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

readOnlyHint=true already tells the agent this is a side-effect-free read, so the bar is lower. The description adds the endpoint mapping (GET /api/custom-placeholders) and the scoping of placeholders as global/booking/property, but discloses nothing about pagination, result size, or auth requirements.

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?

Two tight sentences: the scope/return shape first, the API endpoint second. Zero filler and the most decision-relevant information is front-loaded.

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 no parameters and no output schema, the description does the right thing by enumerating the returned fields (key, default value, global/per-booking/per-property), which is exactly the information a missing output schema would otherwise provide. The only gap is that it does not help an agent rule out the similarly named reservation-placeholder 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 tool takes zero parameters, so there is nothing for the description to disambiguate. Baseline 4 applies: the schema is empty and the description correctly avoids inventing parameter semantics.

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 ('List the account's custom message placeholders') and even enumerates the returned fields (key, default value, scope). It is clear what the tool does, but it never names or contrasts the easily-confused sibling smoobu_get_reservation_placeholders, leaving the account-wide vs reservation-scoped distinction to inference.

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 word 'account's' implies this is the account-level list rather than a per-reservation one, which is the main usage decision an agent faces here. However, there is no explicit when-to-use statement and no named alternative, so the guidance remains implied rather than stated.

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