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

Annotations only declare the safe-read profile; the description goes well beyond, enumerating the possible status values, explaining that ends_at means the moment the CURRENT state ceases to be true (with per-status nuance), noting omission when unknown, and disambiguating 'none'+only_hobby as a free account rather than a missing one. That is exactly the interpretive context annotations cannot supply.

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?

Purpose and scope are front-loaded in the first clause, followed by necessarily dense response semantics. Every sentence carries load for a tool with no output schema, though the packed enum/return detail makes it slightly heavy to parse.

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?

With no output schema, the description assumes full responsibility for return values and does so: exact payload shape, all status enum members, ends_at semantics per state, and the free-account edge case. Nothing needed to call or interpret the tool 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?

Zero parameters, so baseline 4 applies. The description confirms 'Takes no arguments', reinforcing the empty schema, but adds no further parameter meaning because none exists.

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?

Names a specific verb+resource ('The account's plan state') and immediately draws the boundary against the sibling site-scoped tools with 'not a site's'. An agent can select this over any site-level tool without opening a 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?

The scope clarification ('account's plan state — not a site's') tells the agent which situations this tool covers versus site-level siblings, and 'Takes no arguments' confirms invocation shape. There is no explicit when-not guidance, but no sibling overlaps on billing/subscription state, so the routing context is effectively clear.

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