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

Annotations already declare this a safe read (readOnlyHint=true, destructiveHint=false), so the safety burden is met. The description adds genuinely useful behavioral context beyond structured fields: the join semantics (campaign metrics + latest portfolio assignment snapshot) and the rollup grain. It does not cover pagination/rate-limit behavior, but the schema handles pagination.

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 tight sentences with zero waste: purpose, grain, then routing. The scoping/grain information is front-loaded before the alternative-tool pointer.

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 an output schema present, return values need not be explained, and the 100%-covered schema handles parameters. The description covers purpose, grain, join provenance, and the drilldown alternative, which is complete for a read-only rollup 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 description coverage is 100%, so all 12 parameters (including the 'unassigned' portfolio_id convention) are already documented in the schema. The description adds no parameter-level detail, so the baseline 3 for full schema coverage is appropriate.

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 ('Returns Sponsored Products performance rolled up by portfolio ID') and defines the output grain ('one row per portfolio'). It also distinguishes itself from campaign/ad group/keyword drilldown siblings, so an agent can tell it apart without opening a 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?

Explicitly routes the agent away for finer-grained metrics ('For individual campaign/ad group/keyword metrics, use the matching drilldown tool'). It gives a clear condition but names the alternative only generically rather than citing get_campaign_performance / get_keyword_performance, leaving some inference to the agent.

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