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

Annotations already establish readOnly/idempotent/non-destructive, so the safety profile needs no restating. The description adds genuinely non-annotated context: the module must be enabled and all money is denominated in NZD. It stops short of stating caching or freshness behavior, but this is meaningful added 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?

Front-loaded with the resource, then a compact return-content enumeration followed by two short factual sentences on currency and prerequisite. The first sentence is long but every clause enumerates a distinct output field, so little is wasted.

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?

There is no output schema, so the description must carry the return shape, and it does: opening balance, monthly planned vs saved, IRD/ACC payments, projected tax/ACC levy, amount still due, and whether savings are ahead. Combined with currency and the module prerequisite, nothing an agent needs to call and interpret this 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%, so tax_year (with its end-year convention) and organisation_id (with its required-when-multi-org rule) are already fully documented. The description only echoes the tax-year scope and adds no format or syntax beyond the schema, so the 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 specific verb+resource (the savings plan for one NZ tax year) and then enumerates exactly what the plan contains: opening balance, monthly planned/saved amounts, IRD/ACC payments, and taxapp's tax/ACC workings. This is clearly distinguishable from sibling update_savings_plan and copy_savings_plan_from_previous_year without opening any schema.

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?

It discloses a real prerequisite ('Needs the savings module switched on'), which tells the agent when the call will fail, but it gives no guidance on when to read the savings plan versus get_tax_summary or get_tax_year_overview, nor does it name any alternative. Usage is only implied by the domain.

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