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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so safety is covered. The description adds genuine value beyond that by disclosing the returned fields and the specific mono/16 kHz pre-check that drives engine selection. It does not mention failure behavior for unreachable URLs, which would be useful given openWorldHint.

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?

Two sentences, nothing redundant: outputs first, then the call-to-action ordering hint. Every clause carries information an agent needs.

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?

With no output schema, the description usefully enumerates return fields, so the agent knows what it will get back. The only gap is error/unreachable-URL behavior for an open-world read against an arbitrary remote URL.

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?

There is a single parameter with 100% schema description coverage, so the schema already defines mediaUrl fully. The description only echoes 'for a URL' and adds no format, scheme, or size constraints beyond it.

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?

States a specific verb (probe/return) and enumerates the exact resources produced: duration, dimensions, codecs, container, plus a mono/16 kHz flag. This clearly separates it from sibling media tools like lunara_transcribe or lunara_start_subtitle_job, though those siblings are never named.

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?

"Call this before choosing an ASR engine" gives a concrete ordering/precondition, which is real usage guidance. It stops short of naming the alternative tools or stating when to skip it (e.g., when metadata is already known).

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