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

Annotations already declare read-only/idempotent/non-destructive, and the description corroborates without contradicting while adding substantial behavior: Pacific Time, the 'last 28 finalized days' default window, that click change is decomposed into impressions and CTR parts, that filters/search type apply to both periods, and a candid caveat that changes are observed differences and do not identify causes. This is disclosure well 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?

Front-loaded with the triggering scenario, then flows from scope to comparison modes to return shape to caveats. The single dense paragraph is efficient, though it is long enough that the return-value and timezone details could be split for faster 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 13-parameter comparative tool with an output schema and full annotation coverage, the description covers the decision to use it, the comparison-mode semantics, defaults, timezone, and the limits of what the output means. Nothing an agent needs to select or invoke it correctly 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?

Schema coverage is 100%, so parameters are already documented, but the description still adds defaults and cross-parameter semantics the schema lacks: 'previous_period' as the implicit default window, the last-28-finalized-days behavior with no dates, and the rule that explicit previous dates override compare_to. Useful additive context rather than mere repetition.

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 (compares) and resource (clicks, impressions, CTR, average position for a property between two periods), and implicitly distinguishes itself from the single-period sibling get_performance by framing the whole tool around change between periods. An agent can immediately tell this is the 'what changed' tool rather than a raw metrics fetch.

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?

Opens with explicit trigger conditions ('when the user asks what changed between two periods, why traffic went up or down, or which pages/queries/countries/devices gained or lost'), and the compare_to enum plus the 'Not used with explicit previous dates' note disambiguates the comparison modes. It also references get_performance for filter format, anchoring the sibling relationship.

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