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

readOnlyHint=true already tells the agent this is a safe read, so the bar is lower. The description adds the underlying REST mapping (GET /users/{user_id}), which is mild context, but says nothing about not-found behavior, error responses, or result freshness.

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 primary purpose, with the API mapping relegated to a secondary clause. No padding or repetition.

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 one-parameter read tool whose safety profile is covered by annotations and which has no output schema, the description is nearly sufficient. The only real omission is behavior when the user does not exist, which an agent would likely want to know.

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% and the single parameter is fully documented as 'MagicBell user id or external id'. The description restates that same fact rather than adding format, prefix, or resolution-order detail, so 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 (fetch) and resource (a single user) and names the lookup key, which distinguishes it from list_users and create/update_user. It does not explicitly name the sibling it replaces, but scope ('a single user') makes the boundary inferable.

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 implied by 'by MagicBell user id or external id' — an agent understands it needs an identifier — but there is no explicit when-to-use guidance, no statement that it returns nothing for unknown ids, and no mention of list_users as the alternative for enumerating users.

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