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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description still contributes the non-obvious constraint that only verified subscribers are retrievable, which explains zero-result behavior an agent would otherwise misread as a bad email.

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 short sentences: purpose, the key caveat, then the endpoint reference. The most decision-relevant information is front-loaded 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?

With two fully-documented required parameters, no output schema, and annotations covering the read-only nature, the description supplies enough to invoke correctly. Only the absence of guidance on what to do when a subscriber isn't verified keeps it from 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?

Both parameters are fully described in the schema, including the MF-prefixed 12-character uuid format and a cross-reference to referralhero_list_campaigns. The description adds no parameter-level information beyond what the schema already provides, so baseline 3 applies.

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 (find one subscriber of a campaign) plus the lookup key (email), which naturally separates it from get_subscriber, get_subscriber_by_referral_code, and search_subscribers_by_name. It stops short of naming those siblings explicitly, so the differentiation is implied 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 caveat 'Only verified subscribers are found' tells the agent when this lookup will fail, which is genuine usage context. However, it never points to an alternative such as get_subscriber or search_subscribers_by_name for unverified subscribers, so selection among the several subscriber-lookup siblings is left to inference.

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