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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds genuinely new operational context: "Requires sign-in," which tells the agent about an auth precondition the annotations do not express, plus a summary of the returned state.

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?

Two tight sentences: the first front-loads what the caller gets, the second gives the timing rule and the auth prerequisite. No redundant restatement of the name or title.

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?

An output schema exists, so the description needn't specify return structure, yet it still previews the key fields. Combined with the sign-in prerequisite and the pre-purchase timing guidance, nothing an agent needs to invoke this correctly is missing for a zero-parameter read tool.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The description correctly adds no parameter text and instead spends its words on return content.

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 names a concrete resource (the signed-in buyer) and enumerates the exact fields returned: credit balance, subscription status, owned image count, trial availability. An agent can immediately tell this is the account-state reader rather than any of the entitlement or purchase siblings, though the retrieval verb itself is only implied by the name.

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?

"Call this before proposing a purchase, so you know what they can already afford" gives an explicit trigger condition tied to sibling actions like create_checkout and start_trial. It stops short of stating when not to call or naming those alternatives directly, so it is clear context without full routing.

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