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

Annotations already declare readOnly, idempotent and openWorld, but the description adds genuinely new operational behavior: the paid x402 USDC-on-Base model at $0.002 per successful call, the instruction to call without payment to receive the PaymentRequired object, and the no-charge policy for unavailable/private videos and errors. These are things an agent cannot learn from the annotations or schema.

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 tight sentences ordered by importance: use case and returned fields first, accepted input formats second, billing/error behavior last. No filler or repetition of the tool name.

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 appropriately enumerates the returned fields, and the annotations cover the safety profile. Payment requirements, accepted input formats, and failure/charging behavior round out everything an agent needs to invoke it correctly.

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?

Schema coverage is 100%, so the baseline is 3, but the description adds meaning beyond the schema by accepting 'a video URL, Shorts URL or 11-character ID' — the schema only mentions 'YouTube video URL or 11-character video ID', omitting Shorts. It adds nothing for lang/country, which the schema already documents.

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+resource ('the details of one YouTube video') and enumerates the exact data returned (view/like/comment counts, duration, publish date, category, description, tags, live status, thumbnail). The 'one YouTube video' scope cleanly separates it from siblings like youtube_channel, youtube_comments, youtube_playlist and youtube_search.

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?

'Use when you need the details of one YouTube video' gives clear selection context, and the field enumeration reinforces that. However, it never names the alternatives (e.g., use youtube_search to find a video, youtube_transcript for captions) or states when not to use it, so routing still requires inference.

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