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

Annotations already state readOnlyHint, openWorldHint, idempotentHint=false, destructiveHint=false, so the safety profile is covered. The description adds real behavioral context beyond that: no speech-to-text, uncharged when the requested language is missing, and payment-rail constraints ('Not offered on Solana: pay on Base or Polygon'). Does not mention rate limits or pagination, but for a read-only fetch that is acceptable.

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 dense sentences front-loaded with purpose and output shape, followed by constraints. No filler, though it packs many clauses and could be split for readability. Substance outweighs structure issues.

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?

For a 3-param read-only tool with no output schema, the description covers purpose, inputs, outputs, language handling, billing behavior, and payment-rail limits. It lacks explicit notes on rate limits or rate-limit errors and does not name sibling alternatives, but it is complete enough for correct invocation.

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 all three parameters are already documented in the schema, including enum descriptions and defaults. The description restates input forms ('video URL or 11-character id') and format/language options but adds no syntax or edge-case details beyond the schema. Baseline 3 is correct when the schema does the heavy lifting.

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 (fetch subtitles) and resource (YouTube transcript) with scope limits: 'No speech-to-text.' Names the accepted input form (URL or 11-char id) and outputs, so an agent can tell it apart from siblings like youtube or youtube-comments without opening schemas.

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?

Clearly implied usage: send a video URL or id to get subtitles. Gives one exclusion ('No speech-to-text') which routes agents that need transcription elsewhere. Does not name a specific alternative sibling for videos without subtitles or for other YouTube data, so it stops short of a 5.

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