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

With no annotations, the description discloses meaningful behavioral traits: it is free, rollups are kept forever across plans, and results are newest-first. It does not explicitly state read-only behavior or auth requirements, but the read nature is evident from the content.

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, information-dense sentences with no filler. Each sentence contributes distinct value: what it returns, cost, and retention behavior.

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 low-complexity tool with two optional parameters and no output schema, the description covers purpose, ordering, cost, and data retention. It doesn't detail the exact fields returned, but this is not critical for basic use.

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%, with both parameters fully described. The description adds only a loose allusion to the monthly window and 'one agent,' providing no additional syntax or format details beyond the schema.

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?

Clearly states it returns daily spend for one agent over the last month with newest-first ordering. Distinguishes itself from sibling budget tools like budget_status and budget_delegate by focusing on historical spend rather than status or actions.

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?

Provides clear use contexts: tracing cost spikes to their start day and answering historical questions after individual calls expire. Does not explicitly name alternatives or exclusions, but the context strongly implies when this tool is appropriate.

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