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

Annotations already cover read-only, idempotent and non-destructive behavior; the description adds useful context beyond that with auth scope ('account key'), content exclusions and a concrete size failure mode ('may exceed the 1 MiB read limit'). It does not spell out error handling or rate limits, so it is not a 5.

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?

Three compact sentences with the read/auth label and field scope front-loaded. The exclusions and size caveat each earn their place; nothing repeats the title or schema mechanically.

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 single-parameter read tool with annotations and no output schema, the description covers returned fields, excluded content and the read-limit caveat. It does not describe response structure or not-found behavior, but the essentials for selecting and invoking the tool are present.

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?

There is one required message_id parameter with 0% schema description coverage. The description implies it is the submitted message's ID, but does not explicitly document the input or explain its format/constraints, so it only partially compensates for the schema gap.

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 read operation and enumerates the returned fields: ID, sender, recipient, subject, groups and routing metadata. The 'submitted message' scope and exclusions distinguish it clearly from the account/domain/IP list tools in the sibling set.

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 '(account key)' marker signals the auth scope, and exclusions for bodies, attachments, custom headers and recipient custom fields imply when this metadata-only tool is appropriate. It does not name an alternative tool, but no sibling appears to compete for message-level metadata retrieval.

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