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

Annotations already declare non-readOnly, non-destructive, non-openWorld, so the safety profile is covered. The description adds genuinely new behavior: the operation is reversible by re-invoking with the opposite value, and it acts on exactly one flag per call. It omits permission requirements and error behavior, which keeps it short of 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 sentences, front-loaded with the core action and semantics, followed by the undo rule and the two sibling escape hatches. No filler and every sentence carries information an agent 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?

For a single-flag mutation with no output schema, the description covers the action, the value semantics, reversibility, and where to go instead for adjacent tasks. Nothing an agent needs in order to invoke this correctly 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%, so uid, flag, value, folder and mailbox are already documented, including the enum meaning of 'seen' vs 'flagged'. The description restates the same enum semantics (read/unread, star/unstar) without adding format, default, or addressing guidance beyond the schema. Baseline 3 applies.

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 and resource (set or clear one flag on one message) plus the exact scope (single message, single flag). It further disambiguates from siblings by naming label_message and mark_all_read, so an agent can distinguish this from adjacent tools without reading schemas.

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?

Explicitly routes alternatives: free-form tags go to label_message, whole-folder read state goes to mark_all_read. It also states the reversibility condition ('calling it again with the opposite value undoes it'), leaving nothing to inference about when this tool is the right pick.

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