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

Annotations already declare readOnly, idempotent, non-destructive and open-world, so the safety profile is covered. The description adds genuinely useful behavior beyond that: the response is exhaustive across apps, includes verification/plan/billing/usage/refunds, and returns empty for an unknown email — a meaningful edge case an agent must handle.

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 compact sentences: the action first, then return shape and scoping, then the empty-result edge case. Every sentence carries information and nothing is padded.

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?

No output schema exists, so the description must describe the return — and it enumerates the returned detail categories and the empty case. Combined with two well-explained parameters, an agent has everything needed to call and interpret it.

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 0%, so the description must carry the parameter meaning — and it does: email is the lookup key and app_id is an optional scope limiter ('within one app if app_id is given'). It does not state email format or app_id sourcing, leaving a small gap, but the semantics are usable.

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 and resource ('Look up a customer by email') plus the exact scope of what is returned (every subscription/registration across apps). This clearly distinguishes it from sibling list_* tools, which enumerate collections rather than resolving one customer by identifier.

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 description makes the selection context clear: use it when you have an email and need the full customer picture, and pass app_id to narrow to a single app. It stops short of explicitly naming an alternative tool or stating when not to use it, so it falls just below the top band.

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