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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds meaningful value beyond those: the authentication requirement (Authorization: Bearer or X-API-Key header, key minted via the bootstrap script), the guest-call restriction, and the month defaulting to current UTC. No contradiction with the annotations exists.

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?

The core purpose sentence is well-structured and informative, but the two opening sentences about API-key onboarding (sign-up URL, header format, bootstrap script) are verbose and tangential to the tool's core function. They earn their place as needed auth context, but could be tightened. The purpose is front-loaded after the auth preamble.

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 report with one optional parameter and no output schema, the description is thorough: it conveys the full return content, month default behavior, the REST endpoint, and the auth requirement. Nothing an agent needs to call it 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% — the month parameter is already documented in the schema as "Optional month filter (YYYY-MM); defaults to the current UTC month." The description adds only marginal clarity by noting a prior month can be read, which mostly restates the schema. Per the baseline rule, 3 is appropriate since the schema does the heavy lifting.

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 states a precise verb+resource: "Month-to-date spend summary for the calling owner" and enumerates the exact return fields (total USD, purchase count, top merchants, current rules, remaining budget). This clearly differentiates it from its siblings spend_guard_record_purchase (a write), spend_guard_set_rules (configuration), and spend_guard_check_purchase (a single-purchase check). An agent can tell this is the read-only aggregate report tool.

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 gives clear usage context: it is a month-to-date summary, the month parameter reads a prior month, and it defaults to the current UTC month. It also states it is "Not guest-callable" and requires an API key, which guides when the tool can be used. It does not explicitly name sibling alternatives and when to choose them, but the report-vs-action distinction is evident from the content.

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