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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely non-obvious context: results may contain end-user PII and the agent should quote minimally and mask direct identifiers. It stops short of describing result volume, pagination or rate limits.

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?

Two tight sentences: the capability and scope come first, the PII caveat second. No filler, no repetition of the title, and every clause earns its place.

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 needn't explain return values, and it correctly covers the sensitivity of the data plus the scope of the drill-down. What it lacks is sibling routing and any hint that user_intent/reason are required steering parameters, which is the main remaining gap.

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 67% (range and interval are undocumented, and the reason/user_intent conventions live only in the schema), so the description could have compensated but does not mention any parameter. The facet list describes output dimensions rather than inputs, leaving the parameter burden largely on the schema.

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?

Names a specific verb ('drill into') and scoped resource ('one user's activity timeline') and enumerates the facets returned (costs, outcomes, agents, intents, signals, sessions). It clearly distinguishes itself as a per-user drill-down, though it never names the siblings (get_session, list_sessions, aggregate_sessions) it competes with.

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?

There is no explicit when-to-use or when-not-to-use guidance, no mention of prerequisites such as obtaining user_id from list_users, and no routing advice against the many sibling tools. The phrase 'drill into one user's activity' only weakly implies the use case.

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