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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds useful detail about returning the full message list and metadata fields, but does not disclose additional behavioral traits such as ordering, pagination, or thread-scoping nuances beyond what the name and schema imply.

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 compact, front-loaded with the main action in the first clause, and the sibling pointer sentence is useful. The long list of returned fields is somewhat redundant with the output schema, but it helps an agent quickly understand what a 'full thread' includes.

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 simple read-only getter, the description is complete: all required parameters are documented in the schema, the output schema exists, annotations cover safety, and the description gives the core behavior plus an explicit alternative. Nothing critical is missing for an agent to invoke it correctly.

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 all three parameters are already documented. The description mentions thread_id and inbox_id only as returned fields, not as parameter semantics. It adds no meaning beyond the schema, so the baseline 3 is appropriate.

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 uses a specific verb and resource: 'Fetches one thread with its full message list', which clearly identifies what the tool does and distinguishes it from single-message or list tools. It even names the sibling for single messages, making the scope concrete. This is a purpose statement that needs no inference.

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 explicitly tells when to use this tool versus an alternative: use this for a full thread with messages, and 'For a single message use get_agentmail_inbox_message'. This provides a clear selection rule rather than leaving the agent to compare schemas.

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