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

Annotations only cover readOnly and openWorld; the description adds material facts the agent could not otherwise know: single page with no pagination (~20 results), subscriber counts as text rather than exact numbers, data sourced from the public channel-filtered search page, and a $0.002 USDC x402 cost per successful call with failures uncharged. The billing and pagination limits are exactly the kind of operational detail that changes invocation behavior.

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 the primary purpose and result shape before the cost/payment clause, with zero filler. The field enumeration (channel id, name, handle, subscriber text, snippet, link) partially duplicates the output schema, which is mildly redundant but still useful for quick scanning.

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 one-parameter read tool with an output schema, the description covers the remaining unknowns an agent needs: result cap, no pagination, precision limits on subscriber counts, data provenance, and pay-per-call billing with failure handling. Nothing required to invoke or budget for the call 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?

There is a single required parameter and schema coverage is 100%, including the '2-100 characters' constraint, so the schema does the work. The description restates the concept as 'search query' without adding syntax, matching behavior, or operator support beyond what the schema already declares.

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 and resource ('YouTube channels matching a search query') and immediately bounds scope: first page, ~20 results, channel-filtered search page. It explicitly excludes what siblings cover ('No video lists or exact subscriber counts'), letting an agent separate it from social_youtube or social_youtube_edu_channels without reading 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?

The exclusions ('No video lists or exact subscriber counts') give implicit routing guidance toward video-level siblings, and 'first page, about 20' tells the agent this is a shallow lookup, not exhaustive enumeration. It stops short of naming the specific alternative tool to use instead, so it is clear context rather than explicit when/when-not routing.

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