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

With no annotations provided, the description carries the full burden and does so well: it discloses a 1 CRD cost per call, that the charge applies even to rejected calls, the 429 RATE_LIMITED exception where nothing is charged, the 404 USER_NOT_FOUND outcome for missing lookups, the result ordering (newest first), and that errors carry a request_id to quote to support. That is far more than the structured data supplies.

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 dense sentences, each earning its place, with the two modes front-loaded and cost/error behavior following. No filler, no restatement of the tool name, and the parenthetical error codes are compact rather than bloated.

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 9-parameter read tool with zero annotations and no output schema, the description covers modes, ordering, filtering, cost, and error contract thoroughly. What remains thin is the shape of a returned user record and the auth/x402 path, though the latter is documented per-parameter in the schema.

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 is 3, but the description adds mode semantics the schema does not: that email/external_id select a single-record lookup rather than acting as filters, and that signup_host / signup_credit_type_id apply only to the list mode with newest-first ordering. It omits any mention of x402_payment or the api_key/project_id auth params, which the schema does cover.

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 opens with a specific verb and resource ('Look up one end user') and cleanly splits the tool into two modes: single-user lookup by email/external_id and a newest-first listing with filters. The purpose is unambiguous, but it never names a sibling (e.g. create_user, bulk_create_users) to sharpen the boundary, which keeps it just short of a 5.

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 states the concrete trigger for each mode: pass email or external_id to fetch one user, otherwise use signup_host / signup_credit_type_id to list. That is genuinely actionable routing guidance, but there are no exclusions or pointers to alternatives when the caller actually wants to create or mutate users.

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