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

Annotations already declare the safe read profile (readOnly, idempotent, non-destructive), and the description adds genuinely new behavior: no API key required, the paging contract (pass next_start_seconds as start_seconds), and explicit failure semantics (captions_unavailable rather than an empty transcript). This is exactly the kind of context annotations cannot express.

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?

Four sentences, each earning its place, front-loaded with purpose then usage, paging, and error behavior. No filler or 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?

With no output schema, the description carries the return-value burden and does so by naming the segment structure, joined text, language, auto-generated flag, and next_start_seconds. Combined with the error and paging notes, an agent has everything needed to call and consume 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 description coverage is 100%, so the baseline is 3; the description still adds value by tying start_seconds to the returned next_start_seconds, clarifying the paging loop across calls rather than restating the field. max_chars/lang defaults are left to the schema, so it is not a full 5.

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 (get) and resource (transcript/captions) and even enumerates the return shape: timestamped segments, joined text, language, auto-generated flag. This clearly separates it from siblings like yt_video_details or yt_video_comments, which would not return spoken content.

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 clear use context ('summarize, quote or search what is said in a video'), so the agent knows the intent. It does not, however, explicitly exclude or route against the closely related yt_video_details/yt_video_comments siblings, so it stops short of the 5-level routing guidance.

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