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

Annotations cover the safety profile (readOnly, openWorld, non-destructive), and the description goes well beyond them: it discloses the return shape (language + code + creator-uploaded vs auto-generated + default), the edge case that an empty list means captions are off, and the cost of 1 credit. That is meaningful operational context not derivable from the schema or annotations.

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 sentences, each load-bearing: trigger, return contents, cost, and the negative routing to get_youtube_transcript. Front-loaded with the use case rather than padding, with no redundancy.

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?

No output schema exists, but the description describes the returned fields and the empty-list case, so the agent knows what to expect. Combined with a single fully documented parameter and clear annotations, nothing needed to call it correctly is missing.

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 description coverage is 100% for the single video_url parameter, so the schema carries the syntax burden (baseline 3). The description adds cross-tool meaning by explaining that the returned code should be passed as `language` to get_youtube_transcript, which is value beyond the schema.

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 (list caption/subtitle languages for a YouTube video) and explicitly contrasts with the sibling get_youtube_transcript, which also deals with transcripts. An agent can distinguish the two without opening either schema.

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?

Gives both the trigger ('when the user asks which caption or subtitle languages a video has') and a proactive case (before requesting a non-English transcript). It also names the exclusion: 'Not needed for an ordinary transcript request: get_youtube_transcript already reports which language it returned.' This is explicit when/when-not/alternative guidance.

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