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

Annotations already declare the safety profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false, openWorldHint=false). The description adds the copy-source behavior and format conventions but never clarifies whether existing plan entries are overwritten or merged, which is important for a 'fill' mutation.

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 compact sentences, front-loaded with the core purpose. The minor-units/date-format sentence is somewhat tangential since neither parameter is an amount, but it is brief and still useful for interpreting copied values.

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 two-parameter mutation tool with no output schema, the description covers purpose and value formats. The notable gap is overwrite/merge semantics on the destination month, which an agent needs to call it safely.

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 description coverage is 0%, so the description must carry the load. It explains 'kind' (expense or income) and clarifies 'period' as a YYYY-MM budget period, which meaningfully documents both parameters. It leaves some ambiguity about whether 'period' is the target month being filled.

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 specific verb+resource+source: 'Fill a budget month's plan (expense or income) from the month before.' An agent can immediately grasp the copy-forward operation. However, it does not name a sibling (e.g., set_budget) to differentiate from, so it falls short of a 5.

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 no when-to-use or when-not-to-use guidance and does not mention alternatives such as set_budget. The only hint is the implicit source ('from the month before'), which describes mechanics rather than decision context.

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