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

Annotations already cover readOnly, idempotent and openWorld; the description adds cost ('Free, and fast') and the crucial economic context that all other non-signal tools consume credits. That's real value beyond structured fields, though it doesn't discuss whether the balance is workspace-scoped or cached/staleness.

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?

What it returns leads, then cost/latency, then the decision guidance. Every sentence earns its place — no filler, no restating the title verbatim.

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 values needn't be explained, yet the description still names the three figures. For a trivial 0-param tool the description gives everything an agent needs: cost, latency, and when to interleave calls.

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?

Zero parameters, so baseline 4 applies; the description correctly doesn't waste words on inputs and instead explains what the (absent) parameters don't scope, i.e. a single global balance.

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 ('Get the current Datamagnet credit balance') and enumerates the returned fields (remaining, total, used). It's clearly distinguishable from every sibling, which are all search/get-profile/signal 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?

Explicit when-to-use: before a run making many paid calls, and again after a tool reports running out. It also names the exception set ('every other tool except the signal tools spends credits'), which routes the agent correctly against the 18 siblings.

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