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

Annotations already declare readOnlyHint=true, so the agent knows this is a non-mutating read. The description adds that the response includes primary and secondary field definitions, which is useful context beyond the annotations, but it omits error behavior for a missing/invalid list_id and any auth or rate-limit notes.

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 short sentences, front-loaded with the action and the returned content. The trailing 'Castle: GET /v1/lists/{id}' is a useful API mapping, though it slightly duplicates the schema's required list_id.

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 read tool with no output schema and readOnlyHint annotations, the description covers what it does and what the response contains. It would be fully complete with a note on missing-id behavior or how it relates to search_list/count siblings.

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% with only one parameter ('list_id'), and the schema already documents it as 'The list's id.' The description adds nothing param-specific beyond what the schema provides, so the baseline of 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 and resource ('Fetch a single list') and adds scope about what is returned ('primary and secondary field definitions'), which is more specific than the bare title. It does not explicitly differentiate from siblings like castle_search_lists or castle_get_list_item, so the sibling-confusion gap keeps it from a 5.

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 gives no when-to-use guidance, no prerequisites, and does not mention alternatives such as castle_search_lists for discovery or castle_get_list_item for items. An agent must infer usage from the name alone.

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