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

Annotations already declare readOnly, idempotent, non-destructive and open-world behavior, so the bar is lower; the description nonetheless adds meaningful context the annotations lack — live third-party provenance (ProPublica Nonprofit Explorer) and explicit scope disclaimers about ranking and tax/legal advice. It does not cover edge behavior such as a nonexistent or malformed EIN, which is the main remaining gap.

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-loaded with the usage trigger, then a compact inventory of attributes and a scope disclaimer in two sentences with no filler. The long embedded field list is dense and partly duplicates the output schema, which costs a bit of economy but still reads as purposeful rather than padded.

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?

For a one-parameter, read-only lookup with full annotation coverage and an output schema, the description supplies source, scope, and disclaimers, which is close to sufficient for correct invocation. The missing piece is failure semantics (what happens for an unknown or invalid EIN), which an agent would want before calling.

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% with a single required parameter whose format (9 digits, optional hyphen, example 53-0196605) is fully documented in the schema. The description only restates 'by EIN' and adds no format, lookup, or normalization detail beyond it, so the baseline of 3 applies.

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 ('check a US nonprofit by EIN') and enumerates exactly what the profile contains (IRS subsection, foundation status, deductibility, ruling date, multi-year Form 990 financials, filing links, export formats). The single-EIN profile framing plus 'does not rate or rank organizations' clearly separates it from the sibling compare_nonprofits and search_nonprofits tools.

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?

Opens with an explicit trigger ('Use this when the user wants to check a US nonprofit by EIN') and adds scope exclusions ('does not rate or rank', 'no tax or legal advice'), which is real when-to-use guidance. It never names compare_nonprofits or search_nonprofits as alternatives, so the routing decision is left partly to inference.

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