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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context beyond annotations: the cross-account resolution behavior, the fact that a thread id not owned by the caller still resolves, and the instruction to check the returned inbox_id. This is meaningful additional transparency.

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 and front-loaded with the core behavior, then lists the returned fields, then explains the cross-account caveat and names the sibling. Every sentence earns its place. It is slightly dense with the field list, but that list is useful for an agent deciding whether this tool returns what it needs.

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?

The tool has an output schema, so return values are already documented. The description covers the key contextual gap: the whole-account resolution and the shared-account caveat. It also names the scoped alternative. It does not mention pagination or message ordering, but for a single-thread fetch with an output schema, the description is sufficiently complete.

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 schema already documents both parameters (thread_id and Authorization). The description adds context about what fields the thread includes and the cross-account behavior, but it does not add syntax or format details for the parameters themselves. Baseline 3 is appropriate when the schema carries the parameter documentation.

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 states a specific verb ('Fetches'), a specific resource ('one thread by id'), and a distinctive scope ('without naming an inbox... resolves against the whole account'). It also names the scoped twin, get_agentmail_inbox_thread, which distinguishes it from the many sibling thread tools. This is a clear, differentiating definition.

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 explains when to use this tool: when you have a thread id and no inbox context, because it resolves across the whole account. It also warns that other callers' threads are reachable and advises checking the returned inbox_id. It names the alternative (get_agentmail_inbox_thread) as the scoped twin, giving the agent a clear routing decision.

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