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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuine value beyond that by disclosing the return payload (field list) and that the key can be either an id or an e-mail address. It does stop short of describing error behavior for a missing/invalid subscriber, which is a minor gap.

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, front-loaded with the scope and returned fields, with zero filler. The trailing 'GET /subscribers/{id_or_email}' restates the resource path already implied by the name and title, making it mildly redundant rather than harmful.

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?

For a one-parameter read tool with full annotation coverage, the definition does what matters: it names the lookup keys and previews the returned fields in the absence of an output schema. Only the failure mode for an unknown id/e-mail is unaddressed, which is a small omission for this complexity level.

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 sole parameter's description already states 'id or e-mail address', so the description's 'by id or by e-mail address' duplicates rather than extends the schema. Baseline 3 is appropriate — no format, max-length or encoding detail is added.

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 ('Get') and resource ('One subscriber') with explicit scoping to a single record keyed by id or e-mail, and enumerates the returned fields (type, tags, notes, metadata, source, engagement, lifecycle dates). The singular framing plus 'GET /subscribers/{id_or_email}' clearly separates it from buttondown_list_subscribers and buttondown_update_subscriber.

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 implies the fetch-one-subscriber use case through 'One subscriber by id or by e-mail address', which is enough for an agent to infer when to call it versus a list. However, it never explicitly contrasts with buttondown_list_subscribers or buttondown_update_subscriber, and states no prerequisites. Implied usage only.

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