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

Annotations already declare readOnly/openWorld/non-destructive, so the safety profile is covered. The description adds real behavioral context beyond them: limit is capped at 20 and excludes Shorts and live streams, and it enumerates the normalized fields returned per video despite the absence of an output schema. It stops short of covering failure modes or ordering guarantees.

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 tight sentences, front-loaded with purpose and mode routing before the return-shape list. The field enumeration is long but justified given there is no output schema, so every clause earns its place.

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 field list is a valuable substitute and the mode/limit/exclusion details are all present. It omits error handling and what happens with an invalid channel handle, so it is strong but not fully complete for a two-mode 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 coverage is 100% and all three parameters are self-documented in the schema, so the baseline is 3. The description largely restates the schema (mode semantics, channel handle without @, limit range) and adds no syntax or format detail the schema lacks.

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 names a specific verb+resource ('YouTube video search by keyword, or the videos of one channel') and pins down the two operating modes. It clearly distinguishes this from siblings like youtube-comments and youtube-transcript, which cover different resources.

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?

It states which input each mode expects ('mode search takes keywords; mode profile takes a channel handle without the @'), giving clear routing between the two modes. However, it never states when to prefer this tool over the sibling youtube-transcript/youtube-comments tools or any exclusion criteria.

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