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PNG magic-byte check, file discarded

by io.github.sadri-dridi
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png-magic

PNG magic-byte check, file discarded

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
refNoGit ref name; discarded after the shape check
urlNoHTTPS URL to normalize or cite
cityNoCity name for a public weather hint; discarded after the call
feedNoPublic RSS or Atom URL; titles discarded
hostNoPublic hostname
jsonNoJSON text to validate; discarded after the check
pathNoFile path to check; no disk access
zoneNoIANA timezone name
queryNoSearch text; discarded after the length check
websiteNoPublic https website (homepage domain) to check for agent readiness: robots.txt, llms.txt, sitemap.xml, extractable HTML. Bounded, identified, robots-respecting.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • addedInput schema / properties / website
      Added value: +{
+  "description": "Public https website (homepage domain) to check for agent readiness: robots.txt, llms.txt, sitemap.xml, extractable HTML. Bounded, identified, robots-respecting.",
+  "type": "string"
+}
  2. First observed

TDQS

C2.9/5.0
Behavior2/5

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

There are no annotations, so the description carries the full burden of disclosing side effects. It offers only 'file discarded,' which is ambiguous (discarded from memory? deleted from disk?) and does not state whether the operation is read-only, what happens on non-PNG input, or whether any network/disk access occurs. A safe read-only behavior cannot be confidently inferred.

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?

One short sentence with no filler; the core action and the file-disposition note are front-loaded. As a concise statement it is well structured, although the terseness causes completeness problems captured elsewhere.

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 100% schema parameter descriptions, the overall tool behavior is incomplete: no output schema, no return/status description, no guidance for choosing among 10 optional parameters, and no note on whether the check applies to path, url, or another input. An agent would struggle to know what to pass and what the result means.

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?

Input-schema coverage is 100%, with each of the 10 parameters already described (e.g., path, url, website). The main description adds no parameter-level meaning or guidance on which parameter is relevant to the PNG magic-byte check, so it stays at the baseline.

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?

Names a specific operation: checking PNG magic bytes, and adds the disposition ('file discarded'). This is clear enough to distinguish it from generic validation tools, but it does not explicitly contrast with sibling shape-check tools like file-path-ok or fetch-status.

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?

The description gives no guidance about when to select png-magic over sibling tools such as file-path-ok, browser-url-ok, or web-fetch, and no conditions or exclusions. The only usage signal is the phrase itself, which implies 'use for PNG checks' but does not state how to choose among the 10 optional parameters.

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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