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

No annotations are present, so the description carries the full burden. It discloses the return format ('plain text with title and metadata'), input normalization across four URL formats plus bare ID, and how the lang parameter changes caption-track selection. This covers the core observable behavior well, though it omits edge-case behavior such as what happens when a video has no captions or an invalid ID is supplied.

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 earning its place: purpose is front-loaded in the first clause, the second sentence covers both input flexibility and output format, and the third covers the optional parameter. There is no fluff, no repetition of what the schema already states.

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?

Because there is no output schema, it is important that the description states the return shape, and it does ('plain text with title and metadata'). Both parameters are fully documented in the schema and invocation behavior is clear. The only missing context is failure-mode behavior (unavailable transcripts, invalid IDs), which is a minor gap for a simple read tool.

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% — both video and lang are already documented with clear descriptions including BCP-47 examples. The tool description adds marginal value by explicitly enumerating the URL variants (watch, youtu.be, shorts, embed), but the schema already carries the semantic load, so the baseline of 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 opens with a specific verb+resource pair: 'Fetch the full text transcript of a YouTube video.' It is unambiguous about what is retrieved and explicitly states the output form (plain text with title and metadata), leaving no room to confuse it with a summary, metadata-only call, or video download tool even with no siblings present.

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?

With no sibling tools to route between, the description supplies clear usage context instead: it enumerates the accepted input forms (watch, youtu.be, shorts, embed URLs or bare 11-character ID) and explains when to set the optional language parameter versus omitting it for the default track. It provides clear invocation context, though there are no explicit exclusions or when-not-to-use statements since no alternatives exist.

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