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

Annotations already signal readOnly/not destructive, and the description adds meaningful behavioral detail beyond them: exact error conditions (unsupported_provider, master_key_required), the requirement for a connected integration, and the returned fields (id, email address, active status). This is precisely the kind of auth/error context that helps an agent predict outcomes.

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?

Five sentences, each adding a distinct piece of information: scope, return shape, downstream usage, auth/error behavior, and integration prerequisite. Front-loaded with the core listing behavior; no filler or repetition of schema content.

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 2-parameter list tool with no output schema, the description is complete: it covers return fields, supported/unsupported providers, auth requirement, prerequisite integration, and error behavior. The only omitted details (e.g., pagination) are minor and not necessary for correct invocation.

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 coverage is 100%, so the baseline is 3. The description adds value beyond the schema by explaining provider parameterization and that instantly, despite being an enum value, currently returns unsupported_provider. It also ties the returned id to the emailAccountId required elsewhere, giving the provider parameter operational context the schema alone does not provide.

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?

Description opens with a specific verb and resource: 'List an external provider's sending mailboxes / email accounts', then immediately scopes it as provider-parameterized and notes that only Apollo exposes mailboxes today. This clearly distinguishes it from sibling list tools like campaignstack_list_external_lead_lists and campaignstack_list_external_sequences, even without opening the schema.

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?

Clear usage context: it states Apollo is the only supported provider (other providers return unsupported_provider), calls out prerequisites (MASTER API key, connected integration), and explains a downstream use case (Apollo's add-to-sequence needs emailAccountId). However, it does not explicitly name alternative tools or say 'use X instead', so it stops short of a fully explicit when/alternative decision rule.

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