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

With destructiveHint=true and idempotentHint=false already declared, the description earns credit by adding real behavior: the cost is applied asynchronously with a 'queued' result, the day total is spread across that day's clicks so derived ROI/CPC/profit change, and the 90-day/31-day limits bound impact. It does not explicitly warn that 'set' overwrites an existing day total beyond the parenthetical, which keeps it off a 5.

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?

Three dense sentences, front-loaded with the core action, then the accounting effect, then the operational limits. Every clause carries information an agent would otherwise have to guess.

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 non-idempotent mutation with no output schema, it covers the essentials: what is written, how it propagates, the async return ('queued'), and the size/time limits. Remaining gaps are minor — permissions required and whether prior costs are recoverable after an overwrite.

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 only 33%, so the description must compensate, and it does: it establishes that 'set' replaces and 'add' increments the day total, that the value is a per-day total rather than per-click, and that up to 31 items are allowed. It adds less for campaign_id, which stays unexplained but is self-evident.

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 pairs a specific verb ('Set (or add to)') with a precise resource ('a campaign's total traffic cost per day') and scopes it to 'sources without an API integration', which implicitly separates it from import_google_costs/import_meta_costs and from the segment-level set_segment_costs. An agent can recognize the operation without opening the schema.

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 gives clear applicability ('for sources without an API integration') and hard operating constraints ('Up to 31 days per call, 90-day editable window'). It stops short of naming an explicit alternative or stating when NOT to use it, so it is strong context rather than full routing guidance.

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