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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds genuine behavioral context beyond that: the spend order (plan credits first, then bonus), that actions degrade to bonus credits rather than failing, and that content creation depends on subscription status.

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?

Front-loads the return summary, then unpacks the two credit fields, then closes with the usage call-to-action. Dense but every sentence carries meaning; the em-dash clauses require a second read but are not wasteful.

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 and no parameters, the description correctly takes on the job of explaining what comes back (creditsRemaining vs planCredits vs status) and how to interpret it. An agent has everything needed to call it and understand the result.

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?

There are zero parameters, so there is no parameter semantics to carry; baseline is 4. The description is used instead to define return field semantics (creditsRemaining, planCredits, status), which is a reasonable substitute.

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 ('Returns the signed-in user's plan and credits') and scopes it to the current user, which no sibling tool does. An agent can distinguish it from create_post or get_post_metrics immediately.

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?

'Check this before running actions that cost credits' gives a concrete pre-condition, and the note that content creation needs an active subscription identifies the gating scenario. It stops short of naming specific credit-consuming siblings, but the trigger condition is clear.

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