, the description should describe what comes back, and 'returns the faces found' is minimal – no indication of coordinates, count, or confidence. The input side is adequately covered, but the return side is under-specified.

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% and there is only one parameter, so baseline is 3. The description restates that the image comes in the POST body but adds no detail beyond the schema, such as size or format constraints.

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

Purpose4/5

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

States a specific verb and resource ('face detection on an image') and clearly scopes the input. It is distinguishable from the closest sibling 'detectObjects', but only implicitly via the word 'face' – it never names or contrasts the alternative. Clear purpose, no explicit sibling differentiation.

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

Usage Guidelines2/5

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

There is no when-to-use guidance and no reference to the closely related detectObjects sibling that an agent would plausibly confuse this with. 'Takes no query parameters' is a structural note, not usage guidance. Nothing tells the agent when to prefer this over alternatives.

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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