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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely useful behavioral detail beyond that: the body is returned as plain text and attachments come back as names and sizes only, not content.

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 front-loaded sentence that enumerates the returned fields with no filler. The clause 'by the id thunderbird_list_emails or thunderbird_search_emails returned' is slightly convoluted but justified by its routing value.

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?

With a full input schema, an output schema, and read-only annotations in place, the description covers purpose, id source, and content-vs-metadata expectations. An agent could call this correctly without further information; only minor usage-exclusion guidance 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 coverage is 100%, so both id and max_bytes (including the truncation behavior) are already documented in the schema. The description only restates the id provenance and says nothing about max_bytes or truncation, so it adds little beyond the structured fields — baseline 3.

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 (reads) and resource (one Thunderbird message by id), and explicitly names the two sibling tools that produce that id, so it is unmistakable versus thunderbird_list_emails or thunderbird_search_emails.

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 id provenance ('from thunderbird_list_emails or thunderbird_search_emails') tells the agent when this tool applies and implies a prior list/search step. It stops short of stating exclusions (e.g. use search when you don't yet have an id), but the usage context is clear.

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