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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, so the safety profile is covered. The description adds genuinely new behavioral detail: it returns the subscriber object including its uid, and a missing match surfaces as a not-found error rather than an empty result — that shapes agent error handling.

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 short sentences, no filler, and the lookup scope is front-loaded ahead of the return contract. Every clause carries information.

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 no output schema, the description correctly takes on the return-value burden by naming the returned subscriber and its uid. What's missing is edge-case behavior (case sensitivity, whether a not-found error is distinguishable from an auth/list-not-found error), but for a 2-parameter read this is close to sufficient.

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%, but the two required parameters are self-describing by name and the description contributes the key semantic that list_uid scopes the lookup to a single list. It adds nothing about uid format, case-sensitivity of the email match, or duplicate match behavior within a list.

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 subscriber') plus the scoping constraint ('in one list by email address'), which cleanly separates it from get_subscriber (by uid) and list_subscribers. It never names those siblings explicitly, so the differentiation is inferable rather than stated.

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 phrase 'by email address' and 'in one list' implicitly cues when this is the right tool (you have an email, not a uid; you know the list), but there is no explicit when-to-use/when-not or named alternative. The agent must infer the routing decision from the sibling names.

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