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

Annotations already establish readOnly, non-destructive, closed-world behavior, and the description adds the one behavior that matters and is not obvious: an omitted rate silently falls back to a labelled 7% teaching default rather than a lender rate. That disclosure prevents the agent from presenting the output as a real quote. It does not describe what the comparison actually returns, though with no output schema that is a secondary gap for a read-only calculation tool.

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?

Three short sentences, front-loaded with the trigger condition followed by the exclusion and the default behavior. No filler and nothing buried.

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?

The usage boundary and the rate default are well covered, and annotations carry the safety profile. However, with no output schema present, the description never indicates what the comparison returns (figures, a table, savings vs. cost), which an agent needs in order to describe results to a caller.

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 description coverage is 100%, so every parameter including the 7% default for illustrative_annual_rate_percent is already documented in the schema. The description's mention of the 7% teaching default restates the schema rather than adding new meaning, so a baseline 3 is appropriate.

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 first sentence names the specific comparison being performed (cash vs. loan vs. adding a battery later) and the constraint that it operates on figures the caller already has, which separates it from estimate_bill_savings and the explain_* siblings. It stops short of naming a sibling tool explicitly, so the differentiation is inferential rather than stated.

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?

Gives both a positive trigger ('Use this when comparing...') and a clear exclusion ('Do not use this to offer a rate or say they should borrow'), which is stronger guidance than most definitions provide. It does not name which sibling to route to when the caller does want a rate discussion, leaving that inference to the agent.

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