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

Annotations already establish that this is a safe, read-only, idempotent operation, and the description still adds real substance: semiannual compounding per $25, TreasuryDirect as the rate source, and the 3-month interest penalty applied to bonds held under 5 years. It also sets expectations that results are an estimate that can differ from Treasury values by cents, which an annotation could never convey.

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, both front-loaded with the core action and the two return values, followed by the methodology and the accuracy caveat. No filler and no repetition of schema or annotation content.

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 carries the return-value burden and does so by naming both outputs: total accrued value and redemption value after the penalty. Combined with the compounding method and the precision caveat, nothing material is missing for a three-parameter read-only estimator.

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?

Schema coverage is 100%, so the baseline would be 3, but the description adds evaluative meaning beyond the schema: valuation is anchored to "the start of a month" and the output depends on holding period relative to 5 years. That clarifies how issue_month and as_of_month interact in a way the field descriptions do not.

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 uses a specific verb ("Estimate") on a specific resource ("what I bonds are worth") and names the two inputs that drive it, an issue month and a purchase amount. That clearly separates it from the sibling tools, which all deal in rates rather than dollar values.

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?

It scopes the calculation ("at the start of a month", "bonds under 5 years") but never says when to pick this over calculate_i_bond_composite_rate or the rate-lookup siblings, and gives no prerequisites. Usage is implied by the tool's phrasing rather than stated.

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