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

The readOnlyHint annotation already declares this is a safe read, lowering the burden on the description. The description adds the account-vs-subaccount scope but says nothing about the return format or whether an empty/low value has special meaning. Adequate but thin behavioral context.

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 short, front-loaded sentences with no filler. The trailing 'FireText: /credit.' is a REST endpoint reference of marginal value but costs little.

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 simple read-only tool with one optional parameter and no output schema, stating what is returned (remaining message credits) covers the essential need. Only the return shape/format is left implicit, which is minor here.

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 'subaccount' parameter is fully documented in the schema. The description's mention of 'or a sub-account' mirrors the schema without adding syntax, default, or omit behavior guidance, so baseline 3 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?

States a precise verb and resource ('Number of SMS credits (messages) remaining') and clarifies scope ('on the FireText account or a sub-account'). None of the siblings return credit data, so an agent can distinguish it immediately.

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 description describes what the tool returns but gives no explicit when-to-use guidance, no prerequisites, and never names an alternative. Implied usage (check remaining balance) is left entirely to inference.

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