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

Annotations only supply destructiveHint=true; the description adds meaningful context beyond that: the required gmail.modify scope, a reconnect hint for older connections, and an explicit confirmation warning ('This actually changes the mailbox'). It stops short of describing reversibility or partial-failure behavior, but goes well past the annotation.

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 core action and scope are front-loaded in the first sentence, and each subsequent sentence carries distinct load (label rules, account routing, auth, confirmation). It is dense without obvious filler, though the four sentences stack several topics that could be tighter.

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 destructive multi-parameter mutation with no output schema, the description covers what an agent needs: scope, auth requirement, multi-account routing, operational prerequisites, and a confirmation caveat. Nothing critical to correct invocation 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%, and it already documents thread_id format, account conditions, and label-id conventions (including system labels), so the schema carries the parameter burden. The description only adds a cross-reference ('Same label id rules as modify_message_labels'), which is a pointer rather than new semantics, fitting the 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?

The description states a specific verb pair and resource ('Add and/or remove labels on EVERY message of a Gmail thread'), and the phrase 'EVERY message of a Gmail thread' explicitly scopes it apart from the message-level sibling modify_message_labels. An agent can distinguish it from siblings 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?

It gives concrete contextual guidance: pass `account` only when multiple Gmail accounts are connected (routing to list_accounts), and confirm with the user for anything beyond routine triage. It does not, however, explicitly state when to prefer the message-level sibling, leaving that differentiation to inference from the scope phrase.

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