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

Annotations already cover safety (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the bar is lower. The description adds real behavioral context: 'Nothing is copied — the network fetches the URL when it publishes,' which clarifies that this is deferred fetching rather than a transfer. It does not describe the shape of the reported result, keeping it below a 5.

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

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

Three short sentences, front-loaded with the core action, then the no-copy clarification, then the alternative routing. No wasted words.

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

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

For a one-parameter validation tool with no output schema and full annotation coverage, this description covers what happens, what does not happen (no copy), and the correct alternative. Only the nature of the returned report is left vague, which is acceptable given the tool's simplicity.

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

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

With a single parameter and 100% schema description coverage, the schema already documents the url field fully ('A public https URL to an image or video'). The description restates the public-https constraint but adds no syntax, format, or edge-case detail beyond it, so baseline 3 applies.

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

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

The description gives a concrete verb and resource: it validates that a public https URL is fetchable and reports what it is. It also implicitly differentiates itself from request_upload_url, though the name 'register_media' is slightly broader than the 'check and report' behavior described.

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

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

It states the exact condition that selects the alternative: 'If you hold bytes rather than a URL, call request_upload_url instead.' This is an explicit when-to-use / when-to-use-something-else routing rule, leaving nothing to inference.

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