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

Annotations already establish readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds genuine computation semantics beyond that: dates are shifted off weekends and legal holidays, Form 8868 extension applies except for 990-N, and late-filing consequences are included. It omits any note on how the extension is computed or whether the EIN path derives the fiscal year end.

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?

Purpose and returned values are front-loaded, and the two trailing clauses on tax_year and input choice are economical. The opening sentence is heavily comma-loaded with a five-item list, which is slightly dense but each element is substantive rather than filler.

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 burden of describing returns and does so fully: due date, adjusted date, extension, days remaining, penalties. Combined with the tax-year and default-form guidance, an agent has what it needs to call this correctly; only the EIN-to-fiscal-year derivation is left implicit.

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 75% and the description compensates for the one undocumented parameter by defining tax_year precisely (the calendar year the accounting year begins in, defaulting to the latest ended year), which the schema leaves silent. The 'month or EIN' phrasing signals the two alternative lookup paths, but the interaction between ein and fiscal_year_end_month is not resolved.

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 names a specific resource (the Form 990 due date) and enumerates the exact outputs: the due date adjusted off weekends/holidays, the Form 8868 extended date, days remaining, and late-filing consequences. An agent can distinguish this from siblings like which_990_form (form selection) or check_exempt_status without opening any schema.

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?

"Give the fiscal year end month or an EIN" tells the agent exactly what input to supply and implies either identifier path works, which is meaningful since there are zero required parameters. It does not name alternatives or state when not to use it (e.g., versus which_990_form for picking a form type), so it stops short of full routing guidance.

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