pyfastmail-mcp

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

No annotations are provided, so the description carries the full burden. It discloses the return format ('id, name, and email address'), which is useful behavioral context. However, it lacks details on permissions, rate limits, or pagination behavior, which would be helpful for a read operation.

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?

The description is two sentences with zero waste: the first states the purpose, and the second specifies the return values. It is front-loaded and efficiently structured, with every sentence providing essential 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?

Given the tool's simplicity (0 parameters, no annotations, but with an output schema), the description is mostly complete. It explains what the tool does and what it returns, though it could benefit from additional behavioral context (e.g., authentication needs). The output schema likely covers return details, reducing the burden on the description.

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 has 0 parameters with 100% schema description coverage, so no parameter information is needed. The description appropriately does not discuss parameters, earning a baseline score of 4 for not adding unnecessary details.

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 clearly states the specific action ('List all sender identities') and resource ('on this Fastmail account'), distinguishing it from sibling tools like mail_list_mailboxes or mail_list_masked_emails. It precisely defines the scope of what is being listed.

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 usage when needing to retrieve sender identities, but does not explicitly state when to use this tool versus alternatives (e.g., for email-related tasks like sending or searching). No exclusions or prerequisites are mentioned, leaving context to be inferred from the tool name and sibling list.

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