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

Annotations fully cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is low. The description adds the underlying algorithm and that it returns a weekday name, but the return value is also covered by the output schema, so incremental behavioral value is modest.

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?

Four short, front-loaded sentences with the core purpose first. The trailing 'See list_bundles for related fun calculators' is tangential and adds little, but overall it is tight.

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?

With an output schema present, return values need no explanation, and purpose/method are clear. However the incorrect input description and absence of sibling routing leave gaps for what should be a trivially simple one-parameter tool.

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?

The description claims 'Inputs: day, month, year', but the schema defines a single required field 'date' in YYYY-MM-DD format. This mismatch could push an agent to supply three separate values and actively misleads against the schema's 100% coverage.

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: 'Find the day of the week for any date', with the algorithm (Zeller's congruence) named. It is clearly distinguishable from numeric-date siblings like calculate_days_between or calculate_time_difference, though it does not explicitly contrast itself with them.

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?

'Use for historical dates or birthday checks' gives loose usage context but no when-not guidance and no named alternative among the many date-related siblings. The pointer to list_bundles for 'fun' calculators is a weak side reference rather than real routing help.

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