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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds real value beyond that: it discloses the built-in dry_run identity, notes that connected credentials exist but 'their keys are never shown' (a security guarantee an agent would otherwise have to discover), and mentions delivery state in the response.

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?

A single sentence with no filler, and the core subject ('the identities a campaign can send through') is front-loaded before the explanatory colon clause. Slightly dense packaging — the parenthetical about keys sits mid-sentence — but nothing is wasted.

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 zero-parameter, read-only listing tool with no output schema, the description covers what is enumerated (dry_run plus connected identities), a key security property, and the presence of delivery state. It stops short of describing the per-identity fields an agent would receive, which with no output schema is the one 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?

The tool takes no parameters and the schema is an empty object with additionalProperties=false, so there is nothing for the description to explain. Per the zero-parameter baseline this scores 4; the description correctly spends no words on inputs.

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?

The description names the resource precisely — sending identities a campaign can send through — and enumerates its two sources (built-in dry_run plus operator-connected ones), which is concrete enough for an agent to know exactly what is returned. It does not explicitly contrast with sibling list_* tools (list_campaigns, list_suppressions, list_email_status), but the resource is distinctive enough to disambiguate in practice.

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?

Usage is only implied: the framing 'the identities a campaign can send through' hints at pre-launch/pre-send checks, but there is no explicit when-to-use, when-not-to-use, or named alternative. With zero parameters there is little risk of misuse, so the guidance gap is moderate rather than severe.

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