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

With no annotations, the description carries the full burden and does so: it discloses the exact statutory formula, the $500 threshold and $400 per-account cap, that the two accounts are never combined, that the operation is deterministic arithmetic, read-only, and requires no key. Return values are also described, so nothing material about behavior is left implicit.

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 purpose, then the rule, then the return format and safety profile. It is slightly long but every sentence carries distinct information (formula, account independence, output figures, determinism).

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, yet the description states what is returned (truncated whole-dollar figure and exact one-third figure), and it supplies the full rule and constraint set. For a deterministic two-parameter computation tool, an agent has everything needed to call it correctly.

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 baseline is 3. The description goes beyond the schema by clarifying the crucial parameter interaction – the Octopus and AlipayHK amounts are evaluated separately and never summed – which materially affects how the caller should supply them.

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 ('Compute the Hong Kong Public Transport Fare Subsidy Scheme amount') scoped to one month and to two named accounts. This is clearly distinguishable from the sibling tools log_trip, monthly_summary, and mtr_fare.

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 makes the input context explicit ('from the expenses recorded under an Octopus account and an AlipayHK account' this month), which tells the agent when this tool applies. It does not name siblings or state when-not-to-use (e.g., versus monthly_summary), so it stops short of a 5.

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