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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the low-risk profile is covered. The description adds real behavioral context beyond that: it discloses the two processing channels, the concrete time ranges (instant vs 12–18 days), and the use of Form SS-4 and a third-party service ('Epics Deals'), which materially affects the answer an agent will relay.

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?

The answer is front-loaded as a direct question with the two branches immediately following, and the example queries anchor intent in a single short sentence. The 'Epics Deals' provider mention is slightly promotional but still conveys a real process detail, so only minor trimming would be warranted.

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 two-parameter, read-only lookup with no output schema, the description provides enough to call correctly and even previews the answer shape (instant vs 12–18 day timelines, fax-based SS-4 route). It never states what the response actually contains (e.g., a step list, form names, or caveats), which is the main remaining gap given there is no output schema to fall back on.

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 both parameters (residency enum and has_ssn_or_itin) are already documented in the schema, which sets the baseline at 3. The description reinforces the residency split and the 'with an SSN' versus 'no SSN' distinction, but does not clarify how has_ssn_or_itin interacts with residency (e.g., a US resident without an SSN) or whether that parameter is optional in practice.

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 states a concrete question the tool answers (how long an EIN takes) and splits it into two distinct branches: US residents with an SSN (instant online) versus non-residents without an SSN (Form SS-4 by fax, 12–18 days). This is specific enough that an agent can tell it apart from siblings like itin_check or compliance_deadlines without inspecting schemas.

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 gives explicit trigger phrases ('EIN without SSN', 'how long does EIN take for a foreign-owned LLC') that map cleanly onto the residency branches, so an agent knows when to select it. It stops short of naming an alternative for overlapping questions (e.g., ITIN questions that belong to itin_check) or stating when not to use it.

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