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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real context beyond that: the 30-day window over which savings are computed and the fact that each item carries an estimated monthly saving amount, which shapes how the agent should present results.

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 sentences, scope front-loaded in the first and category enumeration kept tight. The final sentence is a usage nudge rather than descriptive content and sits a little awkwardly, but it is short and does not bloat the definition.

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 return values need not be explained, and the annotations plus the stated time window cover the read semantics. What is missing is routing: with siblings like estimate_switch, compare_models and get_spend in the same family, the description should say which question this tool answers versus those.

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 the schema-derived baseline of 4 applies; there is nothing about inputs the description needs to compensate for.

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 the resource concretely — savings opportunities over the last 30 days across four specific categories (prompt caching, batch API, cheaper host, cheaper model) each with a monthly estimate. That is clear enough for an agent to know what comes back, but it never distinguishes itself from close siblings like estimate_switch or compare_models, which plausibly return overlapping advice.

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?

There is no explicit when-to-use or when-not-to-use guidance relative to siblings. The closing line 'Test a cheaper model's answers before switching' gestures toward compare_models without naming it, leaving the agent to infer the relationship itself.

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