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 safety profile is well covered. The description adds value by clarifying there is no financial side effect (no purchase authorization) and no sensitive wallet exposure, which is meaningful given payment-related siblings, but it does not add further context such as data freshness or behavior when credits are exhausted.

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?

Two sentences, each pulling weight: the first states the core function, the second draws a safety boundary relevant to the sibling set. Nothing is redundant or extraneous.

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 parameterless, strongly annotated, read-only inspection tool, the description is nearly complete: it names the exact scope (credits and storage) and excludes the relevant side effects. The only gap is that no output schema exists and the description does not hint at the return shape, leaving the agent unsure whether the response delivers the balance figure it needs.

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?

With zero parameters, there is nothing the description needs to document; the empty schema and 100% coverage fully define the invocation surface. The description correctly implies the tool takes no arguments by not referencing any. Baseline 4 applies.

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 uses a specific verb and resource ('Inspect remaining prepaid credits and current storage'), clearly stating what the tool does. The negation 'Does not authorize a purchase or expose a wallet' helps separate it from purchase/pricing-related siblings like mithrandir_onboarding_pay and mithrandir_quote, though it does not distinguish it from other inspection tools such as mithrandir_inspect or mithrandir_state.

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?

The 'Does not authorize a purchase' clause implicitly tells an agent this tool is for checking status before financial actions, not for performing them. However, it never names an alternative tool to use for purchases nor gives any condition for choosing this tool over the inspect/state/search siblings, so routing is left to inference.

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