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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower, yet the description still adds real scope boundaries: it reports credits and published list price only and 'never reports what a campaign costs to run.' That negative boundary is genuinely useful context beyond the annotations.

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?

Four short sentences, front-loaded with purpose, then usage, then return shape, then a clarifying boundary. Each sentence earns its place and nothing is redundant.

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 description carries the burden of explaining returns and does so by listing the exact fields (email, availableAdRunCredits, requiredAdRunTokens, canStartAdRun, buyUrl), plus usage context and scope limits. An agent has everything needed to call and interpret it correctly.

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 0% and the description never mentions the single optional response_format parameter. However, it is a self-explanatory optional enum (markdown/json) with no required params, so the omission is minor and the baseline of 3 is appropriate rather than a harsher score.

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+resource ('Show the signed-in user's Ad Run credit balance') and adds the decision-relevant scope ('whether they can start a new Ad Run'). It is clearly distinguishable from siblings like get_capabilities or list_ad_runs.

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?

Explicitly says when to reach for it ('Use this before suggesting a launch, so you know whether the user needs to buy a credit first'), giving a concrete trigger condition. It does not name a specific alternative tool or a when-not case, so it falls just 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.