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

readOnlyHint=true already marks this as a safe read, and the description adds real value beyond that: chronological ordering (oldest first), the fact that ads appear as starting/running/stopping with spend and revenue attached, and that unusual days carry a whyId linking out to a breakdown. It does not mention pagination or how far back 'all' reaches, but the behavioral profile is otherwise well sketched.

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?

The usage cue is front-loaded and the whyId pointer is a useful closer, but the middle is a single run-on enumeration of eight event types that is dense to parse. Every item is arguably informative, yet the wall-of-items formatting makes the definition heavier than it needs to be.

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, the description need not document return values, and the readOnly annotation covers safety. Given that backing, it supplies the right residual information: what the list aggregates, its ordering, and the drill-down path to why_did_it_change. Only the absence of any hint about result volume or time span keeps it below a 5.

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%, and the schema already enumerates period formats, source options, and app identification in detail. The description adds no parameter-level meaning; the only added semantics (the whyId chain) is about a return value rather than an input. Baseline 3 applies when the schema does the heavy lifting.

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?

It clearly states the resource: a period viewed as a single chronological list (oldest first) containing ad starts/stops with spend and revenue, campaign changes, releases, unusual days, best days, milestones, and review bursts. The content is specific enough for an agent to recognize this is the timeline/event-history tool rather than a metric or story tool, though the operative verb ('see a period as one list') is softer than a crisp action verb.

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?

The opening clause gives an explicit precondition ('before explaining a chart or a change'), which tells the agent to gather context first, and the closing sentence routes full drill-down to the sibling why_did_it_change using the day's whyId. It does not, however, distinguish itself from the other siblings today, app_story, or how_are_my_apps.

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