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

The readOnlyHint annotation already declares this a safe read. The description adds the underlying API path (GET /v1/email/messages/{message_id}), which is weak extra context, but says nothing about auth requirements, 404/not-found behavior, or rate limits. With annotations covering the safety profile, this meets the minimum bar without exceeding it.

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, front-loaded sentences with the purpose first and no padding. The trailing API-path sentence earns its place only marginally, keeping this just short of a 5.

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 readOnlyHint and no output schema, the description covers what the agent needs to call it correctly. Missing error/not-found behavior is a minor gap given the simplicity.

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% for the single parameter and the schema already documents 'Email message id.' The description's 'by id' adds no format, prefix, or sourcing detail beyond that. Baseline 3 applies when the schema does the heavy lifting.

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 ('Fetch') and resource ('a single email message by id'), which cleanly separates it from the list counterpart bird_list_email_messages and from the SMS/WhatsApp getters by channel. It does not explicitly name those siblings, but the resource phrasing is unambiguous.

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?

'by id' implies the tool is used when a message id is already known, but there is no explicit when-to-use statement, no mention of prerequisites like obtaining the id from bird_list_email_messages, and no exclusions. Usage is only weakly implied.

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