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

Annotations already declare readOnly=true, destructive=false, openWorld=false, so the description is freed from the safety profile. It then adds genuinely non-obvious context: attachments are returned as authenticated v1 API links requiring a Bearer key with the inbound scope, and explicitly are not signed/presigned URLs. That access-model detail is a real behavioral trait an agent could not infer from annotations or schema.

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, front-loaded with purpose and then the returned fields, with no filler. It is dense with parentheticals, but every clause carries information rather than repeating the title.

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 two-param read tool with an output schema and read-only annotations, the definition is essentially complete; the agent knows what it gets back and how attachment links are authorized. It could be strengthened by noting the list-tool alternative for lookup-by-criteria, but nothing needed to call it correctly is missing.

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%, so the 'id' publicId format and the 'context' intent-tracking contract are fully documented in the schema. The description's 'by id' merely restates the schema's required identifier without adding syntax or constraints. Baseline 3 applies.

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 received inbound email message by id') and enumerates the returned payload. An agent can distinguish it from senderkit_inbound_messages_list (plural/list) and senderkit_messages_get on the 'inbound' qualifier without opening any 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?

The 'by id' framing and 'single' contrast with the sibling list tool for the same resource, so usage is implied. However, no explicit when-to-use/when-not guidance or named alternative is provided, so an agent must infer the routing itself.

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