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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds the non-obvious indexing convention (Monday=0), which is genuinely useful, but says nothing about timezone assumptions or behavior on invalid/out-of-range dates.

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?

One short sentence that front-loads the purpose and tucks the output convention into parentheses. No filler, nothing to trim.

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 single-parameter pure function with a 100%-documented schema and an output schema, the description is nearly sufficient. Minor gaps (timezone handling, invalid-date behavior) remain, but nothing that would stop an agent from calling 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 description coverage is 100%: the single 'date' parameter is documented with a 'YYYY-MM-DD preferred' description and an example. The description adds no further parameter meaning, 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 ('Day of the week for a given date') and even defines the output encoding (Monday=0 … Sunday=6). It does not differentiate from near siblings such as calculator_days, calculator_date_difference, or calculator_leap_year, so an agent comparing them gets no disambiguation.

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?

Usage is only implied by the name and description — take a date, get a weekday. There is no explicit statement of when to prefer this over calculator_days or calculator_date_difference, and no note about date-format prerequisites beyond the schema.

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