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

Beyond the readOnlyHint annotation, the description discloses billing scope behavior ('IN A SHARED TEAM WORKSPACE A MEMBER SEES THE PLAN AND THE BALANCE ONLY') and the correct attribution for missing card/auto-reload data. It also reveals the precise pricing semantic (use plan.priceUsd per plan.period, not plan.monthlyUsd). This adds deep context the annotation cannot express. No contradiction with annotations.

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 description is longer than average, but every sentence contributes critical operational guidance: the exact price fields, the admin-rights check, the member-scope restriction, the warning about mail cards. It is front-loaded with the core purpose. A few extra words add safety value, so conciseness remains strong despite length.

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 fully enumerates the returned fields (plan, balance, auto-reload, card, admin rights) and the privacy constraints. It also provides decision-relevant info (when to call, what to expect for members). An agent can use this tool correctly without any additional context. Truly complete.

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 input schema is an empty object with 0 parameters, so schema coverage is trivially 100%. Per the baseline for 0 params, a score of 4 is appropriate. The description doesn't need to explain parameters since none exist; it correctly focuses on output semantics instead.

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 and resource ('Show this account's billing at a glance') and enumerates exact fields (plan id/label/ACTUAL price, credit balance, auto-reload, card on file, admin rights). It distinguishes itself from siblings like upgrade_plan and set_auto_reload by explicitly positioning itself as the pre-check for those. No ambiguity about what the tool does.

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 description gives explicit when-to-use guidance ('Call it before upgrade_plan / set_auto_reload to know what's possible') and even documents who can use it for what (members have read-only billing). It also provides negative guidance: never claim a member has no card, just say you can't see it. This is far beyond typical descriptions.

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