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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description only needs to add cost/auth context — and it does: 'Free' and 'secret key' auth for the end-user scope. It does not discuss rate limits or eventual consistency of the balance, which keeps it out of 5 territory.

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 clauses: what it reads, how scope is selected, when to call it, plus a one-word cost note. Zero filler, and the primary purpose is front-loaded ahead of the usage triggers.

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?

An output schema exists, so return-format details are unnecessary here. Between the description and schema an agent knows both scopes, the auth mode each requires, the trigger conditions, and that the call is free — nothing needed to invoke correctly is missing.

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% and already documents external_user_id's type and auth requirement, so baseline is 3. The description goes beyond the schema by clarifying the default (omitting the parameter returns the pooled account balance rather than failing), which is behavioral meaning the schema does not encode.

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 concrete verb+resource ('Read the remaining credit balance') and immediately disambiguates the two scopes it can serve: the pooled account, or a single end user via external_user_id. An agent can distinguish it from the write-side sibling add_credits 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?

Gives two concrete triggers ('before a large spend', 'after an insufficient-credits error') that make the call condition actionable. It also states the auth precondition for the per-user mode. It stops short of naming an alternative tool or stating when not to use it, so it's strong but not exhaustive.

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