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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuinely non-obvious context beyond that: the result is scoped to "this API key" (not a global/account view) and the call is "Free", i.e. it does not consume the credits it reports. It stops short of describing error behavior or whether plan data includes renewal/billing detail.

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 sentences, roughly a dozen words, with the resource and scope front-loaded and the cost note trailing. Nothing is redundant or padded.

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 zero-parameter read tool with no output schema, the description covers everything an agent needs: what it returns (remaining credits plus plan), whose data it returns (this API key), and that it costs nothing. There is no complexity left unexplained.

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?

The tool takes zero parameters, so the baseline is 4. The description correctly introduces no parameter discussion and instead clarifies that the API key is implicit from the request context, which is the only input-like consideration.

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 ("Show") and resource ("remaining TranscriptYT credits and plan") with an explicit scope ("for this API key"). None of the siblings (get_transcript, search_youtube, summarize_video, etc.) are account/credit tools, so there is no ambiguity about which tool to pick.

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?

Usage is implied rather than stated: "for this API key" and "Free" suggest a lightweight pre-check or budget check, but the description never says when to call it (e.g. before bulk transcript requests) or what happens if the key is invalid or credits are exhausted. With no comparable sibling, the absence of explicit alternatives is tolerable, but the when-to-use guidance is left to inference.

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